GOMX-3 (GomSpace Express-3)
The GOMX-3 mission is a collaboration between ESA (European Space Agency) and GomSpace ApS of Aalborg, Denmark to demonstrate new capabilities of nanosatellites focusing on attitude control, RF sensing, and high-speed data downlink. 1)
Work on the mission began in June 2014 under the IOD (In-Orbit Demonstration) element of the ESA GSTP (General Support Technology Program); the flight model was delivered one year later. The satellite was developed in compliance with the ESA tailored ECSS (European Cooperation for Space Standards) engineering standards and product/quality assurance requirements for IOD CubeSat projects.
Some background on CubeSat communications:
The space agencies are equipped with networks of tracking ground stations compatible with ITU and CCSDS (Consultative Committee for Space Data Systems) standards. These networks use EESS (Earth Exploration Satellite System) TT&C S-band (2025-2100 MHz for TeleCommand; 2200-2290 for Telemetry), and X-band (8025-8400 MHz for high data rate telemetry). More and more private ground tracking networks are also equipped with S-band and X-band ground stations, benefiting from a station cost reduction trend. 2)
CubeSats are presently generally fitted with UHF or S-band payload TM (telemetry) subsystems which allow downloading a few hundreds of Mbit per day. This data volume is limited because the telemetry bit rates are restricted to hundred of kbit/s in UHF and to few Mbit/s in S-band to comply with the CCSDS spectral recommended occupation bandwidth (6 MHz maximum in EESS S-band). The use of a fraction of the ISM S-band (2400-2483.5 MHz) is also possible for telemetry, but this band is subject to interferences, and is not associated to a separate ISM (Industrial, Scientific and Medical) S-band for TC. The diplexer of a 2.4 GHz TT&C equipment is therefore bigger than with a 2/2.2 GHz equipment. To limit the number of antennas and equipments on board and to increase CubeSats are presently generally fitted with UHF or S-band payload TM subsystems, which allow downloading a few hundreds of Mbit/day. This data volume is limited because the telemetry bit rates are restricted to hundred of kbit/s in UHF and to few Mbit/s in S-band to comply with the CCSDS spectral recommended occupation bandwidth (6 MHz maximum in S-band). The use of a fraction of the ISM S-band (2400-2483.5 MHz) is also possible for TM, but this band is subject to interferences, and is not associated to a separate ISM S-band for TC. The diplexer of a 2.4 GHz TT&C equipment is therefore bigger than with a 2/2.2 GHz equipment. To limit the number of antenna and equipments on board and to increase the TC (TeleCommanding) capacity (for software upload for instance), EESS S-band TCs looks appropriate.
Therefore, to increase transmission data rates and to allow compatibility with their existing ground tracking networks, CNES and ESA are interested in a miniature S-band TT&C transceiver, and in a miniature X-band transmitter, both designed for CubeSats. ESA decided to design the OPS-SAT triple CubeSat to test new space operation control concepts using EES S- and X-bands. The launch of OPS-SAT is planned for 2017.
As of 2015, Earth observation or spectrum monitoring or astronomy or technological payloads can be embarked on very small platforms, but they require the capability to download a large volume of data with a high telemetry bit rate subsystem. The Syrlinks' EWC27 HDR-TM X-band transmitter combined with a miniaturized COTS antenna solves this problem and enables to download up to 17 GB/day on a 3.4 m X-band station. CNES and Students are currently developing the EYE-SAT triple CubeSat, provided with an astronomical payload to observe zodialcal light. The needs in term of data rate imposed the use of EWC27 X-band HDR-TM (High Data Rate-Telemetry).
To allow early in-orbit testing of the EWC27 HDR-TM as soon as in the autumn 2015, CNES and ESA co-funded the end of the qualification process of this equipment, and its integration into the GOMX-3 ESA triple CubeSat.
