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TET-1 (Technology Experiment Carrier-1)

Overview    Spacecraft    Launch   Mission Status    Experiment Complement   Ground Segment   References

TET-1 (Technologie Erprobungs Träger-1) is a German technology demonstration microsatellite of DLR (German Aerospace Center) within its OOV (On-Orbit Verification) program. Project funding is provided by the German Ministry for Economics and Technology (Bundesministerium für Wirtschaft und Technologie). The overall objective is to provide industry and research institutes with adequate means for the in-flight validation of space technology. Certain programmatic rules were established for the space segment and the ground segment to realize TET-1 as a low-cost mission within a relatively short timeframe under the leadership of an industrial space company as prime contractor. 1) 2) 3) 4) 5) 6) 7)

Flight opportunities for technology demonstration and verification should ideally be provided on a regular basis in a cost efficient and safe manner. A market survey of German industries' and institutes' technologies has shown that about 75% of the experiments can be verified using a microsatellite.

The OOV-Program is thus structured into two main parts with respect to the flight opportunities offered. The first comprises the microsatellites TET with a planned flight opportunity every two years. For payloads which do not fit on TET microsatellite concept, DLR will cooperate with national and international partners to provide flight opportunities on other carriers. 8) 9)

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Figure 1: TET-1 spacecraft: launch configuration (left), deployed without MLI (right), image credit: Kayser-Threde

Mission duration

1-2 month LEOP and commissioning phase
12 months of mission operations

Orbit (circular, LEO)

Altitude range between 450 – 850 km
Inclination: 53º to sun-synchronous

Launch of S/C

As secondary payload on a flight of opportunity

Reliability

0.9 (for platform and payload support system)

Ground station support

Weilheim for TT&C, Neustrelitz for payload data downlink

TET platform

Should be based on the BIRD satellite (max. use of the BIRD heritage)

TET mass

~120 kg (total for spacecraft), 50 kg (payload)

Table 1: Summary of TET-1 main mission requirements of the bus

Spacecraft:

The small satellites in this series are largely based on the flight-proven BIRD (Bi-Spectral Infra-Red Detection) spacecraft bus launched in 2001. At a size of about 65 cm x 55 cm x 88 cm and a total mass of ~ 120 kg, the TET spacecraft can handle a payload of up to 50 kg in an envelope of 460 mm x 460 mm x 428 mm (Figure 2).

In July 2008, DLR awarded the prime contract to Kayser-Threde GmbH of Munich (a company of OHB-System, Bremen) for the space and ground segment as well as for the launch services of TET-1. The microsatellite bus was designed and developed at AstroFein (Astro- und Feinwerktechnik Adlershof GmbH), Berlin in a subcontract to Kayser-Threde.

The modular multimission spacecraft bus consists of three segments: service, electronics, and payload. In the Service Segment a battery, reaction wheels, the PCU (Power Control Unit) and laser gyro are installed. The Electronics Segment contains the SBC (Satellite Bus Computer) and the payload supply system. In the Payload Segment are the experiments and also satellite bus components (star sensors, magnetic field sensors and antennas (low-gain and GPS). 10) 11) 12) 13) 14) 15) 16)

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Figure 2: The envelope of the generic TET spacecraft bus with the empty payload segment on top (image credit: AstroFein)

A special point in the design of the satellite bus was the interface between satellite bus and payload. To support different kinds of missions, the system contains the nominal satellite bus and a PSS (Payload Supply System). This payload supply system is on its payload interface side adaptable to the data (SpaceWire, RS422/485, CAN-Bus, etc.) and power interface requirements, data storage requirements and payload control requirements.

The nominal satellite bus will remain unchanged for different missions, but of course can be adapted in parts, like an upgrade to X-band system if higher data rates are required. The PCBs (Printed Circuit Boards) of the PSS will be adapted for every new payload accommodation.

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Figure 3: Accommodation of the TET spacecraft bus and several S/C and payload components (image credit: AstroFein)

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Figure 4: Top view of Electronics Segment with attached Service Segment (image credit: AstroFein)

The Service Segment contains the PCU, two IMUs, four reaction wheels and two battery stacks. The Electronics Segment comprises the PSS (Payload Support System), the SBC (Spacecraft Bus Computer), the PDU (Power Distribution Unit), data processing boards of the redundant sun sensor system, the redundant GPS and the driver electronics of the redundant magnetic coil system. All of these components of the Electronics Segment are designed as PCBs in Europe Card size (160 mm x 100 mm, max. 15 PCBs for PSS and 15 PCBs for the satellite bus components) and are connected via two backplanes. Additional to that the complete TM/TC hardware is integrated into the Electronics Segment, except the two low gain antennas (for omni-directional communication).

The PSS (Payload Support System) provides the control function and physical interface between the spacecraft bus and the experiments. The PSS consists of three elements which are realized on PCBs (Printed Circuit Board) connected to the backplane:

1) The power supplies responsible for the supply of the experiments and the PSS itself

2) The processor boards that are used to control all PSS activities

3) The I/O boards that provide the interfaces to the experiments.

The functions provided by the software executed on the PSS processor cover the following tasks:

- Control of experiments with minimal built-in processing logic

- Acquisition of housekeeping data of the experiments and the PSS itself

- Acquisition of measurement data of the experiments

- Temporary storage of all acquired measurement and housekeeping data

- Formatting and packaging of data according CCSDS for downlink

- Reception and execution of commands from the spacecraft bus controller

- Control of PSS devices as switches, circuit breakers, etc.

- Management of thermal control of experiments and PSS

- Boot, self-test and software update management

- Failure detection, isolation and recovery, redundancy management.

 

AOCS (Attitude and Orbit Control Subsystem): The AOCS uses a redundant set of star sensors, gyroscopes, magnetometers, and sun sensors for attitude sensing. Actuation is provided by 4 reaction wheels and a magnetic coil system. The main design driving requirements for the fault-tolerant AOCS are: 17) 18) 19) 20)

- Attitude determination <0.5 arcmin

- Attitude control < 5 arcmin

- Pointing stability < 2 arcmin/s

- Position determination < 150 m

- Single failure tolerance at component level

- 21h RAAN (Right Ascension of the Ascending Node) sun synchronous orbit.

The main torque actuators of TET-1 are four precise RWs (Reaction Wheels) in hot redundant tetrahedron configuration. This redundant configuration allows the RWs to work in lower rpm level which leads to less mechanical abrasion. The three internally redundant MCSs (Magnetic Coil Systems) are drivable independently from each other. 21) 22)

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Figure 5: Overview of the AOCS elements of TET-1 (image credit: DLR)

All sensors for attitude and orbit determination are available as redundant systems as well. The ASC (Autonomous Stellar Compass) of DTU is the primary sensor for precise attitude determination (< 24 arcsec) and comprises a cold redundant DPU and two switchable CHUs. The CHUs boresight axes are tilted 70º away from each other to avoid simultaneous sun, moon or earth blinding. - The CSS (Coarse Sun Sensor) system consists of four internally redundant analog sensor heads which are mounted at the outermost area of the satellite's outer surface to ensure maximum signal availability. The analog signal processing board is arranged redundantly as well.

A further reference sensor is the fluxgate MFS (cold redundant) which is needed for magnetic field determination and MCS control input. The angular rate is determined besides the ASC mainly by the cold redundant IMU. - The orbit determination is performed by a cold redundant GPS system. The position and time data obtained by the GPS is introduced into the orbit estimator in the ONS (Onboard Navigation System) module.

The robustness of the AOCS is realized by redundancy of H/W components and a "configuration management system" to handle the redundancy. The autonomy requires onboard failure detection and diagnosis. Another design aspect for robustness is a robust control loop to react against perturbation and component anomaly with sufficient stability margin. Also essential for a seamless filtering and processing of AOCS data is the autonomous monitoring of the "timings" of all AOCS threads and processes. 23) 24)

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Figure 6: Overview of the S/W and H/W components of the AOCS (image credit: DLR)

The AOCS S/W is build up as an object oriented and modular application with a static architecture. The break down into several layers (Figure 7) allows an autonomous and robust handling of each feature and was extensively proven during the BIRD mission. The S/W runs as an application for the real time operating system BOSS (timing accuracy 1 ms) on the SBC.

- The lowest layer consists of the interface modules to the hardware components and the ONS module. The low level data processing and formatting of actuator and sensor data takes place in this layer.

- A layer above resides the so called EPC (Estimator, Predictor, Controller) layer, in which the actual control task takes place. The encapsulating EPC module is a combination of state estimation, prediction (Kalman filter) and state control. The estimator, predictor and controller modules form together with the mode processor the core of the control S/W.

- The topmost layer contains the TMTC (telemetry and telecommand) interface of the AOCS and its components. A very elementary role in the FDIR is taken by the surveillance module, which is responsible for monitoring the thread timing.

