GHGSat-C1 and C2
GHGSat-C1 and -C2, the next two microsatellites in the GHGSat constellation
In June 2016, GHGSat-D (Claire) was launched, becoming the first high-resolution microsatellite designed to measure greenhouse gas emissions from point sources, such as industrial facilities and power plants. The bus was provided by the UTIAS/SFL (University of Toronto, Institute for Aerospace Studies /Space Flight Laboratory) under contract to GHGSat Inc. of Montreal, Canada. Claire has successfully demonstrated greenhouse gas measurements around the world, and several such measurements of methane emissions have been released publicly in the last year. In order to extend the service capability and as a precursor to a full constellation, GHGSat-C1 and GHGSat-C2 are the next two microsatellites under development. 1)
With a mass of approximately 16 kg each, the design follows its predecessor Claire in leveraging SFL's Next Generation Earth Monitoring and Observation (NEMO) bus. Bus platform modifications such as enhanced electromagnetic compatibility and hardware redundancy will result in increased performance and reliability. Enhancements to the payload include reduced stray light, onboard calibration capability, and additional radiation mitigation. Furthermore, the inclusion of an optical downlink as a technology demonstrator will result in greater data downlink capacity. These upgrades will be entirely accomplished with the same volume and power constraints as Claire. The development of the GHGSat-C1 and GHGSat-C2 satellites is currently underway and the first of the two is scheduled for launch at the beginning of 2019.
Mission objectives: GHGSat's idea started with the implementation of carbon cap-and-trade programs in various Canadian provinces and US states. 2) Several provinces in Canada have carbon pricing systems in place. British Columbia has had a carbon tax in place since 2008, and Quebec and Ontario adopted a cap-and-trade system in 2013 and 2017, respectively. The vision of GHGSat is to be the global standard for emissions across the world. The GHGSat microsatellites have a different observing strategy than other satellites on orbit with the capability of detecting carbon dioxide and methane. Whereas previous satellites have had spatial resolution on the order of kilometers, the spatial resolution of the GHGSat microsatellites is less than 50 m.
Background: The increase of atmospheric concentration of greenhouse gases (GHG), such as carbon dioxide (CO2) and methane (CH4) is one of the factors contributing to Earth's changing climate. Similar to other industrialized countries, in Canada carbon dioxide is the primary GHG emitted through human activities via the combustion of fossil fuels. The second highest emission is methane, sources of which include livestock, landfills, coal mines, and wastewater management. Canada takes significant steps to address climate change by implementing national plans to reduce GHG emissions and by transitioning to a clean growth economy. These goals flow down to all sectors of the economy and a report is submitted annually to the United Nations Framework Convention on Climate Change (UNFCCC) that includes estimates of the CO2 equivalent in six economic sectors, shown in Figure 1.
Figure 1: Canada's GHG emissions by economic sector, breakdown by IPCC (Inter-Governmental Panel for Climate Change) Sector 2015 3)
GHGSat-C1/-C2 Satellite Platform
In order to expand satellite system capacity after the successful launch of its demonstration satellite, GHGSat started the design of two (2) additional high-resolution satellites in January 2017, called GHGSat-C1 and GHGSat-C2 ,respectively. GHGSat-C1/-C2 are intended to have similar designs to GHGSat-D, while applying critical lessons learned to improve performance. 4)
In March 2017, UTIAS/SFL (University of Toronto Institute for Aerospace Studies/Space Flight Laboratory) has been contracted by GHGSat Inc. of Montreal to develop the GHGSat-C1 and C2 greenhouse gas monitoring satellites. 5)
SFL has begun development of the GHGSat-C1 and C2 satellites at its Toronto facility with planned launches in late 2018 and early 2019, respectively. Serving as GHGSat's first two commercially operating satellites, they will be identical to each other but contain incremental, yet significant, enhancements from the demonstration mission.
The design phase for two new, high-resolution satellites was completed in 2017. Changes made to the payload include (Ref. 4):
• Improved stray light / ghosting mitigation
• Addition of onboard calibration features
• Improved radiation mitigation
• Optimized spectroscopy for primary instrument
• Replacement of secondary instrument
• Addition of experimental optical downlink
Overall, GHGSat expects an order-of-magnitude performance improvement from this design, within the same volume, mass and power constraints of the demonstration satellite (Ref. 1).
The Next Generation Earth Monitoring and Observation (NEMO) platform is SFL's first bus designed for microsatellite missions. This standard bus consists of two trays and six panels as seen in Figure 2. This supports a main payload mass of maximum 6 kg with an overall spacecraft mass ranging from 10-20 kg depending on secondary payloads and optional SFL avionics required to meet current and future missions. The GHGSat-C1 will have a mass of ~16 kg.
The nominal volume is 20 x 30 x 40 cm with a peak power between 50-100 W. Higher power is achieved through optional pre-deployed solar wings. The bus design supports a main payload of volume 8000 cm3 with a payload power of up to 45 W and 40% duty cycle.