S-band system architecture: The Syrlinks EWC31 TT&C transceiver has a CubeSat form factor with a height of about 4 cm including the diplexer and 3 dB coupler when needed (its high is about 2.5 cm without diplexer). Operational CubeSats might need such a diplexer, in order to connect two S-band patch Rx/Tx antennas mounted on opposite faces to the transceiver, through a 3 dB coupler. Doing so, whatever the CubeSat's attitude in orbit, there will be always one of the 2 antennas pointing roughly toward the ground. Therefore, such subsystem allows permanent TC and TM connections with the involved 2 GHz tracking ground station(s), even if the satellite is temporary subject to uncontrolled tumbling.
The two S-band Rx/Tx patch antennas shall be provided with a single RF connector, used to input and output S-band TC and TM signals. If, as it is generally the case, the using space agency ground network is provided with dual circular polarization parabolic antennas, the two patch antennas mounted on opposite faces of the CubeSat can have opposite circular polarization (case of EYE-SAT), to minimize the coupling between these 2 patches.
This can be done especially if the tumbling angular velocities are not high. The 2 flat antennas could also have the same polarization (case of OPS-SAT) to facilitate the ground S-band tracking operations, especially, when only single polarization ground stations are available, or when the maximum tumbling angular velocities to cope with are very high. In all cases, a careful global antenna pattern measurement provided by the two patch antennas mounted on a structure representative of the CubeSat is mandatory.
The choices of modulation and coding have a major impact on the S-band subsystems performances regarding the bit rate, consumption, implementation complexity, but also the interoperability possibilities with the ground segment.
X-band system architecture: Existing nanosatellite telemetry systems (UHF or S-band) can dump only a few hundred of Mb to 1 Gb per pass. The emerging need for higher dumping capacity on a CubeSat mission made a new solution (the X-band transmitter) emerge to provide a major improvement (for instance 6 to 14 Gbit/ pass), compatible with X- band stations ( 3.4 m to 5 m diameter in that case), and affordable in the nanosatellite format (3U ): power consumption below 10 W peak , 1 W mean/orbit, 300 g, small antenna. The transmitter RF output power can be tuned up to 2 W (assuming 10 W peak consumption and 10% duty cycle). The telemetry antenna's gain is assumed to be 0 dBi at ± 60°, with 1 dB antenna loss, this giving an EIRP of 2 dBW. The ground station can have for instance a diameter of 3.4 m (25 dB/K G/T) : above 10° elevation, or 5 m (30 dB/K G/T) : above 5° elevation.
The modulation and coding, currently chosen are a power efficient standard rather than spectrum efficient (as required for larger bit rate) , to benefit from a 4 to 5 dB impact. This standard is compatible with usual ground stations and CCSDS: OQPSK (Offset Quadrature Phase Shift Keying) with convolutional coding (k=7, R= ½ + 255/223 Reed Solomon).
Outside its RF performances, OQPSK with k=7 R=½ + RS is also very interesting because convolutional and Reed Solomon coding operations can be split and performed in different locations. The first one can easily be implemented into the transmitter and realized in real time. The second one can be performed with the framing at the mass memory or processor level in real time or by post processing. Such repartition also facilitates the interface between the mass memory or processor and the transmitter because the data flow to modulate is continuous.
Using CBR (Constant Bit Rate), large and unexploited link budget margins occur at elevations higher than 20°. Using VBR (Variable Bit rate) with Nbr possible values of bit rates, the download capacity is multiplied by 1.6 with Nbr=2, and multiplied by 2 with Nbr=3 during a pass. During the bit rate change transition sequences, IDLE sequences are used to avoid data losses. The transition time percentage during the pass is estimated to 5% with 3 bit rates (5 s / commutation). The ground station receiver could receive predictable commands to stations for bit rate switching, if not autonomous.
VBR with OQPSK and k=7, R= ½ convolutional + RS coding is CCSDS compatible, despite the signal spectrum variation during a pass, since CCSDS defines the "mission phases" as the mission period during which the signal parameters are constant. That means that a pass. using VBR is provided with N+1 "mission phases", when there is N bit rate commutations, with N = 2 x (Nbr-1). With a maximum bit rate of 50 Mbit/s, the maximum spectral width of the transmitted telemetry signal is significantly smaller than the 375 MHz available in the EESS X-band.
