SkySat constellation of Terra Bella - formerly SkySat Imaging Program of Skybox Imaging
SkySat is a commercial Earth observation microsatellite of Skybox Imaging Inc. (Mountain View, CA, USA), licensed to collect high resolution panchromatic and multispectral images of the Earth. 1) 2) 3) 4)
Google's Skybox Imaging has a new name and business model as of March 8, 2016 (see Mission Status).
Background: Skybox Imaging (Skybox) provides global customers easy access to reliable and frequent high-resolution images of the Earth by designing and building microsatellites and cloud services. By operating the world's first coordinated microsatellite constellation, Skybox aims to empower commercial and government customers to make more informed, data-driven decisions that will improve the profitability of companies and the welfare of societies around the world. Founded in Silicon Valley in 2009 by four graduate students at Stanford University, Skybox is backed by leading venture firms and comprised of internet and aerospace professionals.
Skybox Imaging is looking at two distinct markets for their imagery and video: various environmental applications, including monitoring agriculture, forestry, and other natural resources; and asset tracking, where spacecraft images help customers monitor various facilities for changes. Those plans have won Skybox a significant amount of VC (Venture Capital) funding. In 2012, the company raised $70 million in a Series C round of financing, bringing the total raised by the company to $91 million. Khosla Ventures, Bessemer Venture Partners, Canaan Partners, and Norwest Venture Partners, VC firms who have a significant Silicon Valley presence, have all invested in the company. 5)
Figure 1: The co-founders of Skybox Imaging (left to right): Dan Berkenstock, Ching-Yu Hu, Julian Mann and John Fenwick (image credit: Skybox Imaging) 6)
• In May 2013, Skybox announced it has entered into a multi-year, strategic partnership with Japan Space Imaging (JSI), a subsidiary of Mitsubishi Corporation, to provide high-resolution imagery and full motion commercial video to the Japanese market. The agreement, subject to U.S. regulatory approval, will enable JSI to directly task, downlink and receive imagery from Skybox's constellation of microsatellites on a reliable and frequent basis. 7) 8) 9) 10)
• After building its first two satellites, Skybox hired SS/L (Space Systems/Loral) to build the next 13 improved spacecraft and Orbital Sciences Corp. to launch six in late 2015 on a Minotaur-C rocket from Vandenberg Air Force Base in California. Skybox plans to offer customers timely access to still imagery, full-motion video and data services. 11)
- SSC Corp.’s ECAPS division will provide propulsion systems for 12 satellites to be built for imagery services of startup Skybox Imaging, the companies announced March 11. ECAPS (Ecological Advanced Propulsion Systems, Inc., Solna, Sweden), a subsidiary of Sweden-based SSC, was already under contract to supply the propulsion system for that satellite, dubbed SkySat-3, and now has an order for the remaining 12. 12)
- SkySat-3 is expected to be launched in the summer of 2016.
• On June 10, 2014, Skybox announced that it had entered into an agreement to be acquired by Google for US$500 million. The acquisition was completed on August 1, 2014. Skybox is now a subsidiary of Google (Ref. 58).
• In January 2016, Arianespace announced it signed a contract with Skybox Imaging to launch four SkySat minisatellites (SkySat-4 though -7) on a Vega vehicle from Kourou in the summer of 2016, along with PeruSat-1 of the Peruvian Armed Forces.
Figure 2: Illustration of the SkySat-1 and -2 microsatellites (left) to the second generation SkySat-3 minisatellite (image credit: Skybox, Ref. 65)
SkySat-3 will be different than SkySat-1, and -2 in these ways:
- smaller pixels
- increased agility to collect more area
- propulsion for orbit stationing.
SkySat-1 and SkySat-2 are microsatellites built and operated by Skybox Imaging that are licensed to acquire high resolution panchromatic and multispectral images of Earth. The spacecraft are three-axis stabilized using an on-board closed-loop control system. Each satellite has a mass of 83 kg and features body-mounted solar panels. The microsatellites feature an aperture cover that protects the imaging payload during launch and initial orbital operations. The cover also hosts the high-data rate antenna of the satellite. The spacecraft will acquire high-resolution images and video of Earth. 13)
Figure 3: Photo of the SkySat-1 microsatellite in the clean room of Skybox Imaging (image credit: Skybox Imaging)
Table 1: Parameters of the SkySat-1 and SkySat-2 spacecraft parameters
Flight qualification of the star trackers conducted on orbit for the SkySat-1 mission: 14)
The performance of the two ST-16 star trackers, developed at Sinclair Interplanetary,fell initially significantly below expectations. Concerned by these results, engineers at Skybox Imaging (SB), Sinclair Interplanetary (SI) and Ryerson University (RU) embarked on an aggressive and comprehensive flight qualification program to understand the causes of these problems and to re-attain the expected performance targets. Two months later (February, 2014), the the investigative project made the last of a sequence of software, catalog and parameter modifications that have met these goals.
Table 2: Key parameters of the ST-16 Star Tracker
Figure 4: Photo of the Sinclair Interplanetary ST-16 Star Tracker (image credit: SI)
Collaborative relationship: Restoring the star trackers to full function was do-or-die for both Skybox Imaging and Sinclair Interplanetary. Skybox had invested in the spacecraft, and Sinclair in the star tracker product, and neither could afford to fail. While stressful, this unity of purpose was in no small part responsible for timely success. Skybox operations was extremely accommodating in collecting and delivering large quantities of data. Sinclair and Ryerson focused exclusively on this problem for a two month period. In a more relaxed and less motivated environment the necessary advances might not have been made.
In summary, there were many improvements being done to sensor processing on the ST-16 that were necessary to bring the sensor performance back up to their intended specifications. These changes included improvements in the logic for star detection, star measurement, rate estimation, and catalog management. Together the algorithmic improvements yielded higher availability, better accuracy, and much-lower bad-match rate. Although new launches may require a short qualification period to tune calibration and operating parameters, the project expects that the core software is stable.
Figure 5: SkySat-1 and SkySat-2 deployed configuration (image credit: SkyBox Imaging) 15)
Launch: The SkySat-1 microsatellite was launched on Nov. 21, 2013 as a secondary payload on a Dnepr launch vehicle from the Dombarovsky (Yasny Cosmodrome) launch site in Russia. The launch provider was ISC (International Space Company) Kosmotras. 16) 17) 18) 19)
The primary payloads on this flight were DubaiSat-2 of EIAST (Emirates Institute for Advanced Science and Technology), a minisatellite of UAE (United Arab Emirates) with 300 kg, and STSat-3, a minisatellite of KARI, Korea (~150 kg).
The secondary payloads on this flight were:
• SkySat-1 of Skybox Imaging Inc., Mountain View, CA, USA, a commercial remote sensing microsatellite of ~83 kg.
• WNISat-1 (Weathernews Inc. Satellite-1), a nanosatellite (10 kg) of Axelspace, Tokyo, Japan.
• BRITE-PL-1, a nanosatellite (7 kg) of SRC/PAS (Space Research Center/ Polish Academy of Sciences of Warsaw, Poland.
• AprizeSat-7 and AprizeSat-8, nanosatellites of AprizeSat, Argentina (SpaceQuest)
• UniSat-5, a microsatellite of the University of Rome (Universita di Roma “La Sapienza”, Scuola di Ingegneria Aerospaziale). The microsatellite has a mass of 28 kg and a size of 50 cm x 50 cm x 50 cm. When on orbit, UniSat-5 will deploy the following satellites with 2 PEPPODs (Planted Elementary Platform for Picosatellite Orbital Deployer) of GAUSS:
- PEPPOD 1: ICube-1, a CubeSat of PIST (Pakistan Institute of Space Technology), Islamabad, Pakistan; HumSat-D (Humanitarian Satellite Network-Demonstrator), a CubeSat of the University of Vigo, Spain; e-st@r-2 (Educational SaTellite @ politecnico di toRino-2), of Politecnico di Torino, Italy; PUCPSat-1 (Pontificia Universidad Católica del Perú-Satellite), a 1U CubeSat of INRAS (Institute for Radio Astronomy), Lima, Peru; Note: PUCPSat-1 intends to subsequently release a further satellite Pocket-PUCP) when deployed on orbit. 20)
- PEPPOD 2: Dove-4, a 3U CubeSats of Cosmogia Inc., Sunnyvale, CA, USA
MRFOD (Morehead-Roma FemtoSat Orbital Deployer) of MSU (Morehead State University) is a further deployer system on UniSat-5 which will deploy the following femtosats:
- Eagle-1 (BeakerSat), a 1.5U PocketQub, and Eagle-2 ($50SAT) a 2.5U PocketQub, these are two FemtoSats of MSU (Morehead State University) and Kentucky Space; Wren, a FemoSat (2.5U PocketQub) of StaDoKo UG, Aachen, Germany; and QBSout-1, a 1U PocketQub testing a finely pointing sun sensor.