The NEMO platform supports various ACS (Attitude Control Subsystem) sensors and actuators from sun sensors, magnetometer and rate sensors to magnetorquers and reaction wheels. This gives an ACS stability of approximately two degrees. When missions demands are higher, a star tracker can be used which increases ACS stability to about 10-60 arcseconds. The bus also supports GPS.
The typical NEMO platform uses an UHF uplink and S-band downlink. The standard uplink rate is 4 kbit/s with a downlink rate between 32 kbit/s and 2 Mbit/s. The platform can be further enhanced to use an S-band uplink.
Depending on deployed appendages, the NEMO platform is compatible with two SFL separation systems: the XPOD Duo and XPOD Delta. These separation systems are compatible with multiple launch vehicles.
Figure 2: GHGSat-D structure exploded view (image credit: UTIAS/SFL)
Improvements for GHGSat-C: The mechanical design of GHGSat-C1/-C2 has been constrained to match the design of GHGSat-D as closely as possible. Changes have been made due to upgrades in SFL hardware, updates to the primary payload, additions of new payloads, and to decrease the EMI sensitivity of the bus. The spacecraft exterior solid model is shown in Figure 3.
Figure 3: GHGSat-C1 exterior solid model (image credit: UTIAS/SFL)
SFL Hardware Upgrades: Several updates to the S-band transmitter have been implemented across all NEMO-class satellites to improve performance. Coaxial cables with improved shielding and filters between the transmitter and the antennas were added. These cables offer reduced insertion loss and, along with the higher powered S-band radio that has been baselined, will increase the transmit gain allowing for increases in data transfer using just S-band communications. These improvements have been proven during ground testing of other SFL spacecraft.
Firecode functionality has also been shifted from each individual onboard computer (OBC) to a separate firecode board, which interfaces with the radio and OBCs directly. This was done to further improve resilience to radiation-induced upsets. Additional upgrades to the OBC have been made but do not affect the mechanical interfaces within the bus.
A fourth reaction wheel has been added as a redundancy. It uses a skew orientation where it has control authority in all three of the principal spacecraft axes and thus can act as a backup. Figure 4 shows the four-wheel configuration.
Figure 4: Four reaction wheel configuration (image credit: UTIAS7SFL)
A permanent magnet has also been included to prevent an undesirable attitude where the payload face – one that has no solar panels – is locked in an attitude that faces the sun. This orientation would result in a power negative state until the satellite is commanded out of that attitude or environmental disturbances cause the satellite's attitude to drift. The magnet was sized to impart a permanent dipole to the satellite which would interact with Earth's magnetic field inducing a torque that would prevent it from getting stuck in that "sun-stare" attitude.
Addition of New Payloads: The addition of Darkstar has resulted in several changes to the GHGSat-C1/-C2 spacecraft. Mechanically speaking a new bracket needed to be designed to simultaneously ensure that the laser downlink had adequate line-of-sight to its ground station while maintaining a sufficient star tracker viewing angle to prevent impingement from the sun or Earth in nominal operations. A much larger cutout in the panel was required to accommodate this and a split panel design was implemented to facilitate both reducing the EMI sensitivity and ease of assembly as seen in Figure 5. By installing each part of the panel around Darkstar the actual size of the panel cutout was minimized reducing the effect of electromagnetic interference on the satellite avionics.
Figure 5: Split panel design to accommodate Darkstar (image credit: UTIAS/SFL)
Due to frequency conflicts between the Q8 processor high speed data transfer connection and other spacecraft components it was necessary to contain the board within an enclosure and shield the associated cabling to prevent undesired interferences. The addition of this payload posed some challenges because of extremely tight clearances especially with moving components such as the reaction wheels. The enclosure placement within the bus can be seen in Figure 6.
Figure 6: Q8 processor enclosure (image credit: UTIAS/SFL)
EMI (Electromagnetic Interference) Sensitivity Reduction: To further improve uplink sensitivity of the bus EMI gaskets were added to the primary spacecraft panels. Adjustments to the panel-to-panel and panel-to-tray interfaces were required to ensure there was sufficient material present for the gasket cutouts. Additionally, EMI gaskets were added to the UHF enclosure, as shown in Figure 7.
Figure 7: EMI gaskets for UHF (shown in red) and panel-tray (shown in green), image credit: UTIAS/SFL
Future Constellation Plans: The second and third GHGSat microsatellites will assure continuity of observations and will expand GHGSat's customer capacity. More satellites will enable more frequent tracking of sites. GHGSat-C1/-C2 are the first two satellites in a planned constellation of GHG monitoring satellites.
Figure 8: Artist's rendition of the GHGSat-C1/-C2 on orbit (image credit: UTIAS/SFL, GHGSat)
Launch: The GHGSat-C1 microsatellite is scheduled for launch as a secondary payload in Q2 of 2019.
