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KH-9 Hexagon

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KH-9 (BYEMAN codename HEXAGON), commonly known as Big Bird or KeyHole-9,[1] was a series of photographic reconnaissance satellites launched by the United States between 1971 and 1986. Of twenty launch attempts by the National Reconnaissance Office (NRO), all but one were successful.[2] Photographic film aboard the KH-9 was stored on RCA Astro Electronic Division take up reel system then sent back to Earth in recoverable film return capsules for processing and interpretation. The highest ground resolution achieved by the main cameras of the satellite was 2 ft (0.61 m),[3] though another source says "images in the "better-than-one-foot" category" for the last "Gambit" missions.[4]

KH-9 HEXAGON
A KH-9 HEXAGON during assembly by Lockheed
Mission typeImagery intelligence
OperatorNational Reconnaissance Office
Spacecraft properties
Manufacturer
Launch mass11400 kg to 13300 kg (with mapping camera)
Dimensions16.2 m × 3.05 m (53.1 ft × 10.0 ft)
Start of mission
RocketTitan III
Launch siteVandenberg Air Force Base, SLC-4E
ContractorMartin Marietta
Orbital parameters
Reference systemSun-synchronous orbit
RegimeLow Earth orbit
Perigee altitude170 km (110 mi)
Apogee altitude260 km (160 mi)
Inclination97°
Main telescope
Typefolded Wright camera
Diameter0.91 m (3 ft 0 in)
Focal length1.52 m (5 ft 0 in)
Focal ratiof/3.0
Wavelengthsvisible light, Near-infrared
A KH-9 HEXAGON main features

They are also officially known as the Broad Coverage Photo Reconnaissance satellites (Code 467), built by Lockheed Corporation for the NRO.[1]

The satellites were an important factor in determining Soviet military capabilities and in the acquisition of accurate intelligence for the formulation of U.S. national policy decisions as well as deployment of U.S. forces and weapon systems. The satellites were instrumental in U.S. National Technical Means of Verification of Strategic Arms Limitation Talks (SALT) and the Anti-Ballistic Missile Treaty (ABMT).[5]

The KH-9 was declassified in September 2011 and an example was put on public display for a single day on 17 September 2011 in the parking lot of the Steven F. Udvar-Hazy Center of the National Air and Space Museum.[6][7][8]

On 26 January 2012, the National Museum of the United States Air Force put a KH-9 on public display along with its predecessors the KH-7 and KH-8.[9]

Development

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KH-9 was, according to many who worked on it, the most sophisticated mechanical satellite in history.[10] It was conceived in the early 1960s as a replacement for the CORONA search satellites. The goal was to search large areas of the Earth with a medium resolution camera. The KH-9 carried two main cameras, although a mapping camera was also carried on several missions. The photographic film from the cameras was sent to recoverable re-entry vehicles and returned to Earth, where the capsules were caught in mid-air by an aircraft. Four re-entry vehicles were carried on most missions, with a fifth added for missions that included a mapping camera.

Between September 1966 and July 1967, the contractors for the Hexagon subsystems were selected. Lockheed Missiles and Space Company (LMSC) was awarded the contract for the Satellite Basic Assembly (SBA), Perkin Elmer for the primary Sensor Subsystem (SS), McDonnell for the Reentry Vehicle (RV), RCA Astro-Electronics Division for the Film Take-Up system, and Itek for the Stellar Index camera (SI). Integration and ground-testing of Satellite Vehicle-1 (SV-1) were completed in May 1971, and it was subsequently shipped to Vandenberg Air Force Base in a 70 ft (21 m) container. Ultimately, four generations ("blocks") of KH-9 HEXAGON reconnaissance satellites were developed. KH9-7 (missions 7 to 12) was the first to fly a Block-II panoramic camera and SBA. Block-III (missions 13 to 18) included upgrades to electrical distribution and batteries. Two added tanks with ullage control for the Orbit Adjust System (OAS) and new thrusters for the Reaction Control System (RCS) served to increase KH-9's operational lifetime. In addition, the nitrogen supply for the film transport system and the camera vessel was increased. Block-IV (missions 19 and 20) was equipped with an extended command system using plated-wire memory.[11] In the mid 1970s, over 1,000 people in the Danbury, Connecticut area worked on the secret project.[12]

