NASRDA (National Space Research & Development Agency) of Abuja, Nigeria has continued its association with SSTL, with two parallel projects - the NX and NigeriaSat-2. Like NigeriaSat-1, NX is based on the SSTL-100, but is being developed by a team of 26 Nigerian trainee engineers at SSTL's facilities in England. The Nigerian engineers will completely manage the total lifecycle of the NX and will be responsible for the delivery of the satellite to full flight specification.
Capacity building is central to the implementation of the Nigeria Space Program. As part of the Know-How Technology Training (KHTT) on the NigeriaSat-2 satellite project is the development of a training model (TM) named NigeriaSat-X. The TM will be used to give the KHTT’s hands on experience in the requirements specification, project management, system engineering, manufacture, test, assembly / integration and final system testing of a spacecraft. Unlike the NigeriaSat-1 TM, NigeriaSat-X will be built to flight specification and will be launched along with NigeriaSat-2. Twelve Nigerian engineers and scientists are currently involved in the building of the satellite using SSTL facilities since September 2006.
The key features of the NigeriaSat-X include:
• 22 m next generation imager with improved resolutions and optics over the same swath areas as DMC+
• Swath of 600 km @ 8 bit
• High rate X-band downlink set to 20 Mbit/s
• Low rate S-band at 8 Mbit/s
• 2 x 2 GByte data recorders.
The NigeriaSat-X sensor will provide 22 m multi-spectral (R, G, NIR) imagery. The data volume is estimated to be a handful of images per day. The onboard storage is different between NX and N2 (NigeriaSat-2); NX uses SSDRs (Solid State Data Recorders), while N2 uses HSDRs (High Speed Data Recorders).The processing is also different. 1) 2)
The microsatellite is based on the SSTL-100i platform and is closely related to the Deimos-1 satellite platform. The overall satellite wet mass is ~ 87 kg, and the dimensions are approximately 0.6 m x 0.6 m x 0.6 m. 3)
The AODCS (Attitude and Orbit Determination and Control Subsystem) provides 3-axis momentum-biased attitude control. Dual-axis sun sensors on the four sides of the spacecraft and dual redundant vector magnetometers provide attitude knowledge by measuring the sun angle and Earth's magnetic field. Three microsatellite reaction wheels and three magnetorquer rods are used to control the attitude and attitude rates to maintain nadir attitude pointing. In addition, the AODCS features an experimental star tracker head and electronics that is integrated into the –Y panel.
The SSTL GPS receiver SGR-10 (Space GPS Receiver-10) is used to determine the orbit and to provide accurate timing information.
A heritage DMC butane cold gas monopropellant propulsion subsystem is provided for to deliver ~ 20 m/s ΔV. This can be used for small orbit change maneuvers if required. The propulsion subsystem comprises 2 cylindrical tanks holding a combined total of 2.35 kg of Butane propellant, and a single low-thrust thruster augmented by an electric resistojet to achieve a high efficiency. The system is mounted internally on the spacecraft space facing facet (-Z panel), alongside the propulsion-controller electronics. The thruster is mounted externally aligned with the satellite center of mass.
EPS (Electric Power Subsystem): Power generation is provided by three body-mounted GaAs solar panels on the –X, +X and +Y axes of the satellite. This power is managed by a BCR (Battery Charge Regulator), PCM (Power Conditioning Module), and PDM (Power Distribution Module). A raw 28 V and a regulated 5 V power bus are provided to all modules on the satellite.
A Li-ion battery with a capacity of 15 Ah provides power storage for eclipse durations and periods of peak power demand. The EPS delivers over 30 W orbit-average power (OAP) to the platform and payloads, with ~ 12 W for the platform, and ~ 16-20 W for payload operations.
Figure 1: Computer image of the NX satellite showing the Earth facing facet (+Z direction) with the six channel multispectral imager payload (image credit: SSTL)
Figure 2: Image of the NX satellite showing space facing facet (-Z direction) and the two solar arrays in the +X and +Y directions (image credit: SSTL)
DHS (Data Handling Subsystem): Two redundant Intel 386 based OBCs (On-board Computers) are used for onboard data handling. The OBCs provide communications between subsystems, monitor the temperature and current consumptions, maintain log files and execute the imaging schedule. The data handling subsystem uses a CAN (Controller Area Network) bus for onboard data exchange.