The redundant GPS system is referred to as AGPS-1 (Astrofein GPS-1) which is being space-qualified on this mission. AGPS-1 is based on the the Phoenix-Sensor and calculation algorithm of DLR/GSOC and fulfills the following technical specifications: 25)

- 12 channels

- Code L1 C/A and carrier

- Position accuracy 10 m (3D 1σ)

- Velocity accuracy 0.1 m/s (3D 1σ)

- Time signal accuracy 0.2 µs

- Warm start TTFF < 2 min

- Cold start TTFF < 15 min (90%)

- 1 Hz update rate of navigation data

- Operation temperature -20 to +50ºC, storage temperature -30ºC to +70ºC

- 1.1 W power consumption (for one branch)

- 230 g (for PCB version)

- 160 x 100 x 25 mm (for PCB version).

The system offers a built-in orbit propagator to aid the initial acquisition and to allow a short time-to-first-fix. Due to a separate provided memory for the GPS catalog, the system can make a warm start, also in the case when it was nearly shut down.

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Figure 7: EPC module within a control cycle

EPS (Electric Power Subsystem): The EPS consists of a triple junction GaAs solar array, PCDU (Power Control and Distribution Units), and a NiH2 cell battery of 240 Ah capacity. The PCDU in turn consists of PCU+PDU. The solar array has one fixed panel and two deployable panels. Electrical power of 220 W (maximum power point) is provided. Use of regulated and unregulated power modules.

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Figure 8: Photo of the NiH2 cells and PCDU (image credit: AstroFein)

The SBC (Spacecraft Bus Computer) controls all activities of the subsystems and the satellite bus. The SBC consists of 4 identical boards (2 in hot, 2 in cold redundancy) and watchdog circuits for failure detection and recovery. The architecture of the redundant SBC boards (nodes) is totally symmetric; each board is able to execute all control tasks. One node (the worker) is controlling the satellite while a second node (supervisor) is supervising the correct operation of the worker node. The two other node computers are spare components and are disconnected.

TCS (Thermal Control Subsystem): A semi-active TCS was developed to keep the temperature of the satellite and all of its components within the parameters of normal operating temperatures. It consists mainly of a MLI (Multi-Layer Insulation), heat pipes, the radiator, temperature sensors and heaters. The MLI detaches the satellite thermally from its environment, which means that an exchange can only take place via the radiator. The radiator is coated with a special white paint, which remains stable over long periods of time and has a good absorption-emission ratio. The radiator is mounted on the underside of the satellite, which generally does not face either the sun or Earth.- The heat pipes lead waste heat emanating from the payload directly to the radiator to avoid compromising the satellite bus components. Additional heat can be generated by the heater, should the satellite cool down excessively.

Spacecraft mass

Total: 120 kg; bus: 70 kg; payload: ~50 kg

Spacecraft size (envelope)

880 mm (H) x 580 mm (W) x 670 mm (L)

Stabilization

3-axis stabilized

Pointing accuracy

2 arcmin (5 arcmin required for TET-1)

Pointing knowledge

~ 10 arcsec

Jitter

~ 12 arcsec/sec (2 arcmin/sec required for TET-1)

Position knowledge

10 m

Alignment of payloads or solar arrays

Sun, Earth, nadir, zenith, in flight direction and deep space

Payload power consumption

20 W (for TET-1, up to 80 W with adapted radiator for follow-up missions)

Payload peak power

160 W per 20 minutes

Bus voltage/max. current

20 VDC ( min. 18 V; max. 24 V) / 8 A

Data rate

4 kbit/s (uplink), 2.2 Mbit/s (downlink, when S-band is used)

Bus reliability (14 month mission)

0,95 (0,92 required for TET-1)

Table 2: Overview of key parameters of the TET spacecraft bus

RF communications: An S-band system is used with hot redundant receivers and cold redundant transmitters. The downlink transmission rates are 137.5 kbit/s or 2.2 Mbit/s. The transmitter and receiver channels are redundant and can be switched to the omni-directional low gain antenna system or to the high gain antenna.

The system can emit the telemetry via the redundant transfer switches and the omni-directional low gain antennas, or the directional high gain antenna. The entire system is designed in accordance with the international CCSDS standard (Consultative Committee for Space Data Systems), and in its current configuration allows uplink speeds of 4 kbit/s and downlink speeds of 2.2 Mbit/s.

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Figure 9: Block diagram of the TM/TC subsystem (image credit: AstroFein)

In 2010, the TET bus is a commercially available product of Astro- und Feinwerktechnik that is offered to the microsatellite community. Due to market requests there are plans for a bus upgrade to include the following items (Ref.10) :

- Higher payload mass (≥ 70 kg) and increased envelope

- More power for the payload

- X-band downlink for payload data

- Propulsion subsystem (as optional piggyback system, will be part of payload mass).

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Figure 10: Photo of TET-1 in January 2011 prior to shipment to the launch site (image credit: Kayser-Threde)

 

Launch: The TET-1 spacecraft was launched on July 22, 2012 as a secondary payload to the Kanopus-V-N 1 primary spacecraft of Roskosmos/Roshydromet/Planeta on a Soyuz-FG Fregat launch vehicle. The launch site was the Baikonur Cosmodrome, Kazakhstan. The launch provider was Starsem. 26) 27)

Note: The TET-1 spacecraft has been ready for launch since the beginning of 2011. The last launch date for Kanopus-Vulkan was scheduled for September 2011. However, the Soyuz launch vehicle experienced a failure on August 24, 2011, carrying a Progress M-12M capsule filled with supplies for the ISS (International Space Station). This event resulted in a failure investigation and in a follow-up delay of all planned Soyuz launches. - In particular, the long launch delay was due to the still incomplete readiness status of the primary spacecraft, a fate often experienced by the secondary payloads.

The secondary payloads on this flight were:

• BelKa-2 (Belarusian space apparatus-2), a minisatellite imaging mission of Belarus (NASRB) with a mass of ~ 400 kg

• TET-1 (Technologie Erprobungs Träger-1), a technology probe of DLR, Germany with a mass of 120 kg

• Zond-PP, a microsatellite of IRE (Institute of Radiotechnology and Electronics), Moscow, Russia for technology demonstrations.

• exactView-1 [ formerly ADS-1B (AIS Data Services-1B)], a communicvation microsatellite with a mass of 100 kg, [AIS (Automatic Identification System) application] of exactEarth (COM DEV), Canada.

Orbit: Sun-synchronous circular orbit, altitude of ~ 510 km (of TET-1), inclination = 97.8º, period = 98 minutes, LTAN (Local Tine on Ascending Node) = 11:27 UTC.

Kanopus-V-N 1, BelKa-2 and TET-1 were released into an orbit of ~ 510 km. Afterwards, Fregat had to maneuver to a higher ~800 km orbit to deploy Zond-PP and the exactView-1 payloads. All satellites were successfully deployed and Fregat made a deorbit maneuver.

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Figure 11: Photo of the Soyuz-FG Fregat payloads prior to launch (image credit: Roskosmos)

 


 

Mission status:

• October 30, 2015: Indonesia is on fire – the island state is currently facing a bitter struggle against forest and peat fires on Sumatra and Borneo, most likely caused by illegal 'slash and burn' farming to clear the land for palm oil or timber plantations. The extremely dry conditions resulting from the El Niño weather phenomenon exacerbate this problem. Reliable information on the source and spread of the fires is needed to initiate suitable countermeasures and to estimate the environmental consequences. Although satellite images can provide this information, the thick clouds of smoke concealing the exact locations of the fires often prevent them from delivering sharp pictures. This is why the microsatellite TET-1 (Technology Testbed-1), built by DLR, has been deployed. "The high sensitivity and spatial resolution of the camera installed on TET-1 allows us to detect and monitor very small fires, even through thick smoke," says Eckehard Lorenz from the DLR Institute of Optical Sensor Systems and Principal Investigator of the mission, who was involved in the development of the camera. 28)

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Figure 12: The DLR TET-1 microsatellite delivers detailed images of the fires in Indonesia, acquired on Sept. 24, 2015, as shown in the inset (image credit: DLR)

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Figure 13: Top: A series of TET-1 images of fires in Indonesia, acquired in September and October 2015; Bottom: equivalent MODIS Hotspot events (image credit: DLR)

- The fires in Indonesia are emitting large quantities of carbon that shroud the neighboring countries of Malaysia, Singapore and Thailand in smoke and obstruct fire detection from space. The Pollutant Standards Index indicates the quantity of atmospheric contaminants, and it clearly shows how dangerous the particulate pollution is for humans and the environment – the Province of Kalimantan logged a value of 1800 at the start of October, with 300 considered the threshold for seriously harmful air pollution.

- Although satellites operated by NASA, such as Aqua and Terra, can use their MODIS (Moderate Resolution Imaging Spectroradiometer) instruments to detect irregularities in fire and heat in their satellite images, the irregularities themselves must be sufficiently strong for the sensors to detect them. In some cases, clouds and smoke will influence or prevent the identification of fires. This is where DLR's TET-1 satellite can provide a solution: "What makes TET unique is the special design of its instruments. The two infrared cameras are able to detect and analyze objects on Earth using infrared radiation," explains Lorenz.

