Minimize GHGSat-C1 and C2

GHGSat-C1 and -C2, the next two microsatellites in the GHGSat constellation

Spacecraft    Launch    Sensor Complement    References

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Figure 8: Artist's rendition of the GHGSat-C1/-C2 on orbit (image credit: UTIAS/SFL, GHGSat)

 

Launch: The GHGSat-C1 (Iris) nanosatellite was launched as a passenger payload on Vega's rideshare service flight VV16, using the Small Spacecraft Mission Service (SSMS) dispenser for light satellites, launched from Europe's Spaceport in Kourou, French Guiana at 01:51 UTC, 03:51 CEST on 3 September, 2020 (22:51 local time on 2 September in Kourou). 6)

Orbit: Sun-synchronous orbit; target orbit for the 7 microsatellites: altitude of 515 km, inclination of 97.45º; target orbit for the 46 nanosatellites: altitude of 530 km, inclination = 97.51º. The nominal mission duration (from liftoff to separation of the 53 satellites) is: 1 hour, 44 minutes and 56 seconds.

In April 2019, GHGSat announced that the firm's second satellite (namely GHGSat-C1) will be known as Iris — the naming follows a company tradition to name their satellites after the firm's team's children as a symbol of the importance of its mission to future generations. Iris is expected to build on Claire's success by making it possible to monitor even more sites, more frequently, at a fraction of the cost of other technologies. 7)

Passenger payloads (53) of the Vega rideshare mission VV16

Arianespace has realized the first European "rideshare" mission for small satellites, with 53 satellites onboard the Vega launcher for 21 customers from 13 different countries. With this new SSMS (Small Spacecraft Mission Service) shared launch concept, Arianespace demonstrates its ability to respond – in an innovative and competitive manner – to institutional and commercial requirements of the growing market for small satellites. The total satellite launch mass was 1,327 kg. 8)

With the demonstration of its new SSMS service, Arianespace is strengthening its position in the growing market for small satellites. This service will soon be supplemented by the MLS (Multi Launch Service) – a similar offer available on Ariane 6, allowing Arianespace to increase the number of affordable launch opportunities for small satellites and constellations.

• ESAIL is a maritime microsatellite with a mass of 112 kg for AIS (Automatic Identification System) ship tracking operated by exactEarth. Is was built by a European manufacturing team led by the satellite prime contractor Luxspace. ESAIL features an enhanced multiple antenna-receiver configuration for global detection of AIS messages and high-resolution spectrum capture, which will enable the demonstration of advanced future services such as VDES (VHF Data Exchange System) message reception. 9)

• Lemur-2, eight 3U CubeSats built by Spire Global Inc., San Francisco, CA . These satellites carry two payloads for meteorology and ship traffic tracking. The payloads are: STRATOS GPS radio occultation payload and the SENSE AIS payload.

• TriSat is a 3U CubeSat (5 kg) imaging mission led by the University of Maribor, Slovenia. The mission is focused on remote sensing by incorporating a miniaturized multispectral optical payload as the primary instrument, providing affordable multispectral Earth observation in up to 20 non-overlapping bands in NIR-SWIR (Near to Short Wave Infrared) spectrum.

• The launch integrator company Spaceflight Inc. of Seattle WA is providing its services for four different customers with a total of 28 satellites. These are:

a) NewSat-6 (also written as ÑuSat-6), is a low Earth orbit commercial remote sensing microsatellite (43.5 kg) designed and manufactured by Satellogic S.A. with HQs in Argentina, a vertically integrated geospatial analytics company that is building the first Earth observation platform with the ability to remap the entire planet at both high-frequency and high-resolution. This is Satellogic's 11th spacecraft in orbit, equipped with multispectral and hyperspectral imaging capabilities and it will be added to the company's growing satellite constellation.

b) 14 Flock-4v, 3U CubeSats, next-generation SuperDove satellites of Planet Inc., San Francisco, they will join its constellation of 150 Earth-imaging spacecraft.

c) SpaceBEE, 12 (.25U) picosatellites of Swarm Technology which provide affordable global connectivity.

d) Tyvak-0171, an undisclosed minisatellite of Tyvak, developed by Maxar with a mass of 138 kg.