• Uplink telecommand communications is supported in the S-band frequency at 9.6 kbit/s uplink rate. The receiver subsystems are comprised of two receiver electronics modules and four receiver patch antennas providing an omni-directional antenna pattern to maintain the RF link at all times when in view of the communicating ground station.
• Downlink telemetry communications is supported in the S-band frequency at 38.4 kbit/s downlink rate. The transmitter downlink modules are comprised of one dedicated low-rate transmitter with two monopole antennas. A redundant low-rate downlink is provided by the 8 Mbit/s payload transmitter that can operate, if required, in a low-rate mode of operations.
Payload data is recorded onto two SSDRs (Solid State Data Recorders) of 2 GByte capacity per SSDR. Payload data is downlinked via either the high rate S-band transmitter operating at 8 Mbit/s, or the high rate X-band transmitter operating at 20 Mbit/s.
Figure 3: Photo of a Nigerian engineer's work on NigeriaSat-X (image credit: SSTL)
Figure 4: Photo of Nigerian engineers working on NigeriaSat-X (image credit: SSTL)
Launch: The NX spacecraft along with NigeriaSat-2 as secondary payloads was launched on August 17, 2011 on a Dnepr-1 launch vehicle using the SHM (Space Head Module) configuration from the Yasny/Dombarovsky launch site located in the Orenburg Region, Russia. In Sept. 2009, a contract was signed between ISC Kosmotras and SSTL for the launch of NigeriaSat-2 and NigeriaSat-X spacecraft. 4)
The primary payload of the cluster launch will be the Sich-2M spacecraft of NSAU (National Space Agency of Ukraine) with a launch mass of 175 kg.
The secondary payloads on this flight are:
• N2 (NigeriaSat-2) of NASRDA, Abuja, Nigeria, spacecraft mass = 270 kg
• NX (NigeriaSat-X) of NASRDA, Abuja, Nigeria, spacecraft mass = 87 kg
• EduSat of the University of Rome (Sapienza), Italy, spacecraft mass = 10 kg
• RASAT of Tubitak Uzay, Ankara, Turkey, spacecraft mass = 95 kg
• AprizeSat-5 and AprizeSat-6 of AprizeSat, Argentina built by SpaceQuest, Fairfax, VA, USA, each microsatellite features a next generation AIS (Automatic Identification System) payload. Each spacecraft has a mass of 14 kg.
• BPA-2 (Blok Perspektivnoy Avioniki-2 — or Advanced Avionics Unit-2) of Hartron-Arkos, Ukraine. The BPA-2 experimental payload remained attached to the upper stage of the Dnepr-1 launch vehicle.
Figure 5: Photo of some payloads in the payload bay of the Dnepr Launch Vehicle (image credit: GAUSS)
Deployment sequence: AprizeSat-5, AprizeSat-6, EduSat, NigeriaSat-X, RASAT, Sich-2M, and NigeriaSat-2.
Orbit: Sun-synchronous near-circular orbit, altitude = 663 km x 700 km, inclination = 98.25º, the orbital period is about 98 minutes, the equatorial nodal crossing time is at 10:15 LTAN (Local Time on Ascending Node).
• The NigeriaSat-X spacecraft and its payload are operating nominally in 2014. 5)
• NigeriaSat-X is being operated by NASDRA and is fully operational. In March 2012, SSTL handed over control of NigeriaSat-X to NASDRA, although before that there was already a gradual handover of operations. The official handover ceremony between SSTL and NASDRA took place in July 2012. 6)
- In the spring of 2012, NigeriaSat-X is already participating in a number of commercial imaging campaigns organized through DMCii. 7)
• NigeriaSat-X acquired its first satellite image just three days after the successful launch on 17 August 2011. Currently two joint NASRDA/SSTL teams are working in parallel in Abuja and Guildford to commission NigeriaSat-X and NigeriaSat-2. After the initial commissioning phase is complete, the NASRDA team in Guildford will return to Nigeria to continue NigeriaSat-X operations from the Abuja ground station. 8) 9)
Figure 6: First image of NigeriaSat-X (22 m MS) of Auckland, New Zealand observed on August 21, 2011 (image credit: SSTL, NASRDA)
• Following confirmation of separation from the launch vehicle, ground stations in Guildford (UK) and Abuja (Nigeria) established contact with NigeriaSat-2 and NigeriaSat-X, respectively, and commissioning of the satellites in their respective orbits is now progressing.