Led by GomSpace in Denmark, GOMX-3 is a 3U CubeSat mission to demonstrate aircraft ADS-B (Automatic Dependent Surveillance-Broadcast) signal reception and geostationary telecommunication satellite spot beam signal quality using an L-band reconfigurable software defined radio payload. A miniaturized high data rate X-band transmitter developed by Syrlinks and funded by the French space agency CNES will also be flown as a third party payload. The satellite is planned to be deployed from the ISS (International Space Station).
Figure 1: Illustration of the deployed GOMX-3 nanosatellite (image credit: GomSpace, ESA)
The satellite was developed, integrated, tested, and delivered over a period of 13 months using off the shelf components available from GomSpace. GOMX-3 uses the next generation of CubeSat OBC (NanoMind A3200) and UHF radio (NanoCom AX100). Both of these subsystems use a motherboard-daughterboard system designed to minimize stack height to fit more capability in a smaller volume. A P31us EPS and a BP4 battery pack complete the CubeSat bus. The 3U CubeSat features body-mounted triple-junction solar cells employing maximum power point tracking to optimize the performance of the arrays for the given illumination and thermal environment Ref. 2). 5)
Pointing is key to the GOMX-3 mission; the satellite uses a combination of advanced sensors (coarse sun sensors, IR horizon sensors, magnetometers, fine sun sensors, NovAtel GPS receiver) and actuators (in-panel magnetorquers, Astrofein momentum wheels) controlled by a dedicated ADCS A3200 computer. The GomSpace ADS-B receiver, a GomSpace SDR (Software Defined Radio) module, and the Syrlinks EWC27 CubeSat X-band transmitter complete the internal payload stack.
The overall satellite layout is shown in Figure 2. The bottom 1U of the satellite is dedicated to the OBC, COM, and EPS subsystems, while the middle 1U houses the ADCS, the ADS-B receiver, and the SOFT (aka SDR) radio. The upper 1U contains the X-band transmitter and the further ADCS support hardware. Externally, the satellite uses interstage boards to mount the fine sun sensors and collect sensor data from the solar panels. The stack breakout boards are used to electrically connect the 1U stacks. Additionally, GOMX-3 uses five RF antennas mounted on various external faces.
Figure 2: Illustration of the GOMX-3 instrument stack (image credit: GomSpace)
The primary communications system of the satellite is a half-duplex UHF transceiver operating in the 435-438 MHz range to reach a nominal data rate of 9.6 kbit/s. The satellite makes use of a CubeSat Space Protocol, a network-type protocol implemented by all subsystems across the space link and the ground system, easing the integration and testing of systems as well as simplifying operations since every component is assigned a node with access to command resources available within the network.
X-band link: The first in-orbit test of the Syrlinks EWC27 X-band transmitter will occur aboard GOMX-3. Syrlinks (Bruz, France) developed an X-band patch antenna considering the following initial requirements: Frequency band between 8.025 and 8.4 GHz, RHCP polarization, <3dB axial ratio, <3 dB between ±30°, >0 dBi Gain between ±30° from boresight, <12 dB return loss in a bandwidth of 400 MHz. The available volume on the GOMX-3 platform for the antenna was limited, especially the thickness was restricted to 7 mm. The as-built dimensions of the active part of the antenna are 73.5 x 73.5 m x 6.8 mm. The measured performances are in line with simulations results. Finally some qualification tests (temperature cycling, vibration, thermal vacuum) were made in order to check the evolution of the performances after environmental testing.