• Delfi-n3Xt, a nanosatellite (3.5 kg) of TU Delft (Delft University of Technology), The Netherlands.
• Triton-1 nanosatellite (3U CubeSat) of ISIS-BV, The Netherlands
• CINEMA-2 and CINEMA-3, nanosatellites (4 kg each) developed by KHU (Kyung Hee University), Seoul, Korea for the TRIO-CINEMA constellation.
• GATOSS (former GOMX-1), a 2U CubeSat of GomSpace ApS of Aalborg, Denmark
• NEE-02 Krysaor, a CubeSat of EXA (Ecuadorian Civilian Space Agency)
• FUNCube-1, a CubeSat of AMSAT UK
• HiNCube (Hogskolen i Narvik CubeSat), a CubeSat of NUC (Narvik University College), Narvik, Norway.
• ZACUBE-1 (South Africa CubeSat-1), a 1U CubeSat (1.2 kg) of CPUT (Cape Peninsula University of Technology), Cape Town, South Africa.
• UWE-3, a CubeSat of the University of Würzburg, Germany. Test of an active ADCS for CubeSats.
• First-MOVE (Munich Orbital Verification Experiment), a CubeSat of TUM (Technische Universität München), Germany.
• Velox-P2, a 1U CubeSat of NTU (Nanyang Technological University), Singapore.
• OPTOS (Optical nanosatellite), a 3U CubeSat of INTA (Instituto Nacional de Tecnica Aerospacial), the Spanish Space Agency, Madrid.
• Dove-3, a 3U CubeSats of Cosmogia Inc., Sunnyvale, CA, USA
• CubeBug-2, a 2U CubeSat from Argentina (sponsored by the Argentinian Ministry of Science, Technology and Productive Innovation) which will serve as a demonstrator for a new CubeSat platform design.
• BPA-3 (Blok Perspektivnoy Avioniki-3) — or Advanced Avionics Unit-3) of Hartron-Arkos, Ukraine.
Deployment of CubeSats: Use of 9 ISIPODs of ISIS, 3 XPODs of UTIAS/SFL, 2 PEPPODs of GAUSS, and 1 MRFOD of MSU.
Orbit: Sun-synchronous near-circular orbit, altitude = 600 km, inclination = 97.8º, LTDN (Local Time on Descending Node) = 10:30 hours.
Launch: The SkySat-2 microsatellite was launched as a secondary payload on July 8, 2014 (15:58:28 UTC) with a Soyuz-2.1b/Fregat launch vehicle of NPO Lavochkin. The launch site was the Baikonur Cosmodrome, Kazakhstan. The primary payload on this flight was the Meteor-M-2 spacecraft of Roskosmos/Roshydromet/Planeta (Moscow, Russia). 21) 22) 23) 24)
Secondary payloads on this flight were:
• MKA-PN2 (Relek), a microsatellite of Roskosmos, S/C developer NPO Lavochkin on the Karat platform (59 kg, study of energetic particles in the near-Earth space environment (ionosphere) including the Van Allen Belts.
• DX-1 (Dauria Experimental-1), the first privately-built and funded Russian microsatellite (22 kg) of Dauria Aerospace, equipped with an AIS (Automatic Identification System) receiver to monitor the ship traffic. 25)
• TechDemoSat-1 of UKSA/SSTL, UK with a mass of 157 kg
• SkySat-2 of Skybox Imaging Inc. of Mountain View, CA, USA, a commercial remote sensing microsatellite of 83 kg.
• M3MSat dummy payload of 80 kg.
• AISSat-2, a nanosatellite with a mass of ~7 kg of FFI (Norwegian Defense Research Establishment) Norway, built by UTIAS/SFL, Toronto, Canada.
• UKube-1, a nanosatellite (~3.5 kg) of UKSA/Clyde Space Ltd., UK.
Orbit of Meteor-M2: Sun-synchronous circular orbit , altitude of ~ 825 km, inclination = 98.8º, period = 101.41 minutes, LTAN (Local Time on Ascending Node) at 9:30 hours.
Orbit of the secondary payloads: Sun-synchronous near-circular orbit, altitude of ~ 635 km, inclination = 98.8º. The MKS-PN2 (Relek) was released first of the secondary payloads into an elliptical orbit of 632 km x 824 km.
Launch: The SkySat-3 microsatellite was launched as a secondary payload on June 22, 2016 (03:56 UTC) aboard a PSLV vehicle of ISRO (PSLV-C34 flight) from SDSC (Satish Dhawan Space Center) SHAR (main launch center of ISRO on the south-east coast of India, Sriharikota). The CartoSat-2C mission was the primary payload on this flight with a launch mass of 727.5 kg. The total mass of all satellites onboard was 1288 kg. 26)
Orbit: Sun-synchronous orbit, altitude = 515 km, inclination = 97.56º.
The secondary payloads (19 satellites) on this flight were:
• SkySat-3, also referred to as SkySat-C1, an imaging minisatellite of Terra Bella of Mountain View, CA, USA. The first satellite of the SkySat constellation with a HPGP (High Performance Green Propulsion System).
• GHGSat, a microsatellite (15 kg) of GHGSat Inc., Montreal, Canada
• BIROS (Bi-spectral InfraRed Optical System), a minisatellite 130 kg) of DLR, Germany.
- BIROS carries onboard the picosatellite BEESAT-4 (Berlin Experimental and Educational Satellite-4) of TU Berlin(1U CubeSat, 1 kg) and release it through a spring mechanism [ejection by SPL (Single Picosatellite Launcher) after the successful check-out and commissioning of all relevant BIROS subsystems]. After separation, it will perform experimental proximity maneuvers in formation with the picosatellite solely based on optical navigation.
• M3MSat (Maritime Monitoring and Messaging Microsatellite) of DRDC (Defence Research and Development Canada) and CSA (Canadian Space Agency).
• LAPAN-A3, a microsatellite (115 kg) of LAPAN (National Institute of Aeronautics and Space of Indonesia) Jakarta, Indonesia.
• SathyabamaSat, a 2U CubeSat of Sathyabama University (1.5 kg), India.
• Swayam, a 1U CubeSat of the College of Engineering (1 kg), Pune, India.
• 12 Flock-2p Earth observation satellites (3U CubeSats) of Planet Labs (each with a mass of 4.7 kg), San Francisco, CA.
Figure 6: The 20 satellites, with the primary payload CartoSat-2C on top) are packaged inside the PSLV’s payload fairing. The number marks the most satellites ever launched by India on a single flight (image credit: ISRO)
Figure 7: Photo of the SkySat-3 minisatellite with a mass of ~ 120 kg and the integrated HPGP propulsion system (red boxes), image credit: Terra Bella
Launch: Four SkySat minisatellites (SkySat-4 through SkySat-7) of Terra Bella, secondary payloads to PeruSat-1 (primary payload of the Peruvian Armed Forces), were launched on September 16, 2016 (01:43:35 UTC) on a Vega vehicle of Arianespace from Kourou. 27)
Orbit: Sun-synchronous orbit, altitude = 695 km, inclination = 98.3º.
• SkySat-4 to -7. The four imaging minisatellites of TerraBella (former SkyBox Imaging, Mountain View, CA, USA) are part of this mission. The four secondary payloads are integrated in the upper position atop the VESPA (Vega Secondary Payload Adaptor) dispenser system, and will be released one-by-one during the flight sequence's 40-minute mark, to be followed by PeruSat-1's separation approximately one hour and two minutes later. 28)
The SkySat satellites, each with a mass of approximately 110 kg, will be used to provide very-high-resolution maps of the entire Earth, augmenting the existing three on orbit satellites for new Arianespace customer Terra Bella, a Google company.
Terra Bella’s satellites — SkySat-4, -5, -6 and -7 — separated from the Vega rocket’s upper stage over a ground station in South Korea about 40 minutes after liftoff into an orbit about 500 km above Earth. 29)
Figure 8: Artist’s concept of the four SkySat satellites deploying one after another from the Vega rocket’s upper stage (image credit: Arianespace)
Launch: On Oct. 31, 2017 (21.37 UTC), six SkySat minisatellites of Terra Bella (a Planet Labs company) and 4 Dove (Flock-3m) nanosatellites of Planet Labs were launched on a Minotaur-C vehicle of Orbital ATK from VAFB, CA (SLC-576E). The Minotaur-C is an upgraded, renamed version of the Orbital Sciences Taurus rocket. Approximately 12 minutes into flight, the ten commercial Planet spacecraft deployed into their targeted sun synchronous orbit of 500 km altitude. 30) 31)
Orbit: Sun-synchronous near-circular orbit, altitude of ~500 km, inclination of ~97º.