- Although the microsatellite cannot cover the same area as a larger satellite, its flexible control compensates for this to a certain degree. TET-1 delivers comprehensive data that scientists like Florian Siegert, a biologist at the GeoBio-Center at Ludwig Maximilian University in Munich, use in their work. Siegert analyses the impact of peat fires on the global climate and advises organizations such as Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) and the World Wildlife Fund (WWF) about the ecological state of Indonesia. His analyses of time series images help to identify the source of fires, their expansion patterns as they spread, as well as the speed at which the hotspots move. Researchers analyze the temperatures of the fires to detect their potential spread and to calculate their thermal output. They can therefore determine whether they are looking at a low-energy peat fire or a 'hot' fire spreading across vegetation. "The TET data and other field measurements conducted on the ground allow us to estimate the greenhouse gas emissions associated with peat and forest fires with much greater precision than before. This is an important contribution to researching the causes of climate change," explains Siegert. Therefore, a combination of two or more TET satellites could deliver important information and provide an early warning system for fires breaking out in the tropical belt.

- TET-1 has now become a part of the FireBIRD mission, which in the future will use two satellites working in tandem, orbiting Earth to acquire remote sensing data for the detection of fires. Their task is to identify and measure high temperature incidents and to deliver remote sensing data for scientific research conducted by DLR and other partners. DLR will launch the BIROS (Berlin Infrared Optical System) satellite in April 2016 to achieve these goals and will work with TET-1 to provide the necessary data. In particular, BIROS will be tasked with transmitting information on currently active or nascent fire sources directly to mobile communications devices. The satellite is equipped with a propulsion system that can be used to alter its constellation position with TET -1 to enable repeated imaging of a specific location from various angles (Ref. 28).

• May 2015: The TET-1 mission is operating 'nominally' in 2015 , completing its 3rd year on orbit in July. - More than 10 years after the launch of DLR's first small satellite BIRD, a follow-on project, called FireBird was started. Based on the success of the BIRD mission, the main scientific goal- the investigation of high temperature events and their impact on the climatic processes- will be continued, but in consideration to the advantages given by the operation of a constellation of two small satellites. The first of these satellites — TET-1, was launched on June 22, 2012. The launch of the second satellite, BIROS (Bi-spectral Infrared Optical System), is scheduled for spring 2016. 29)

• Since November 2014, the OOV-TET1 Satellite is operated by DLR as part of the FireBird mission. The detection of high temperature events is one of the main features of the TET-1 satellite. Therefore, the focus lies on the detection of fires and events with temperatures around 1000 K. In order to avoid saturation for events with high energy, two samples per ground pixel are taken, with the second sampling is performed using a shorter integration time. With TET-1, it is even possible to detect fires with areas much smaller than the actual ground mapped pixel size. 30)

To achieve this sub-pixel accuracy, it is necessary to have at least two distinct infrared bands which are provided by TET. One band is sensitive in the mid infrared range (3.4 to 4.2 µm) where typical fires have their radiative maximum. The second band required is sensitive in the thermal infrared range (8.5 to 9.3 µm), providing the background temperature. Using these two infrared bands as input data for the dual-band approach, the effective temperature and the effective sub-pixel area of a detected high temperature event can be estimated simultaneously. Additionally, the red band of TET is used for basic classification for background pixel assessment and false alarm rejection e.g. due to sun light reflections.

Before applying the dual-band approach, the three different spectral bands have to be co-registered and potential fire pixels have to be identified. The red band and the thermal infrared band are mapped to the mid infrared band by a half-automatic method. A rough manual mapping is refined by a cross-correlation approach. After this, the three bands are co-registered with a precision of less than a pixel. Then, background pixels are determined by interpreting these three bands, e.g. classify and exclude clouds, sunglints, and hot objects. These background pixels define thresholds which are used to identify fire pixels. After clustering neighbor fire pixels, the dual-band method estimates fire temperature and area for all found fire clusters. Furthermore and finally, the fire radiative power (FRP) and geographic coordinates are calculated for every fire cluster. Since the active fire front covers often only a small fraction of a pixel less than 10% it is essential to have a good estimation of the background temperature. Canopy layers, e. g. forests and bush lands are often very heterogeneous, which can lead to larger margins in fire parameter estimations especially in the case of small fires.

- Validation activities: In order to obtain a correct interpretation of the radiances, it is essential to compare the fire parameters derived from the observations with parameters obtained from ground based measurements under defined conditions. For this, a test fire design has been made and deployed at the calibration side DEMMIN® (Durable Environmental Multidisciplinary Monitoring Information Network) in north-east Germany. A test burn has been performed on August 17, 2013. The aim of the test was to create a fire at the low detection threshold of the sensor with a well specified dimension and a homogeneous surface temperature. The estimated FRP, an abstract parameter to describe the fire with respect to biomass consumed, was 2.24 MW compared to 1.36 MW calculated.

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Figure 14: Test fire site in the DEMMIN area (image credit: DLR)

HTE (High Temperature Event) observation, detection and quantification:

In the time that the IR camera system aboard TET-1 has been active, approximately 1450 scenes have been captured containing wildfires and other HTE, which are volcanos and industrial sites, like power plants, offshore gas and oil rigs, refineries, and mine sites. HTE occur in all continents within a wide range of land cover types, from grasslands in South Africa, Eucalyptus forests in Australia, Boreal forests in Canada and even volcanos in Iceland.

- Detection of Hotspots in Australia: The climate in Australia is conducive to the spread of fire across the large regions of the continent. From the hot dry summers (November to March) in the southern temperate zones to the tropical dry season (April to September) in the north, fire events occur across the continent over the entire year. This provides an opportunity for the TET-1 satellite system and the FIREBIRD project in the future, with a target area with the potential to provide a wide range of fire types and sizes, over a wide range of surface land cover varieties.

Figure 15 shows a site west of Sydney and was the site of a catastrophic wildfire commencing between 17th and 20th October and ending on 19th November. This fire resulted in two people dying and 248 buildings destroyed, with a total cost of $94 million. In total, all of the fire instances in this area during this time burnt over 65,000 hectares.

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Figure 15: Detected fire fronts in Australia on 26.10.2013 (image credit: DLR)

- Monitoring Sea Surfaces Temperatures: Although monitoring of water surfaces is not the designed task of TET-1, a good example of different water brightness temperatures obtained from the LWIR band is shown in Figure 16. The dark blue areas indicate upwelling cold water in the Baltic Sea between Germany and Denmark. In order to separate flat water areas from land additional information is needed, such as a water mask obtained from the NIR band. Brightness temperatures might be also used to detect maritime pollution, e. g. oil spills, since they affect the emissivity of the surface in the thermal infrared range. This subject, however, hasn't been investigated due to a lack of either clear scenes and, fortunately lack of polluted areas to observe.

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Figure 16: Sea surface temperatures on the Southern Baltic Sea, acquired on July 10, 2014 (image credit: DLR)

• TET-1 is operating in 2014. 31)

The TET-1 satellite is powered by nickel-hydrogen (NiH2) batteries during eclipse phases of the orbit. Although unexpectedly confronted with high battery temperatures after launch, the OOV (On-Orbit Verification) mission could successfully be completed.

The operations team became aware of several issues shortly before and after the launch which created some challenges for battery operation. Some battery cells had undergone reversal prior to launch which seems to have created an imbalance between them. Also, the overall temperature within the satellite and that of the batteries turned out to be higher than predicted and the battery voltage slightly exceeded the limit for payload operations.

Regulating the battery temperature was at first attempted by lowering the amount of charge put into the battery. However, this could not be done indefinitely due to worries of battery damage (memory effect) and the constricted charge available for payload operations. Hence, other approaches were employed. The SPRM (Sun Pointing Rotate Mode was one of the methods used. The satellite was reoriented in such a way that the bus radiator was pointed away from the Earth and the Sun by more than 90º. Consequently, its ability to radiate heat away into deep space was improved.

Another method was a different charging scheme called "ratchet charging", known to have been successfully employed on the HST (Hubble Space Telescope). In this method the battery is charged more than once during a Sun phase and the EoC (End–of–Charge) value is raised every week. A software update during the latter stage provided more flexible control over the solar panel strings, allowing the satellite to fly without a pitch offset which was used earlier to lower the voltage.

Note: The NiH2 battery is the operational battery set of TET-1, while the Li-Polymer battery is one of the 11 technological experiments on TET-1.

• In the spring of 2014, the Firebird mission is prepared with most of the focus on the infrared camera. The bottleneck with regard to the execution of payload operations will be the ability to downlink the infrared pictures. Only as many pictures will be taken as the downlink capacity permits. Power is no constraint. In case the temperature becomes an issue again, additional actions could be taken by switching off units such as the star cameras or the magnetic coil system for certain periods.