CNES also participates in the GOMX-3 project by providing two X-band ground stations. The first station is new and is located at the French CSG ( Guyana Space Center), in Kourou, and will be used for experimental passes until the end 2015. It has an antenna diameter of 11 m. The second one is a quite "old" 3 CNES X-band station provided with a 3.4 m antenna, and has been transferred from the Operation directorate to the JANUS Student/CNES nanosatellite project. It is now physically implemented at the site of ENAC (Ecole Nationale de l'Aviation Civile), the French national civil aviation engineering school, and presently being refurbished. It will be used in a second step, after the test period with the station in Kourou. The S/X-band architecture is able to deliver a considerable increase in data rate for nanosatellites.
Syrlinks works jointly with CNES, ESA, TU Graz, and GomSpace to develop advanced radio solutions for CubeSats. Syrlinks proposes CubeSat RF equipment using EESS (Earth Exploration Satellite System) X-band, or EESS or ISM (International Manufacturing Services) S-band, or both S-band and X-band (Figure 3), and study also solution using other bands. The EWC27 X-band transmitter reuses an important part of the ESA PROBA-V telemetry hardware, and provides up to 100 Mbit/s. The EWC31 S-band equipment is also fully CCSDS compatible with ranging and coherent transmission capabilities.
Figure 3: Photo of the nano S/X TT&C : EWC27 X-band transmitter from Syrlinks, integrating the S-band receiver — extracted from the EWC31 S-TT&C transceiver from Syrlinks (image credit: Syrlinks)
The CSP (CubeSat Space Protocol) is defined for a variety of physical buses; some subsystems support redundant buses and may be switched on-orbit. Figure 4 shows the data interfaces used aboard GOMX-3. CSP is the core of these interfaces and is responsible for the majority of the core subsystem communication. The ADCS sensors rely on the GomSpace Sensor Bus (GSSB), allowing interstage boards to act as intermediaries between sensors and the ADCS computer. Additionally, serial communication is used for some specialized payloads.
Figure 4: GOMX-3 data interfaces (image credit: GomSpace)
Figure 5: Technology testing of GomX-3 under construction (image credit: ESA, GomSpace) 6)
Table 3: GomSpace methods of reducing development time before delivery (Ref. 12)
Launch: The GOMX-3 nanosatellite was a secondary payload on the HTV-5 service mission of JAXA (Japan Aerospace Exploration Agency) nicknamed Kounotori 5) to the ISS. HTV-5 was launched on August 19, 2015 at 11:50:49 UTC from the Tanegashima Launch Center, Japan, on the H-IIB vehicle of MHI (Mitsubishi Heavy Industries, Ltd.). The launch vehicle flew smoothly, and, at about 14 minutes and 54 seconds after liftoff, the separation of the Kounotori-5 was confirmed. 7)
Orbit: Near-circular orbit, altitude of ~400 km to ISS, inclination =51.6°.
JAXA astronaut Kimiya Yui, a Flight Engineer for Expedition 44 and 45 aboard the ISS, was assigned to manipulate the SSRMS (Space Station Remote Manipulator System) for the operation of capturing Kounotori-5. It is the first time for Japanese astronauts to capture a HTV vehicle.
JAXA astronaut Koichi Wakata served as lead CAPCOM (Capsule Communicator) for the HTV-5 mission at the NASA MCC (Mission Control Center) in Houston, TX.
Secondary CubeSat payloads of the HTV-5 mission cargo. All CubeSats are part of the PLC (Pressurized Logistics Carrier): 8)
- 14 Flock-2b nanosatellites (3U CubeSats) of Planet Labs, San Francisco, to provide high-resolution (3-5 m) imagery of the Earth. Each nanosatellite has a mass of 5 kg.
- GOMX-3, a 3U CubeSat mission (~3 kg) of ESA developed by GomSpace in Aalborg, Denmark. Payload: SDR receiver and an ADS-B receiver to receive signals broadcast by civilian aircraft. The SDR is used to receive signals from communication satellites in GEO for an assessment of signal quality in the L-band range. 9)
- AAUSAT-5, a 3U CubeSat student satellite demonstration of Aalborg University (AAU), Denmark. AAUSAT-5 is to receive AIS (Automatic Identification System) beacons from ships. The beacons are used to identify and locate vessels to support collision avoidance and search and rescue efforts.