Figure 9: Illustration of the launch sequence (image credit: Orbital ATK) 32)
The launch was the first time Planet was the primary customer for a launch, having relied on secondary payload accommodations for all its previous launches. That meant that, for this mission, the company was able to choose the orbit and time of the launch, said Mike Safyan, senior director for launch and global ground stations at Planet, in post-launch statement.
“We sent these 10 satellites to an afternoon crossing time of approximately 13:30 hour to further diversify our product offering,” said Safyan. Most remote sensing satellites, he said, operate in morning-crossing sun synchronous orbits, including the company’s other Dove and SkySat spacecraft. “Having the world’s largest fleet of medium and high-resolution assets in both morning and afternoon crossing times enables a dataset never before provided in the commercial market at this scale,” he said. 33)
Figure 10: The six SkySats and four Doves were enclosed inside the Minotaur-C’s payload fairing earlier in October (image credit: FAA/Orbital ATK)
Launch: The SkySat-14 and -15 microsatellites (100 kg each) of the SSO-A rideshare mission of Spaceflight were launched on 3 December 2018 (18:34 GMT) on a SpaceX Falcon-9 Block 5 vehicle from VAFB (Vandenberg Air Force Base) in California, 34)
SpaceX statement: On Monday, December 3rd at 10:34 a.m. PST (18:34 GMT), SpaceX successfully launched Spaceflight SSO-A: SmallSat Express to a low Earth orbit from Space Launch Complex 4E (SLC-4E) at Vandenberg Air Force Base, California. Carrying 64 payloads, this mission represented the largest single rideshare mission from a U.S.-based launch vehicle to date. A series of six deployments occurred approximately 13 to 43 minutes after liftoff, after which Spaceflight began to command its own deployment sequences. Spaceflight’s deployments are expected to occur over a period of six hours. 35)
This mission also served as the first time SpaceX launched the same booster a third time. Falcon 9’s first stage for the Spaceflight SSO-A: SmallSat Express mission previously supported the Bangabandhu Satellite-1 mission in May 2018 and the Merah Putih mission in August 2018. Following stage separation, SpaceX landed Falcon 9’s first stage on the “Just Read the Instructions” droneship, which was stationed in the Pacific Ocean.
Orbit: Sun-synchronous circular orbit with an altitude of 575 km, inclination of ~98º, LTDN (Local Time of Descending Node) of 10:30 hours.
• June 9, 2020: Over the past year, Planet has seen increased demand for its SkySat imagery to fulfill customers’ needs for timely, accurate and frequent information across the decision cycle. The COVID-19 pandemic has intensified this trend, as traditional surveying and inspection methods are not currently possible. 36)
- To meet the present moment, and demonstrate our commitment to rapidly deliver more value to customers every year, Planet is excited to unveil three new releases as part of our overall tasking offerings. Combined, these releases not only enhance the core imagery for analysis, but also reduce friction to acquire that data.
Figure 11: The Old City of Tripoli, Libya imaged at 50 cm per pixel from an altitude of 456 km (image credit: © 2020, Planet Labs Inc., All Rights Reserved)
- Higher resolution 50 cm imagery: In just six months, we successfully lowered our SkySat constellation to enhance the spatial resolution of our SkySat imagery from 80 cm to 50 cm for our ortho product. This improvement enables customers to get a more precise view of changing conditions on the ground and adds more granular context to decision-making. This is particularly important for commercial and government mapping use cases, where seeing smaller features like road surface markings are key.
Figure 12: 50 cm SkySat imagery of the Mundra Power Plant in Gujarat, India, acquired on April 4, 2020 (image credit: © 2020, Planet Labs Inc. All Rights Reserved)
- Planet’s imaging pipeline and delivery infrastructure have been built in the cloud and the Tasking Dashboard and API are the latest results of that foundation. The Tasking Dashboard is a new user interface that allows customers to request SkySat collections, while our new API provides efficient, automated access. Instead of spending precious time going back and forth with a human rep, with the Tasking Dashboard and API, customers can autonomously submit, modify and cancel SkySat imagery requests. This enables visibility into the end-to-end experience, from order to fulfillment, so expectations can be managed with analysts and teams.
Figure 13: Screenshot of Planet’s Tasking Dashboard (image credit: © 2020, Planet Labs Inc. All Rights Reserved)
Rapid Revisit, with up to 12x revisit capabilities
- Planet guarantees sub-daily revisit and the upcoming launch of six new SkySats will allow the company to image certain locations up to 12 times per day, at a global average of 7 times per day. This unprecedented capability will provide more rapid response to global events and enable imaging at times of the day previously unseen by satellites.
- At Planet's SkySat offerings support the company's growing customer base, from federal and civil governments, to commercial forestry to energy and more. These product advances are key components of Planet's overall mission to democratize access to satellite imagery, providing critical intelligence to customers and organizations when they need it most.
Figure 14: The first SkySat image (taken in the morning) and second SkySat image (taken in the afternoon) were collected on May 20, 2020, and show the remains of the Edenville Dam, breached after heavy rainfall over Michigan. The silvery appearance of the water in the morning is due to sunglint, which is the reflection of light directly into the satellite’s telescope (image credit: © 2020, Planet Labs Inc., All Rights Reserved)
• September 4, 2019: ESA and Planet are pleased to announce the opportunity to freely access, up to the end of the year, PlanetScope and SkySat data over three European demonstration sites: Demmin, Wilhelmshaven and Berlin. 37)
- In the framework of ESA's Earthnet program, the possibility of integrating new Third Party Missions (TPM) is assessed.
- ESA is promoting this opportunity as a data familiarisation phase before the potential formal integration of PlanetScope and SkySat into the Third Party Missions program.
- The Demmin test site is located some 220 km north of Berlin near the city of Demmin in Mecklenburg- Western Pomerania. This area is intensively used for agriculture.
- The area is part of the TERENO initiative and is cultivated by IG-Demmin (ca. 30,000 ha). It is well suited to remote sensing science applications since the site is heterogeneous with respect to landscape, soil cover and hydrology and the average size of the fields, ca. 80 ha, is very high for Germany.
- The main crops cultivated are winter wheat, barley, and rye, which cover almost 60% of the fields. The area devoted to sweet corn, sugar beets and potatoes amounts to about 13%.
- Since 2011 the Demmin calibration and validation facility is formally recognised as part of the ESA SMOS Mission (Soil Moisture and Ocean Salinity).
- The Wilhelmshaven area is located in the Wattenmeer area of Germany and a typical sample for coastal and urban application.
- The Berlin city test area covers a highly dense urban region with a variety of urban structures.
Figure 15: Lacations of the three test stations used by ESA (image credit: ESA)
• August 9, 2019: SkySat is a constellation composed 15 optical satellites operated by Planet. The SkySat Level 2B Basic Scene, Level 3B Ortho Scene and Level 3B Consolidated full archive and new tasking products are available as part of Planet imagery offer. 38)
- The SkySat Basic Scene product is uncalibrated and in a raw digital number format, not corrected for any geometric distortions inherent in the imaging process. Rational Polynomial Coefficients (RPCs) is provided to enable orthorectification by the user.
- The available processing levels includes Analytic (unorthorectified, radiometrically corrected, multispectral BGRN), Analytic DN (unorthorectified, multispectral BGRN) and Panchromatic DN (unorthorectified, panchromatic).
- The SkySat Ortho Scene is sensor- and geometrically-corrected (by using DEMs with a post spacing of between 30 and 90 meters) and is projected to a cartographic map projection; the accuracy of the product will vary from region to region based on available GCPs.
• August 2019: A common goal for satellite operations is to achieve a level of automation that minimizes human interaction, especially as constellation sizes increase. Planet’s SkySat fleet is a constellation of high resolution Earth imaging smallsats that has grown from three to fifteen satellites in three years. This rapid expansion, along with Planet’s goal of improving operational reliability, has necessitated automating operations to reduce manual effort to maintain the health and safety fleet. To address the growing amount of work required for anomaly triage, systems were created for automated anomaly response. These systems have removed the need to actively monitor satellite health and safety. Instead, operators rely on interrupt-driven alerts to inform them of an anomaly. With the goal of further decoupling fleet size from operator effort, the mission operations team is working to automate routine maintenance tasks. As a result, the number of person-hours needed to actively operate the fleet has seen a three-fold reduction per week while enabling a five-fold increase in on-orbit assets. The systems developed have enabled an operational posture that removes the need for 24/7 staffing at a dedicated operations center. 39)
The SkySat Fleet
- The SkySat fleet is comprised of fifteen small satellites capable of sub-meter resolution imagery. Unlike the Dove flock, which provides near-continuous imagery along each satellite’s orbit, the SkySats capture on-demand imagery of targets requested by customers. In addition, while the Doves are 3U CubeSats numbering in the hundreds, the SkySat fleet consists of a few dozen small satellites in the 100 kg range. The SkySat fleet is comprised of satellite buses with both maneuverable and non-maneuverable capabilities. Apart from this difference, the various SkySat buses are largely the same and operate under the same concept of operations.