• Planet Inc. of San Francisco launches a total of 26 Flock 4v SuperDoves on this mission. They will be split into two batches on the same launch: 14 of them will be housed inside and deployed from ISL's QuadPack deployers and the remaining 12 will be deployed from D-Orbit's InOrbit Now (ION) freeflying deployment platform. 10)

• Athena, a communications minisatellite mission (138 kg) of PointView Tech LLC, a subsidiary of Facebook. The objective is to provide broadband access (internet connectivity) to unserved and underserved areas throughout the world.

• AMICalSat, a 2U CubeSat, an educational mission, developed by CSUG (University of Grenoble Alpes, France) and MSU-SINP (Lomonosov Moscow State University-Skobeltsyn Institute of Nuclear Physics, Russia). The objective is to take pictures of the Northern light in order to reconstruct the particle precipitation into the polar atmosphere. The payload is a very compact, ultra-sensitive wide filed imager (f=23mm, aperture f/1.4). Firstly, AMICal Sat will observe auroras using nadir pointing, i.e. by determining the centre of the Earth to map and link the geographical position of the auroral oval and its internal structures with solar activity. Secondly, the cubesat will perform image capture ‘in limbo' through tangential orientation with the Earth to capture the vertical profile of the auroras and match an altitude to their various emissions.

• PICASSO, a 3U CubeSat mission (mass of 3.8 kg) developed for ESA ( European Space Agency) led by BISA (Belgian Institute for Space Aeronomy), in collaboration with VTT Technical Research Center of Finland Ltd, Clyde Space Ltd. (UK) and the CSL (Centre Spatial de Liège), Belgium. The goal is to develop and operate a scientific 3U CubeSat.

• GHGSat-C1 of GHGSat Inc., Montreal, Canada, is the first of two nanosatellites (~16 kg) as the commercial follow-on to the GHGSat-D (CLAIRE) demonstration satellite developed and launched by UTIAS/SFL of Toronto in 2016. GHGSat monitors industries greenhouse gas (GHG) and air quality gas (AQG) emissions, including: oil & gas, power generation, mining, pulp & paper, pipelines (natural gas), landfill, chemicals, metals & aluminum, cement, agriculture, and transportation.

• NEMO-HD of SPACE-SI (Slovenian Center of Excellence for Space Sciences and Technologies) is a microsatellite (65 kg) developed at UTIAS/SFL of Toronto, Canada in cooperation with SPACE-SI. The NEMO-HD (Next-generation Earth Monitoring and Observation-High Definition) satellite is a high precision interactive remote sensing mission for acquiring multispectral images and real time HD video.

• FSSCat (Federated Satellite Systems on Cat) is the winner of the 2017 Copernicus Master "ESA Sentinel Small Satellite Challenge (S3)". Proposed by the Universitat Politèctica de Catalunya (UPC) and developed by a consortium composed of UPC (ES), Deimos Engenharia (PT), Golbriak Space (EE), COSINE (NL) and Tyvak International (IT).

• Phi-Sat-1 (Φ-Sat-1) is the first on-board ESA initiative (6U CubeSat) on Artificial Intelligence (AI) promoted by the Φ Department of the Earth Observation Directorate and implemented as an enhancement of the FSSCat mission. Among mission objectives, scientific goals are Polar Ice and Snow monitoring, soil moisture monitoring, terrain classification and terrain change detection (i.e. hazard detection and monitoring, water quality), while technological goals are optical Inter-Satellite Link (OISL) demonstration.

