Planet - Flock Imaging Constellation
On Nov. 26, 2013, Planet Labs, a private start-up company of San Francisco, CA, announced that it successfully launched its most recent nanosatellites, Dove 3 and Dove 4, into orbit on a Dnepr vehicle (launch on Nov. 21, 2013 from the Yasny Cosmodrome, Russia), completing a series of four prototype nanosatellites in 2013. Those proved successful, enabling the company to quickly follow up with the production of a 28-member network. The launch of Planet Labs' "Flock 1" fleet of 28 nanosatellites in December/January, which will be the largest constellation of Earth-imaging satellites ever launched. 1) 2)
The latest additions to the Planet Labs fleet offer improvements in the capability provided by the firm's first nanosatellites launched in April, Dove 1 and Dove 2, which also were 3U CubeSats. Dove 3 and Dove 4 will demonstrate the firm's latest technology, including upgraded communications, attitude control and observation technology.
Background: Planet Labs was founded in 2010 by Chris Boshuizen, Will Marshall and Robbie Schingler, three NASA alumni interested in altering the space industry. Their "Dove" nanosatellites are meant to be low-cost and rapidly deployable, and capable of taking pictures of Earth that provide a spatial resolution of 3-5 m. — On March 17, 2014, Planet Labs announced that it has confirmed launches for more than 100 satellites over the next 12 months. This full constellation of nanosatellites will allow Planet Labs to image the entire Earth every day. 3)
The Flock 1 constellation nanosatellites (all 3U CubeSats) were designed and built by Planet Labs Inc. They feature mostly COTS (Commercial-off-the-Shelf) components, including their imagers. Each nanosatellite has a mass of ~5 kg and a size of 10 cm x 10 cm x 34 cm.
Figure 1: Photo of the proud developers and a close-up view of a Flock 1 nanosatellite (image credit: Planet Labs) 4)
It can be expected, that the Flock 1 nanosatellite design corresponds closely to the design of the prototype Dove series, which demonstrated that the company's engineers can accurately position the orbiters and capture a continuous stream of imagery with a resolution of 3-5 m.
ADCS (Attitude Determination and Control Subsystem): The attitude is sensed by magnetometers, gyros and photodiodes. The attitude is being controlled by magnetorquers and reaction wheels. The B-dot controller makes use of the B field to reduce the angular rate of the satellite. In this control mode, Dove-1 therefore behaves as a permanent magnet, remaining locked and axis-aligned to the Earth's magnetic field. - Dove-1 will be nadir pointing twice per orbit. The alignment of the magnetic field is known to about 1º at any point.
EPS (Electrical Power Subsystem): The bus provides central power control through a power supply to the camera, the flight computer and the magnetorquers. The power supply regulates the voltages and ensures a stable power supply to each component. Power storage is provided by 8 Lithium-ion cells, providing 20 Ah of charge at full capacity. The batteries will be recharged by body-mounted TASCs (Triangular Advanced Solar Cells).
C&DH (Command & Data Handling) subsystem: C&DH is controlled by the single board computer. Additionally there will be a discrete watchdog board that will be able to reboot the flight computer in the event of errors.
RF communications: The communication subsystem consists of a VHF radio beacon for transmitting telemetry and an S-band frequency hopping spread spectrum modem for two-way communication and as the primary radio for data downloading. After powering up, the first mission event is to transmit telemetry data over the VHF beacon. The beacon will transmit health packets (including temp/power supply/current/RSSI/solar vector/acceleration) at 1200 baud AFSK approximately every 30 seconds (AX.25 protocol, 145.825 MHz). The beacon can transmit at up to 1 W and will use a quarter wave monopole antenna cut from tape measure.
The S-band radio will operate in the 2.4 GHz half-duplex ISM (Industrial, Scientific and Medical) band at a wireless link rate of 115 kbit/s using a patch antenna.
In Nov. 2013, Planet Labs' Flock 1 nanosatellites were delivered to NASA's Wallops Flight Facility in Virginia for launch on board an Antares rocket in December 2013. These satellites were built in production at the Planet Labs headquarter offices in San Francisco. Planet Labs is on track to launch 32 satellites on four different launches in 2013. 5)
Radio Development History(update to RF communications): 6)
In order to meet Mission 1 data volume objectives, Planet built a custom high-speed X-band radio using COTS components that is tightly integrated with the rest of the CubeSat bus. Several radio architecture decisions were made in order to meet the size, weight, and power (SWaP) constraints of the CubeSat platform while also improving the overall system efficiency. For example, the final stage RF power amplifier was integrated adjacent to the antenna to improve the total system power efficiency. This allowed the use of antenna solutions with 10-12 dBi gain, but still achieve data rates comparable to much larger class satellites that typically have very high gain antennas.
The custom development solution also allowed for rapid prototyping and repeated iterations that led to continuous improvements to the radio subsystem. Planet's first satellite launch hosted the sixth iteration (Build 6) of the spacecraft that transmitted 2 W of RF power through a 3 dBi gain patch antenna. A 6.1 m dish at the Chilbolton Observatory in the UK served as the ground station antenna. On April 25, 2013, Planet achieved its first successful X-band downlink at 4 Mbit/s data rate, which set a new data rate record for CubeSat class satellites.
The success of the early launches and tech demos proved that Planet could meet its Mission 1 goals by following the \agile aerospace" philosophy of rapid prototyping, repeated iterations, and continuous improvements. Less than three years after the first launch, the thirteenth iteration (Build 13) of the satellites was launched on the Indian Polar Satellite Launch Vehicle (PSLV) in June 2016. This constellation code named Flock 2P or F2P included twelve \Build 13" satellites (launch in June 2016). Further improvements were made to this build and Flock 3P or F3P consisting of 88 satellites was launched in February 2017. With the F3P launch, Planet set a record for the most number of operational satellites (100 Build 13 satellites and several B10, and B12 satellites). Planet has demonstrated 220 Mbit/s peak data rates with the Flock 3P constellation and average data rates of approximately 160 Mbit/s. Cumulatively, these satellites generate several TB of imagery data daily that is downlinked to eight geographically diverse ground station sites. The active imaging satellites and the ground stations are shown in Figure 36.
Improvements to the spacecraft HSD (High-Speed Data) system have followed Planet's iterative design approach. Higher gain antennas have been added and improved, RF circuit impairments have been addressed, amplifier settings have been optimized, and the radio firmware and software has undergone constant development. Table 1 provides a summary of key HSD development milestones.
Table 1: Summary of key HSD development milestones
The HSD radio operates at X-band with a center frequency of 8150 MHz and 70 Mbaud symbol rate. This frequency is within the 8025-8400 MHz (X-band) range where EESS (Earth Exploration Satellite Service) has a primary spectrum allocation.
