Minimize SkySat

SkySat constellation of Terra Bella - formerly SkySat Imaging Program of Skybox Imaging

Spacecraft     Launch    Mission Status     Sensor Complement    Propulsion Subsystems    References

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. 44).

• 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. 51)

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)

Spacecraft mass

83 kg (microsatellite)

Spacecraft size (stowed configuration)

60 cm x 60 cm x 80 cm

Spacecraft power

120 W OAP (Orbit Average Power), use of body mounted solar panels

Attitude control accuracy


RF communications

X-band downlink of payload data: 470 Mbit/s
S-band uplink: 16 kbit/s
Onboard data storage capacity: 768 GB

Design life

4 years

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.


< 7 arcsec RMS cross-boresight, < 70 arcsec RMS around boresight



Size, mass

59 x 56 x 31.5 mm, ~90 gram

FOV (Field of View)

7:5º (half axis)

Exposure time

100 ms


3746 stars

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.

Secondary payloads:

• 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)



Mission status:

• 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 vehicleof 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. 30)

- 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. 31)

- 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 3.


Number of maneuvers

Total impulse (as of 1 June 2017



1,732 Ns



57 Ns



150 Ns



269 Ns



317 Ns

Table 3: SkySat constellation propulsive maneuver summary

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 9 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 9, 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.


Figure 9: Comparison of On-Orbit Steady-State Performance vs. Pre-Flight Predictions (image credit: ECAPS)

- A comparison plot showing the reactor temperature of Thruster B on SkySat-3 during regular orbit maintenance maneuvers is provided in Figure 10, with the end of each maneuver indicated by a sharp decrease in reactor temperature.


Figure 10: SkySat-3 Thruster B (image credit: ECAPS)

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. 32)

- 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 11: 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 12 to 15) of Google headquarters in Mountain View, Rome, Amsterdam, and Algeciras, Spain are untuned and uncalibrated. 33)


Figure 12: SkySat-4 image over Google Headquarters in Mountain View, CA on September 23, 2016 (image credit: Terra Bella)


Figure 13: SkySat-5 image over Rome, Italy on September 23, 2016 (image credit: Terra Bella)


Figure 14: SkySat-6 image over Amsterdam, Netherlands on September 19, 2016 (image credit: Terra Bella)


Figure 15: SkySat-7 image over Algeciras, Spain on September 23, 2016 (image credit: Terra Bella)

Legend to Figure 15: 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. 34)

• 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. 35)


Figure 16: 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." 36)

- 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. 37)

- 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. 38)

- 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. 39)

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 17: The Tower of London (bottom center) acquired by SkySat-1 on November 10, 2014 (image credit: Skybox) 40)


Figure 18: SkySat-1 image of the Helkeim Glacier in Greenland, acquired on Aug. 18, 2014 (image credit: Skybox Imaging) 41)

In the summer of 2014, Skybox Imaging has entered into an agreement to be acquired by Google! 42) - 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." 43)

- 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." 44)

• 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. 45)

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 19: SkySat-2 image of Port-au-Prince, Haiti, acquired on July 10, 2014 within 48 hours after launch (image credit: Skybox Imaging) 46)


Figure 20: SkySat-2 image of Bangor, Maine, USA, acquired on July 10, 2014 (image credit: Skybox Imaging) 47)


Figure 21: SkySat-1 image of Zayed University in Abu Dhabi, UAE (United Arab Emirates), acquired on Dec. 7, 2013 (image credit: Skybox Imaging)


Figure 22: 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. 48)


Figure 23: SkySat-1 image of Beaton Park in Perth, Australia acquired on Dec. 4, 2013 (image credit: Skybox Imaging)



Sensor complement:

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. 49)

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.

Spatial resolution

Pan (Panchromatic): 90 cm at nadir
MS (multispectral): 2.0 m at nadir

Nominal swath width

8 km at nadir

Spectral bands

Pan: 450-900 nm
MS: Blue = 450-515 nm
MS : Green = 515-595 nm
MS: Red = 605-695 nm
MS: NIR = 740-900 nm

Video data

Duration up to 90 seconds
Frame rate = 30/s
GSD = 1.1 m at nadir
FOV: No smaller than 2.0 km x 1.1 km

Table 4: Specification of the optical imager

Each SkySat satellite is equipped with a Ritchey-Chretien Cassegrain telescope 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. 50)

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 24. 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.


Figure 24: SkySat-1 focal plane, as projected to the ground (image credit: Skybox, DLR) 51) 52)

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 25).

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.


Figure 25: Fos sur Mer industrial zone as seen by SkySat-1. The bounding boxes show the individual frames after coregistration and multi-image fusion (image credit: Skybox, DLR)

Legend to Figure 25: 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. 50).



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. 53) 54)

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. 55)

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: 56) 57)

• 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. 58)


Figure 26: 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. 59)

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. 59).

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 27. 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.


Figure 27: The SkySat HPGP system architecture (image credit: ECAPS/SSC)

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 28: 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 29: 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 30 and 31, permits four (4) 1N HPGP thrusters to be mounted in parallel.


Figure 30: Four 1N thrusters mounted in TS-2 (image credit: ECAPS/SSC)


Figure 31: TS-2 with multiple 1N thrust balances mounted (image credit: ECAPS/SSC)

SkySat HPGP propulsion modules: As shown in Figure 32, 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.


Figure 32: Photo of multiple SkySat HPGP systems in various stages of production (image credit: ECAPS/SSC)


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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 (

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