X-37B OTV (Orbital Test Vehicle) a classified program of the USAF
On May 7, 2017, the USAF X-37B Orbital Test Vehicle mission 4 (OTV-4), the Air Force's unmanned, reusable space plane, landed at NASA's Kennedy Space Center Shuttle Landing Facility, Fla. Managed by the Air Force Rapid Capabilities Office, the X-37B program is the newest and most advanced reentry spacecraft that performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies. 1)
Figure 1: The X-37B Orbital Test Vehicle-4 landed at Kennedy Space Center (image credit: USAF)
"Today marks an incredibly exciting day for the 45th Space Wing as we continue to break barriers," said Brig. Gen. Wayne Monteith, the 45th SW commander. "Our team has been preparing for this event for several years, and I am extremely proud to see our hard work and dedication culminate in today's safe and successful landing of the X-37B."
The OTV-4 conducted on-orbit experiments for 718 days during its mission (May 20, 2015 to May 7, 2017), extending the total number of days spent on-orbit for the OTV program to 2,085 days.
"The landing of OTV-4 marks another success for the X-37B program and the nation," said Lt. Col. Ron Fehlen, X-37B program manager. "This mission once again set an on-orbit endurance record and marks the vehicle's first landing in the state of Florida. We are incredibly pleased with the performance of the space vehicle and are excited about the data gathered to support the scientific and space communities. We are extremely proud of the dedication and hard work by the entire team."
"The hard work of the X-37B OTV team and the 45th Space Wing successfully demonstrated the flexibility and resolve necessary to continue the nation's advancement in space," said Randy Walden, the director of the Air Force Rapid Capabilities Office. "The ability to land, refurbish, and launch from the same location further enhances the OTV's ability to rapidly integrate and qualify new space technologies."
Figure 2: Alternate view of the X-37B OTV-4 mini shuttle after landing at KSC, FL (image credit: USAF)
The stubbed-winged orbiter will be towed later today from the Florida runway to a modified NASA shuttle hangar, left over from retirement of the civilian program, for post-flight deservicing. 2)
Early this morning, the craft received the ground-issued command to return to Earth, initiating a fully autonomous sequence of events to fire its propulsion system and brake from orbit. Plunging through the atmosphere for a super-hot reentry, the spaceplane executed a series of turns to dissipate speed as it flew towards the Cape.
With Runway 15 fast approaching the glider dropped its landing gear with dinner plate-sized wheels for a tire-smoking touchdown some time around 8 a.m. EDT (1200 GMT) while flying on a sophisticated autopilot fed with GPS navigation.
Some background of the X-37 Program
The Boeing X-37, also known as the OTV (Orbital Test Vehicle), is a reusable unmanned spacecraft. It is boosted into space by a launch vehicle, then reenters Earth's atmosphere and lands as a spaceplane. The X-37 is operated by the USAF (United States Air Force) for orbital spaceflight missions intended to demonstrate reusable space technologies. 3) It is a 120%-scaled derivative of the earlier Boeing X-40. 4)
The X-37 began as a NASA project in 1999, before being transferred to the U.S. Department of Defense in 2004. It conducted its first flight as a drop test on 7 April 2006, at Edwards Air Force Base, California. The spaceplane's first orbital mission, USA-212, was launched on 22 April 2010 using an Atlas V rocket. Its successful return to Earth on 3 December 2010 was the first test of the vehicle's heat shield and hypersonic aerodynamic handling. A second X-37 was launched on 5 March 2011, with the mission designation USA-226; it returned to Earth on 16 June 2012. A third X-37 mission, USA-240, launched on 11 December 2012 and landed at Vandenberg AFB on 17 October 2014. The fourth X-37 mission, USA-261, launched on 20 May 2015 and landed on 7 May 2017 at Kennedy Space Center.
In 1999, NASA selected Boeing Integrated Defense Systems to design and develop an orbital vehicle, built by the California branch of Boeing's Phantom Works. Over a four-year period, a total of $192 million was spent on the project, with NASA contributing $109 million, the U.S. Air Force $16 million, and Boeing $67 million. In late 2002, a new $301-million contract was awarded to Boeing as part of NASA's Space Launch Initiative framework.