- SERPENS (Sistema Espacial para Realização de Pesquisa e Experimentos com Nanossatélites), a 3U CubeSat developed by a consortium of Brazilian Universities for technology demonstrations.
- S-Cube (Shootingstar Sensing Satellite) is a 3U CubeSat (4 kg) of PERC (Planetary Exploration Research Center) at the Chiba Institute of Technology and Tohoku University, Japan. The objective is meteor observation.
Status of mission:
• November 4, 2016: ESA's first technology CubeSat has burned up in the atmosphere after a year in orbit (reentry on Oct. 18, 2016). Following six months of demonstrating new technologies, the tiny satellite's working life was extended in order to gather wind data from aircraft in flight – a notable first, essentially making it Europe's smallest weather satellite. 10)
- Released from the International Space Station in October 2015, GomX-3 – built for ESA by GomSpace in Denmark – carried an experimental receiver to detect ‘ADS-B' tracking signals broadcast by aircraft in flight, as well as a software-defined radio for evaluating spot beams from telecom satellite signals, and an X-band receiver for high-bandwidth data downloads. — GomX-3's extended mission involved a collaboration with the Met Office, the UK's national weather service, trying out a new spaceborne method of wind measurement.
- Aircraft threading their way through the sky are increasingly regarded as a useful resource for weather forecasting. The Met Office has established a network of five ground receivers that gather wind and temperature data from aircraft across the country's airspace in response to air traffic control radar. "The information in these messages can be used to derive high-quality wind information explains Edmund Stone, Met Office observations scientist. "This works by comparing the reported ground and air speeds and directions. We also retrieve lower-quality temperature data.
- Research is under way to assimilate these data into UK numerical weather models, to help improve forecast accuracy. "These messages are becoming more common across the globe," Edmund Stone adds. "This information will be useful in the future in increasing the global observation network coverage, both from the ground and also from orbit, which led us to the collaboration with ESA and GomSpace."
- GomSpace reconfigured the ADS-B receiver on GomX-3 to record the information in the aircraft signals needed by the UK Met Office, and collected the data over a two-week period. "By processing the data, we were able to confirm that these could also be used to acquire good-quality, high-volume wind information. On a global scale it would improve global weather forecasting, particularly for areas where wind data are relatively scarce."
Figure 6: GomX-3 orbit and weather data retrievals: The blue lines show the GomX-3 orbital track over the two week period of the trial. The red dots are locations where Mode-S wind and temperature observations were derived and highlight the regions of the world where these data would be available currently. The volume of data obtained was strongly limited by the temporal coverage available from a single satellite (image credit: UK Met Office) 11)
Legend to Figure 6: The Met Office already uses ground-based receivers that gather wind and temperature data from aircraft in flight across the country's airspace, using ‘ADS-B' and ‘Mode-S Enhanced Surveillance' messages can be used to derive high quality wind information, by comparing the reported ground and air speeds and directions, plus lower quality temperature data. GomX-3 was used to investigate the feasibility of doing the same from orbit.
• August 2016: The satellite continues to operate with no loss of functionality. It is a success because of the vast reconfigurability of its subsystems, which use a variety of tools (parameter system, on-orbit image upload, watchdogs, distributed network topology) to ensure mission success given tight schedule constraints. The satellite has remained active enough to necessitate the development of an optimization tool to best determine payload scheduling given geometric and target constraints, which are realized via rapid attitude maneuvering with up to 7 target changes per orbit. 12)
- Operations summary: The GOMX-3 3U CubeSat deployed from the ISS on October 5, 2015 and immediately started a compressed commissioning schedule for its bus and advanced payloads. After 24 hours on-orbit, the downlink was increased to 19.2 kbit/s to take advantage of the excellent communication capability. Its helical ADS-B antenna was deployed on day 2 of on-orbit operations and began collecting thousands of aircraft positions each day. After 96 hours on orbit, the satellite entered 3-axis control. In the weeks that followed, GOMX-3 used its 1 degree pointing capability to track nadir, ram, ground stations, and geostationary satellites. In addition, it successfully demonstrated its high-speed X-band downlink capability (based on a transmitter & antenna from Syrlinks and funded by CNES) using a CNES ground station located in Kourou, French Guinea. Finally, GOMX-3 successfully demonstrated its powerful SDR (Software Defined Radio) via a spectrum analysis of L-band signals in its ISS-like orbit.