- The star cameras had already been switched off for a portion of the orbit relatively early in the mission. This was effective temperature wise (~2ºC). However, since the IMU (Inertial Measurement Unit) was then the only source of accurate attitude information and that unit had failed several times throughout the mission it was decided to not continue this course of action. The software update had essentially fixed the IMU issue, though, and therefore the approach could be attempted again. The attitude requirements for the infrared camera will need to be taken into account, though.

- Building on the experience gained from operating GRACE would also be beneficial. At the moment, the battery treatment on that mission is continuously optimized. Although the rationale behind the treatment is understood the data necessary to do the same on TET is not available. If this changes in the future a similar optimization could possibly be performed. Implementing the Grace treatment much earlier in its life on TET could significantly extend its life expectancy.

Table 3: Battery operations for the TET–1 spacecraft 32)

• Starting on November 1, 2013, TET-1 is being used as the first satellite of the FireBird mission of DLR (German Aerospace Center). 33)

All payloads are being operated successfully. During the 3rd TET Customer Days in March 2013, the payload providers presented also their scientific and technology (interim) results. The occasion provided information on the on-going TET-1 mission through current results and feedback from the experimenters. Further improvements to mission operations have been discussed, and a programmatic outlook on future OOV (On-Orbit Verification) missions completed the program.

The FireBird mission consists of the two constructed DLR spacecraft TET-1 and BIROS. The main goal of this mission is the detection and monitoring of so called high temperature events (HTE), e.g. forest fires or other hot spots.

1) The TET-1 spacecraft (launched June 22, 2012) was in the first year of operations used by the OOV (On-Orbit Verification) mission of the DLR Space Administration. After one year in space, the objectives changed to the goals of the FireBird mission, supported by the R&D program of the DLR Directorate Space.

2) The BIROS (Bi-spectral Infrared Optical System) spacecraft, with a launch expected in 2015, is the second satellite of the two satellite constellation FireBird. BIROS is also supported by the R&D program of the DLR Directorate Space. The constellation phase of FireBird is expected to last one year. The project "FireBird" ends in December 2015, but if the satellites are still working fine it is perhaps not the end of the mission.

3) BIROS has the same size as TET-1 (but with a mass of 140 kg) and an IR payload for fire detection. The secondary payloads on BIROS are:

• AVANTI, an optical navigation experiment with BEESat-4, a CubeSat of the TUBerlin. The CubeSat seves as non-cooperative target for optical navigation and proximity operations (100 m to 10 km distance). The ISL (Intersatellite Link) BEESat-4-BIROS permits the usage of the CubeSat GPS receiver as a BIROS "remote device" for AVANTI verification.

• VAMOS (Verification of Autonomous Mission Planning On-board a Spacecraft). VAMOS, a GSOC experiment, will be used to schedule and (re-)command tasks. 35)

• OSIRIS, an experimental optical communication system for small satellites. 36)

• HTW, a high agility experiment.

Table 4: Some elements of the FireBird mission 34) 35) 36)

• The TET-1 spacecraft and its payload are operating nominally in 2013. Since October 16, 2012 TET-1 is fully operational and is undergoing routine mission operations. 37) 38)

• October 16, 2012: The TET-1 satellite started its operational phase after a review board gave its permission. 39)

• August 10, 2012: Continuation of the commissioning phase (testing of all subsystems). The LEOP phase was ended 1 week after launch. 40)

• July 24, 2012: In LEOP (Launch and Early Orbit Phase), DLR/GSOC conducted initial test with the spacecraft subsystems involving in particular the various attitude modes. 41)

 


 

Experiment complement in the Payload Segment:

The payload segment contains the payload itself, parts of the AOCS and one of the low gain antennas. Due to the design the sole mechanical and thermal interface between payload and satellite bus is the payload platform, which allows an easy and fast integration of the pre-assembled payload. - This payload platform can be designed as an optical bench (like on BIRD), but in the TET-1 mission it was not required. 42)

For TET-1, a total of 11 experiments have been selected which are covering a range of technologies. The experiments are developed by the individual providers according to specific TET standards and are thoroughly acceptance tested at Kayser-Threde before integration in the payload segment of the satellite.

• Three different types of next generation solar cells including thin-layer technology (Figure 25).

• Lithium polymer battery

• Two GPS receiver systems

• Sensor bus system

• Demonstration of a picosatellite propulsion system: Aquajet

• Test of a new infrared camera as follow-up of the BIRD camera system

• Test of new computer hardware system

• RF communication system.

Number

Experiment

Payload provider

1

N1 Lithium Polymer battery

ASP Equipment GmbH

2

N2 Flexible thin-layer solar cells

Solarion GmbH

3

N3 Sensor bus system

Kayser-Threde GmbH

4

N7 Picosatellite propulsion system: Aquajet

AI (Aerospace Innovation GmbH), Berlin

5

N8 Next generation solar cells / Azur

Azur Space GmbH, Heilbronn

6

N9 Next generation solar cells / Astrium

EADS Astrium GmbH

7

N15 IR-Camera (Optical payload)

DLR-OS

8

N16 Two-frequency GPS (NOX)

DLR/GSOC

9

N17 HW-BOSS

Fraunhofer-Gesellschaft FIRST, Berlin

10

N18 Keramis-2 (Ceramic Microwave Circuits for Satellite Communication)

IMST GmbH (Institut für Mobil- und Satellitenfunktechnik), Kamp-Lintfort

11

N19 MORE (Memory Orbit Radiation Experiment)

IDA TU Braunschweig

Table 5: Overview of the TET-1 experiment complement 43)

The accommodation of the experiments in the Payload Segment is shown in Figure 20. The accommodation has been driven by the various experiments needs as e.g. viewing directions, unobstructed view angle for antennas, heat dissipation, etc. Thermal analyses, power budget and data handling considerations were performed to establish mission scenarios which are fulfilling the individual operational requirements of the experiments as e.g. number of experiment cycles, durations and timelines which are compliant with the satellite resources. The solar cell experiments have been accommodated on the sun pointing side of TET-1 next to the satellite bus solar arrays (Figure 25).

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Figure 17: Accommodation of the payload onto TET-1 (image credit: DLR) 44)

Data Handling Modules (Ref. 14):

Figure 18 shows the block diagram of the CPU (Central Processing Unit) module and Figure 19 the top side of the CPU board. The central parts are the radiation tolerant LEON-III-CPU and a radiation tolerant system FPGA (RTAX-FPGA). These two components contain with exception of the memory devices and the physical interfaces all essential logic elements.

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Figure 18: Block diagram of the CPU module (image credit: Kayser-Threde)

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Figure 19: Top side view of the CPU module (image credit: Kayser-Threde)

The CPU LEON3-FT, CID-7 type with FPU of Gaisler Research has been used. This is a failure tolerant version of the LEON-3 CPU, which is integrated in an Actel RTAX-2000 FPGA This configuration has a performance of 20 MIPS. The LEON3-FT CPU is based of the SPARC-V8 architecture.

The system-FPGA contains the following functions:

- Memory interface for SDRAM with Reed-Solomon error correction and DMA controller for die image data of the SpaceWire interfaces.

- 2 asynchronous serial interfaces (UART)

- 2 synchronous serial interfaces for the telemetry-IF of the CPU modules

- 1 fast serial interface (SpaceWire) for control of the digital boards

- 2 interfaces for the PPS signals of the CPU modules

- 1 SpaceWire core for the communication with the digital-IO module

- Latch-up protection logic.

The SDRAM memory on the CPU module amounts to 768 MByte and is split into two banks. The useful payload data memory amounts to 512 MByte; the remaining 256 MByte are used for error correction. The CPU module contains two EEPROMS 28LV011 from Maxwell with a memory size of 1 Mbit each. In one EEPROM the Bootloader is located, in the other one the Flashloader. Two flash memory modules with a capacity of 2 GB are on the CPU module. The second one is for redundancy purpose.

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Figure 20: Integrated payload compartment (image credit: Kayser-Threde)

 

PSS (Payload Supply System):

PSS (N6) provides the interface between the satellite bus and the experiments, a fixed and a flexible part that can easily be adjusted to the experiments needs. The overall objective of the experiment are: 45)

• Reduction of mass

• Provision of more flexibility regarding design and integration

• Simplified assembly, integration and test procedures.

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Figure 21: Schematic view of PSS (image credit: Kayser-Threde)

PSS, developed by Kayser Threde, is a modular system consisting of a backplane and set of different types of boards in Europe Card size (160 mm x 100 mm):

- Two processor boards (1 main, 1 cold redundant) used for all PSS activities, for control of the experiments and for communication with the satellite

- One digital and one analog I/O board providing the electrical interfaces to the experiments The range of interfaces comprises serial communication with RS-422, I2C and SpaceWire, general purpose digital inputs and outputs, outputs for the distribution of PPS-signals, high precision differential analog inputs and inputs for temperature sensors of type AD590 or PT1000.

- Five power boards for regulated for unregulated voltages for supply of the experiments and the needs of the PSS. All power channels can be switched on/off independently by relays, all channels are protected by latched current limiters and they are equipped with sensor circuits for monitoring voltages and currents. For TET-1 regulated (6) and unregulated (8) voltages with up to 8 A of current are provided.