The X-37's aerodynamic design was derived from the larger Space Shuttle orbiter, hence the X-37 has a similar lift-to-drag ratio, and a lower cross range at higher altitudes and Mach numbers compared to DARPA's Hypersonic Technology Vehicle. An early requirement for the spacecraft called for a ΔV of 3.1 km/s to change its orbit. An early goal for the program was for the X-37 to rendezvous with satellites and perform repairs. The X-37 was originally designed to be carried into orbit in the Space Shuttle's cargo bay, but underwent redesign for launch on a Delta-4 or comparable rocket after it was determined that a shuttle flight would be uneconomical. — NASA's X-37 Orbital Vehicle was never built: but its design was the starting point for the Air Force's X-37B Orbital Test Vehicle program.
The X-37 project was transferred from NASA to the DARPA (Defense Advanced Research Projects Agency) on 13 September 2004. 5) Thereafter, the program became a classified project. DARPA promoted the X-37 as part of the independent space policy that the United States Department of Defense has pursued since the 1986 Challenger disaster.
On 17 November 2006, the U.S. Air Force announced that it would develop its own variant from NASA's X-37A. The Air Force version was designated the X-37B OTV (Orbital Test Vehicle). The OTV program was built on earlier industry and government efforts by DARPA, NASA and the Air Force, and was led by the U.S. Air Force Rapid Capabilities Office, in partnership with NASA and the AFRL (Air Force Research Laboratory). Boeing was the prime contractor for the OTV program. The X-37B was designed to remain in orbit for up to 270 days at a time. 6) The Secretary of the Air Force stated that the OTV program would focus on "risk reduction, experimentation, and operational concept development for reusable space vehicle technologies, in support of long-term developmental space objectives". 7)
The X-37 Orbital Test Vehicle is a reusable robotic spaceplane. It is a 120%-scale derivative of the Boeing X-40, measuring 8.8 m in length, 2.9 m in height and features two angled tail fin with a wingspan of 4.5 m. Power is provided by GaAs (Gallium Arsenide) solar cells with Li-ion batteries. The launch mass of OTV-1 is 4,990 kg. The X-37 launches atop an Atlas-V version 501 rocket of ULA with a Centaur second stage.
X-37B is powered by a single Aerojet engine using storable propellants. The Main Propulsion System provides a total thrust of about 700 N and X-37B's propulsion system is capable of providing a total ΔV of nearly 3.1km/s over the course of a mission. Power generation is accomplished with a solar array consisting of Gallium Arsenide solar cells.
The spacecraft features an autonomous guidance system as well as modern avionics and flight computers. OTV is a testbed for advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, reentry and landing. According to Boeing, X-37B is capable of operating in orbits with altitudes from 200 to 925 km.
Figure 3: The first X-37B Orbital Test Vehicle waits in the encapsulation cell of the EELV (Evolved Expendable Launch Vehicle) April 5, 2010, at the Astrotech facility in Titusville, Fla. Half of the Atlas-V 5 m fairing is visible in the background. The OTV-1 launch on April 22, 2010 on a ULA vehicle from KSC, FL (image credit: USAF)
As of early May 2015, the Air Force has successfully flown three X-37B missions, OTV-1 through OTV-3, beginning with its first launch on April 22, 2010 from Cape Canaveral Air Force Station, Florida. All three have landed successfully at Vandenberg AFB, California, with the latest landing of OTV-3 occurring on Oct. 17, 2014, after 674 days in orbit. the first three OTV missions have spent a total of 1,367 days in orbit, successfully checking out the x-37B's reusable flight, reentry and landing technologies.
Table 1: As of October 2018, the operational X-37Bs have completed five orbital missions, spending a combined 2,856 days in orbit (Ref. 13)
Orbit of OTV-1, altitude of 401 km x 422 km (mean altitude of 410 km), inclination of 39.99º, period of 90 minutes.
Orbit of OTV-2, altitude of 278.5 km x 289.3 km, inclination of 41.9°, period of 90.2 minutes.
Orbit of OTV-3, altitude of 320 km x 333 km, inclination of 43.5, period of 90.93 minutes.
Orbit of OTV-4, altitude of ~310 km (initial altitude), later it raised its altitude to 350 km where it spent the majority of its mission. On Feb. 5, 2017, the spaceplane maneuvered into an orbit of 307 x 312 km.