- The rapidly changing operational needs required to maintain the fleet dictated that the SkySat Mission Operations (SMO) team develop tooling around the ground software, described below. These tools would give the team greater control over features while staying as independent as possible from the deployment of new ground software releases. By decoupling this development, SMO could experiment with various automation strategies while maintaining the inherently stable ground software. The ultimate goal of automation was to minimize the effort required to operate the fleet and maximize the amount of time operators could be absent from the operations center, in a so-called lights-out operational posture.
Ground software and tooling
- The base ground system initially provided to operate the SkySat fleet was a web-based user interface with basic features such as scripted command execution and telemetry monitoring and charting. As the number of satellites in orbit grew and operational needs changed, this base infrastructure acted as the foundation for additional tools built to facilitate SkySat mission operations.
- Scripting Engine: One of the primary features of the ground system allows operators to execute prepared scripts, written in Python. The purpose of this feature was to improve upon on-orbit command execution by allowing for complex logic and telemetry verification. The use of scripts has significantly reduced the occurrence of operator error and as a result increased fleet uptime.
- The first scripts developed were simple wrappers for available command and single-point telemetry checks. In these scripts, operators still adjusted the logical flow of execution via command line prompts. Over time layers were added to the script library, logic became more complex, and the need for operator prompting diminished. Much of this was due to the cumulative experience of both operators and engineers in dealing with anomalies on-orbit.
- Telemetry Monitoring and Charting: Operators are able to create custom screens of telemetry points for monitoring specific aspects of the satellite, as well as chart, in real-time, downlinked telemetry data from the satellite. Historical data is kept in a database to be retrieved for long-term trending of satellite data.
- Additionally, the ground system continuously monitors incoming telemetry against a set of thresholds defined by the satellite engineering team. These violations are then recorded and a visual indicator is presented to the operator for further action.
- Organization and Planning Tools: In order to facilitate executing satellite activities, SMO developed the pass planning tool. With this tool, operators may plan and display which operator-initiated activities would be executed in future contacts for each satellite. Additionally, this tool gathers and displays information about the fleet including what activities a satellite will be performing in upcoming orbits, the location of the satellites, and the health and safety effects of upcoming activities. This organization and planning tool became the foundation for many of the future tools that SMO would build, as it evolved into a one-stop location for all of the data the operations team needed.
Procedural and tasking algorithms
- As the fleet expanded, a multi-prong approach was taken to decouple operator effort from fleet size. This included building upon existing script libraries for automated anomaly response, moving to an alert-driven operational posture, and reducing the burden of planning and executing maintenance activities.
- Automated Anomaly Response: Over time, a set of common anomalies has emerged across the fleet that has lead to a thorough understanding of how to triage and resolve said anomalies. These well-defined responses to known anomalies have been implemented into scripts, which can be executed any time an anomalous state is detected. This development eliminated the need for operators to monitor real-time telemetry in order to respond to on-orbit issues.
- Autonomous anomaly response is an integral part of SkySat operations. Responses to common anomalies are entrusted to automation instead of operator intervention. By laying the foundation of autonomous anomaly response, the operations team was able to transition from focusing attention on one satellite at a time to considering and investigating anomalies at a fleet-wide scale. This enabled the operations team to more effectively manage a growing fleet. By automating the most critical responses, human attention could be more evenly spread between satellites competing for resources.
- Back-orbit Activities: A majority of commanding for all satellites in the SkySat fleet is done procedurally via a timestamped sequence of commands. These sequences are prepared and validated on the ground prior to being loaded to the onboard software for execution. These activities, such as image captures, data downlinks, and maintenance tasks, are typically performed in this manner.
- Some maintenance activities with specific constraints, such as onboard storage cleanup, must be executed when the satellite is not in contact with a ground station. To execute these “back-orbit” activities, a suitable execution time must be found and the sequence of commands must be loaded to onboard software prior to that time. Traditionally these tasks were manually performed by operators, which required a thorough understanding of not only when the satellites would be collecting images, but also the orbital state and satellite attitude. For example, would the satellite be passing through the South Atlantic Anomaly (SAA) and will the satellite be in an unsafe orientation if the activity is performed? Automating this process required defining a clear set of constraints that could then be implemented into the logic for determining the appropriate time to execute an activity. Once a suitable window was generated, the sequence of commands could then be embedded into the upcoming activities performed by the satellite.
Table 3: This table shows a simplified representation of the possible timing of a task that must be done while the satellite is in eclipse, not in the SAA, and not interfering with images or contacts. The final row indicates that there are two blocks near the end of the eclipse period that meet these criteria. If the length of these blocks is sufficient to complete the task, it will be added to the sequence of commands loaded to the satellite for that time frame.
- Alert and Paging System: One of the greatest enablers of lights-out operations was the development of an alert and paging system. If an anomaly occurs on orbit or within the ground infrastructure, the alerting system generates a summary of information surrounding the event to inform the on-call operator of the incident. Tying the alerts into Planet’s existing ticketing system meant issues could be tracked from discovery to resolution.
- Historically, visual indicators signifying that the value of a specific telemetry point was outside a defined threshold were used by operators to identify an anomalous scenario on-orbit. These, alongside logged messages from the onboard computer, were used by operators to triage and respond to the anomaly. At the core of its design, the alerting system collates related messages and telemetry into a single notification to send to operators.
- The precision and completeness of this information allows operators to develop a plan of action without requiring the depth of investigation needed in the past. Combined with changes to the operational posture, specifically extending the time in which operators must respond to an anomaly, this information enabled operators to work outside the operations center and respond to anomalies when convenient.
- After implementing automated scheduling of back-orbit activities, the last thing operators were responsible for was the planning and execution of activities that required communication with a ground station. These activities require ground services for a number of reasons, such as needing a link for telemetry verification or data exchange. These “in-pass” activities are frequently used for anomaly investigation and resolution, software updates, and nominal maintenance. In order to further reduce the person-hours needed to manually plan these activities, the SkySat Mission Operations team needed an automated workflow to determine what activities needed to be executed. In addition, this workflow needed to schedule the activities and to verify their successful completion.
- With the introduction of a so-called “automated operator”, all non-anomaly planning and scheduling could be handled without operator intervention. Tasks in this system were separated into three categories: default plans, run when no other activity is required; recurring plans, maintenance activities run at a regular interval; and triggered plans, activities that are planned in response to on-orbit conditions. The core functionality of the automated operator is scheduling these activities and verifying their successful execution.
- Default Plans: A majority of contacts across the fleet perform nothing beyond routine satellite health and safety checks. These routine tasks are encapsulated in a single script, removing the need for real-time telemetry verification by operators.
- Before the implementation of the automated operator, the “default script” was scheduled via a temporary workaround added to the pass planning tool. This occasionally led to scheduling conflicts and activities that had been carefully planned by operators could be overwritten without notice. This was an acceptable occurrence when the fleet was smaller, since the time it would take for operators to investigate why an activity did not occur did not interrupt nominal fleet operations. As the fleet grew, it became apparent that the system needed a new way to schedule these default tasks. By centralizing all contact scheduling into the automated operator, there was no longer a requirement for operators to manually ensure that a planned task would not be overwritten.
- In order to not interfere with operators manually planning a contact, the automated operator will only schedule recurring or triggered plans if a default plan was there previously. Human operators still have the authority to schedule and execute activities independently of the automated operator.
- Recurring Plans: There are certain tasks that must be executed at defined intervals as prescribed by the responsible subsystem engineer. Many of these tasks require ground assets and cannot be done while the satellites are not in contact. Some of these include loading updated orbit information or loading software configurations to the satellite. The intervals at which these tasks are executed range from weekly to quarterly.
- The automated operator is responsible for all aspects of these activities, including the following:
a) Tracking the status of all individual activities to determine if a task needs to be scheduled (activity scheduled; activity failed; activity recently completed successfully)
b) Tracking constraints for scheduling (minimum contact duration; ground station exclusions)
c) Tracking templates for activities approved for automated scheduling.
- Triggered Plans: There are certain on-orbit states that are typically dealt with in a well-defined manner. These activities are necessary for performing on-orbit tasks such as routine cleanup of satellite storage and enabling certain subsystem configurations. By defining both the on-orbit state and the necessary response to that state in a script, the automated operator schedules these activities as needed without operator intervention. As with default and recurring plans, these triggered plans are monitored from schedule planning through execution verification by the automated operator.
Personnel and staffing
- Despite the fleet size growing from one to fifteen satellites, staffing has remained relatively constant. This is due to evolving operational postures as well as the growing role that automation has played.
- Traditional Operations: Historically, satellite operations has involved staffing a dedicated operations center 24 hours a day, seven days a week. With the launch of SkySat-1, this was the posture taken, with three operators on console monitoring satellite health and safety. This was then reduced to two personnel when commissioning was complete. Outside of commissioning activities, 24/7 two-person operations (2PO) was maintained until 2017.
