US Spy Satellites at Diffraction Limit for Resolution Since 1971

The image tweeted by Trump was at about 10-centimeter resolution. It was likely a picture taken by the US224 satellite. This was one of the upgraded keyhole satellites that have the same mirror as the Hubble Telescope (2.4 meters). It was widely known for many years that the Keyhole spy satellites were using the same mirror as the Hubble Space Telescope.

The Wikipedia entry on the KH-11 satellites has a resolution of 6-centimeters listed. The sources listed were public documents from 1966 for the theory on the resolution and a 1989 book by astronomer Clifford Stoll.

A perfect 2.4-meter mirror observing in the visual (i.e. at a wavelength of 500 nm) has a diffraction-limited resolution of around 0.05 arcsec, which from an orbital altitude of 250 km corresponds to a ground sample distance of 0.06 m (6 cm, 2.4 inches). Operational resolution should be worse due to effects of the atmospheric turbulence. Astronomer Clifford Stoll estimates that such a telescope could resolve up to “a couple inches. Not quite good enough to recognize a face”

Ted Molczan at a satobs.org discussion list had speculated on the resolution of current US spy satellites.

There were closes passes of satellite. They could have taken images with 5-7 centimeter resolution.

There was a 3U cubesat images of the Iran rocket pad explosion taken by Planet Labs. This had 3 meter resolution.

Maxar provided better resolution in the 0.5 to 1.5 meter resolution range. DigitalGlobe is an American commercial vendor of space imagery and geospatial content, and operator of civilian remote sensing spacecraft. On 5 October 2017, Maxar Technologies completed its acquisition of DigitalGlobe.

WorldView-2, launched October 2009, is the first high-resolution 8-band multispectral commercial satellite. Operating at an altitude of 770 kilometers, WorldView-2 provides 46 cm panchromatic resolution and 1.85 meter multispectral resolution.

The U.S. government regulates the resolution of imagery sold by commercial imaging satellite owners. DigitalGlobe claims panchromatic 31 cm resolution from 617 km altitude, for its Worldview 3 and 4 satellites, launched in 2014 and 2016, respectively. Commercial satellites have now achieved approximately the resolution of the Hexagon satellite, that KH-11 replaced by raising its apogee to 1000 km.

NROL-71 launched recently. It is believed to be a new generation of spy satellite.

The Gambit-3 (aka KH-8, launched in 1971) provided the NRO with images at the limit of resolution imposed by atmospheric turbulence. Allen Thomson indicated that scientific papers published in the mid-1960s by David Fried and, independently, by John C. Evvard, revealed that atmospheric turbulence limits the imaging resolution achievable from Earth orbit to about 5 cm to 10 cm. Larger mirror would not have improved the resolution of Gambit-3, at its minimum altitude of 130 km. A larger mirror would only have been required if this distance were to be increased.

The 2.4-meter, and the resolution limit imposed by atmospheric turbulence, it is reasonable to assume that the lowest altitude would be about the same as the present, 250 km.

A big 9.6-meter mirror could get maximum resolution from an orbit about four-time higher (1000 km).

SOURCES- Cees Bassa, Planet Labs, Maxar, Dae Schmerler, Donald Trump, Scott Manley Youtube
Written by Brian Wang, Nextbigfuture.com

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