Figure 2: Photo of the 28 Flock 1 nanosatellites before being sent to the launch site (image credit: Planet Labs) 7)
Figure 3: Planet Dove nanosatellite in operational configuration with solar panels deployed and communications antenna flap opened (image credit: Planet)
Launch: The Cygnus CRS-1 logistics flight of Orbital Sciences to the ISS was launched on Jan. 09, 2014 on an Antares-120 Vehicle of OSC from MARS (Mid-Atlantic Regional Spaceport), Wallops Island, VA. 8)
Cygnus CRS-1 (Commercial Resupply Services) Orb-1 logistics flight of Orbital Sciences is the second Cygnus flight to the ISS (International Space Station) and the third launch of the company's Antares launch vehicle from MARS (Mid-Atlantic Regional Spaceport), Wallops Island, VA. The flight is the first of 8 under the CRS (Commercial Resupply Services) contract to NASA. The mission is scheduled to launch on December 18, 2013. Cygnus is expected to deliver 550 kg of cargo to ISS and dispose of about 1,000 kg through destructive reentry.
Orbit: Near-circular orbit of the ISS, altitude between 370-430 km, inclination = 51.6º. — Planet refers to a group of Doves deployed simultaneously into a single orbit as a flock.
Secondary payloads: commercial payloads of Orbital Sciences.
Part of the Cygnus payload consists of 33 CubeSats which will be deployed in early 2014 with the NanoRacks Smallsat Deployment Program using the J-SSOD (JEM-Small Satellite Orbital Deployer), located in the airlock of the JEM/Kibo module of JAXA.
The CubeSats are:
• 28 Flock 1 3U CubeSats, the first generation of an Earth observation constellation of Planet Labs. They will be placed into 400 km circular orbits (inclination of ~52º), providing imagery with a resolution of 3-5 m. 9)
• LituanicaSAT-1, a CubeSat which will carry a VGA camera, a GPS receiver and a voice transponder. The satellite has been developed at the Vilnius University (Lithuania) and has been named after the aircraft named Lituanica that flew across the Atlantic Ocean 80 years ago.
• LitSat, a 1U CubeSat developed by the Lithuanian Space Federation. The satellite will carry an onboard VGA camera and a GPS receiver.
• UAPSAT, a 1U CubeSat developed by UAP (Universidad Alas Peruanas), Lima, Peru, as a student educational project. Once in orbit the satellite can be accessed by radio amateurs; UAPSAT will test the behavior of electronic design communication, orientation and stabilization and verify the implementation of the technology and methodology used in the manufacture of the satellite.
• SkyCube, a 1U CubeSat developed by the Southern Stars Group LLC (San Francisco, CA) and funded by thousands of sponsors and mobile app users around the world (crowd funding). Its objective is to facilitate global grassroots public outreach and educational effort whose purpose is to make space exploration accessible as never before by allowing participants to send simple broadcasts - "tweets from space". The satellite is also fitted with a camera for on-demand pictures of Earth. At the end of the 90 days mission, SkyCube will inflate an onboard balloon that will make it visible to the unaided eyes and assist in de-orbiting the satellite cleanly through atmospheric drag. 10)
• Ardusat-2, developed by Nanosatisfi LLC, which is similar to the Ardusats that were sent to ISS earlier this year.
On January 12, 2014, the Cygnus CRS-1 spacecraft arrived at the ISS where Expedition 38 astronauts grappled the spacecraft and berthed it in a flawless operation (Figure 4).
Figure 4: Image of Cygnus grappling with Canadarm2 and berthing to the ISS (image credit: NASA)
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. 11) 12)
Orbit: Sun-synchronous orbit, altitude of ~500 km, inclination of ~97º.
Figure 5: Illustration of the launch sequence (image credit: Orbital ATK) 13)
Status of the Planet Flock Nanosatellite Constellation — and the fleet acquisitions:
• January 21, 2018: Today, Rocket Lab's Electron vehicle "Still Testing" successfully reached orbit and deployed a Dove Pioneer Earth-imaging satellite for launch customer Planet, as well as two Lemur-2 satellites for weather and ship tracking company Spire Global. The Electron vehicle of Rocket Lab lifted off on 21 Jan. 2018 at 01:43 UTC from the Rocket Lab Launch Complex 1 on the Mahia Peninsula in New Zealand. 14) 15)
- Following successful first and second stage burns, Electron reached orbit and deployed customer payloads at 8 minutes and 31 seconds after lift-off.
- "Today marks the beginning of a new era in commercial access to space. We're thrilled to reach this milestone so quickly after our first test launch," says Rocket Lab CEO and founder Peter Beck. "Our incredibly dedicated and talented team have worked tirelessly to develop, build and launch Electron. I'm immensely proud of what they have achieved today."
- "Reaching orbit on a second test flight is significant on its own, but successfully deploying customer payloads so early in a new rocket program is almost unprecedented. Rocket Lab was founded on the principal of opening access to space to better understand our planet and improve life on it. Today we took a significant step towards that," he says.
- Rocket Lab's commercial phase will see Electron fly already-signed customers including NASA, Spire, Planet, Moon Express and Spaceflight. Rocket Lab's offices are in Huntington Beach, California; Washington DC, and Auckland, New Zealand.
• November 30, 2017: A new partnership with the pioneering satellite-imaging and data company Planet will enable Human Rights Watch to increase substantially the organization's use of satellite imagery to document rights violations around the world. 16)
- Satellite imagery plays an increasingly important role in exposing abuses, especially in countries where access for human rights investigators is blocked or heavily restricted, such as Burma or Syria.
- Planet is an integrated aerospace and data analytics company that operates history's largest fleet of Earth-imaging satellites, collecting a massive amount of information about the changing planet with a core mission of "using space to help life on Earth." Decision makers in business, government, and nonprofit organizations use Planet's data and machine learning-powered analytics to develop new technologies, drive revenue, power research, and solve some of the world's toughest challenges. In November, Planet met its goal of imaging the entire Earth's landmass every day.
- In recent months, satellite images have played a key role in documenting widespread burning of ethnic Rohingya villages in Burma, attacks against schools in Syria, and airstrikes in populated neighborhoods of Mosul, Iraq. Beyond its use in areas of armed conflict, Planet's data is uniquely suited for research on abuses related to land rights and the environment.
- Kenneth Roth, executive director of Human Rights Watch, mentioned that Planet's generous arrangement with Human Rights Watch, with access to its vast collection of high-resolution satellite data, will help provide timely evidence of unfolding abuses in some of the world's most dangerous spots. This partnership will boost our ability to monitor complex events as they occur and help identify those responsible for atrocities.