Secondary payloads on the OTV-4 flight (that were deployed from the spaceplane):
• LightSail-A, a 3U CubeSat of The Planetary Society, Pasadena, CA.
• GEARRS-2, a 3U CubeSat built by NearSpace Launch in collaboration with the AFRL (Air Force Research Laboratory).
• OptiCube-1, -2, -3, three 3U CubeSats developed by Cal Poly to provide on-orbit targets for ground assets to calibrate sensors for orbital debris studies and small-object tracking improvements.
• USS (Unix Space Server) Langley, a 3U CubeSat built by US Naval Academy.
• BRICSat-P, a 1.5 CubeSat of the USNA (US Naval Academy) and George Washington University.
• AeroCube-8A and AeroCube-8B, a 2U CubeSat mission of the Aerospace Corporation of El Segundo, CA.
• ParkinsonSat, a 1.5U CubeSat of USNA.
Payloads aboard the OTV-4:
• According to Aerojet Rocketdyne, the XR-5A Hall Thruster has completed initial in-orbit validation testing aboard the X-37B space plane. XR-5A is an electric propulsion experiment to enable in-space characterization of the design modifications that are intended to improve performance to the units onboard the AEHF (Advanced EHF (Extremely-High Frequency) for RF communications), a military communications spacecraft. 8)
- The XR-5A Hall Thruster is an enhanced version of the Aerojet Rocketdyne XR-5 Hall Thruster. Both thrusters are 5 kW class Hall Thrusters; however, the XR-5A incorporates modifications that improve performance and operating range. Aerojet Rocketdyne has manufactured and delivered 16 XR-5 Hall Thrusters and flown 12 to date. As with most new product introductions, Aerojet Rocketdyne is introducing a product upgrade to incorporate improvements identified after the initial low-rate production and flight programs.
Figure 4: Aerojet Rocketdyne's upgraded XR-5A Hall thruster demonstrates successful on-orbit operation (image credit: Aerojet Rocketdyne)
• METIS (Materials Exposure and Technology Innovation in Space) experiment of NASA to expose nearly 100 different materials samples to the space environment for more than 200 days. METIS is based on the MISSE (Materials on International Space Station Experiment), which flew more than 4,000 samples in space from 2001 to 2013.
Figure 5: Illustration of the X-37B Orbital Test Vehicle elements (image credit: NASA, ULA) 9)
OTV-5 (Orbital Test Vehicle-5) mission
Launch: The OTV-5 mission was launched on 7 September 2017 atop a SpaceX Falcon-9 vehicle from Launch Complex 39A at NASA's Kennedy Space Center in Florida. 10)
The X-37B is heading back to space later this year, and this time the space plane will launch for the first time ever by a SpaceX Falcon-9, the Air Force announced late Tuesday. 11)
The fifth mission of the X-37B, sometimes referred to as the orbital test vehicle, involves carrying the Advanced Structurally Embedded Thermal Spreader (ASETS-II) created by the Air Force Research Laboratory, the service stated in a news release.
Although the Air Force gave scant details on the nature of the mission or the capability of the new payload, generally speaking, the X-37 is used to help validate emerging technologies and concepts of operations related to reusable spacecraft. In a statement, it explained the ASETS-II would "test experimental electronics and oscillating heat pipes in the long duration space environment."
Figure 6: Artist's rendition of the X-37B spaceplane (OTV-5) with the deployed solar panels (image credit: Boeing Phantom Works)
Payload aboard the OTV-5
Most X-37B payloads and activities are classified. The only OTV-5 payload revealed to date by Air Force officials is the ASETS-II (Advanced Structurally Embedded Thermal Spreader-II).
Developed by the U.S. Air Force Research Laboratory (AFRL), this cargo is testing experimental electronics and OHP (Oscillating Heat Pipes) for long-duration stints in the space environment. According to AFRL, the three primary science objectives are to measure the initial on-orbit thermal performance, to gauge long-duration thermal performance and to assess any lifetime degradation.12)
The OHP is a simple, wickless heat pipe capable of rejecting more than 200 times the maximum heat load of an axially grooved heat pipe, and transporting more than 45 times more heat than copper.