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Figure 22: Testing of the payload supply system in a test rack (image credit: Kayser-Threde)

The PSS is accommodated in the Electronic Segment of the satellite bus next to the satellite bus controller. The electronic segment provides space for 12 boards in 3 stacks as shown in Figure 22. Thus, it was possible to integrate one of the experiments, the N17 board within the Payload Supply System. The block diagram (Figure 23) shows the PSS with the various interfaces.

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Figure 23: Block diagram and external interfaces of the PSS (image credit: Kayser-Threde)

 

BOSS (Embedded Operating System design for dependability and for formal verification)

BOSS is a real-time operating system of BIRD microsatellite heritage (launch Oct. 22, 2001) which also provided the original name, namely BOSS (BIRD Operating System 'Simple') of the system. FhG/FIRST [Fraunhofer Gesellschaft/Institut für Rechnerarchitektur und Softwaretechnik (Institute of Computer Architecture and Software Technology), Berlin Adlershof, Germany] is the developer of the BOSS system. The SBC (Spacecraft Bus Computer) of TET-1 employs an an advanced version of the original BOSS system that assumes the transmission, processing, and memory of all data on board the satellite. In addition to data acquisition from individual subsystems, such as the experiments conducted on board or the power supply, it also takes over communications with ground control. With its multitude of interfaces to subsystems, components, and application software, the SBC constitutes a highly complex system with the following features: 46)

• Redundancy: A highly redundant architecture with a quadruple computer node is being used. Each computer node is providing redundant processing and memory structures. To prevent data loss in case of memory errors, each computer node is equipped with a shadow storage. The content of the shadow storage will replace the memory content should an error be detected in a node. The control and communication unit is located in a FPGA (Field-Programmable Gate Array).

• Latch-up protection: Radiation in particular presents a major challenge to computer systems on board satellites. High-energy particles are constantly impinging on the satellite computer's components which can lead to a SEU (Single Event Upset). In a SEU, a so-called bit flip, in which the state of a bit is altered, can cause a malfunction of the affected component. To avoid this, FhG/FIRST is employing radiation-tolerant components as well as a latch-up protection: through continuous voltage metering in various modules of the computer, a short-term shutoff of the affected component is carried out in case of disproportionately high voltage rises, thus preventing destruction.

• EDAC (Error Detection and Correction subsystem): The EDAC of the SBC works according to the monitor-worker principle: two active computer nodes work together and monitor each other. A malfunction is for instance detected by one computer node not sending data at all, or sending obviously false data, that do not match the other computer node's results. The computer node receiving such false data or none at all, detects this and prompts a reset of the respective other computer node. - Furthermore, each computer node can opt for a reset itself when it detects contradictory data. In the extreme case of one or both computer nodes delivering differing results even after a reset, the remaining computer nodes take over control. As long as they are not needed, they form the so-called emergency reserve: during this time, they are inactive and thus not subject to radiation-induced aging.

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Figure 24: Functional block diagram of the TET-1 SBC (image credit: FhG/FIRST)

The microprocessor board has dimensions of 100 mm x 170 mm x 20 mm, a mass of 0.166 kg, a radiation hardness of > 13 kRad (tested), a power consumption of 4 W (max), and a temperature range of -40º to 80ºC.

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Figure 25: Accommodation of the three solar cell experiments (image credit: DLR)

 

NOX (Navigation and Occultation Experiment):

The objective of NOX (N16) is to demonstrate and validate the use of a COTS-based dual-frequency GPS receiver, originally designed for terrestrial use only, can be successfully employed in Low-Earth Orbit (LEO) satellite applications. As a secondary objective, the technical characterization of the receiver is supplemented by the collection of scientific GPS measurements and their use in two key applications: precise orbit determination (POD) and occultation measurements. 47) 48)

The NOX payload comprises a PolaRx2 GPS receiver developed by Septentrio, Belgium. The PolaRx2 receiver is built around the GNSS Receiver Core (GreCo), which represents an advanced version of the earlier AGGA0 chip. It offers a total of 48 channels and can thus track C/A code, P1 code and P2 code for up to 16 GPS satellites. The tracking and navigation software is executed on a MachZ system-on-a-chip computer that includes an i486 core and can be operated at up to 128 MHz.

For use within the NOX experiment, the PolaRx2 receiver is complemented by a dedicated interface board (Figure 26), which performs the power conditioning and latch-up protection and provides the necessary line drivers for serial communication and discrete signal. The total mass of NOX is 1 kg, power consumption of 8 W.

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Figure 26: Architecture of the NOX system (image credit: DLR)

Aside from the electronic box, the NOX payload comprises a switchable pair of GPS antennas, one pointing to the zenith and used for POD, and a second one oriented in anti-flight direction (pointing at the Earth horizon) devoted to occultation measurements. The switching between these antennas is performed via an R/F relay. A dual-frequency low-noise amplifier is used to amplify the signals received by the passive antennas to a level suitable for processing by the GPS receiver.

 

Aquajet:

Aquajet is a small satellite propulsion system designed and developed at AI (Aerospace Innovation GmbH), Berlin in close cooperation with the Aerospace Institute (ILR) at TUB (Technical University Berlin). The objective is on-orbit qualification/verification of the Aquajet system performance on the TET-1 mission. The Aquajet micropropulsion device is an enabling system, small enough to provide its services to future pico- and nanosatellite missions. In particular, the micropropulsion device is an enabler for the positional control of nanosatellite constellations. 49)

The following parameters pertain to Aquajet:

• The resistojet propulsion system is based on environmentally benign ("green") and safe propellants (water & anti-freeze)

• The system is ECSS (European Cooperation for Space Standards) compliant in development and testing procedures

• Instrument size: 100 mm x 100 mm x 30 mm; total mass: ~0.5 kg

• Highly integrated propulsion module including PMD (Propellant Management Device), redundant sensor systems and space qualified interface electronics

• The modular design leads to a flexible, easy-to-handle device capable to support future PnP (Plug and Play) applications

• A customized design of the propulsion module is also possible. An Aquajet version for microsatellites is currently under development (development model finished, EM in preparation)

• The Aquajet ground qualification for TET-1 was completed in 2009. The PFM was delivered in September 2009.

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Figure 27: PFM (Proto Flight Model) of Aquajet (image credit: AI)

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Figure 28: Accommodation of Aquajet on TET-1 (image credit: Kayser-Threde)

 

Optical IR payload:

The optical payload, designed and developed at DLR, consists of an assembly of three pushbroom cameras, one in the VNIR (Visible Near Infrared) range and two imagers in the infrared region. The overall objective is the detection and quantitative analysis of HTE (High Temperature Events) like wildfires and volcanoes. 50) 51) 52)

Time is essential to support most effectively the decisions of fire managers in fire suppression planning, crew mobilization & movement. Therefore, on-board processing of fire front attributes, including geo-referencing and their direct transmission to the user on ground is a challenging task for small satellites, but it shall be technically feasible.

Key procedures for on-board fire detection and analysis are pre-processing and extraction of fire attributes. The pre-processing includes:

- Radiometric correction (using system correction files)

- Inter-channel co-registration (using system correction files), and

- Geo-referencing (using on-board navigational information).

The fire detection and analysis extraction of fire attributes includes:

- Background classification for threshold adaptation: land, water, clouds, sun glints

- Hotspot detection (based principally on the BIRD algorithm)

- Consolidation of hot pixels in hot clusters

- Extraction of attributes of hot clusters, such as coordinates, FRP (Fire Radiative Power) and, optionally, fire line strength, effective fire temperature and area.

Parameter

VNIR camera with 3 CCD-line FPA

Bi-spectral infrared camera with cooled linear detector arrays

Spectral wavelengths

Line 1: 460-560 nm
Line 2: 565-725 nm
Line 3: 790-930 nm

MWIR: 3.4 - 4.2 µm
LWIR (TIR): 8.5 - 9.3 µm

Focal length

90.9 mm

46.39 mm

FOV (Field of View)

19.6º

19º

F number

3.8

2.0

Detector type

CCD-line array

CdHgTe line arrays

Detector cooling

Passive, 20ºC

Stirling, 80-100 K

Detector element size

7 µm x 7 µm

30 µm x 30 µm

No of pixels

3 x 5164

2 x 512 staggered

Data quantization

14 bit

14 bit

Ground pixel width

42.4 m

356 m

Sample width

42.4 m

178 m

Swath width

211 km

178 km

In-flight calibration

None

Use of a removable calibration flap

Data interface

LVDS SpaceWire

LVDS SpaceWire

Data rate

44 Mbit/s (max), 11 Mbit/s (nominal)

350 kbit/s

Data volume

330 MByte/min, 83.4 MByte (nominal)

5 MByte/min

Instrument mass

< 12 kg

Table 6: Specification of the optical payload

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Figure 29: Photo of the optical payload (image credit: DLR)

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Figure 30: Schematic view of the optical payload of TET-1 and BIROS (image credit: DLR)

The on orbit verification of on-board fire detection and analysis will be tested on Germany's first Technologie Erprobungs Traeger (TET-1). The optical payload of TET-1 is of BIRD mission heritage (launch of BIRD in 2001) together with a special signal processor to demonstrate the on-board processing of fire attributes based on the sensor data obtained over wildfires.