- Andrew Zolli, who leads the global impact initiatives at Planet, furthered Roth's comment by adding that Planet seeks to use their groundbreaking technology to promote human rights, protect the Earth, and diminish suffering. As one of the first satellite companies to originate entirely from the civilian sector, Planet is uniquely suited to support Human Rights Watch's mission of documenting rights abuses wherever and whenever they occur.
• October 31, 2017: Today, 6 SkySats (SkySat 8-13) and 4 Doves (Flock 3m) successfully launched on an Orbital-ATK Minotaur-C rocket destined for a Sun Synchronous, 500 km orbit. Planet is happy to report that all 10 satellites were deployed successfully and that our Mission Control team has made contact with all satellites and begun the commissioning process. 17)
- This marks the 20th launch for Planet and our first-ever dedicated launch, which gave us the ability to choose our orbital parameters and timing of the launch. We sent these 10 satellites to an afternoon crossing time of approximately 13:30 hour to further diversify our product offerings. Most remote sensing satellites operate in morning-crossing configurations, including our currently on orbit set of 160+ Doves and 7 SkySats, and 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.
- Furthermore, the launch of these 6 SkySats will double the sub-1 meter imaging capacity of our current SkySat fleet, providing a deeper understanding about our changing planet. With this expanded capability, decision makers will gain smarter, more timely insights to solve our world's toughest challenges.
• October 17, 2017: Newly released satellite images reveal that at least 288 villages were partially or totally destroyed by fire in northern Rakhine State in Burma since August 25, 2017, Human Rights Watch said today. The destruction encompassed tens of thousands of structures, primarily homes inhabited by ethnic Rohingya Muslims. 18)
Figure 6: Multiple villages on fire along the coast of Maungdaw Township, Burma on the morning of September 15, 2017. Analysis Human Rights Watch (image credit: Planet Labs)
- Analysis of the satellite imagery indicates both that the burnings focused on Rohingya villages and took place after Burmese officials claimed security force "clearance operations" had ceased, Human Rights Watch said. The imagery pinpoints multiple areas where destroyed Rohingya villages sat adjacent to intact ethnic Rakhine villages. It also shows that at least 66 villages were burned after September 5, when security force operations supposedly ended, according to a September 18 speech by State Counselor Aung San Suu Kyi. The Burmese military responded to attacks on August 25 by the Arakan Rohingya Salvation Army (ARSA) with a campaign of ethnic cleansing, prompting more than 530,000 Rohingya to flee across the border to Bangladesh, according to the United Nations refugee agency.
Figure 7: Complete destruction of Rohingya villages in close proximity to an intact Rakhine village, Maungdaw township, recorded on 21 September 2017. Analysis by Human Rights Watch (image credit: Digital Globe)
• August 2017: Planet's Flock 3p is a constellation of 88 Dove Earth observation nanosatellites launched on a single Indian PSLV (Polar Satellite Launch Vehicle). Commissioning this fleet of satellites is a unique and challenging task that requires significant planning, automation, and operations. At Planet, a small team of operators is charged with shepherding the flock of satellites from initial deployment through full sensor and radiometric calibration. 19)
- Since its founding, Planet has defined and embraced agile aerospace. Planet's culture encourages iterative development based on experimentation. The operations team has grown and refined the commissioning process through experience on eight different launches.
- Spacecraft operations at Planet also demands a focus on automation. By rejecting the idea that satellites require large teams to operate, the team has built one-of-a-kind automated systems for commissioning and operating large fleets of satellites. Operators focus on improvements and anomalies, not day-to-day operations.
- With 88 satellites deployed from a single rocket, Flock 3p is the largest launch in history. Despite requiring an extensive list of calibration and checkout activities, Planet's small spacecraft operations team was able to meet its deadlines and have Flock 3p producing imagery for customers in just over three months.
Table 2: Historical Planet launch manifest, including RapidEye and SkySat launches (Ref. 19)
- How Planet Launches: Planet's ideal imaging orbit is morning sun-synchronous (like most other remote sensing missions). However, Planet's strategy is to aggressively procure regular launches even if they are not to an ideal orbit. Variability in orbits, such as in power and thermal budgets, is managed through operations and extensive software automation. For this reason, Planet has historically utilized commercial launch services through Nanoracks to provide convenient access to space through the ISS (International Space Station). Flocks are launched as cargo to the ISS and deployed through the J-SSOD (JEM Small Satellite Orbital Deployer) in groups of two. Because the Dove satellites do not have active propulsion, these satellites have an orbit very similar to that of the ISS.
- Planet has also done several direct launches with Doves as a secondary payload to sun-synchronous orbits, which provide the benefit of a consistent sun angle.
Table 3: ISS versus SSO Orbits
- Planet strives for diversity in its launch manifest for a number of reasons. Vehicle family diversity is important because launch failures will usually delay the manifest of that vehicle family by a year or more while an accident investigation is underway. Geopolitical diversity has also proven to be very important to accommodate unpredictable changes in regulations and the global political environment.
• August 2017: Planet has deployed a constellation of low-cost, state of the art small satellites that will effectively act like a line-scanner of the Earth's surface. Planet's current mission is to provide images of the entire Earth's landmass one time per day, every day. The work with the U.S. Navy seeks to expand this global monitoring capability from the land to the sea, starting with coastal regions and expanding into the oceans. Planet's innovative approach to space is also observed in its image pipeline and rectification processes. Rectification and interpolation approaches are evaluated over water collects to observe how these impact relative geo-accuracies out to 10 km, 60 km, and open water (>=100 km). Analysis revealed relative geo-accuracies of 220 m at 10 km, 221 m at 60 km, and >~1.4 km in open water. 20)
- To date, Planet has launched more than 200 Dove satellites, operates more than 100 Dove spacecraft, and collects and processes more than 150 million km2 of satellite imagery per day. Toward the end of the second quarter of 2017, Planet's constellation of Doves will revisit every spot on the Earth daily which will include open water areas. Planet complements its imagery collection backbone with an internally developed, web-based, imagery processing platform that enables automated image processing and orthorectification as well as data discovery and delivery.
- In an effort to understand the accuracy of Planet's products over water, Planet was under contract with the Navy's Program Executive Office Space Systems via the Rapid Innovation Fund program. The goal is to measure the relative geo-accuracy of the image data sets over littoral (10 km), coastal (60 km) and open water (>100 km). The purpose is to provide an understanding and drive confidence on Planet's processing pipelines for the addition of maritime imaging to Planet's commercial product line.