- Hence, ESA maintains an IOD (In-Orbit Demonstration) element of its technology program, which "finds flight opportunities for innovative technologies". In this way, ESA reduces risk stepwise, allowing advanced technologies to trickle up (into larger programs which have a lower risk ceiling) or across (into other demonstration missions which rely on them to test more advanced technologies).
Advanced ADCS: Because of the advanced goals of GOMX-3, some degree of on-orbit calibration was required for the ADCS. The attitude determination (and thus also control system) relies on a variety of sensors uses as inputs: fine and coarse sun sensors, magnetometers, and a rate gyro. Each sensor was calibrated using a variety of techniques unique to each sensor. The satellite inertia matrix and magnetic dipole moment were also empirically determined by fitting to on-orbit data. This process took a period of about one month, as large datasets are sometimes necessary for calibration.
The UKF (Unscented Kalman Filter) itself also requires tuning to ensure that its estimates of attitude uncertainty are in line with reality. This was performed using standard methods. Figure 7 shows the attitude control and determination performance after calibration while flying in nadir-tracking mode. As shown, the satellite is capable of periods of 1 degree pointing (1σ), but suffers from worse performance when the orientation vectors (magnetic and sun) are close together or during eclipse, when the sun vector is lost entirely and the magnetometer and gyro are used to propagate the satellite attitude.
The stability of the ADCS system is also of note. After proving momentum dumping via the magnetorquers, the GOMX-3 was set to nominally point its 1U face toward the local ram vector, a minimum drag configuration. The consistency of the ADCS in maintaining this attitude has extended the expected orbit lifetime from 6 months to well over 1 year, providing much more utility from the ISS orbit.
This ADCS consistency is shown in Figure 8, which shows the satellite's historical Bstar drag term, an estimate of the satellite ballistic coefficient normalized for altitude. After the satellite was set for ram pointing in early December 2015, the Bstar drag was significantly lowered. Note that the attitude is sometimes varied to allow for mission operations such as Earth object tracking or nadir-pointing.
Figure 8: GOMX-3 historical Bstar drag term, showing the effect of its primarily ram-pointing attitude after commissioning is complete in Dec 2015 (image credit: GomSpace, ESA)
Figure 9 shows a three-week period (May to June 2016) of uninterrupted ram pointing. The onboard ADCS filters the onboard sensors to determine the perceived angular offset from the local ram vector. The mean perceived error is just 0.69º over this period.
Figure 9: GOMX-3 perceived angular offset over a 3-week period; the mean offset is 0.69 degrees (image credit: GomSpace, ESA)
Figure 10 shows the results of a verification of the NovAtel GNSS receiver aboard GOMX-3. In both tests, the receiver maintained lock for a period of 20 to 30 minutes. Each of these attempts was about 12 hours away from a TLE epoch which was used to determine a relative position error between the SGP4-based (Simplified General Perturbation 4) position and the receiver's reported position. As shown, the Euclidean difference between the SGP4- and GNSS-reported positions varies from 4 to 10 km with an average of 7 km. This is in good agreement with previous nanosatellites comparing TLE-to-GPS position errors.
Figure 10: The GNSS receiver position estimation as compared with the TLE/SGP4 estimation (image credit: GomSpace, ESA)
ADS-B receiver: After initial communication checks to the ADS-B receiver, the next step was deployment of the helix antenna designed for data collection at 1090 MHz. Immediately after antenna deployment, the satellite recorded its first ADS-B signals. To date, the ADS-B receiver has regularly collected thousands of plane positions per day and continues to operate nominally.