BIROS (Berlin InfraRed Optical System) is a follow-on fire detection mission of DLR with a planned launch in 2016. BIROS will be based on the satellite bus as developed for TET-1.

The BIROS sensor system is nearly identical to the optical payload of TET-1. It will be built – as the BIRD and TET-1 sensors which were designed by the DLR Institute of Robotics and Mechatronics / Dept. of Optical Information System (DLR-RM-OS) in Berlin-Adlershof.

 

Keramis-2 payload (Ceramic Microwave Circuits for Satellite Communications):

KERAMIS (Keramische Mikrowellenschaltkreise für die Satellitenkommunikation) stands for the continued R&D activities of an industrial-academic consortium of partners experienced in the design, development, and fabrication of compact microwave modules based on the ceramic multilayer technology LTCC (Low-Temperature Cofired Ceramic) for satellite communications at Ka-band frequencies. The overall objective is the development of innovative and inexpensive components for future applications in multimedia satellite communications. This technology project (N18) is funded by DLR and BMWi (Bundesministeriums für Wirtschaft und Technologie) to be flight-validated on TET-1. 53) 54) 55)

The LTCC technology offers great potential for cost-efficient three-dimensional hybrid-integrated hermetic modules of high functional density incorporating active and passive sub-modules and circuitry. Specific challenges of the project are:

• to control the high structural precision required for industrial components for spaceborne microwave applications

• to balance fabrication issues and microwave performance

• to demonstrate full space qualification.

For these reasons, specific flight experiments have been developed and implemented on board of a test satellite for remote testing in space. A contribution to the competitiveness of the German-European space program as well as the approval of innovative modular microwave design can be considered essential outcomes of the satellite flight.

Three experiments have been developed as payload equipment for OOV (On-Orbit-Verification) in a national R&D LEO satellite (TET-1):

1) Experiment 1: Switch Matrix

The TU (Technische Universität) Ilmenau has developed an LTCC module incorporating a reconfigurable 4 x 4 switch matrix. The double-sided module integrates space-qualified PIN-diode SP4T switch ICs with broadband transmission lines and signal distribution networks. Between 18 and 27 GHz, a complete signal path displays return loss > 20 dB, insertion loss < 6 dB (without amplification), and path isolation > 50 dB. The module measures less than 2.5 cm x 2.5 cm x 1 cm and weighs about 20 g. To monitor performance and reliability on orbit, the flight module will be equipped with a digital controller, redundant signal sources, power detectors, and temperature sensors. The industrial partners of the consortium support the fabrication, implementation and space qualification of the payload module.

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Figure 31: Illustration of the 4 x 4 matrix (TU Ilmenau)

2) Experiment 2: Synthesizer

IMST GmbH (Institut für Mobil- und Satellitenfunktechnik) of Kamp-Lintfort is responsible for the design and development of tow Ka-band (20 GHz) payload boards. These experimental units are composed of modular LTCC components in hermetically sealed housings. Standard LGA or wire bond transitions are applied for HF transitions. The LTCC components are mounted on a multilayer PTFE (Polytetrafluorethylene - Teflon ®) substrate. The heart of the module is a fractional-N synthesizer in BiCMOS and CMOS technology with a SiGe VCO (Voltage Controlled Oscillator). Additional components are hybrid and MMIC (Monolithic Microwave Integrated Circuit) amplifiers, mixers, SPDT (Single Pole Double Throw) switch and power detectors. The modular concept allows the implementation of reliable, flexible and space qualified transceiver units for multimedia satellite applications at low costs. The multilayer LTCC is manufactured by MSE (Micro Systems Engineering GmbH), while RHE Microsystems GmbH of Radeberg processes the housing as well as the assembly and integration techniques.

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Figure 32: Photo of the SiGe fractional-N synthesizer in the LTCC housing (image credit: IMST)

3) Experiment 3: Transceiver

The University Hamburg-Harburg is responsible for conversion of a transmitted analog S-band signal to Ka-band and vice versa; this conversion concept test is the stated objective of the payload module. To this end, different transceiver configurations are designed, which consist of previously developed basic LTCC modules like mixers, signal sources and power amplifiers. The general suitability and versatility of advanced LTCC packaging in combination with hybrid integration methods for high reliability applications will be verified.

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Figure 33: Photo of the flip-chip components on multilayer PTFE (image credit: University Hamburg-Harburg)

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Figure 34: Photo of the open Keramis payload box with 6 experimental boards (image credit: IMST)

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Figure 35: Photo of the Keramis-2 payload (image credit: IMST)

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Figure 36: Block diagram of the TET-1 communications system with the integrated Keramis payload (image credit: IMST)

 

MORE (Memory Orbit Radiation Experiment)

MORE is an experiment of IDA (Institut für Datentechnik und Kommunikationsnetze) at the TU (Technical University) Braunschweig with the following objectives: 56) 57) 58)

• In-orbit measurement of radiation effects in NAND (Not AND logic) flash devices

- Static /dynamic SEUs (Single Event Upsets)

- SELs (Single Event Latchups)

- SEFIs (Single-event Functional Interrupts)

• Measurement on different device types

- 32 Gbit Samsung K9WBG08U1M

- 8 Gbit Micron MT29F8G08AAAWP

• Comparison with available test results at heavy ion accelerators

• Qualification (TRL > = 5) for use of NAND flash devices in future solid state mass memories

- ESA + national missions: EO + science platforms

- Instrument applications: Image buffers for space cameras.

The test philosophy is to subject the specimens to an operational environment which is similar to real mass memory.

• Long-term static error collection in storage mode (biased /unbiased)

• Verification of countermeasures against SEFIs.

The following design features are implemented:

• Single PCB (Printed Circuit Board) layout

• Complete control logic including the processor in one FPGA

• Processor softcore @ 16 MHz

• Reed-Solomon ECC (Error Correction Code) protected processor SDRAM (Synchronous Dynamic Random Access Memory)

• 4 independent memory partitions equipped with

- 96 32 Gbit NAND flash devices (3.375 Tbit)

- 32 8 Gbit NAND flash devices (0.25)

- Total 3.625 Tbit = 464 GByte

• All partitions are directly connected to FPGA

• Each partition with 8 device groups (7 Samsung + 1 Micron) and individual LU protection switch

• Standard RS 422 I/F

• Integrated power converter.

The main features of the software are:

• Bootloader hard-wired in FPGA

• Software upload supported

• Autonomous test operation of NAND flash devices in software

• Autonomous error recording & evaluation.

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Figure 37: Block diagram of MORE (image credit: TU Braunschweig)

An Actel ProASIC AEPE3000L in FG896 ball grid housing contains the whole digital logic. This is mainly a LEON processor with its periphery. The high pin count allows the direct connection of the whole memory array with its 4 data and control busses independent for each memory partition. So, each partition can be switched on and off independently. The width of the data bus of 32 bits is chosen to fit to the bus width of the processor. The LU switches are controlled by the microprocessor.

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Figure 38: Photo of the MORE PFM without cover (image credit: TU Braunschweig)

 

Qualification of Lithium-Polymer battery:

The Li-Po battery test is being implemented by ASP (Advanced Space Power Equipment GmbH) of Salem-Neufrach, Germany. The objective of the experiment is to space qualify the following items: 59)

• Space qualification of German Li-Po (Lithium Polymer) cells

- Cycle stability of the cells

- Operation under vacuum conditions for 1 year

• Space qualification of ASP () Battery Management System

- Balancing of the Li-Po cells

- Monitoring of the relevant battery data.

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Figure 39: Photo of the PFM battery during integration (image credit: ASP)

Battery parameters:

- Nominal voltage: 14.4 V

- End of charge voltage: 16.0 V

- Rated capacity: 7.5 Ah

- Rated energy density: 85 Wh/kg.

A redundant battery management system is provided to monitor all aspects of battery operations.