Figure 7: Cutaway of an oscillating heat pipe (OHP) showing its microchannel pattern (image credit: AFRL)
In addition to outperforming traditional thermal management technologies, OHPs enable low-cost manufacturing techniques due to the lack of an internal wick structure. OHPs provide a low cost method to alleviate electronics thermal constraints and allow for increased processing power, or bandwidth, for commercial and military users.
Experiment: The ASETS-II experiment is made of three low-mass, low-cost OHPs and an electronics/experiment control box. The three OHPs are of varying configuration (center heating with single- and double-sided cooling) and working fluids (butane and R-134a) in order to isolate specific performance parameters of interest. In addition to serving as a science experiment, the ASETS-II flight experiment serves as a pathfinder for incorporating high performance OHP-based thermal spreaders into flat plate structures such as electronics chip carriers, thermal ground planes, and spacecraft panels.
Microgravity influence on fluid flow, especially two-phase flow, is significantly different than in a terrestrial environment. This is true for steady-state operations, but is more important for transient operation. In addition, the combined effects of the space environment (e.g. thermal cycling, high vacuum, charging, vibration) are required to verify the performance of the system on orbit for long durations.
The three primary science objectives are to measure the initial on-orbit thermal performance, to measure long duration thermal performance, and to assess any lifetime degradation. Flight data will be used to validate microgravity portions of an OHP operating limits model recently published by members of the team. Returned flight experiment hardware will be subjected to post-flight testing to assess the presence of any non-condensable gas that may have formed on orbit.
Figure 8: Photo of ASETS-II (image credit: AFRL)
Collaboration: The AFRL Space Vehicles Directorate has been investigating OHPs since 2008. In 2012, the original ASETS experiment measured the performance of an AFRL-developed OHP aboard a Space Test Program (STP) funded and NASA managed the microgravity aircraft flight.
The ASETS-II OHPs were developed by ThermAvant Technologies, LLC (Columbia, MO) under a Department of Defense Small Business Innovation Research (SBIR) contract. The flight hardware was designed and built by AFRL's Space Vehicles Directorate, with support from LoadPath (Albuquerque, NM), Applied Technology Associates (Albuquerque, NM), Gulfview Research Inc. (Fort Walton Beach, FL), and Odin Engineering (St. Petersburg, FL).
Located at Kirtland Air Force Base, New Mexico, the Space Vehicles Directorate serves as the Air Force's "Center of Excellence" for space technology research and development. The directorate develops and transitions space technologies to provide spaceborne capabilities.
• October 27, 2019: The Air Force's X-37B spaceplane made a pre-dawn landing Oct. 27 2019 at NASA's Kennedy Space Center in Florida, after spending 780 days in orbit, breaking its own record by 62 days. 13)
- The Air Force said the X-37B Orbital Test Vehicle-5 (OTV-5) mission officially ended at 3:51 a.m. Eastern with a successful runway landing at NASA's Shuttle Landing Facility. The mission originally launched on a SpaceX Falcon 9 rocket Sept. 7, 2017.
Figure 9: The Air Force X-37B spaceplane landed at NASA's Kennedy Space Center Shuttle Landing Facility on Oct. 27, 2019 (image credit: USAF)
- The Boeing-built X-37B that landed Oct. 27 is one of at least two such autonomous spaceplanes in the Air Force's fleet. First launched in 2010, the reusable X-37Bs have accumulated 2,856 days in orbit. The spaceplane originally was designed to fly for just 270 days.
- "The X-37B continues to demonstrate the importance of a reusable spaceplane," Air Force Secretary Barbara Barrett said in a news release.
- The spaceplane program, managed by the Air Force Rapid Capabilities Office, has been used for science experiments to test technologies in a long-duration space environment.
- "With a successful landing today, the X-37B completed its longest flight to date and successfully completed all mission objectives," said Randy Walden, director of the Air Force Rapid Capabilities Office. "This mission successfully hosted Air Force Research Laboratory experiments, among others, as well as providing a ride for small satellites."
- One of the experiments on OTV-5 is the U.S. Air Force Research Laboratory's second Advanced Structurally Embedded Thermal Spreader (ASETS-II). This experiment will measure the long term performance of an oscillating heat pipe on orbit. Oscillating heat pipes are capable of transporting more than 45 times more heat than copper and are one of many technologies that the Air Force is testing to help advance space vehicle designs, AFRL said.