NanoCom SDR: On-orbit tests began by simply powering the device on and monitoring its behavior. As shown in Figure 11, the highly capable SDR maintains temperatures within operational bounds, even over long time spans. Next, the spectrum monitoring capabilities proceeded to monitor the UHF environment during GOMX-3 transmissions. With this sanity check complete, the system was used to record the spectrum in L-band while tracking specific geostationary satellites. The patch antenna used by the GOMX-3 SDR is centered at 1592 MHz with a VSWR (Voltage Standing Wave Ratio) ≤3 bandwidth of 175 MHz.
Figure 11: On-orbit active-SDR temperatures over a 24-hour period (image credit: GomSpace, ESA)
Mission Success: Over eight months after the deployment from the ISS, GOMX-3 is still fully operational and has fulfilled all its mission requirements. It has been a complete mission success. In fact, the operations of the satellite have been consistent enough to allow for the development of an automated experiment scheduling tool. This tool maximizes the utility of the payloads while maintaining the battery charge-level above a critical threshold. - The satellite now continues operations in its extended mission.
• April 7, 2016: GomX-3's distinctive helical antenna has detected millions of signals from aircraft, building a detailed map of global aviation traffic. These signals are regularly broadcast from aircraft, giving flight information such as speed, position and altitude. All aircraft entering European airspace are envisaged to provide such automatic surveillance in the coming years. Built for ESA by GomSpace in Denmark, the GomX-3 CubeSat was ejected from the International Space Station on 5 October 2015, along with a Danish student satellite. 13)
- "This 3U GomX-3 is ESA's very first technology CubeSat to fly. We were able to make it operational within only 96 hours of its release from the Space Station, with a wide variety of tests taking place during the following months," explains Roger Walker, overseeing ESA's technology CubeSat effort. "Being small and low-cost, they make ideal platforms for rapidly flight testing experimental technologies."
- GomX-3 also carries a miniaturized X-band transmitter, developed by Syrlinks in France, which has demonstrated the rapid download of data.
- In addition, the CubeSat is measuring radio signals emitted by telecom satellites to assess their overall transmission efficiency and how their signal quality changes with respect to distance from their target footprints.
- "GomX-3 has in contrast to many other CubeSats demonstrated three-axis control, so it can be pointed as required, whether downwards or upwards, to an accuracy of 3º," explains Roger. "A success in terms of planning, speed of development and technical achievements, GomX-3 has now completed its planned six-month technology demonstration mission and continues to operate normally. With its orbit naturally decaying from atmospheric drag, the satellite is predicted to reenter and burn up in September of this year." Meanwhile, GomSpace is developing a follow-up 6U CubeSat called GomX-4B, also supported by ESA, scheduled for launch in the second half of 2017.
Figure 12: Detections of aircraft in flight made by ESA CubeSat GomX-3 during the last six months, since it was released from the International Space Station on 5 October 2015 (image credit: ESA/GomSpace) 14)
• October 16, 2015: ESA's first technology-testing CubeSat, released last on Oct. 5, 2015 from the International Space Station, is in good health and is set to start work on its six-month mission. The project is taking first steps towards putting its technology payloads through their paces. 15)
- Despite its small size of 10 x 10 x 30 cm, the nanosatellite precisely controls its orientation by spinning miniaturized ‘reaction wheels' at varying speeds. This precision is an important factor in the effectiveness of the mission's technology-testing payloads. One task will see GOMX-3 pointing up towards to detect radio signals from telecom satellites in geostationary orbit to assess their overall transmission efficiency. The processing software can be changed in flight, allowing the receiver to be reconfigured and used in extremely flexible ways, of wider interest for future ESA missions.
- The CubeSat also carries a miniaturized version of a transmitter being flown on ESA's PROBA-V minisatellite for downloading data rapidly at X-band radio frequencies. Developed by the French Syrlinks company in cooperation with France's CNES space agency, the antenna will aim at X-band ground stations in the CNES network. Once the communications link has been tested over the coming months, the transmitter will be available to fly on future nanosatellites to boost their amount of downloaded data.