Orbit: Sun-synchronous orbit, altitude of ~500 km, LTDN (Local Time on Descending Node) of 10:30 hours.
Sensor complement (Imaging Spectrometer, Optical Downlink)
The Imaging Spectrometer, operating in the SWIR (Short-Wave Infrared) region, is based on a Fabry-Perot (FP) interferometer. The optical design of the instrument includes three lens groups, in addition to the Fabry-Perot interferometer, as well as beam folding mirrors required to fit the telescope within a microsatellite bus. The payload avionics includes a Q7 hybrid processor provided by Xiphos Systems Corporation.
GHGSat-D flew the first iteration of this payload, shown in Figure 9. Each target observation produces approximately 200,000 measurements of the atmospheric radiance in the SWIR region.
Figure 9: Photo of the GHGSat-D fully integrated payload (image credit: GHGSat)
Several changes were made to the payload for GHGSat-C1/-C2 to improve upon the performance of GHGSat-D. These include upgrades to further mitigate stray light, ghosting, spectral bandpass inefficiencies, and radiation effects.
Stray light is extra light observed by the optical payload which was not intended to be observed. During nominal satellite operations, it was determined that up to 5% stray light was encountered on GHGSat-D with much of that being from off-axis incoming light. To improve upon this, several elements of the optical system were redesigned.
Ghosting is rotated, reflected, zoomed, or translated copies of the intended image. To improve upon this, anti-reflective (AR) coatings were either updated or newly applied to various surfaces in the optical system.
The GHGSat-D payload restricted the incident spectral passband to a wavelength region between 1600-1700 nm, which was selected for the presence of spectral features of methane and carbon dioxide, as well as relatively little interference from other atmospheric species. However, attempting to capture both methane and carbon dioxide lines proved to be inefficient. Therefore, the passband was narrowed in order to focus on methane.
Radiation in the space environment affected the detector. To help increase the lifetime of the detector radiation shielding was introduced around the IR camera for GHGSat-C1/-C2.
GHGSat-D also had a cloud and aerosol camera secondary payload, which ultimately did not contribute significantly to the mission. Therefore it was replaced with a visible light auxiliary camera, providing higher resolution imagery to improve image alignment and georeferencing.
Darkstar (Experimental Optical Downlink)
The downlink is currently the system bottleneck, given that the payload can generate data faster than it can be downlinked to the ground. GHGSat-D, along with GHGSat-C1/-C2, have an S-band transmitter for downlinking payload data and satellite bus telemetry. This system can achieve downlink rates of up to 2 Mbit/s. GHGSat-C1/-C2 are therefore testing an experimental laser downlink system that is intended to achieve downlink rates of up to 1 Gbit/s. Designed by Sinclair Interplanetary, the wavelength of the laser is785 nm, in the near-IR range.
The optical downlink is built on the reverse side of the optical bench of an expanded Sinclair Interplanetary ST-16RT2 star tracker, shown in Figure 10. Its mass is less than 400 g and its outer dimensions are 100 x 68 x 68 mm.
Darkstar interfaces with a Q8 processor designed by Xiphos Systems Corporation. The Q8 processor is a new design and is also included on GHGSat-C1 in order to gain flight heritage. The Q8 is connected to the payload avionics via an Ethernet connection for high speed data transfer. Although the primary means of downlinking payload data will still be over the S-band transmitter, the connection between the Q8 and the payload avionics exists in order to test and characterize the optical downlink system with the large volume of data generated by the main payload.
1) Laura M. Bradbury, Michael Ligori, Robert Spina, Daniel Kekez, Pawel Lukaszynski, Robert E. Zee, Stephane Germain, "On-Orbit Greenhouse Gas Detection with the GHGSat Constellation," Proceedings of the 69th IAC (International Astronautical Congress) Bremen, Germany, 1-5 October 2018, paper: IAC-18.B4.4.8
2) Stephane Germain, Berke Durak, Jason McKeever, Vincent Latendresse, Cordell Grant, James J. Sloan, "Global Monitoring of Greenhouse Gas Emissions," Proceedings of the 30th Annual AIAA/USU SmallSat Conference, Logan UT, USA, August 6-11, 2016, paper: SSC16-III-11, URL: https://digitalcommons.usu.edu/cgi/
3) "Canada's 7th National Communication and 3rd Biennial Report," Gatineau, QC: Environment and Climate Change Canada, 2017, URL: https://unfccc.int/files/national_reports/national_
4) "GHGSat-C1/C2," GHGSat, January 2017, URL: http://www.ghgsat.com/who-we-are/our-satellites/satellite-2/
6) Doug Sinclair, Kathleen Riesing, "The Rainbow Connection -Why Now is the Time for SmallSat Optical Downlinks," Proceedings of the 31st Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 5-10, 2017, paper: SSC17-II-06, URL: https://digitalcommons.usu.edu/cgi/viewc
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 (firstname.lastname@example.org).