- Under contract with the Navy's Program Executive Office Space Systems, Planet determined its data relative accuracy over littoral, coastal and open water regions. Planet demonstrated its technical approach and evaluated the relative geo accuracy for the images produced by the Dove spacecrafts. The goal was to drive confidence in Planet's capabilities to collect over water or in areas where GCPs (Ground Control Points) are not available to provide accurate rectification for the imagery.
- Known relative geo-accuracies over water enables Planet to make collects beyond the Earth's landmass available to users like the Navy that can benefit from these for its day-to-day operations and mission sets by providing them with unprecedented coverage over water, significantly increasing maritime domain awareness. As a result of the work done through the Rapid Innovation Fund, Planet is in the process of releasing an Open Oceans commercial product. The technical preview for the Open Oceans program went live in June 2017 through which Planet has made several open ocean areas available via the platform.
• May 2017: The Geological Remote Sensing Group (GRSG) has expanded its membership once again with the recent news that Planet — the owners and operators of the world's largest commercially-operated fleet of satellites — has joined as the Group's latest corporate member. 21)
- Founded in 2010 by a team of ex-NASA scientists, Planet has designed, built and launched fleets of smallsats to make images of the Earth from space, affordable and widely available, in pursuit of their Mission 1 — to image the entire Earth, every day. With an agile approach to satellite development and Planet's acquisition of BlackBridge's fleet of five RapidEye satellites (in October 2015) and recently, TerraBella's seven high-resolution SkySat satellites, the company has grown their presence in the geospatial industry incredibly quickly.
- Blanca Payas, Planet Sales Director for Europe, Russia and Central Asia explained why Planet decided to join the GRSG by stating that, in a very short time, Planet has become a global company with offices in Europe and the US. More and more, we are finding that our capabilities are supporting projects in the domain of geology. The companies hope to develop new initiatives and activities together.
- GRSG Chairman, Charlotte Bishop, added that Planet's data and access model starts to challenge the way we in the industry think about and use satellite data in a way not done before.
• 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. 22)
- 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 8: 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)
• March 5, 2017: Located at the gateway to the Sahara desert, within the confines of the fertile zone of the Sudan and in an exceptionally propitious site near to the river Niger, Timbuktu (also spelled Timbuctoo) is one of the cities of Africa whose name is the most heavily charged with history. It is an UNESCO World Heritage Site. - The town is the capital of the Timbuktu Region, one of the eight administrative regions of Mali with a current population of about 55,000. 23)
- Founded in the 5th century, the economic and cultural apogee of Timbuktu came about during the15th and 16th centuries. It was an important center for the diffusion of Islamic culture with the University of Sankore, with 180 Koranic schools and 25,000 students. It was also a crossroads and an important market place where the trading of manuscripts was negotiated, and salt from Teghaza in the north, gold was sold, and cattle and grain from the south.
- The three big Mosques of Djingareyber, Sankore and Sidi Yahia, sixteen mausoleums and holy public places, still bear witness to this prestigious past. The mosques are exceptional examples of earthen architecture and of traditional maintenance techniques, which continue to the present time.
Figure 9: The Flock constellation acquired this view of the famous city of Timbuktu on March 5, 2017 (image credit: Planet)
• Feb. 23, 2017: The 88 nanosatellites of Planet, launched on Feb. 15, 2017, joined dozens already in orbit, bringing the constellation of "Doves," as these tiny imaging satellites are known, to 144. Six months from now, once the Doves have settled into their prescribed orbits, the company says it will have reached its primary goal: being able to image every point on Earth's landmass at intervals of 24 hours or less, at resolutions as high as 3.7 meters — good enough to single out large trees. It's not the resolution that's so impressive, though. It's getting a whole Earth selfie every day. 24)
- The news has already sparked excitement in the business world, which is willing to pay a premium for daily updates of telltale industrial and agricultural data like shipping in the South China Sea and corn yields in Mexico. But scientists are realizing that they, too, can take advantage of the daily data—timescales that sparser observations from other satellites and aircraft could not provide.
- "This is a game changer," says Douglas McCauley, an ecologist at the University of California, Santa Barbara, who wants to use Planet imagery to map coral bleaching events as they unfold. At present, coral researchers often rely on infrequent, costly reconnaissance airplane flights. "The previous state of the science was, for me, like taking a family photo album and shaking out all the photos on the floor and then being asked to haphazardly pick up three images and tell the story of the family."
- McCauley is participating in Planet's Ambassadors Program, which provides free satellite imagery to researchers as it is collected, with no lag time, under an agreement that prohibits them from reselling the data. Joe Mascaro, a tropical ecologist who runs the program, says it was created in the fall of 2015 in response to queries from scientists yearning for access to the company's growing archive of data. Over the course of 2016, Planet approved the applications of about 160 researchers across a range of fields. "We anticipate there will be many new applications of our data that we didn't anticipate," Mascaro says. The company intends to expand the program in the months ahead, and says it is looking for projects that have social, humanitarian, and environmental impacts—and that have the potential for rapid publication in peer-reviewed journals.
Figure 10: With the launch of 88 tiny Doves on Feb. 15, 2017, the satellite company Planet now has 144 at work, which will permit daily images of the entire Earth (image credit: Jonathan McDowell, Harvard-Smithsonian CfA)
- Andreas Kääb, a geoscientist at the University of Oslo, applied to the program to obtain additional data for his work on glaciers, including an investigation into a massive glacial avalanche in Tibet last July that killed nine herders and hundreds of sheep and yaks. Kääb already had before-and-after imagery from Landsat and Sentinel-2, U.S. government and European Space Agency satellites that have, respectively, 30 m and 10 m resolution and revisit intervals of 16 and 10 days. But higher resolution Planet images provided Kääb with valuable, timely clues. The appearance of large crevasses before the avalanche indicated the glacier was "surging," although surges, typically somewhat slow, don't usually lead to avalanches. But Kääb also saw water pooling on the surface of the glacier—a sign of heavy rainfall or unusually high temperatures. That water might have seeped through the crevasses, soaking the sediments below the glacial bed and creating a lubricant that triggered the sudden slip. When he saw a second nearby glacier with similar patterns, "We warned Chinese authorities, but when our warning arrived the glacier had already collapsed," Kääb says (no people, or yaks, were hurt).
- Kääb also used Planet images to study surface displacements along fault lines in New Zealand following the country's 7.8-magnitude earthquake last November. Though high-resolution GPS ground stations are typically used for this, not all faults have dense GPS networks monitoring them. He used Planet images to determine that two fault lines had slipped between 6 and 9 m — showing that medium-resolution optical satellites can fill the gap.