A reentry vehicle from the first Hexagon satellite sank to 16,000 ft (4,900 m) below the Pacific Ocean after its parachute failed. The USS Trieste II (DSV-1) retrieved its payload in April 1972 after a lengthy search, but the film had disintegrated during the nine months underwater, leaving no usable photographs.[13]

Over the duration of the program, the lifetime of the individual satellites increased steadily. The final KH-9 operated for 275 days. The satellite mass with and without the Mapping Camera System was 13,300 and 11,400 kg (29,300 and 25,100 lb), respectively.

NRO intended to replace HEXAGON with ZEUS, later DAMON—HEXAGON's camera flown on the Space Shuttle—but DAMON was canceled in December 1980.[10][14] In December 1976 NRO launched the first KH-11 KENNEN. While its electro-optical digital imaging had a smaller field of view than HEXAGON, by not needing film KENNEN was usable for years.[10]

Main KH-9 components

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Satellite Control Section

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Satellite Control Section

The Satellite Control Section (SCS), which forms the aft part of the SBA, started as Air Force Project 467. SCS was intended as a more capable replacement for the on-orbit propulsion, which had been provided by the Agena upper stage for previous generations of reconnaissance satellites. The SCS featured an increased diameter of 10 ft (3.0 m) (compared to 5 ft (1.5 m) for the Agena) and a length of 6 ft (1.8 m). It housed hydrazine propellant tanks for the pressure fed Orbital Adjust System (OAS) and the Reaction Control System (RCS). OAS and RCS were connected by a transfer line to facilitate propellant exchange. The tank pressure was maintained within the operational range by additional high pressure nitrogen tanks. The SCS incorporated a freon gas system for backup attitude control inherited from the Agena, commonly referred to as "lifeboat".[15] SCS was equipped with deployable solar panels and an unfurlable parabolic antenna for high data rate communication.[16]

Main camera

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A main camera optical path
 
A achieved ground resolution of Satellite Vehicles (SV) 1 to 18 main cameras.
 
A KH-9 image of the Soviet Kubinka airfield

The main camera system was designed by Perkin-Elmer to take stereo images,[17] with a forward looking camera on the port side, and an aft looking camera on the starboard side. Images were taken at altitudes ranging from 90–200 mi (480,000–1,060,000 ft; 140–320 km). The camera optical layout is an f/3.0 folded Wright camera, with a focal length of 60 in (1,500 mm). The system aperture is defined by a 20 in (510 mm) diameter aspheric corrector plate, which corrects the spherical aberration of the Wright design. In each of the cameras the ground image passes through the corrector plate to a 45°-angle flat mirror, which reflects the light to a 0.91 m (3 ft 0 in)-diameter concave main mirror. The main mirror directs the light through an opening in the flat mirror and through a four-element lens system onto the film platen. The cameras could scan contiguous areas up to 120° wide, and achieved a ground resolution better than 2 ft (0.61 m) during the later phase of the project.[3][18] Dwayne Allen Day calculated, using disclosed specifications, that HEXAGON was capable of 0.2 m (7.9 in) at nadir, and 0.4 m (1 ft 4 in) at apogee.[10]

Mapping camera

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Missions 1205 to 1216 carried a "mapping camera" (also known as a "frame camera") that used 9 in (230 mm) film and had a moderately low resolution of initially 30 ft (9.1 m), which improved to 20 ft (6.1 m) on later missions [19] (somewhat better than LANDSAT). Intended for mapmaking, photos this camera took cover the entire Earth with images between 1973 and 1981.[20] Almost all the imagery from this camera, amounting to 29,000 images, each covering 3,400 km2 (1,300 sq mi), was declassified in 2002 as a result of Executive order 12951,[21] the same order which declassified CORONA, and copies of the films were transferred to the U.S. Geological Survey's Earth Resources Observation Systems office.[22]

Scientific analysis of declassified KH-9 satellite images continues to reveal historic trends and changes in climate and terrestrial geology. A 2019 study of glacial melt in the Himalayas over the past half-century used data collected by KH-9 satellites throughout the 1970s and 1980s to demonstrate that melt rates had doubled since 1975.[23]