- Lights Out and On-call: The transition to on-call operations started with allowing the nighttime operators to monitor the fleet remotely, relying on the paging system to alert of any anomalies that needed operator response. This was referred to as nighttime lights-out operations (NLO). As confidence in the systems grew, nighttime staffing was reduced to one person on-call, with another support engineer available to be contacted as needed. This 24/7, one person on-call stance was eventually extended to include weekends in what is referred to as weekend lights-out operations (WLO). After on-call operations was implemented, operators were in the operations center twelve hours a day on weekdays with the remaining hours covered by on-call personnel.
- The team operated with these twelve-hour on-call shifts until in-office hours were eventually reduced to follow a standard eight-hour workday. Operators who began their shift on a weekday morning would finish their shifts remotely for a total of twelve hours on-shift. Operators assigned to weekend shift conducted shift activities, such as anomaly resolution, remotely.
- Alerting Periods: Despite moving to an on-call, interrupt driven stance, the SkySat Mission Operations team was still faced with the burden of having operators on-call 24/7 to respond to satellite anomalies. As the systems in place matured, the operations team relied on automation to take over more of the responsibilities of responding to night and weekend anomalies. With a growing number of the fleet, overall performance would not be greatly impacted by downtime on a few satellites.
- After the initial deployment of the alert and paging systems, there was a period of time where operators closely scrutinized and double checked that anomalies were being caught as expected. As confidence in the automation among operators reached a sufficiently high level, a change in operational stance was made to allow automation to be the primary response at night. Engineers could then triage anomalies in the morning and take action as necessary. This was the first time where there was a period of the day when no personnel were actively monitoring the fleet. This so-called “muting period” was then extended to 16 hours, turning the on-call period into an extension of in-office hours as opposed to staffing a separate shift.
- As it currently stands, the nighttime on-call period has been completely eliminated, with operators in-office only for a normal workweek and on-call personnel available during the day on weekends. Introducing the automated operator has further reduced human operator effort by covering all routine maintenance tasks. Although on-console for a full day, a typical day sees operators performing on-orbit activities such as anomaly response only a few hours per day.
- The overall reduction in staffing hours to monitor the fleet has reduced from 336 person-hours per week to 96 person-hours per week, with on-console operators spending less than half of their shift fully devoting attention to the fleet. This reduction in staffing occurred in parallel with fully supporting fleet expansion from one to 15 satellites. As stated previously, staffing levels have remained relatively constant. With the reduction in effort required to maintain the fleet, the mission operations team has provided support to other subsystems of the SkySat platform, including flight software, ground software, and manufacturing. The SkySat Mission Operations team has evolved into a group of subject matter experts on many aspects of the SkySat platform. This cross-training has made the level of knowledge related to the SkySat fleet at Planet resilient to changes in the overall team composition. Many members of the team have transitioned to other teams at Planet, specializing in disciplines ranging from electrical engineering to project management. Without the continued improvement in automation and relaxation of the operational posture, these changes would not have been possible.
- In six years, the SkySat Mission Operations team has gone from operating one to fifteen satellites, with a further fleet expansion on the horizon. This has been accomplished while not only maintaining a constant and lean team size, but while also scaling imaging capabilities to full capacity and maintaining rigorous mission uptime requirements. The introduction of the automated operator freed human operators of the burden of routine maintenance tasks, only requiring intervention during anomalous scenarios. The key factor of the team’s success was reducing the amount of work operators needed to perform per satellite, shifting the perspective from one of maintaining individual satellites to one of maintaining a fleet. This shift represented an understanding that imagery products were resilient to temporary outages on satellites.
- Lessons Learned: Automation is not something that happens all at once. Since the launch of SkySat-1, the mission operations team has incrementally developed systems and processes to assist in the routine maintenance of the growing fleet. Had automation been designed into the system upfront, the operational cadence and risk posture would have been rigid and unresponsive to the ever-evolving needs of the mission. Instead of dictating the automation needs of the operations team, the ground software systems are flexible, centering around a scripting engine and application interface.
- Even if a piece of automation is
designed and works perfectly, time is still required after the initial
deployment for the full benefit to be realized. Before operators fully
unburden themselves of the task to be automated, there will be a period
of time where operators double check and question the efficacy of the
automation. It is important for the software team responsible for
deploying new automation tactics to
• October 25, 2018: SSL, a Maxar Technologies company, has shipped two Earth Observation (EO) satellites to Vandenberg Air Force Base where they will be launched on Spaceflight’s first Sun Synchronous dedicated rideshare mission (SSO-A) aboard a SpaceX Falcon-9 launch vehicle. 40)
- SSL manufactured SkySat 14 and 15 for commercial EO company Planet, advancing SSL’s leadership in the manufacture of innovative, small form-factor satellites. The imaging satellites feature 72 cm. resolution and will be added to Planet’s SkySat constellation, which currently includes 11 SSL-built smallsats.
- The SkySat constellation complements Planet’s Dove constellation, with the most satellites on orbit from a commercial imagery provider. Six of Planet’s SSL-built satellites were launched in 2017 and five were launched in 2016. SSL continues to manufacture additional SkySats for Planet in its state-of-the-art SmallSat manufacturing facility, integrating improvements and increasing the cadence of delivery.
Figure 16: Photo of SkySats-14 and -15 in SSL's smallsats manufacturing facility (image credit: SSL)
• June 2018: SkySat Concept of Operations: There are two different SkySat bus types that satisfy the mission operating in orbit: two Generation A satellites which are non-propulsive (SkySat-1 and -2), and eleven Generation C satellites with propulsion (SkySat-3 to -13). Apart from the propulsive capabilities, the overall concept of operations remains largely the same. 41)
Table 4: SkySat satellite bus details
Table 5: Launch history of SkySat constellation
SkySat commissioning includes:
1) Initial contact and downlink of launch data
2) Utilizing onboard GPS for initial orbit determination
3) Detumbling and stabilizing the satellite using guidance navigation and control hardware
4) Initial checkout of the imaging system and door deployment
5) Calibration of the satellite and payload
6) Orbit phasing maneuvers.
- To-date, 13 SkySats have successfully launched and continue to operate in space, performing their mission to capture high resolution imagery of the Earth. The fleet has maintained continuous imaging operations since SkySat-1’s launch in 2013. The optimization of this fleet is ongoing; by leveraging the existing operations expertise from Planet’s numerous launches and management of its 200+ satellite constellation, the team is well poised to achieve the company’s goals for this one-of-a-kind, heterogeneous Earth Observation fleet.
• May 15, 2018: A year ago, Planet and Google completed a strategic partnership to acquire Terra Bella, making Google a Planet customer and investor. In that time, the Terra Bella and Planet teams have become a cohesive unit, with a shared mission and vision for the future. 42)
- The strength of our partnership was put to test last fall when we launched six SkySat satellites. Today, we’re happy to share that these new SkySat satellites are now fully operational and their data commercially available through Planet APIs (Application Programming Interfaces). This brings the total number of SkySat satellites in orbit to 13 – making it the largest constellation of high-resolution satellites in the market.
- This is a significant milestone for Planet and our global base of customers. Now, the SkySat constellation can not only image any location on Earth’s landmass at sub-meter resolution and twice-daily frequency, but also collect the data faster and at a lower cost point than traditional imagery providers. These capabilities ensure that Planet’s customers have the information they need to make critical business decisions.
- SkySat satellites are instrumental to Planet’s unprecedented daily global dataset, giving customers greater access to high-resolution imagery of the places they care about.
- The SkySat constellation supports off-nadir collection types and now offers a variety of delivery formats, including Basemaps, which are compatible with OpenStreetMaps or as a web mapping tile format.
• November 1, 2017: Planet has confirmed its ground team has contacted the SkySat (SkySat-8 to -13) and Dove satellites launched by the Minotaur-C rocket earlier this afternoon, and the spacecraft are in a good orbit. This confirms the final phase of the Minotaur-C mission occurred as planned, with a normal fourth stage motor burn and a good separation of all six payloads. 43)
• Sept. 5, 2017: Six high-resolution SkySat satellites for Planet, built by SSL (Space Systems Loral) arrived at VAFB, scheduled for launch in mid-October on a Minotaur-C vehicle of Orbital ATK . The satellites will double Planet’s high resolution imaging capabilities and help deliver information to users about our physical world that impacts decision making. 44)
- The satellites, called SkySat 8 through 13, are each about 60 x 60 x 95 cm with a mass of about 100 kg and capture sub-meter color imagery and up to 90-second clips of HD video with 30 frames per second. Working together with the seven SkySats already on orbit, the satellites will dramatically increase Planet’s high resolution imaging capabilities, enabling multiple imaging passes in a single day. These capabilities, combined with Planet’s over 170 Dove satellites and their advanced software analytics platform, make it possible to derive timely insights from any location in the world. The Planet constellation provides a broad range of data, tools, and analytical services that help leaders in business and humanitarian sectors solve complex problems.