- The original X-37 program was led by NASA and ran from 1999 to 2004, when the Defense Advanced Research Projects Agency took possession of the X-37A vehicle and conducted a series of approach and landing tests over the next two years. In 2006, the Air Force tapped Boeing to build the follow-on X-37B. A second X-37B was built in 2010.
- The Oct. 27 landing marked the second time an X-37B touched down at Kennedy Space Center's Shuttle Landing Facility. The first was OTV-4, which landed May 7, 2017, after 718 days in orbit. The first three X-37B missions landed at Vandenberg Air Force Base, California.
- The Air Force plans to launch the sixth X-37B mission from Cape Canaveral in 2020 on a United Launch Alliance Atlas 5 rocket. The Air Force Space and Missile Systems Center expects the launch to occur sometime between April and June.
• August 8, 2019: The robotic X-37B launched its fifth and latest mission, known as Orbital Test Vehicle 5 (OTV-5), hitting the 700-day mark, and is just a few weeks short of breaking the vehicle's spaceflight-duration record. 14)
- The reusable spacecraft, which looks like a miniature version of NASA's space shuttle, is on a mission that has been a topic of speculation since its start on September 7, 2017, as the solar-powered spacecraft's missions, and most of its payloads, remain classified. The US Air Force keeps stressing that the space plane tests technologies for future reusable spacecraft and takes experiments up to space and back.
- Air Force officials, however, have revealed that its payloads include the Advanced Structurally Embedded Thermal Spreader experiment (ASETS-II), which is measuring how oscillating heat pipes and certain electronics perform in the space environment.
- USAF currently possesses at least two X-37B vehicles, both of which were built by Boeing. Each space plane is 8.8 meters long and 2.9 m tall, with a wingspan of almost 4.6 m and a payload bay the size of a pickup-truck bed. Like the space shuttle, the X-37B launches vertically and lands on a runway like a plane.
- The X-37B missions have been in use since 2010, with the first mission (OTV-1) starting in April and ending in December after 224 days in space. OTV-2 lasted from March 2011 through June 2012, racking up 468 days of orbital flight.
- The 675-day OTV-3 mission launched in December 2012 and landed in October 2014, and OTV-4 wrapped up in May 2017 after 718 days. As each mission is longer than the previous one, there is a good chance that OTV-5 will remain in orbit after surpassing the record in 19 days.
• January 28, 2019: The secretive mission of a U.S. Air Force X-37B mini-space plane has winged past 500 days of flight. This robotic drone is performing classified duties during the program's fifth flight. 15)
- This mission – tagged as Orbital Test Vehicle (OTV-5) — was rocketed into Earth orbit on 7 September 2017 atop a SpaceX Falcon 9 booster from Launch Complex 39A at NASA's Kennedy Space Center in Florida.
- The missions of the X-37B space planes are carried out under the auspices of the Air Force Rapid Capabilities Office, and mission control for OTV flights are handled by the 3rd Space Experimentation Squadron at Schriever Air Force Base in Colorado. This squadron oversees operations of the X-37B Orbital Test Vehicle.
- This Schriever Air Force Base unit is tagged as the Air Force Space Command's premier organization for space-based demonstrations, pathfinders and experiment testing, gathering information on objects high above Earth and carrying out other intelligence-gathering duties.
- And that may be a signal as to what the robotic craft is doing — both looking down at Earth and upward.
- On-orbit testing: On this latest clandestine mission of the space plane, all that's known according to Air Force officials is that one payload flying on OTV-5 is the Advanced Structurally Embedded Thermal Spreader, or ASETS-II.
- Developed by the U.S. Air Force Research Laboratory (AFRL), this cargo is testing experimental electronics and oscillating heat pipes for long duration stints in the space environment.
- According to AFRL, the payload's three primary science objectives are to measure the initial on-orbit thermal performance, to measure long duration thermal performance, and to assess any lifetime degradation.
• October 22, 2018: The US Air Force's unmanned X-37B space plane has passed its 400-day mark, inching its way toward setting a new flight duration record for the OTV (Orbital Test Vehicle) mission. 16)
- The spacecraft, the fifth of its kind, was initially rocketed into orbit on September 7, 2017, aboard a SpaceX Falcon 9 rocket, according to Space.com. All previous OTV missions established new flight records, with the fourth spacecraft spending 718 days in orbit.