The KH-9 was never a backup project for the KH-10 Manned Orbital Laboratory. It was developed solely as a replacement for the Corona search system.[15]

Reentry vehicles

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The forward section of KH-9 housed four McDonnell Douglas Mark 8 satellite reentry vehicles (RV), which were fed film exposed by the main cameras. Each RV had an empty mass of 434 kg. It housed a film take-up assembly (built by RCA Astro Electronic Division) with a mass of 108 kg, and could store about 227 kg of film. The twelve mapping missions were equipped with an additional General Electrics Mark V RV, which could store about 32 kg of film for a total mass of 177 kg.[15]

High-altitude atmospheric density

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Missions 1205 to 1207 carried Doppler beacons[24] to help map the atmospheric density at high altitudes in an effort to understand the effect on ephemeris predictions.[25][26] The measurements of the atmospheric density were released through NASA.[27]

ELINT subsatellites

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Stereo pair of KH-9 imagery (Los Angeles-1968/06)

Missions 1203, 1207, 1208, 1209, and 1212 to 1219 included Ferret ELINT sub-satellites, which were launched into a high Earth orbit to catalogue Soviet air defence radars, eavesdrop on voice communications, and tape missile and satellite telemetry. Missions 1210 to 1212 also included scientific subsatellites.[28][29][30][31][32][33][34][35][36]

IRCB (S73-7)

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IRCB (Infra-Red Calibration Balloon) was an 66 cm diameter inflatable calibration sphere orbited in the Space Test Program. It was a piggy-back payload on KH9-8 (1208) boosting it to a 500 mile (800 kilometers) circular orbit. It disappeared from ground-based sensors in the 1990s, and was found again in 2024.[37][38]

KH-9 missions

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Name Block [11] Mission no. Launch date NSSDC ID
NORAD # 		Other Name Launch vehicle Orbit Decay date
KH9-1 I 1201 15 June 1971 1971-056A [39]
05297 		OPS 7809 Titan IIID 184.0 km × 300.0 km, i=96.4° 6 August 1971 [39]
KH9-2 I 1202 20 January 1972 1972-002A [40]
05769 		OPS 1737 Titan IIID 157.0 km × 331.0 km, i=97.0° 29 February 1972 [40]
KH9-3 I 1203 7 July 1972 1972-052A [41]
06094 		OPS 7293 Titan IIID 174.0 km × 251.0 km, i=96.9° 13 September 1972 [41]
KH9-4 I 1204 10 October 1972 1972-079A [42]
06227 		OPS 8314 Titan IIID 160.0 km × 281.0 km, i=96.5° 8 January 1973 [42]
KH9-5 I 1205 9 March 1973 1973-014A [43]
06382 		OPS 8410 Titan IIID 152.0 km × 270.0 km, i=95.7° 19 May 1973 [43]
KH9-6 I 1206 13 July 1973 1973-046A [44]
06727 		OPS 8261 Titan IIID 156.0 km × 269.0 km, i=96.2° 12 October 1973 [44]
KH9-7 II 1207 10 November 1973 1973-088A [45]
06928 		OPS 6630 Titan IIID 159.0 km × 275.0 km, i=96.9° 13 March 1974 [45]
KH9-8 II 1208 10 April 1974 1974-020A [46]
07242 		OPS 6245 Titan IIID 153.0 km × 285.0 km, i=94.5° 28 July 1974 [46]
KH9-9 II 1209 29 October 1974 1974-085A [47]
07495 		OPS 7122 Titan IIID 162.0 km × 271.0 km, i=96.7° 19 March 1975 [47]
KH9-10 II 1210 8 June 1975 1975-051A [48]
07918 		OPS 6381 Titan IIID 157.0 km × 234.0 km, i=96.3° 5 November 1975 [48]
KH9-11 II 1211 4 December 1975 1975-114A [49]
08467 		OPS 4428 Titan IIID 157.0 km × 234.0 km, i=96.7° 1 April 1976 [49]
KH9-12 II 1212 8 July 1976 1976-065A [50]
09006 		OPS 4699 Titan IIID 159.0 km × 242.0 km, i=97.0° 13 December 1976 [50]
KH9-13 III 1213 27 June 1977 1977-056A [51]
10111 		OPS 4800 Titan IIID 155.0 km × 239.0 km, i=97.0° 23 December 1977 [51]
KH9-14 III 1214 16 March 1978 1978-029A [52]
10733 		OPS 0460 Titan IIID 172.0 km × 218.0 km, i=96.4° 11 September 1978 [52]
KH9-15 III 1215 16 March 1979 1979-025A [53]
11305 		OPS 3854 Titan IIID 177.0 km × 256.0 km, i=96.3° 22 September 1979 [53]
KH9-16 III 1216 18 June 1980 1980-052A [54]
11850 		OPS 3123 Titan IIID 169.0 km × 265.0 km, i=96.5° 6 March 1981 [54]
KH9-17 III 1217 11 May 1982 1982-041A [55]
13170 		OPS 5642 Titan IIID 177.0 km × 262.0 km, i=96.4° 5 December 1982 [55]
KH9-18 III 1218 20 June 1983 1983-060A [56]
14137 		OPS 0721 Titan 34D 163.0 km × 224.0 km, i=96.4° 21 March 1984 [56]
KH9-19 IV 1219 25 June 1984 1984-065A [57]
15063 		USA 2 Titan 34D 170.0 km × 230.0 km, i=96.5° 18 October 1984 [57]
KH9-20 IV 1220 18 April 1986 1986-F03 Launch failed [2] Titan 34D — —