- Dave Petley, who studies landslides at the University of Sheffield in the United Kingdom, has not joined the Ambassadors Program yet, but says that access to the images would be "transformational" for his research. Orbital imagery has revealed some 80,000 landslides in the wake of the New Zealand earthquake. Aftershocks are likely responsible for many of them. But because available images can be weeks apart, "we just have to assume that everything happened in the main shock," Petley says. Daily images during the sequence of aftershocks would show how the landscape responds to different amounts of shaking, Petley says, and help with disaster response. "You want to know how many of your roads are damaged, how many valleys might be blocked."
- Planet's images are also finding a niche among researchers who deal with human-caused calamities, like deforestation. Matt Finer, a researcher at the Amazon Conservation Association in Washington, D.C., gets weekly deforestation alerts based on Landsat images, but says they are too course to determine whether the damage is natural or human-caused. He now turns to Planet data to decide whether an event is concerning. He recalls one incident when his group spotted 11 hectares of forest loss in Peru, accompanied by extensive dredging—signs of an illegal gold mining operation. "The Peruvian government was on the ground within 24 to 48 hours, kicking the miners out," he says. In previous years, Finer says, hundreds of hectares might be lost before anyone acted.
- Micah Farfour, a special adviser on remote sensing at Amnesty International in New York City, is using Planet images to monitor humanitarian crises as they unfold. Timely images can help her corroborate witness testimony or pinpoint emerging refugee crises. "It's a really, really amazing tool for narrowing down time frames," Farfour says. Still, images acquired from other private satellite companies, like DigitalGlobe, remain crucial to Amnesty's work, because they can offer the 30 cm resolution needed to, say, identify mass graves or count the buildings destroyed in a village that's been burned to the ground.
- Another limitation of Planet's Doves is that they only have four spectral bands—red, green, blue, and near-infrared—compared with Landsat's 11 bands. "Planet's daily observation frequencies are incredibly useful," says David Roy, a remote sensing scientist at South Dakota State University in Brookings and co-leader of the Landsat science team. "But there are lots of things .... that are probably not doable with Planet labs data." A major missing component, he says, are thermal bands in the far infrared, which enable Landsat to monitor the evaporation of water from plants. That's "quite important if you're looking at drought monitoring or water consumption, particularly in agriculture," Roy says. The Doves also lack a shortwave infrared band, which on Landsat can distinguish between different types of vegetation.
- These concerns have not slowed the juggernaut of Planet. In early February, it made two major announcements: It had folded Landsat-8 and Sentinel-2 data into its archive and it had initiated a deal to acquire Google's Terra Bella satellite imaging division and its seven SkySats, which have the capability to image at 0.7 m resolution. However, a spokesperson for Planet declined to say whether scientists will have access to those higher resolution images once the deal is completed.
- In the meantime, as more scientists publish their papers using Planet imagery, word is getting around. Mascaro says he was at a meeting of the American Geophysical Union in December 2016 when Kääb showed how Planet data were enabling the monitoring of glaciers. "Not surprisingly, I got a few Ambassadors applications from people who were in the room."
Table 4: Planet Launches Satellite Constellation to Image the Whole Planet Daily 25)
• On February 15, 2017 (UTC), ISRO (Indian Space Research Organization) launched the CartoSat-2D primary mission from SDSC (Satish Dhawan Space Center) on the east cost of India on the PSLV-C37 vehicle, along with a record number of 103 secondary payloads, among them 88 3U CubeSats, (Doves, 4.7 kg each) of Planet (formerly Planet Labs), San Francisco. The Dutch nanosatellite company ISIS (Innovative Solutions In Space), Delft, The Netherlands, provided the integration service for 101 nanosatellites on this flight. The total launch mass was 1378 kg into a sun-synchronous orbit of 506 km altitude. 26)
- Less than a minute after reaching orbit, the fourth stage released the CartoSat-2D environmental satellite — the mission's primary payload — about 17.5 minutes into the mission. Ten seconds later, two experimental Indian nanosatellites separated to test new types of sensors to observe Earth's surface, atmosphere and the conditions in the harsh environment of space. 27)
- Then came a carefully-choreographed deployment sequence for the remaining 101 payloads stowed inside 25 Dutch-built "QuadPacks" for the ride into orbit.
- The QuadPacks opened two at a time to eject their CubeSat passengers. Most of the CubeSats separated while the PSLV was flying over a remote stretch of the Indian Ocean between ground stations in Mauritius and Antarctica. Once the PSLV passed in range of the receiving antenna in Troll, Antarctica, launch controllers at the Satish Dhawan Space Center confirmed all 104 satellites separated as planned.
- The launch of 88 Dove satellites came less than two weeks after Planet announced the acquisition of Terra Bella from Google, which has a constellation of seven higher-resolution spacecraft capable of recording high-definition video during passes over ground targets.
Figure 11: A view of the 25 "QuadPacks" holding 101 CubeSats preparing for launch on the PSLV-C37 mission (image credit: Innovative Solutions in Space)
Figure 12: The SDSC (Satish Dhawan Space Center) in Sriharikota, India, imaged by the in-orbit Dove constellation on Feb. 13, 2017, two days prior to the launch of the PSLV-C37 vehicle with 88 Doves onboard (image credit: Planet) 28)
Figure 13: A clear view amid California's rainy winter of Planet's Headquarters, reveals more than just expanse of the Golden Gate Bridge, Park, and Bay Bridge, but the dramatic sediment pulled from the 11900 km2 of the San Francisco Bay watershed. The image was acquired on Feb. 11, 2017 (image credit: Planet)
• September 29, 2016: The U.S. NGA (National Geospatial-Intelligence Agency) continues to expand its use of commercial satellite imagery, exemplified by the agency's recent contract award to Silicon Valley's startup firm Planet, according to Robert Cardillo, the head of NGA. An "introductory" seven-month, $20 million contract to San Francisco-based Planet, formerly Planet Labs, will give defense and intelligence agencies access to the company's global imagery content. This allows NGA to obtain imagery of at least 85% of the Earth's landmass every 15 days from Planet. The imagery has many operational uses, including environmental monitoring, augmenting higher resolution capabilities, change detection, and answering intelligence questions. 29) 30)
- NGA has partnered with other "NewSpace" providers over the last year, including BlackSky Global and the Google subsidiary Terra Bella. In addition, NGA and the NRO (National Reconnaissance Office) recently created the joint CGA (Commercial GEOINT Activity) to evaluate new commercial GEOINT data and services.