• August 2017: Five SkySat satellites with HPGP propulsion systems were launched in 2016, from two different launch sites. SkySat-3 was launched from SDSC of ISRO in India on June 22, 2016, while SkySat-4 to -7 (4 satellites) were launched from VAFB in CA on September 16, 2016. Each satellite’s HPGP system has been successfully commissioned and is now being operated in-orbit. 45)
- Propulsion System Commissioning: Following separation from the launch vehicle upper stage, the same propulsion system commissioning activities were performed on each SkySat. Depending on ground station contact scheduling, the process took approximately 8 hours per satellite. First, the thruster catalyst bed heaters were activated and allowed to operate within their pre-heating temperature setpoints of 330-370°C for 1 hour in order to thoroughly drive off any residual moisture and ensure complete and uniform heating of the entire reactor assembly.
- Recurring Propulsive Operations: The SkySat propulsion systems are used to maintain proper station keeping, maintain inclination and compensate for drag. As of the date of publication, a total of forty (40) propulsive maneuvers have been executed across the entire fleet, for normal operations and both propulsion and other subsystem tests. A summary for each SkySat is shown in Table 6.
Legend to Table 3: SkySat-4 is currently being used as the ‘reference’ for maintaining constellation phasing (and has thus required fewer maneuvers than all of the other satellites).
- The SkySat propulsion maneuvers are executed via an automated sequence with a pre-defined start time and duration. Prior to opening the FCVs (Flow Control Vales), the maneuver sequence configures the satellite state and enables the required 30 minutes of thruster catalyst pre-heating. When the satellite time reaches the programmed maneuver time, the sequence allows the ACS algorithm to slew the satellite to the firing attitude and then dynamically control the individual thruster duty cycles to maintain satellite orientation throughout the bun. Following a successful maneuver, the sequence cleans up the satellite state and slews the attitude back to the nominal cruise orientation.
- System Performance: The on-orbit performance of the SkySat HPGP propulsion systems corresponds well with the pre-flight predictions. Figure 17 shows the as-measured performance of “Thruster B” (which is fired at 100% duty cycle) on the SkySat-3 satellite for all closed loop maneuvers performed to date.
- As seen in Figure 17, the steady-state Isp achieved in orbit is higher than the thruster acceptance test data (due to the thrusters only reaching quasi-steady state temperatures during ground testing at higher feed pressures) and is consistent with the analytical model.
- A comparison plot showing the reactor temperature of Thruster B on SkySat-3 during regular orbit maintenance maneuvers is provided in Figure 18, with the end of each maneuver indicated by a sharp decrease in reactor temperature.
• April 19, 2017: As Planet of San Francisco announced, it has completed its acquisition of rival satellite imaging company Terra Bella on April 18, it confirmed that Google is now a shareholder in Planet as part of that deal. 46)
- Planet announced on February 3 that it had reached an agreement with Google to acquire Terra Bella. Google had purchased Terra Bella, then known as Skybox Imaging, in 2014 for an estimated $500 million. At the time, both Planet and Google declined to disclose the terms of the deal other than that Google signed a multi-year deal to purchase imagery from Planet.
- The deal, though, was rumored to include Google taking a stake in Planet. In an April 18 blog post announcing that the deal had closed, Planet co-founder and chief executive Will Marshall confirmed that. “We’re also delighted to welcome Google as a shareholder and customer,” he wrote.
- Planet spokesperson Rachel Holm said in an April 18 email that Google took an equity stake in Planet, in addition to the previously announced multi-year imagery contract. Neither company, though, has said how much of Planet that Google now owns.
- The deal closed after receiving regulatory approvals from several federal agencies. “Over the last several weeks, we received all necessary regulatory approvals from NOAA (National Oceanic and Atmospheric Administration), FTC (Federal Trade Commission) and FCC (Federal Communications Commission),” Holm said. The NOAA licenses commercial remote sensing systems in the United States, while the FCC licenses satellite communications.
- The FTC, with the Department of Justice, reviews large acquisitions under the 'Hart-Scott-Rodino Act' for any antitrust issues, setting a waiting period for that review before such deals can close. The FTC issued “early termination” notices March 16 for Planet’s acquisition of Terra Bella and Google’s acquisition of part of Planet, ending that waiting period early and allowing the deal to proceed.
- Planet will now work to integrate the high-resolution imagery from Terra Bella’s fleet of seven SkySat satellites with Planet’s own constellation of nearly 150 satellites that provide medium-resolution images. That fleet includes 88 satellites launched in February on an Indian Polar Satellite Launch Vehicle.
- “This ‘close’ is also the beginning—the beginning of a new chapter at Planet, and of a lot of work across our organization over the next year to make SkySat imagery available on the Planet platform,” Marshall said in his statement. - Holm said that a “significant portion” of Terra Bella’s employees will remain with Planet. The company, headquartered in San Francisco, will maintain an office in Mountain View, California, where Terra Bella was based.
Figure 19: An illustration of four of the SkySat high-resolution imagery satellites developed by Terra Bella. Planet announced April 18 it has completed its deal announced in February to acquire Terra Bella from Google (image credit: Space Systems Loral)
• On Sept. 27, 2016, Terra Bella released the first images from the four newest high-resolution imaging satellites, SkySat-4-7, which were successfully launched aboard an Arianespace Vega rocket from French Guiana on September 16, 2016. The following images (Figures 20 to 23) of Google headquarters in Mountain View, Rome, Amsterdam, and Algeciras, Spain are untuned and uncalibrated. 47)
Figure 21: SkySat-5 image over Rome, Italy on September 23, 2016 (image credit: Terra Bella)
Figure 22: SkySat-6 image over Amsterdam, Netherlands on September 19, 2016 (image credit: Terra Bella)
Legend to Figure 23: Algeciras is a port city in the south of Spain, and is the largest city on the Bay of Gibraltar (Bahía de Algeciras). The Port of Algeciras is one of the largest ports in Europe and in the world in three categories: container, cargo and transhipment.
• The launch of SkySat-4, -5, -6 and -7 on Sept. 16, 2016 expanded a growing satellite fleet operated by Google’s Terra Bella company, giving the Silicon Valley firm seven spacecraft fitted with high-resolution cameras that can take rapid-fire pictures many times a second, allowing processors on the ground to string together video clips (Ref. 29).
- The Terra Bella satellites add to Google’s vast imagery catalog, which help improve popular applications such as Google Maps, according to Luc Vincent, director of GEO imagery at Google.
• August 3, 2016: ECAPS announced that the HPGP (High Performance Green Propulsion) system on SkySat-3 has been successfully commissioned on-orbit and declared fully operational. Commissioning of the HPGP propulsion system was completed approximately 48 hours after launch. All initial data from the propulsion system has indicated nominal performance and the HPGP system is now being used for recurring orbit maintenance operations. 48)
• July 1, 2016: SkySat 3, the third satellite of Terra Bella (formerly Skybox Imaging) has downlinked its first images following its June 22 launch aboard a PSLV (Polar Satellite Launch Vehicle) from the ISRO. The satellite launched with 19 other co-passengers and was released into a sun-synchronous orbit of ~ 500 km. 49)
Figure 24: Chicago’s Soldier Field stadium as seen by SkySat-3, acquired on June 25, 2016 (image credit: Terra Bella)
• March 8, 2016: Google today announced its satellite subsidiary Skybox Imaging has been renamed to Terra Bella. The name comes with a new vision: “As Google revolutionized search for the online world, we have set our eyes on pioneering the search for patterns of change in the physical world.” 50)
- Two years ago, Skybox Imaging launched its first satellite, SkySat-1, and has since taken 100,000 images. Terra Bella now has “more than a dozen satellites under development” that are “scheduled to launch over the next few years.” But in today’s announcement, founders Dan Berkenstock, John Fenwick, and Ching-Yu Hu explained they want to go beyond satellite imagery: ”As we have engaged with thousands of potential users, we have been struck over and over again by a simple truth. There is an incredible opportunity for geospatial information to transform our ability to meet the economic, societal, and humanitarian challenges of the 21st century, but satellite imagery represents only one part of the puzzle.”
- In addition to relying on satellite imagery, Terra Bella is now working with a wide array of geospatial data sources, machine learning capabilities, and experts “that we could not have imagined as an independent startup company.” The broader goal is to convert raw imagery into data that can help people and organizations make more informed decisions.
- In other words, Terra Bella will soon be launching new products that don’t depend solely on satellites. These will be revealed “over the coming year,” the Google subsidiary promises.