(NSSDC ID Numbers: See COSPAR)

Cost

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The total cost of the 20 flights KH-9 program from FY1966 to FY1986 was US$3.262 billion in respective year dollars (equivalent to 17.47 billion in 2023, with an average reference year of 1976).[11]

Specifications

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Data source: The Encyclopedia of US Spacecraft [1] and NSSDC

  • Launch vehicle: Titan IIID/34D
  • Total weight: 11,400 kg (25,100 lb), with mapping camera 13,300 kg (29,300 lb)
  • Reentry weight: 5,330 kg (11,750 lb) [58]
  • Max. diameter (main body): 3.05 m (10.0 ft) [58]
  • Length (with mapping camera): 16.21 m (53.2 ft)[58]
  • Orbit: elliptical, 160 km × 240 km (99 mi × 149 mi)
  • Scanners: television, radio, and high resolution camera

Declassification

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The HEXAGON images have been declassified in 2011 as a continuation of Executive Order 12951. [59][60] The declassified imagery has since been used by a team of scientists from Dartmouth College to detect Roman forts in Syria, [61] and the Catalan Institute of Classical Archaeology (ICAC) to detect qanat irrigation systems. [62]

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  •  
    The reaction control system of a KH-9, showing the propellant tanks.
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    A KH-9 in scaffolding, being prepared for launch.
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    A technician despools one of the re-entry modules which contained the film used by the cameras.
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    A diagram showing the assembly, testing and launch preparations of a KH-9.
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    The forward section of a KH-9.
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    A KH-9 re-entry module hanging from its parachute, ready to be retrieved.
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    Another view of the KH-9's reaction control system.
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    A KH-9 being transported by truck in a special canister.
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    The shroud and base of a KH-9 undergo vibration tests in an acoustic chamber.
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    This painting of a KH-9 shows the two different camera types that the satellite carried.
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    Basic specifications and diagram of the KH-9.
  •  
    This diagram shows the various sections of the KH-9 and their dimensions.
  •  
    A KH-9 at the National Museum of the United States Air Force (front view).
  •  
    A KH-9 at the National Museum of the United States Air Force (forward section).
  •  
    A KH-9 at the National Museum of the United States Air Force (film recovery unit).
  •  
    A KH-9 at the National Museum of the United States Air Force (center section).
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    A KH-9 at the National Museum of the United States Air Force (left camera).
  •  
    A KH-9 at the National Museum of the United States Air Force (rear view without solar panels).