- NGA is also working with the GSA (General Services Administration) to set up the CIBORG (Commercial Initiative to Buy Operationally Responsive GEOINT) program, which will use GSA schedules and other government-wide contracts "to provide efficient, rapid access" to new commercial imagery, data, analysis and services, Cardillo said. CIBORG is slated to begin executing in early 2017.
- The "centerpiece" of NGA's commercial imagery program remains EnhancedView, which longtime industry partner DigitalGlobe supports.
• September 2016: The Flock-1 constellation acquired the snow covered Old Crow Flats, deep inside the Arctic Circle, a vast wetland residing within Canada's Vunut National Park (Figure 14). From caribou to grizzly bears, the remote and unspoiled national park plays temporary host to different migratory animals, including waterfowl. 31)
Figure 14: The meandering Yukon River is seen in this Flock-1 image from 19 September 2016. Notice in particular how the September snows cover the landscape, but the various bodies of water are still free of ice (image credit: Planet Labs, eoPortal team)
• July 14, 2016: Off the north-eastern coast of Australia, you'll find one of the seven natural wonders of the world, nearly 1,500 miles long, the Great Barrier Reef (Figure 15). One can see from this image the long veins that look like underwater rivers streaming down, giant avenues for all manner of wildlife (and divers) to explore and navigate – established over thousands of years from the seabed up. 32)
Figure 15: This Flock 1 image, acquired on 8 July 2016, shows the Great Barrier Reef in Australia (image credit: Planet Labs)
• June 17, 2016: The Nazza Lines in Peru are man-made lines and figures in the desert, which are estimated to be between 1500 and 2500 years old. The hundreds of figures vary in form, some of them are simple lines as featured in this image, while others form the shape of animals, people and other life. 33)
- These lines were created by digging shallow trenches in the desert, and removing the red-brown pebbles that coat the Nazca Desert. This made the clear white clay underneath stand out in stark contrast against the ground around the lines. Due to the desert's remote location and arid and windless climate - the Nazca Desert is one of the driest places on Earth - the lines have remarkably been mostly undisturbed in the centuries following their creation.
- The purpose behind the formation of the lines and figures is unknown, leaving scholars with many theories, ranging from astronomical to religious.
Figure 16: This Flock 1 image, acquired on 8 June 2016, shows the enormous Nazca Lines etched into the arid coastal desert in southern Peru (image credit: Planet Labs) 34)
• April 14, 2016: The Saint Lawrence River is one of North America's major shipping channels. The river flows from the Great Lakes Basin into Lake Saint Pierre (Figure 17) — a protected biosphere, home to unique waterfowl — before draining into the Northern Atlantic's Gulf of Saint Lawrence. 35)
- The cities of Trois-Rivières and Sorel-Tracy sit at either end of the lake, where it rejoins the Saint Lawrence River, and part of Trois-Rivières is visible on the mouth of the river here. Lake Saint Pierre is also a Ramsar site, recognized biosphere reserve and wildlife and bird sanctuary, which is an important area for migrating birds.
Figure 17: Lake Saint Pierre in Quebec, Canada, is pictured in this Flock-1 image, acquired on 14 April 2016 (image credit: Planet Labs Inc.)
• Feb. 2016: Figure 18 of the Flock-1 constellation shows a winter scene in the Sumy Oblast region of the Ukraine. This region is known for its rich earth and agriculture. Come summertime, these fields will produce grains, sunflowers, sugar beets, and potatoes. 36)
Figure 18: This Flock-1 image, acquired on 17 February 2016, shows icy fields during winter in Sumy Oblast, Ukraine (image credit: Planet Labs)
• October 26, 2015: Niagara Falls is a famous collection of three waterfalls on the border between the United States and Canada and is situated in the Canadian province of Ontario and the American state of New York. The three individual waterfalls are the Horseshoe, American and Bridal Veil Falls. 37)
- The waterfalls pour forth from the Niagara River, which flows North from the United States into Canada. The river splits into three waterfalls as the water flows around Goat Island (in the center of Figure 19) and a series of smaller islands on its north side. Niagara Falls has become a popular tourist destination over the past 150 years. Some visit the location to witness the power of the torrent of water and enjoy its nearby parks, while others take a more active (and risky) interest, notably including daredevils who have gone over the waterfalls in barrels or crossed the gorge on wires.
- Horseshoe Falls is the widest and deepest of the three waterfalls, and water drops 57 m here. Due to the strength of the current at the waterfalls, this natural source of energy has been harnessed over the years. Power plants have been constructed on or near the falls to take advantage of this, though a treaty was signed in 1950 to limit the use of the falls for this purpose out of concern for conservation of the site.
Figure 19: The Flock 1 constellation acquired this image of Niagara Falls on 26 October 2015. — Niagara Falls is a 12,000-year-old relic of the last Ice Age. Since then, the falls have moved more than 6 miles upstream from their original location near Lake Ontario.
• October 15, 2015: The acquisition of the BlackBridge RapidEye constellation by Planet Labs is now complete. Our energy is now focused on going to market as one team, under a common brand and executing a single global strategy. Over the coming months we'll role out incremental changes, which will improve our product offerings, service levels and empower our partners and customers to do even more. 38)
• March 1, 2015: The area shown in Figure 20 is in Jeollanam-do, the southernmost province on the Korean Peninsula. This region has some of the most favorable weather for farming, which has resulted in the area producing large amounts of crops. In the small town of Hanja Ri, patchwork grain fields line the shore of an inlet, while intricate rows of seaweed are farmed in the protected waters of the Myeongnyang Strait. 39)
Figure 20: This Flock 1 image, acquired on 01 March 2015, shows aquaculture off the coast of South Korea (image credit: Planet Labs)
Legend to Figure 21: Massive tankers are berthed in an oil terminal in the port of Fujairah, a major logistical hub for the United Arab Emirates. It sits on the Gulf of Oman, giving cargo ships easy access to major shipping lanes leading to China and India—two of the world's largest petroleum importers.
Figure 22: Illustration of a fully deployed Flock 1 nanosatellite (image credit: Planet Labs)
• Feb. 2015: The Chinese city of Ordos (Figure 23), located in Inner Mongolia of China (39°36'N 109°47'E), has gained a reputation for being a "ghost town" due to the small population currently living in the city. Built with the capacity to house over a million people when construction began in 2003, this brand new city was expected to reach that total by 2010, but currently only hosts a population of approximately 20,000 instead. 42)
Due to a variety of reasons, the city was never completely finished, leaving a dichotomous mix of new construction and abandoned sections of the city which were left uncompleted. The price of homes in the city was so high when the construction began that few people could afford it. It is estimated that 98% of the city is either still under construction or completely uninhabited. 43) 44)
Figure 23: This Flock 1 image, acquired 24 January 2015, shows the city of Ordos in Inner Mongolia, China (image credit: Planet Labs)
Legend to Figure 23: In this image one can see tall apartment buildings and frozen lakes in the Kangbashi New Area, located close to the Ordos Desert.