 

TFSC (Thin Film Solar Cell) experiment:

The TFSC test is developed by Solarion AG, Leipzig, and HTS GmbH, Coswig, Germany (actually a larger partnership from an academic and industrial background). The objective is to use flexible CIGS (Copper Indium Gallium Selenide) cells on a plastic substrate for power generation and to test their performance in the space environment. Of all thin film technologies, those based on CIGSe have the highest potential to reach attractive photovoltaic conversion efficiencies and combine these with low weight in order to realize high power densities on solar cell and generator level. The use of a flexible substrate ensures a high packing density. The demonstration of this enabling technology in space is of great interest because it offers possible applications in such niches as for space power satellites, solar sails or exploration missions. 60) 61)

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Figure 40: General layout of the TFSC experiment (image credit: Solarion AG)

 


 

Ground segment:

The TET-1 spacecraft is being monitored and controlled at DLR/GSOC using the DLR-owned S-band ground stations in Weilheim, Neustrelitz and SKT (Saskatoon) of CSA (Canadian Space Agency), the latter for the LEOP phase only. A PDC (Payload Data Center) will be provided at the location of the data reception station in Neustrelitz. The main tasks are acquisition, extraction, processing (formatting to a generic ASCII format) archiving and provision of payload data. The 11 users with their experiments onboard TET-1 retrieve (download) their data via a secure FTP interface (Ref. 42). 62) 63)

For the first year of operations the routine operations network is based on WHM and NST ground stations with 4 scheduled contacts per day, 1 contact over WHM and another 3 over NST. The WHM contact is the contact also used for uploading of telecommands while the 3 other contacts are used for payload data dump and telemetry only. If required, stations can be used interchangeable - increasing reliability of the ground segment.

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Figure 41: Overview of the TET ground segment (image credit: DLR)

During the first year, all 11 payloads are operated by GSOC according to their respective requirements – which may be e.g. number of activation cycles, hours of operation or certain attitudes. Requirements have been collected by KTH (Kayser-Threde) from the payload owners. To generate a valid operations concept also the satellite bus capabilities must be collected and analyzed. All information together is stored in an Excel sheet, containing all payload activities for the first year of operation.

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Figure 42: Functional architecture of the TET-1 ground system (image credit: DLR)

It is expected that the satellite will be significantly longer operational than the first planned year. Operations during an extended mission phases are not yet defined but the focus is placed on operations for the N15 infrared camera. Some other experiments may also be operated but only on request of the payload owner and if the payloads still operational.

Experiment types

Specific operations requirement

Experimental battery

High energy consumption during battery loading

Different types of solar cells

Sun pointing

Memory cells

Nothing specific

Picosatellite propulsion system

Specific attitude to avoid satellite bus contamination

Radio transmission experiment

No regular TM/TC transmission possible during experiment

Experimental GPS receivers

Operations with good GPS satellite view which requires an attitude different from sun pointing

Infrared camera

Nadir pointing, high power consumption and high data generation during operations, satellite rotation about nadir axis by 180º for star camera operations

Sensor and software experiments

Operations as long as possible. No specific other requirements

Table 7: Different types of experiments on TET-1

 


1) Stefan Foeckersperger , Klaus Lattner, Clemens Kaiser, Silke Eckert, Wolfgang Bärwald, Swen Ritzmann, Peter Mühlbauer, Michael Turk, Philipp Willemsen, "The Modular German Microsatellite TET-1 for Technology On-Orbit Versification" Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008, ESA SP-660, August 2008, IAC-08.B4.7.3, URL: http://www.kayser-threde.de/tet/downloads/Papers/IAC-08.B4.7.3.pdf

2) Peter Mühlbauer, Herbert Wüsten, Jens Richter, Michael Turk, Philip Willlemsen, Stefan Föckersperger, Sven Müncheberg, "Mission Operation, Ground Segment and Services for the German TET-1 Microsatellite (Technology Experiments Carrier)," Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008, ESA SP-660, August 2008

3) http://www.dlr.de/rd/desktopdefault.aspx/tabid-2274//3396_read-5085/

4) Stefan Föckersperger, "On-Orbit Verifikation mit dem deutschen Kleinsatelliten TET-1," Darmstadt, Sept. 23, 2008, URL: http://www.kayser-threde.de/tet/downloads/Papers/TET-1_Mission_230908.pdf

5) Stefan Foeckersperger, Clemens Kaiser, Klaus Lattner, Silke Eckert, Swen Ritzmann, Michael Turk, Robert Axmann, "The TET-1 Mission - current project status of the small satellite mission and outlook for a one year mission operation phase," Proceedings of the 61st IAC (International Astronautical Congress), Prague, Czech Republic, Sept. 27-Oct. 1, 2010, IAC-10.B4.6A.9

6) S. Föckersperger, "The TET-1 Mission for Technology On-Orbit Verification," Kayser-Threde, July 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/1_TET%20KT%20General/3_TET-1_mission_pdf_version.pdf

7) Clemens Kaiser, Stefan Föckersperger, Timo Stuffler, "Future SMALL Satellite EO Missions based on TET," Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, Oct. 3-7, 2011, paper: IAC-11-B4.4.13

8) "On-Orbit Verification TET," Kayser-Threde, May, 2010, URL: http://www.kayser-threde.de/tet/downloads/2010-05_TET_2s_engl.pdf

9) Stefan Föckersperger, Gavin Staton, Michael Turk, "Future Small Satellite EO Missions Based on TET," Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Portoroz, Slovenia, June 4-8, 2012

10) S. Eckert, S. Ritzmann, S. Roemer , W. Bärwald, "The TET-1 Satellite Bus – A High Reliability Bus for Earth Observation, Scientific and Technology Verification Missions in LEO," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010

11) S. Eckert, S. Ritzmann, C. Schultz, S. Roemer, W. Bärwald, "A German Microsatellite for On-Orbit -Verification," Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, May 4-8, 2009, URL: http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv7/Presentations/1303_IAA-B7-1303_TET-1.pdf

12) S. Eckert, "The TET-Satellite Bus," 2nd TET Customer Day, Kayser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/1_TET%20KT%20General/7_AFW_TET.pdf

13) S. Roemer, S. Eckert, "The TET Satellite Bus – A high reliability bus for LEO missions," 8th IAA (International Academy of Astronautics) Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2011; URL of the presentaion IAA-B8-1001: http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv8/Presentations/IAA-B8-1001.pdf

14) Heinz-Volker Heyer, Stefan Föckersperger, Jörg Würker, Alfred Hönle, Steffen Grieser, Michael Turk, Stephan Römer, "The On-Orbit-Verification Satellite TET-1 with the Payload Supply System Ready for Launch," Proceedings of the DASIA (DAta Systems In Aerospace) 2011 Conference, San Anton, Malta, May 17-20, 2011, ESA SP-694, August 2011

15) TET-1 Satellite Bus," Astrofein, URL: http://www.astrofein.com/2728/dwnld/admin/Brochure_Satellite_TET-1.pdf

16) "Small satellite TET-1," Astrofein, Sept. 14, 2010, URL: http://www.astrofein.com/2728/dwnld/admin/AstroFein_TET_EN.pdf

17) Sergio Montenegro, Lutz Dittrich, "The core avionics system for the DLR compact-satellite series," Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008, ESA SP-660, August 2008

18) Zizung Yoon, Thomas Terzibaschian, Christian Raschke , Olaf Maibaum, "Robust and fault tolerant AOCS of the TET satellite," Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, May 4-7, 2009, IAA-B7-0704

19) Zizung Yoon, Thomas Terzibaschian, Christian Raschke, Olaf Maibaum, "Development of the Fault Tolerant Attitude Control System of OOV-TET Satellite," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010

20) Sebastian Löw, Jaap Herman, Daniel Schulze, Christian Raschke, "Modes and More - Finding the Right Attitude for TET-1," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL: http://www.spaceops2012.org/proceedings/documents/id1274662-Paper-002.pdf

21) Stephan Stoltz, Christian Raschke, Katrin Courtois, "RW 90, a smart reaction wheel – Progress from BIRD to TET-1," 8th IAA (International Academy of Astronautics) Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2011; URL of presentation, IAA-B8-1204, http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv8/Presentations/IAA-B8-1204.pdf

22) S. Stoltz, C. Raschke, S. Roemer, "Advanced sensors for small satellites," Proceedings of GNC 2011, 8th International ESA Conference on Guidance, Navigation & Control Systems, Carlsbad (Karlovy Vary), Czech Republic, June 5-10, 2011

23) Zizung Yoon, Thomas Terzibaschian, Christian Raschke , Olaf Maibaum, "Fault Tolerant Attitude Control System of OOV-TET Satellite - Design and Test Results," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010

24) Z. Yoon, T. Terzibaschian, C. Raschke, O. Maibaum, "Development and verification of the fault tolerant attitude control system of OOV-TET satellite," Proceedings of GNC 2011, 8th International ESA Conference on Guidance, Navigation & Control Systems, Carlsbad (Karlovy Vary), Czech Republic, June 5-10, 2011

25) Staphan Stoltz, Stephan Roemer, "The AGPS-1 - A redundant GPS-unit for nano-satellites," Second Nanosatellite Symposium, University of Tokyo, Tokyo, Japan, March 14-16, 2011

26) "German TET-1 small satellite launched," DLR, July 22, 2012, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-4318/

27) "Soyuz FG Launch Updates - July 2012," Spaceflight 101, July 22, 2012, URL: http://www.spaceflight101.com/soyuz-fg-launch-updates-kanopus-belka-exactview-zond-tet.html

28) "Peering through smoke – DLR satellite TET-1 delivers detailed images of the fires in Indonesia," DLR, October 30, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-15669/year-all/#/gallery/21089