• August 2015: The Skybox Flight Operator program trains rotating cohorts of college students and recent graduates to fly the current constellation of microsatellites, namely SkySat-1 and -2. This program has provided significant benefits for Skybox Flight Operations. First, it attracts highly motivated, energized people, who are interested in the many short-term growth opportunities offered by the role, but who may not be interested in a shift-based role with few long-term growth opportunities. 51)
- The Flight Operations team at Skybox is responsible for commissioning the SkySat satellites after launch and keeping them healthy, robust, and productive throughout their lifetimes. To achieve this mission, Skybox staffs its operations center 24 x 7 with two Satellite Controllers (SatCons) who are responsible for monitoring telemetry, responding to anomalies, and executing maintenance procedures and calibrations.
- Skybox developed an intern staffing program, that draws from aerospace undergraduate and graduate programs at local universities. The first class of nine student interns began flying the Skybox satellite fleet in December 2013, right after the launch of SkySat-1. Since then, Skybox has recruited and trained two more SatCon cohorts. To date, a total of 16 personnel have participated in this intern program.
- Skybox-1 and -2 are operated in Skybox’s MOC (Mission Operations Center), which has been staffed 24 x 7 continuously for over 1.5 years, with the majority of shifts filled by the SatCon interns. This effort has been successful due to the thorough certification program, sourcing and hiring the appropriate personnel for an agile operations environment, and constant drive to reduce operational risk.
• The SkySat-1 and -2 satellites are operating nominally in February 2015.
- The SkySat-3 satellite is scheduled to launch as a secondary payload in the summer 2015 on a PSLV-XL vehicle of ISRO from SDSC (Satish Dhawan Space Center) SHAR on the south-east coast of India. 52)
- ECAPS (Ecological Advanced Propulsion Systems, Inc.) of Solna, Sweden, a division of SSC Corporation, provides propulsion systems for 12 satellites to be built for imagery services startup Skybox Imaging. The contract is the largest ever for ECAPS’s environmentally friendly High Performance Green Propulsion system for small satellites. 53)
Skybox recently signed a contract with manufacturer Space Systems/Loral (SSL) for 13 small imaging satellites, the first of which is being built at Skybox’s Mountain View, Calif., facilities in a collaborative effort between the two companies. ECAPS was already under contract to supply the propulsion system for that satellite, dubbed SkySat-3, and now has an order for the remaining 12 microsatellites.
Skybox ultimately plans a 24-satellite constellation occupying four different polar-orbit planes that will provide high-resolution imagery and full-motion video for commercial sale.
Figure 25: The Tower of London (bottom center) acquired by SkySat-1 on November 10, 2014 (image credit: Skybox) 54)
Figure 26: SkySat-1 image of the Helkeim Glacier in Greenland, acquired on Aug. 18, 2014 (image credit: Skybox Imaging) 55)
• In the summer of 2014, Skybox Imaging has entered into an agreement to be acquired by Google! 56) - Technology giant Google and the satellite Earth imaging startup Skybox Imaging on June 10, 2014 announced that Google is purchasing Skybox and hopes to use Skybox’s imaging technology “over time ... to improve Internet access and disaster relief — areas Google has long been interested in.” 57)
- Google acquired Skybox Imaging for $500 million and started a revolution in space that has been solely enabled by the capabilities of small satellites. Google stated “Skybox’s satellites will help keep Google Maps accurate with up-to-date imagery. Over time, we also hope that Skybox’s team and technology will be able to help improve Internet access and dis-aster relief — areas Google has long been in-terested in.” 58)
• On July 10, 2014, Skybox Imaging released the first images from SkySat-2. The project team progressed already through initial commissioning activities. The SkySat-2 system tuning and calibration is expected to continue for several months. 59)
SkySat-1 and SkySat-2 operations are conducted from the Skybox MOC (Mission Operations Centerour) on a 24 hour/7 day basis in Mountain View, CA.
Figure 27: SkySat-2 image of Port-au-Prince, Haiti, acquired on July 10, 2014 within 48 hours after launch (image credit: Skybox Imaging) 60)
Figure 28: SkySat-2 image of Bangor, Maine, USA, acquired on July 10, 2014 (image credit: Skybox Imaging) 61)
Figure 29: SkySat-1 image of Zayed University in Abu Dhabi, UAE (United Arab Emirates), acquired on Dec. 7, 2013 (image credit: Skybox Imaging)
Figure 30: SkySat-1 sample image of Crown Perth in Perth, Australia, acquired on Dec. 4, 2013 (image credit: Skybox Imaging
• On Dec. 11, 2013, Skybox Imaging released the first high-resolution images acquired with SkySat-1. 62)
Figure 31: SkySat-1 image of Beaton Park in Perth, Australia acquired on Dec. 4, 2013 (image credit: Skybox Imaging)
The optical imager covers a panchromatic band from 450 to 900 nm achieving a Pan resolution of 0.90 m at nadir. Four multispectral channels are covered by the satellite (Blue 450-515, Green 515-595, Red 605-695, and Near Infrared 740-900 nm) achieving a multispectral resolution of 2 m at nadir. A ground swath of 8 km is covered at nadir. Stereo imaging is supported by the satellite. The instrument is a staring 2D imaging device. 63)
The satellite acquires high-definition video in its Pan channel with durations of up to 90 seconds in which the satellite can keep looking at the ground target by slewing to compensate for the movement in its orbit. Video is acquired at 30 frames/s with a resolution of 1.1 m at nadir and a minimum FOV (Field of View) of 2.0 km x 1.1 km.
Skybox images are commercially marketed and find application in a variety of monitoring operations, land use planning, environmental assessment, resources management, tourism, mapping and for scientific use.
Table 7: Specification of the optical imager
Each SkySat satellite is equipped with a Ritchey-Chretien Cassegrain telescope (35 cm ∅) with a focal length of 3.6 m, and a focal plane consisting of three 5.5 Mpixel CMOS imaging detectors. Images are compressed with JPEG 2000 and then stored or downlinked to the ground station. 768 GB of on board storage are available and the data downlink rate is 450 Mbit/s. 64)
SkySat-1and -2 use 3 CMOS frame detectors with a size of 2560 x 2160 pixels and a pixel size of 6.5 µm. The upper half of the detector is used for panchromatic capture, the lower half is divided into 4 stripes covered with blue, green, red and near infra-red color filters. A schematic of the focal plane layout is shown in Figure 32. The native resolution at nadir of the SkySat-1 and SkySat-2 is around 1.1 m. Further satellites will be placed in lower orbits, leading to increased image resolution.
The Raw Video and Frame products contains both a physical camera model and a RPC (Remote Procedure Call) for each individual frame. The interior orientation is given by the location (X,Y,Z) and tilts the CMOS detector planes with respect to the projection center of the telescope. The unconventional interior orientation with 3D rotation of the focal plane with respect to the telescope requires extension of the ordinary frame camera geometry routines.
For the video product, the panchromatic part of a single detector records a video with 30 frames/s while the spacecraft pointing follows the target. Video sequences up to 90 seconds in length can be recorded. The video product can be delivered in different formats, a stabilized Full HD video in MP4 format, where all video frames have been coregistered, and an unstabilized video without coregistration. The video size of both products is 1920 x 1080 pixels. A raw video product with individual TIFF files with 11 bit of radiometric resolution and per frame orbit and attitude parameters and RPCs is also available. The raw video frames are available at the full panchromatic detector area size of 2560 x 1080 pixels.
Frame product: In addition to the video product, larger areas can be covered by strips with a swath width of 8 km. These are acquired in a ”pushframe” mode, where all three detectors acquire a highly overlapping video sequence, for example at 40 Hz (Smiley et al.,2014). All pan and multi-spectral images overlapping with a single panchromatic ”master” frames are coregistered and fused using a super-resolution algorithm. During the fusion, a super-resolution process is used to increase the resolution from 1.1 m to 90 cm. Panchromatic, multispectral and several variants of pansharpened images are delivered.
The master images are chosen to have some overlap in the along track direction, and there is a small across-track overlap between detector 2 and detectors 1 and 3 (Figure 33).
As handling and mosaicking of the individual frames is not a straightforward operation for most imagery customers, Skybox will offer an mosaicked Geo product in the future.
Legend to Figure 33: Fos-sur-Mer is situated about 50 km north west of Marseille, on the Mediterranean coast, and to the west of the Étang de Berre.
With the first civil VHR video products, the SkySat satellites offer very interesting possibilities for future applications. The ”pushframe” architecture and the super-resolution approach reduce the complexity of the SkySat satellites and will allow launch of a constellation with multiple daily visits. A drawback of the constellation is the comparably small footprint of the still and video products, Skybox is thus primarily suited for monitoring applications and not for the mapping of large areas (Ref. 64).