See also

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Other U.S. imaging spy satellites:

References

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  1. ^ a b c Yenne, Bill (1985). The Encyclopedia of US Spacecraft. Exeter Books (A Bison Book), New York. ISBN 0-671-07580-2. p.32 Big Bird
  2. ^ a b "34D-9: Titan rocket with last KH-9 explodes after liftoff (18 April 1986) (F)". 16 December 2012. Archived from the original on 21 December 2021 – via YouTube.
  3. ^ a b Gerald K. Haines (1997). "Critical to US Security: the development of the GAMBIT and HEXAGON satellite reconnaissance system". National Reconnaissance Office (NRO). Archived from the original on 15 September 2012. Retrieved 24 September 2011.   This article incorporates text from this source, which is in the public domain.
  4. ^ Deutsch, Ricky (12 November 2020). "Controlling Hexagon". Retrieved 19 April 2022.
  5. ^ "Assessment of Intelligence Gain Provided by KH-9 over KH-4 and KH-8" (PDF). NRO. Retrieved 7 January 2023.  This article incorporates text from this source, which is in the public domain.
  6. ^ "NRO Observes 50th Anniversary with Declassification".
  7. ^ "Declassified US Spy Satellites Reveal Rare Look at Secret Cold War Space Program". Space.com. 18 September 2011.
  8. ^ Doyle, John M., Big Bird, uncaged, Air and Space, December 2011/January 2012, p.10
  9. ^ Cohen, Aubrey, "Three former spy satellites go on display" Seattle Post-Intelligencer 26 January 2012
  10. ^ a b c d Day, Dwayne Allen (30 January 2017). "Black ZEUS: The top secret shuttle mission that never flew". The Space Review. Retrieved 29 October 2024.
  11. ^ a b c "The HEXAGON story" (PDF). National Reconnaissance Office (NRO). 1988. Archived from the original (PDF) on 16 September 2012. Retrieved 6 October 2011.   This article incorporates text from this source, which is in the public domain.
  12. ^ "Decades Later, a Cold War Secret Is Revealed". Fox News. Associated Press. 25 December 2011.
  13. ^ Walthrop, David (28 May 2013). "An Underwater Ice Station Zebra: Recovering a Secret Spy Satellite Capsule from 16,400 feet Below the Pacific Ocean" (PDF). CIA (Historical Collection Publications). Archived from the original (PDF) on 28 October 2013. Retrieved 29 June 2013.   This article incorporates text from this source, which is in the public domain.
  14. ^ Day, Dwayne Allen (1 July 2019). "Top Secret DAMON: the classified reconnaissance payload planned for the fourth space shuttle mission". The Space Review. Retrieved 29 October 2024.
  15. ^ a b c Burnett, M.G. (1 December 1982). "Hexagon (KH-9) Mapping Camera Program and Evolution" (PDF). NRO. Retrieved 12 May 2021.
  16. ^ Hall, R. Cargill (1988). "The Air Force and the National Security Space Program 1946 - 1988" (PDF). USAF Historical Research Center.   This article incorporates text from this source, which is in the public domain.
  17. ^ Doyle, John M., Big Bird, uncaged, Air & Space, December 2011/January 2012, p.10
  18. ^ Richard J. Chester (1985). "A history of the HEXAGON program". National Reconnaissance Office (NRO). Archived from the original on 15 September 2012. Retrieved 24 September 2011.   This article incorporates text from this source, which is in the public domain.
  19. ^ "NRO's Review & Redaction Guide (RRG), Version 2, 2012" (PDF). NRO. Retrieved 7 January 2023.   This article incorporates text from this source, which is in the public domain.
  20. ^ NARA ARC database description of "Keyhole-9 (KH-9) Satellite Imagery", accession number NN3-263-02-011
  21. ^ "National Archives Releases Recently Declassified Satellite Imagery". National Archives and Records Administration press release. 9 October 2002.   This article incorporates text from this source, which is in the public domain.
  22. ^ "NIMA Sponsors Historical Imagery Declassification Conference America's Eyes: What We Were Seeing". SpaceRef. 15 October 2002. Archived from the original on 14 May 2023. Retrieved 8 January 2018.
  23. ^ Harvey, Chelsea (20 June 2019). "Cold War Spy Satellites Reveal Substantial Himalayan Glacier Melt". Scientific American. Retrieved 20 June 2019.
  24. ^ Barbara Pope (28 April 2006). "NIMS file by satellite name". NASA.   This article incorporates text from this source, which is in the public domain.