Figure 24: Irrigated fields in Pinal County, Arizona captured by Flock 1 nanosatellites in August 2014 (image credit: Planet Labs) 45)
• Once the 28 Flock 1A nanosatellites are operational, Planet Labs will have the largest fleet of orbiting commercial imaging systems on orbit capable of imaging the entire Earth at least once per week.
• Gradually the 28 Flock 1 Earth imaging satellite that were sent to ISS on January 9, 2014 have been deployed from the Kibo module in batches of two over the period Feb. 11-28, 2014 (all were deployed using the NanoRacks deployer system with the JEMRMS (JEM-Remote Manipulator System).
Built and operated by Planet Labs of San Francisco, the Flock 1 small satellites will capture imagery of Earth for use in humanitarian, environmental and commercial applications.
• The Flock1-27 and -28 nanosatellites were deployed on Feb. 28, 2014 (04:20 UTC)
• The Flock1-25 and -26 nanosatellites were deployed on Feb. 27, 2014 (07:40 UTC)
• The Flock1-23 and -24 nanosatellites were deployed on Feb. 27, 2014 (01:50 UTC)
• The Flock1-19 and -20 nanosatellites were deployed on Feb. 26, 2014 (07:35 UTC)
• The Flock1-21 and -22 nanosatellites were deployed on Feb. 26, 2014 (04:20 UTC)
• The Flock1-17 and -18 nanosatellites were deployed on Feb. 25, 2014 (17:00 UTC)
• The Flock1-9 and -10 nanosatellites were deployed on Feb. 15, 2014 (10:55 UTC)
• The Flock1-7 and -8 nanosatellites were deployed on Feb. 15, 2014 (07:00 UTC)
• The Flock1-15 and -16 nanosatellites were deployed on Feb. 14, 2014 (11:45 UTC)
• The Flock1-13 and -14 nanosatellites were deployed on Feb. 14, 2014 (04:15 UTC)
• The Flock1-11 and -12 nanosatellites were deployed on Feb. 13, 2014 (08:20 UTC)
• The Flock1-5 and -6 nanosatellites were deployed on Feb. 12, 2014 (08:30 UTC)
• The Flock1-3 and -4 nanosatellites were deployed on Feb. 11, 2014 (12:41 UTC)
The Flock 1 nanosatellites will be released, two at time, over a span of one to two weeks early this year. Flock 1 will orbit beneath the station's ~400 km altitude to prevent any potential collisions. Like the station, the satellites will circle Earth in an orbit inclined by ~51.6º north and south of the equator, flying over most of the planet at some point. From this altitude, the Flock 1 nanosatellites provide imagery with a spatial resolution of 3-5 m. 48)
Figure 25: The Flock 1 nanosatellites are prepared for deployment on board the ISS. The photo shows four NRCSDs (NanoRacks CubeSat Deployers), each containing two Doves (image credit: Astronaut Koichi Wakata)
Figure 26: Deployment of the first two Flock 1 nanosatellites from the NanoRacks deployer system attached to the Kibo robotic arm (image credit: NASA)
Figure 27: Deployment of the first two Flock 1 nanosatellites from the NanoRacks deployer system of the ISS (image credit: NASA, Universe Today)
News update: In November 2014, the FCC (Federal Communications Commission) cleared Planet Labs' request to launch and operate non-geostationary orbit (NGSO) Earth Exploration Satellite Service satellites. The company plans to launch up to 500 additional NGSO satellites from the ISS (International Space Station) that are physically and technically identical to those previously authorized. 49)
Planet Labs has deployed more than 30 such satellites from the ISS, all of which operate under Call Sign S2912, into circular orbits at altitudes between 380 km and 410 km and an inclination of 51.6º.
The additional satellites were authorized to transmit remote sensing and telemetry data to fixed Earth stations in the 8025-8400 MHz frequency band, receive command signals in the 2025-2110 MHz band, and may use the 401-402 MHz and 449.75-450.25 MHz bands for early phase and emergency-backup telemetry, tracking and command operations.
Payload design: Planet satellites each carry a telescope and a frame CCD camera equipped with Bayer-mask filter. The CCD sensor converts filtered photons into electrons, which are then amplified in order to produce a digital number corresponding to each pixel in each color band. 50)
Figure 28: Planet Optical System and Camera (image credit: Planet)
Planet has flown three generations of optical instruments: Planet Scope 0 (PS0), Planet Scope 1 (PS1), and Planet Scope 2 (PS2). Images have different attributes depending on satellite altitude and instrument type.
PS0 features a 2 element Maksutov Cassegrain optical system paired with an 11MP CCD detector. Optical elements are mounted relative to the structure of the spacecraft.
PS2 features a five element optical system that provides a wider field of view and superior image quality. This optical system is paired with a 29MP CCD detector.
Table 6: Spectral band and FOV (Field of View) information for PS0, PS1 and PS2 instruments flown at various altitudes
PS0 and PS1 optical systems are designed to collect data in the visible portion (red, blue and green) of the electromagnetic spectrum. The following figures show the expected RGB spectral characteristics of the PS0 and PS1 systems:
Figure 29: RGB spectral bands for PS0/1 (image credit: Planet)
Figure 30: RGB spectral bands for PS2 (image credit: Planet)
Figure 31: Expected spectral response for PS2 (with NIR capability), image credit: Planet
Table 7: Orbit, constellation and satellite specifications
Throughout 2014 and the first half of 2016, Planet focused collection capacity on North America, Asia, and South America. Focusing on these areas allowed Planet to establish initial imagery archive to develop and refine imagery processing, imagery quality, imagery mosaic, and API capabilities.
Figure 32: Heat map as of June 2015, showing percent of quad tiles covered by Planet's imagery (image credit: Planet)
Figure 33: Percent of quad covered (image credit: Planet)
Ground Stations and Network
Planet has developed its own global network of ground stations to support both spacecraft mission operations and image data downlink. Each ground station consists of an antenna and a Radio Frequency (RF) system, coupled with a local computer server, connected via secured VPN (Virtual Private Network) access to centralized services. Downlinked image files are transferred from local ground station servers to Planet's cloud infrastructure for ingestion into Planet's data processing and distribution pipeline.
The ground stations use COTS components where possible to reduce complexity and cost. In total, Planet operates equipment at 15 sites using a combination of rented equipment, co-located antenna systems, and fully independent sites. The diversity in assets allows the ground network to better scale to meet the demands of the on-orbit constellation. Relatively rapid flexibility in downlink capacity is especially important in an industry where a constellation can be built, launched, and deployed in the time needed to obtain licensing for a ground site (Ref. 6).