Propulsion subsystems for the SkySat constellation
The declared goal of Terra Bella, formerly Skybox Imaging, is to provide the world's first coordinated constellation of high-resolution EO satellites. After the successful demonstration of the SkySat-1 imaging performance and the development of the SkySat-2 spacecraft, Skybox Imaging of Mountain View, CA, awarded a contract to SSL (Space Systems/Loral) of Palo Alto, CA in February 2014, to build an advanced constellation of LEO (Low Earth Orbit) satellites for Earth imaging. The contract award helps SSL, which is best known for its high-power geostationary communications satellites, to further expand its capabilities building LEO imaging satellites and solutions. 67) 68)
SSL is building 13 small LEO satellites, each about 60 x 60 x 95 cm with a mass of ~120 kg, to be launched in 2015 and 2016. These satellites, based on a Skybox design, will capture sub-meter color imagery and up to 90-second clips of HD video with 30 frames/s. Once the 13 satellites are launched, Skybox will be able to revisit any point on Earth three times per day.
As part of the agreement, Skybox granted SSL an exclusive license to the satellite design. This provides SSL with a unique platform to address the growing demand for small satellites and related services.
The contract with SSL, a subsidiary of MDA Corp. of Richmond BC, Canada, raises the possibility that Skybox could receive backing from Export Development Canada, the country’s export credit agency, one industry source said. Export credit agency financing has become a major factor in the space industry and often helps determine who wins satellite manufacturing and launch contracts. 69)
One of the critical requirements identified in the evolution towards a constellation was the need for a capable propulsion system. Adding propulsion to future SkySat satellites enables the following capabilities: 70) 71)
• Constellation relative phase management: The compact size of the SkySat platform enables enormous cost savings by utilizing a single launch vehicle to launch multiple spacecraft. However, once on orbit, propulsion will be required to phase the spacecraft within each orbit plane and maintain their relative spacing in the face of orbital perturbations.
• Mission flexibility to better serve the EO market: The commercial EO market is relatively new and evolving. High performance propulsion will enable Skybox to meet market demands for increased resolution, collect volume or spacecraft lifetime by adjusting the spacecraft’s orbits.
• Launch vehicle diversity: High performance propulsion will enable Skybox to take advantage of a wide range of future secondary launch options as they become available, while maintaining tight coordination of one-off launches with the rest of the constellation.
Already in late 2012, Terra Bella, formerly Skybox Imaging, became the first commercial company to baseline the HPGP (High Performance Green Propulsion) technology of ECAPS (Ecological Advanced Propulsion Systems, Inc.) of Solna, Sweden — implementing a propulsion system design with four 1N thrusters in their second generation small satellite platform (~120 kg). The initial propulsion module, to be delivered in 2013, will serve to qualify the system design for use in an entire constellation of small satellites intended to provide customers easy access to reliable and frequent high-resolution images of the Earth.
The selection of the HPGP system of ECAPS, an SSC (Swedish Space Corporation) Group company, resulted from a system study of various propulsion options in support of Skybox’s mission to provide high quality and timely earth observation data from a small satellite constellation. Two key technical requirements for the propulsion system were to provide the maximum ΔV achievable (for continued orbit maintenance and mission flexibility) within a considerably limited internal volume typical of many microsatellites. Additionally, in light of the commercial nature of the project, the overall life-cycle cost was considered to be of utmost importance.
A detailed trade study of various propulsion technologies and vendors was conducted by Skybox during the selection process. The results of that study showed that the HPGP solution selected provides nearly twice the on-orbit ΔV of the more traditional monopropellant systems, at the lowest projected life-cycle cost of the liquid propulsion technologies evaluated.
The higher performance of the HPGP system will give the SkySat constellation of small satellites significantly improved mission flexibility, enabling collection and delivery of higher quality and more timely data to customers. Furthermore, the handling and transportation advantages of the environmentally benign ADN (Ammonium Dinitramide) based LMP-103S monopropellant provide reductions in logistics costs and enable more responsive launch preparation. 72)
Figure 34: Photo of a 1 N thruster of the HPGP propulsion subsystem (image credit: ECAPS/SSC)
SkySat-3 will be the first microsatellite of the SkySat constellation which features an HPGP propulsion subsystem with four 1N thrusters, fuel LMP-103S and refueling of the satellite at the launch base.
During 2013, ECAPS worked to design a complete, compact and “modular” HPGP propulsion system; the first (protoflight) version of which was delivered in 2014. A total quantity of nineteen such HPGP propulsion system modules have now been ordered by Terra Bella, and “assembly line” manufacturing is ongoing at ECAPS – with multiple deliveries accomplished in 2015, and continuing into 2016 & 2017. 73)
As a result of the schedule adjustments that are common within the satellite and launch industries, up to eleven of the aforementioned HPGP modules are currently planned to launch in 2016, on three different launch vehicles; from three different launch sites (on three different continents). Collectively, these launches will represent the “commercial debut” for HPGP technology; with the entry point being a large constellation.
SkySat HPGP propulsion system design:
As successfully demonstrated in-space on the PRISMA mission of Sweden (2010-2015), HPGP (High Performance Green Propulsion) technology provides numerous benefits over monopropellant hydrazine, including: 32% higher volumetric efficiency and 8% higher mission-average specific impulse, significantly reduced transportation/handling hazards and costs, and greatly simplified/shortened pre-launch operations (Ref. 73).
The PRISMA HPGP propulsion system was the first in-space demonstration of the ”green” storable monopropellant HPGP technology, based on ADN LMP-103S, and was used for providing the required ΔV for the PRISMA main satellite maneuvers, together with the hydrazine system. The PRISMA mission was concluded in May 2015; by which time the HPGP system had been successfully operated in space for five years.
The architecture of the complete HPGP propulsion system developed by ECAPS for the SkySat platform is shown in Figures 35. The system design consists primarily of four 1N HPGP thrusters, three propellant tanks (with expulsion via Propellant Management Devices) connected in series, two service valves, a latch valve, a pressure transducer and a system filter. All of the components selected have flight heritage from previous missions.
The design and function of the thrusters developed for ADN-based monopropellant blends have several similarities with hydrazine thrusters. The FCV (Flow Control Valve) is a normally closed series redundant valve with independent dual coils. The FCV is manufactured by Moog and has extensive flight heritage. In the HPGP thruster, the propellant is thermally and catalytically decomposed and ignited by a pre-heated reactor. Nominal pre-heating is regulated between 340-360ºC which requires an average power consumption of about 7.3 W per thruster in the PRISMA application. For thermal control, the thruster is equipped with redundant heaters and thermocouples.
Figure 36: Left: The SkySat HPGP system layout; right: The SkySat-3 HPGP flight system (image credit: ECAPS/SSC)
Importantly, from the standpoint of other companies developing small satellites which will require propulsive capability, ECAPS can offer the existing design (or modified derivatives thereof) as a compact (55 x 55 x 15 cm) “drop-in”/off-the-shelf solution for other customers interested in high performance propulsion at a reduced life-cycle cost.
The nineteen complete HPGP propulsion system modules ordered by Terra Bella represent a total quantity of seventy-six (76) 1N HPGP thrusters. In order to achieve the associated production rates, ECAPS has scaled up its capabilities in the areas of both manufacturing and hot-fire acceptance testing of HPGP thrusters.
Figure 37: Photo of 1N HPGP flight thrusters (image credit: ECAPS/SSC)
In support of increased thruster manufacturing rates, ECAPS has invested in additional vacuum braze stations. Additionally, in order to enable an improved thruster acceptance testing timeline, ECAPS’ Test Stand number2 (TS-2) has been modified to support multiple thrusters simultaneously. The new TS-2 configuration, shown in Figures 38 and 39, permits four (4) 1N HPGP thrusters to be mounted in parallel.
SkySat HPGP propulsion modules: As shown in Figure 40, the complete SkySat HPGP propulsion system modules are being manufactured in an “assembly line” manner as well. By implementing standardized procedures and support equipment, multiple systems are able to exist in various stages of production simultaneously – thus streamlining the flow of incoming components into their respective systems, and minimizing the likelihood of key tooling sitting “idle” due to the individual integration schedule of any particular system.
SkySat Mission and Ground Segment
Planet’s Mission Operations team largely focuses on automation to manage nominal operations of the fleet of satellites. Rather than building manual/human processes and then trying to replace them with automation, Planet builds automation first and then iteratively improves it. This workflow has been paramount to operating the large fleet of Dove nanosatellites. 74)
The SkySat Mission Operations (SMO)
team is responsible for the safe and stable space segment operations
for Planet’s thirteen SkySat satellites. This includes all
satellite operations as well as the development of tools, processes,
and systems for these operations. SMO supports ground station
operations, image collection planning, radiometric calibration,
software development, and mission systems engineering. SMO takes
ownership of the satellite immediately after launch and is responsible
for all commissioning, nominal, maintenance, special, and
The SkySat ground system supports satellite commanding and real-time telemetry display, analysis, and trending in a 100% web browser based solution. The majority of planning, analysis, and production tools used operationally are also browser-based. Telemetry storage and computation occurs on centralized servers.
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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 (firstname.lastname@example.org).