- GOMX-3 also sports a receiver to detect navigation signals from aircraft. The satellite points its distinctive helical antenna to Earth and has already picked up a tens of thousands of ADS-B (Automatic Dependent Surveillance – Broadcast) signals from aircraft since the day after deployment from the ISS. - ESA's 2013-launched PROBA-V first confirmed the feasibility of ADS-B detection from orbit, opening up the prospect of a global aircraft monitoring system incorporating remote regions not covered by ground-based air traffic control.
- Due to the near-ISS orbit of GOMX-3, its Aalborg, Denmark ground station has an average of 5.0 passes per day, with an average pass length of 7.4 minutes. After the first 37 minutes of communication with the on-orbit satellite, the GOMX-3 bus was confirmed to be healthy in all aspects: power, communication, and attitude determination & control. This was made possible by designing for on-orbit operation, as well as careful planning and rehearsing of critical ground passes (Ref. 12).
Figure 13: The GOMX-3 spacecraft in deployed configuration (image credit: GomSpace, NASA)
• Deployment of AAUSAT-5 and GOMX-3: On October 5, 2015, at 15:55 CEST (Central European Standard Time), two ESA CubeSats, the student-built AAUSAT-5 of Aalborg University in Denmark and the professional technology demonstrator GOMX-3, were deployed rom the ISS Japanese Kibo module airlock using the Kibo robotic arm.
Figure 14: Photo of the AAUSAT-5 and GOMX-3 CubeSats after their deployment from the ISS. The CubeSats moved away from the ISS with a relative motion of about 1 m/s (image credit: NanoRacks, NASA)
Sensor complement: (ADS-B receiver payload)
ADS-B (Automatic Dependent Surveillance-Broadcast)
Background: The main payload of GOMX-3 is the SDR ( Software Defined Radio) that can receive ADS-B signals from commercial aircraft. These signals represent periodic transmissions of data by an aircraft's Mode-S transponder at the 1090 MHz frequency (L-band), containing the aircraft ID, its position, altitude and intent. 16)
The ADS-B signals are used by air traffic control for areas in which a ground receiving architecture is present, but given the short range of the ADS-B signals, they are not useful over land areas with poor infrastructure and oceanic coverage is very limited. Nevertheless, ADS-B has become a significant part of air traffic control, being used in the same manner as information provided by radars. ADS-B will become mandatory for all aircraft in the near future and there is a strong desire to ultimately phase out the traditional radars and purely rely on ADS-B since the receivers are much easier to maintain.
The ADS-B system is today standard equipment on new commercial aircraft and it is estimated that 70% of the current fleet is equipped. Recent decisions taken by the various aviation authorities such as Eurocontrol (Brussels) and FAA (Federal Aviation Administration, Washington DC) means that ADS-B will become mandatory equipment on all high performance aircraft from 2015 and 2020, respectively.
ADS-B receiver payload:
The ADS-B receiving payload on the GOMX-3 satellite consists of a helical antenna deployed after launch, providing a 10 dB gain at the desired 1090 MHz frequency. Furthermore, the payload is comprised of an RF front end interfacing with the antenna, a FPGA (Field Programmable Gate Array) used as receiver and a Main Controller handling data acquired by the system and transmitting it for storage in the spacecraft memory. Signal decoding is provided by the FPGA, receiving and digitizing the ADS-B signal blocks that consist of a preamble for time synchronization and 112 bits of data sent at a symbol rate of 1 MHz.
Note: The ADS-B payload was also flown on GOMX-1, launched on Nov. 21, 2013. The ADS-B payload is described in the GOMX-1 file.
Figure 15: The ADS-B block diagram (image credit: GomSpace)
The RF front-end provides amplification and initial down-conversion of the signal. To compensate for the increased path-loss due to the receiver location in space in contrast to the 80 NM nominal range of the system, the RF front-end has carefully been designed to provide the required sensitivity to be able to decode the signal.
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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 (email@example.com).