Nearly all Planet ground station sites include UHF radios for telemetry, tracking, and control (TT&C). These systems are used for scheduling, basic health, and ranging for orbit determination. Eight of the ground station sites also include X-band (HSD) capabilities. These eight geographically diverse sites contain a total of twenty-two dishes of varying diameters, from 4.5 meter to 7.6 meter. All dishes are designed for at least 29 dB/K G/T (Gain to noise Temperature) to optimize both the satellite downlink data rates and ground system cost.
The payload data is downlinked from space at X-band, down-converted to L-band using a low noise block downconverter (LNB), and demodulated using a COTS DVB-S2 receiver. Typical ground stations passes average at 160 Mbit/s and peak at 220 Mbit/s data rates with data volumes of 12-15 GB downloaded per 7-10 minute ground station pass. The receiver deframes DVB-S2 packets, and sends raw IP frames to the ground station server via gigabit Ethernet interface. The ground station server monitors the link status (SNR, link margin, received RF power level, bit error rate), reassembles the picture files, and uploads the pictures and metrics to the cloud. Additionally, many Planet ground antenna systems have a built-in feedback loop for basic open-loop power-tracking using link statistics provided by the DVB-S2 receiver. The power tracking introduces random offsets and tests and checks the feedback look for a positive or negative response. Planet's Dove ground network is nominally autonomous and remotely monitored to enable scalability and improve operational efficiency.51) 52) Figure 34 shows the internals of Planet's S-band/X-band ground station system, and Figure 35 shows a picture of a 4.5 m diameter dish at Keflavik, Iceland.
Figure 34: Ground station system functional blocks (image credit: Planet Labs)
Figure 35: A 4.5 m diameter ground station antenna at Keflavik, Iceland (image credit: Planet Labs)
Figure 36: Location of X-band ground station antennas (black circles) and ground tracks of representative Dove satellites per flock (Flock 2P = Black, Flock 3P = Red, Flock 2E = Blue, Green), image credit: Planet Labs (Ref. 6)
During downlink, images are temporarily buffered locally on a server co-located with the receive antenna and added to an Internet upload queue. The imagery gets uploaded to cloud storage over secured Internet connections, which kicks off a per-image processing pipeline. The local storage decouples imagery downlink from network interruptions or congestion and allows sizing the leased line for average bandwidth (as opposed to peak bandwidth or latency). The imagery is also kept stored on the satellite until the next pass so that it can be confirmed as having successfully entered the processing pipeline before being deleted.
Upon ingestion, images are individually de-mosaiced (de-bayered), color-corrected, flat-fielded, and orthorectified. Orthorectification aligns the image to within 10 m (RMSE) horizontally and maps the data to a fine digital elevation model. Further analysis generates a cloud cover mask and ensures that images released to customers satisfy a series of quality metrics. During the month of May 2017, the 90th percentile of the imagery that rectified and passed quality metrics was published within 5.5 hrs of being downlinked by a ground station.
Successfully rectified images are available to customers as soon as processing has completed. To satisfy the multiple use cases of different customers, imagery is available in a variety of forms:
• Orthorectified images (\scenes", which consist of one full-frame capture) are available via REST HTTP API for customers.
• \Tiles" are produced, similar to the OSM \slippy map" architecture, on a grid that is fixed for all Planet products by rectifying and stacking the RGB and NIR bands of scenes from a single strip of imagery.
• All of the rectified imagery that meets minimum quality conditions is made available on a web-based application open to all for browsing and new imagery alerting based on user-defined areas of interest.
Data Analysis and Metrics:
Planet's mission control infrastructure monitors and controls the satellites and the ground network handles over 650 HSD passes per day. Due to the large number of passes and the limited engineer time available, it is impossible to assess each pass manually. Therefore, Planet has created an automated metrics gathering and failure detection system to monitor constellation-wide high speed downlink performance.
The automated metrics system has been built using open source tools, shown in Figure 37 and runs at regular intervals. Pass relevant information, that includes information from the satellite, the ground station, and physical characteristics (orbit-related), is first consolidated. An automated pass classifier assesses the quality of the pass and identifies a possible reason for failure. The combined pass information, along with the failure diagnosis, is written to a database. A dashboard aggregates information from the passes and creates interactive visualizations. The interactive visualizations allow easy access to pass performance information to any engineer without the need to code new tools.
Figure 37: Internals of the metrics generation and reporting system (image credit: Planet Labs)
The pass classifier automates the decision making process of domain experts in order to automatically diagnose possible failure modes for sub-optimal passes. An HSD pass is considered to be a failure if the amount of image data downloaded is less than 80% of the total downlinked data volume predicted using a link budget model. The prediction uses a formula that depends on the transmitter and receiver characteristics, slant range, and orbital geometries during the pass to compute the RF link margin. Once a pass is judged to be a failed pass (which only implies sub-optimality and not an outright failure due to the conservative assumptions in the data rate predictions) the automatic classification begins.
An automated classified has been developed to classify failures. Every week, domain experts from RF and Communication Systems, Orbit Operations, Satellite Operations, System Integrations, and Ground Station Operations teams discuss, classify, and debug failed passes. After manually classifying the passes, the diagnosis is automated by creating a hierarchical tree of failures. The hierarchy of the failure mode is primarily based on the certainty with which that failure mode can be identified.
This automated approach has allowed for the classification of thousands of passes into discrete buckets with well defined problem statements. The total number of failures and the severity of the failure modes provides the direction for problem solving. Using this approach, the HSD performance was improved by ~50% from 110 Mbit/s average to 160 Mbit/s average over a few month period and pass-to-pass variance was reduced.
In summary, using the \agile aerospace" philosophy, Planet has rapidly prototyped and iteratively developed an end-to-end high speed satellite communication solution that not only included the development of the spacecraft and ground station hardware and software, but also the automated monitoring, scheduling, and control systems, and data metrics and analytics solutions. These co-optimized systems are tightly coupled and work together to achieve record downlink speeds of 220 Mbit/s and data volumes of over 4 TB/day. Figure 38 shows monthly aggregated historical downlink data rates and Figure 9 shows constellation wide daily downlink data volume along with the number of active satellites.
Figure 38: Data rate quantiles for HSD1 and HSD2 annotated with key milestones, aggregated monthly. (image credit: Planet Labs)
Figure 39: Top: Constellation-wide daily downlink data volume with key launch dates. Bottom: Number of active satellites (image credit: Planet Labs)
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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).