Sensor complement: (SLIM6)
SLIM6 (Surrey Linear Imager Multispectral 6 channels - but 3 spectral bands):
The upgraded SLIM6 consists of two bore-sighted instrument banks. The SLIM6 design provides for a nadir-viewing, three-band multispectral scanning camera capable of providing mid-resolution image information of the Earth's surface.
Three spectral bands are provided in the ranges: 0.52-0.62 µm (green), 0.63-0.69 µm (red), and 0.76-0.9 µm (NIR). The SLIM6 bands come closely to those of Landsat-7 bands 2, 3, and 4. SLIM6 employs the pushbroom imaging technology using two cameras per band (mounted in a double-barrel cross-track configuration - or in two banks) thus providing a dual (slightly overlapping) swath with a combined swath width of > 600 km with a spatial resolution 22 m GSD (Ground Sampling Distance) at nadir.
Figure 7: Schematic illustration of the SLIM6 configuration (image credit, SSTL)
The imager comprises six lens and sensor pairs, configured in two banks as part of an overall optical bench assembly. The banks are mounted angled away from nadir by approximately 13º, to double the swath width, but with a small overlap of approximately 5% to aid image stitching. The FOV (Field of View) from each bank is 26.6º.
The instrument design has been improved since the first generation, in order to support data quality, radiometry, and calibration. The lens system has been modified to improve the GSD from 32m to 22m, and by employing a larger dimension linear array, the same swath width can be maintained. The SNR (Signal-To-Noise) ratio goal has also increased.
On-orbit calibration techniques have evolved given experience with the first generation system, the radiometric accuracy prior and following launch, and the relative radiometric stability are specified.
Table 1: Parameters of the SLIM6 (SLIM6) instrument
Each SLIM6 imager channel has a solid-state detector at the focal plane. The spectral filters for the bands are located in front of each channel lens. To protect the filter a fused silica radiation protection window is set in front of the filter. 10)
The detector output is digitized to 12 bits and processed either to 10 bits or 8 bits radiometric resolution. The pushbroom system is capable of providing continuous imagery in flight path direction. The source data are stored in an onboard solid-state memory of 2 x 2 GByte capacity. SLIM6 features also a windowing capability. This function was introduced to avoid a saturation of the storage units and add more flexibility during satellite operations.
The operations strategy targets maximum imaging time in the sunlit phase of the orbit. Operating modes with imaging and downlinking data during each orbit, and imaging orbits followed by downlinking orbits have been developed.
The SLIM6 instrument operates in store-and-forward mode, so that images can be taken out of range of any control stations. Payload data remain stored on-board until the spacecraft is commanded to return the data once in contact with one of the network ground stations. The payload therefore comprises two banks of channels, two solid-state data recorders, and two high-speed downlinks. A balance has been struck between cost and performance. Payload operations are fundamentally resource limited by the on-board power available at the end-of-life, and by on-board storage.
In practice, the system is well balanced between these constraints, but there are cases where any of these becomes the limiting factor. Data storage becomes the limiting factor when the spacecraft does not transit stations on several successive orbits, and power is the limiting factor when the spacecraft transits several stations in a single orbit.
1) “NigeriaSat-X Mission Objectives,” URL: http://www.sst-us.com/nigeriasat-x-mission-objectives
2) Godstime Kadiri James, Joseph Akinyede, Shaba Ahmad Halilu, “The Nigerian Space program and its economic development model,” Proceedings of IAC 2011 (62nd International Astronautical Congress), Cape Town, South Africa, Oct. 3-7, 2011, paper: IAC-11-E3.3.4
3) Information provided by Joelle Sykes of SSTL (Surrey Satellite Technology Ltd.), Guildford, Surrey, UK
4) “Highly advanced NigeriaSat-2 small satellite launch date announced,” SSTL, May 11, 2010, URL: http://www.sstl.co.uk/News_and_Events/Latest_News/?story=1569
5) Information provided by Audrey Nice of SSTL and by Dave Hodgson of DMCii, Guildford, UK
6) “Nigeria receives two Earth Observation Satellites,” July 10, 2012, URL: http://www.channelstv.com/home/2012/07/10/nigeria-receives-two-earth-observation-satellites/
7) Information provided by Alex da Silva Curiel of SSTL, Surrey, UK
9) “Nigerian-built satellite acquires first image just days after launch,” SSTL, August 26, 2011, URL: http://www.technologynewsroom.com/press_releases/files/1877/11-400%20PR%20First%20NigeriaSat-X%20image%20LG%20JAS%20vPB.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.