• The RTAFSAT-1 (Royal Thai Air Force Satellite-1) mission, also referred to as NAPA-1, is a 6U CubeSat, the first remote sensing CubeSat mission for Thailand. The satellite will carry out an Earth Observation Demonstration mission with SCS Gecko Camera and Simera TriScape-100 payloads; the designed lifetime is 3 years.

• DIDO-3, a commercial 3U CubeSat mission of SpacePharma. The objective is to gather data by researching the effects of a microgravity environment on biological materials. SpacePharma from Israel will be is on board of SSMS POC with DIDO-3 Nanosatellite to perform biological experiment under Microgravity for several customers involved in pharmaceutical business, supported by Italian Space Agency (ASI) and Israeli Space Agency (ISA). Dido-3 will be monitored from the Ground Station developed by SpacePharma in Switzerland.

• SIMBA (Sun-Earth Imbalance), a 3U CubeSat mission led by the Royal Meteorological Institute Belgium, The objective is to measure the TSI (Total Solar Irradiance) and Earth Radiation Budget climate variables with a miniaturized radiometer instrument. This mission will help in the study of the global warming. This science mission will have a design lifetime of 3 years and the satellite performances will be monitored from ground station located in The Netherlands.

• TARS-1, a 6U CubeSat of Kepler Communications, developed at ÅAC Clyde Space for IoT (Internet of Things) applications. TARS-1 features deployable solar arrays, software defined radios (SDR), a narrowband communications payload and high gain antennas.

• OSM-1 Cicero, the first nanosatellite developed in Monaco by OSM (Orbital Solutions Monaco engineers, a 6U CubeSat with a mass of ~10 kg) based on the Tyvak Nano-Satellite Systems design. OSM plans to build nanosatellites to gather environment and climate data.

• TTU100, a 1U CubeSat developed at the Tallin University of Technology, Estonia. The objective is to test earth observation cameras and high-speed X-band communications. It will perform remote sensing in the visible and IR electromagnetic spectrum.

• UPMSat-2 (Universidad Politecnica de Madrid Satellite-2), a microsatellite (45 kg) of UPM.

 


 

Sensor complement (Imaging Spectrometer, Optical Downlink)

Imaging Spectrometer

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.

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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.

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Figure 10: Darkstar internal space terminal layout (image credit: Sinclair Interplanetary) 11)

 


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/viewcontent.cgi?article=3355&context=smallsat

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_communications_and_biennial_
reports/application/pdf/82051493_canada-
nc7-br3-1-5108_eccc_can7thncomm3rdbi-report_en_04_web.pdf

4) "GHGSat-C1/C2," GHGSat, January 2017, URL: http://www.ghgsat.com/who-we-are/our-satellites/satellite-2/

5) "Space Flight Laboratory (SFL) to Develop Microsatellites for Greenhouse Gas Monitoring," UTIAS/SFL, 24 March 2017, URL: https://www.utias-sfl.net/?p=2541

6) "Vega return to flight proves new rideshare service," ESA Enabling & Support, 3 September 2020, URL: https://www.esa.int/Enabling_Support/Space_Transportation/Vega/
Vega_return_to_flight_proves_new_rideshare_service

7) "GHGSat's Second Satellite Named and to Launch in August," Satnews Daily, 4 April 2019, URL: http://www.satnews.com/story.php?number=812237862

8) "With Vega, Arianespace successfully performs the first European mission to launch multiple small satellites," Arianespace Press Release, 3 September 2020, URL: https://www.arianespace.com/press-release/with-vega-arianespace-successfully-
performs-the-first-european-mission-to-launch-multiple-small-satellites/

9) "Vega flight VV16," Wikipedia, URL: https://en.wikipedia.org/wiki/Vega_flight_VV16

10) Mike Safyan, "Planet's First Launch of 2020: 26 SuperDoves on a Vega," Planet, 13 February 2020, URL: https://www.planet.com/pulse/planets-first-launch-of-2020-26-superdoves-on-a-vega/

11) 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/viewcontent.cgi?article=3605&context=smallsat
 


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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