- Though details of the space plane are kept on a need-to-know basis by officials, it has been reported that the craft is carrying in its payload an Advanced Structurally-Embedded Thermal Spreader.
- In August, the space plane was spotted by Marco Langbroek, a Netherlands-based satellite tracker. Langbroek previously told Space.com that X-37B was flying at a very low altitude, somewhere between 193 and 202 miles up.
- "Basically, only one type of object fits this: X-37B," he said of the craft he observed. "Previous X-37B missions we tracked also orbited at such very low altitudes. The object also has a similar brightness to previous OTV missions." Langbroek works alongside a small group that tracks the plane's path.
- Although it's unclear where the OTV-5 will land once its mission comes to a close, website Space Flight Insider reported that it's likely to land at the Shuttle Landing Facility in Florida, the same location where OTV-4 touched down in May 2017.
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2) Justin Ray, "X-37B spaceplane returns to Earth and makes autopilot landing in Florida," Spaceflight Now, May 7, 2017, URL: https://spaceflightnow.com/2017/05/07/x-37b-spacepl
3) "X-37B Orbital Test Vehicle," USAF, April 15, 2015, URL: http://www.af.mil/About-Us/Fact-Sheets/Display/Article/104539/x-37b-orbital-test-vehicle/
5) Brian Berger, "NASA Transfers X-37 Project to DARPA," Space News, Sept. 15, 2004, URL: http://www.space.com/337-nasa-transfers-37-project-darpa.html
6) Stephen Clark, "Air Force X-37B spaceplane arrives in Florida for launch," Spaceflight Now, Feb. 25, 2010, URL: https://spaceflightnow.com/atlas/av012/100225x37arrival/
7) Leonard David, "U.S. Air Force Pushes For Orbital Test Vehicle," Space.Com, November 17, 2006, URL: http://www.space.com/3124-air-force-pushes-orbital-test-vehicle.html
8) "Aerojet Rocketdyne's Modified XR-5 Hall Thruster Demonstrates Successful On-Orbit Operation," Aerojet Rocketdyne, July 1, 2015, URL: http://www.rocket.com/article/aerojet-rocketdyne%E2%8
9) Leonard David, "Air Force's Mysterious X-37B Space Plane Wings by 600 Days in Orbit," Space.com, January 10, 2017, URL: http://www.space.com/35272-x-37b-military-space-plane-600-days.html
10) Leonard David, "X-37B Military Space Plane Wings Past 400 Days on Latest Mystery Mission," Space.com, 18 October 2018, URL: https://www.space.com/42175-x-37b-space-plane-otv5-400-days-orbit.html
11) Valerie Insinna, "X-37B space plane to get lift from SpaceX," Defence News, 7 June 2017, URL: https://www.defensenews.com/space/2017/06/07/x-37b-space-plane-to-get-lift-from-spacex/
12) "Advanced Structurally Embedded Thermal Spreader II (ASETS-II)," Air Force Research Laboratory - Space Vehicles Directorate, July 2017, URL: https://www.kirtland.af.mil/Portals/52/documents/ASETSII.pdf?ver=2017-08-10-125001-450
13) Sandra Erwin, "Air Force X-37B secret spaceplane lands after 780 days in orbit," SpaceNews, 27 October 2019, URL: https://spacenews.com/air-force-x-37b-secret-spaceplane-lands-after-780-days-in-orbit/
14) "USAF X-37b military space plane's mystery mission circling Earth hits 700 days," Space Daily, 8 August 2019, URL: http://www.spacedaily.com/reports/USAF_X_37B_Military_Space_Planes_
15) Leonard David, "U.S. Air Force Space Plane Wings Past 500 days of Earth Orbiting (Updated)," 28 January 2019, URL: http://www.leonarddavid.com/u-s-air-force-space-plane-wings-past-500-days-of-earth-orbiting/
16) "US Air Force's X-37B space plane marks 400 days in orbit," Space Daily, 22 October 2018, URL: http://www.spacedaily.com/reports/US_Air_Forces_X_37B_
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 (email@example.com).