  25. ^ James N. Bass; Krishin H. Bhavnani; Isabel M. Hussey (1 April 1975). "Atmospheric Density Determination from Analysis of Doppler Beacon Satellite Data". Air Force Cambridge Research Labs, Hanscom AFB. Archived from the original on 17 July 2011. Retrieved 16 January 2011.   This article incorporates text from this source, which is in the public domain.
  26. ^ K. S. W. Champion; J. M. Forves (1976). "Atmospheric drag analyses of low-altitude Doppler beacon satellites". Satellite Doppler Positioning. 1. New Mexico State University: 343. Bibcode:1976sdp..conf..343C.
  27. ^ Scott F. Large (9 October 2002). "National Reconnaissance Office Review and Redaction Guide: Version 1.0 2008 Edition". NRO. Archived from the original on 6 May 2009.   This article incorporates text from this source, which is in the public domain.
  28. ^ "1972-052C". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  29. ^ "1973-088B". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  30. ^ "1974-020B". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  31. ^ "1974-020C". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  32. ^ "1974-085B". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  33. ^ "1976-065B". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  34. ^ "1976-065C". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  35. ^ "1984-065C". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  36. ^ Day, Dwayne (27 April 2009). "Robotic ravens: American ferret satellite operations during the Cold War". thespacereview.com.
  37. ^ Krebs, Gunter Dirk. "IRCB (S73-7)". Gunter's Space Page. Retrieved 5 May 2024.
  38. ^ Passant Rabie / Gizmodo (1 May 2024). "A satellite that was lost in space for 25 years has finally been found". Qz.com. Retrieved 5 May 2024.
  39. ^ a b "KH 9-01 1971-056A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  40. ^ a b "KH 9-02 1972-002A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  41. ^ a b "KH 9-03 1972-052A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  42. ^ a b "KH 9-04 1972-079A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  43. ^ a b "KH 9-05 1973-014 A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  44. ^ a b "KH 9-06 1973-046A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  45. ^ a b "KH 9-07 1973-088A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  46. ^ a b "KH 9-08 1974-020A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  47. ^ a b "KH 9-09 1974-085A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  48. ^ a b "KH 9-10 1975-051A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  49. ^ a b "KH 9-11 1975-114A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  50. ^ a b "KH 9-12 1976-065A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  51. ^ a b "KH 9-13 1977-056A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  52. ^ a b "KH 9-14 1978-029A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  53. ^ a b "KH 9-15 1979-025A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  54. ^ a b "KH 9-16 1980-052A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  55. ^ a b "KH 9-17 1982-041A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  56. ^ a b "KH 9-18 1983-060A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  57. ^ a b "1984-065A 1984-065A". NASA. 8 October 2010.   This article incorporates text from this source, which is in the public domain.
  58. ^ a b c Stern, Richard G. (5 August 2008). "Reentry Breakup and Survivability Characteristics of the Vehicle Atmospheric Survivability Project (VASP) Vehicles". dtic.mil. Archived from the original on 17 July 2011. Retrieved 13 February 2011.   This article incorporates text from this source, which is in the public domain.
  59. ^ Executive Order 12951
  60. ^ USGS (2017), USGS EROS Archive - Declassified Data - Declassified Satellite Imagery - 3 [Dataset], U.S. Geological Survey
  61. ^ Casana, J., Goodman, D. D., Ferwerda, C. (December 2023). "A wall or a road? A remote sensing-based investigation of fortifications on Rome's eastern frontier". Antiquity. 97 (396): 1516–1533. doi:10.15184/aqy.2023.153.
  62. ^ Buławka, N., Orengo, H. A., Berganzo-Besga, I. (2024). "Deep learning-based detection of qanat underground water distribution systems using HEXAGON spy satellite imagery". Journal of Archaeological Science. 171: 106053. doi:10.1016/j.jas.2024.106053. S2CID 272454830.
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