29) E. Lorenz, S. Mitchell, T. Säuberlich, C. Paproth, W. Halle, O. Frauenberger, "Remote Sensing of High Temperature Events by the FireBird Mission," Proceedings of ISRSE (36th International Symposium on Remote Sensing of Environment), Berlin, Germany, May 11-15, 2015, Editor(s): G. Schreier, P. E. Skrovseth, and H. Staudenrausch, URL: http://www.int-arch-photogramm-remote-sens-spatial-inf-sci.net/XL-7-W3/461/2015/isprsarchives-XL-7-W3-461-2015.pdf

30) O. Frauenberger, E. Borg, A. Börner, W Halle, E. Lorenz, S. Mitchell, C. Paproth, T. Säuberlich, T. Terzibaschian, J. Wohlfeil, "Results on verification and validation of OOV-TET1 multispectral camera observations within the FireBIRD project," 10th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 20-24, 2015, paper: IAA-B10-0802, URL of presentation: http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv10/pdf/0802.pdf

31) Norbert M. K. Lemke, Stefan Föckersperger, Silke Eckert, Hans Gronert-Marquardt, Eckehard Lorenz, "Mission Success of TET‐1 for On‐Orbit Verification," Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Port Petro, Majorca Island, Spain, May 26-30, 2014

32) Kay Müller, Arvind Kumar Balan, "Battery Operations for the TET–1 Spacecraft," SpaceOps 2014, 13th International Conference on Space Operations, Pasadena, CA, USA, May 5-9, 2014, paper: AIAA 2014-1732, URL: http://arc.aiaa.org/doi/pdf/10.2514/6.2014-1732

33) Norbert M. K. Lemke, Stefan Föckersperger, Peter Hofmann, Timo Stuffler, Silke Eckert, Michael Turk, Eckehard Lorenz, "The TET-1 On-Orbit Verification Mission – Status and Future Opportunities," Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B4.3.13

34) Hubert Reile, Eckehard Lorenz, Thomas Terzibaschian, "The FireBird mission – a scientific mission for Earth observation and hot spot detection," DLR, April 8, 2013, URL: http://elib.dlr.de/83866/1/IAA-B9-0201_DLR_FireBird_Mission_extended_abstract.pdf

35) Benjamin Wille, Maria Theresia Wörle, Christoph Lenzen, "VAMOS – Verification of Autonomous Mission Planning On-board a Spacecraft," URL: http://elib.dlr.de/84313/1/IFAC2013_VAMOS_03.pdf

36) Martin Brechtelsbauer, Christopher Schmidt, Peter Becker, Christian Fuchs, "Application of an optical data link on DLR's BIROS satellite," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL: http://www.spaceops.org/images/spaceops/5-OSIRIS.pdf

37) Information provided by Robert Axmann, DLR/GSOC, Oberpfaffenhofen, Germany.

38) Robert Axmann, Tobias Lesch, Marcin Gnat, Heiko Damerow, Jens Richter, "The TET-1 Operational Ground System," Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013, paper: IAA-B9-0801

39) "Deutsche Kleinsatellitenmission TET-1 nimmt Regelbetrieb auf," Kayser-Threde Press Release, Oct. 16, 2012, URL: http://www.kayser-threde.de/download/press/pr-2012-10-16-232-de.pdf

40) Michael Turk, "TET-1, weitere Fortschritte," DLR, Aug. 10, 2012, URL: http://www.dlr.de/blogs/desktopdefault.aspx/tabid-8029/13766_read-592/

41) Carsten Dietrich, "TET-1, Der Satellit wird ausgetestet," DLR, July 24, 2012, URL: http://www.dlr.de/blogs/desktopdefault.aspx/tabid-8029/13766_read-577/

42) Stefan Föckersperger, Klaus Lattner, Clemens Kaiser, Silke Eckert, Swen Ritzmann, Robert Axmann, Michael Turk, "The On-Orbit Verification Mission TET-1 - Project Status of the Small Satellite Mission & Outlook for the One Year Mission Operation Phase," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010, URL: http://www.kayser-threde.de/tet/downloads/Papers/4S_Paper_TET-1_Madeira-2010.pdf

43) http://www.kayser-threde.de/tet/tetdownloads.html

44) Sebastian Löw, Jaap Herma2, Daniel Schulze, Christian Raschke, "Modes and More," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012

45) Christoph Tiefenbeck, "TET-1 Payload N6 Sensorbus system," 2nd TET Customer Day, Kayser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N6_2nd_tet_customerday.pdf

46) Friedrich Schön, "Hardware BOSS," 2nd TET Customer Day, Kyaser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N17_20100705-HW-BOSS-engl.pdf

47) M. Markgraf, C. Renaudie, O. Montenbruck, "The NOX Payload-Flight Validation of a low-cost Dual-Frequency GPS Receiver for Micro- and Nanosatellite Applications," Proceedings of the IAA Symposium on Small Satellite Systems and Services (4S), Rhodes, Greece, May 26-30, 2008, ESA SP-660, August 2008 URL: http://www.weblab.dlr.de/rbrt/pdf/ESA4S_08.pdf

48) M. Markgraf, P. Swatschina, "The Navigation and Occultation eXperiment (NOX) onboard TET-1," 2nd TET Customer Day, Kayser-Threde, Munich, Germany, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N16_2ndTetCustomerDay_N16-NOX_Markgraf.pdf

49) Harry Adirim, Norbert Pilz, Matthias Kreil, Michael Kron, Andrei Mitrofanow, "On-Orbit-Verification of Small Satellite Propulsion System Aquajet on TET-1," 2nd TET Customer Day, Kayser-Threde, Munich, July 5.-6., 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N7_2010-07-01%20OOV%20of%20Small%20Satellite%20Propulsion%20System%20AQUAJET.pdf

50) Stephan Roemer, Winfried Halle, "TET-1 and BIROS A semi-operational Fire Recognition Constellation," UN/Austria/ESA Symposium on Small Satellite Programs for Sustainable Development: Payloads for Small Satellite Programs, Sept. 21-24, 2010, Graz, Austria

51) "Exposé, BIROS (Berlin InfraRed Optical System)," information provided by Winfried Halle of DLR

52) Eckehard Lorenz, "IR Payload auf TET (N15, N15.1)," July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N15_Nutzlast_N15%20.pdf

53) http://www.ltcc.de/en/rd_ke2.php

54) Reinhard Kulke, Christian Hunscher, "KERAMIS-2, Ceramic Microwave Circuits for Satellite Communication," TET Customer Day, Kayser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N18_Keramis-TET1.pdf

55) Siegfried Voigt, "KERAMIS-2," DLR, URL: http://www.dlr.de/rd/en/desktopdefault.aspx/tabid-4161/3338_read-5033/

56) T. Fichna, D. Walter, H. Michalik, "MORE, Memory Orbit Radiation Experiment," TET Customer Day, Kayser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N19_MORE_Presentation.pdf

57) T. Fichna, H. Michalik, F. Gliem, F. Bubenhagen, "MORE – Radiation measurement of NAND flash devices on TET-1," 8th IAA (International Academy of Astronautics) Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2011

58) K. Grürmann, T. Fichna, F. Bubenhagen, F. Gliem, H. Michalik, "MORE – radiation measurement of NAND-Flash devices on TET-1," Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013, paper: IAA-B9-0905P

59) Payload N1: Lithium Polymer Battery," 2nd TET Customer Day, Kayser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N1_2nd%
20TET%20Customer%20Day%20N1.pdf

60) "On Orbit Verification of thin film solar cells," 2nd TET Customer Day, Kayser-Threde, Munich, July 5, 2010,, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/2_TET%20Payloads/N2_Praesentation_N2_Solarion_2010eps.pdf

61) Sebastian Brunner, Kai Zajac, Michael Nadler, Klaus Seifart, Christian A. Kaufmann , Raquel Caballero , Hans-Werner Schock , Lars Hartmann, Karten Otte, Andreas Rahm, Christian Scheit, Hendrik Zachmann, Friedrich Kessler, Roland Würz, Peter Schülke, "Recent Progress Towards Space Applications of Thin Film Solar Cells – The German Joint Project "Flexible CIGSE Thin Film Solar Cells for Space Flight" and OOV," Proceedings of the 9th European Space Power Conference, Saint Raphael, France, June 6-10, 2011, ESA SP-690

62) Robert Axmann, Peter Mühlbauer, Andreas Spörl, Michael Turk, Stefan Föckersperger , Jürgen Schmolke, "Operations Concept and Challenges for 11 Different Payloads on the TET-1 Mini-Satellite," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010

63) Peter Mühlbauer, Robert Axmann, "TET-1 Bodensegment und Missionsbetrieb," 2nd TET Customer Day, Kayser-Threde, Munich, July 5, 2010, URL: http://www.kayser-threde.de/tet/downloads/Praesentationen/1_TET%20KT%20General/6_TET-GSC-HO-0019_Bodensegment%20Praesentation%20zum%20TET%20Customer%20Day.pdf
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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