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LACROSSE / ONYX

Despite its many advances, the KH-12 suffers the shortcoming common to all photographic intelligence satellites, the inability to see through clouds. With much of the (former) Soviet Union and other areas of interest frequently covered with clouds, this has always posed a problem for intelligence collection. However, in the past, this problem was primarily one of directing the satellite's coverage toward cloud-free areas, and awaiting improved visibility in cloudy regions. While this procedure may have been adequate for peace-time operations, it is clearly inadequate for war-time target acquisition.

A typical radar (RAdio Detection and Ranging) measures the strength and round-trip time of the microwave signals that are emitted by a radar antenna and reflected off a distant surface or object. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (in the range 1 cm to 1 m, which corresponds to a frequency range of about 300 MHz to 30 GHz) and polarizations (waves polarized in a single vertical or horizontal plane).

For an imaging radar system, about 1500 high-power pulses per second are transmitted toward the target or imaging area, with each pulse having a pulse duration (pulse width) of typically 10-50 microseconds (us). The pulse normally covers a small band of frequencies, centered on the frequency selected for the radar. At the Earth's surface, the energy in the radar pulse is scattered in all directions, with some reflected back toward the antenna. This backscatter returns to the radar as a weaker radar echo and is received by the antenna in a specific polarization (horizontal or vertical, not necessarily the same as the transmitted pulse). Given that the radar pulse travels at the speed of light, it is relatively straightforward to use the measured time for the roundtrip of a particular pulse to calculate the distance or range to the reflecting object.

The chosen pulse bandwidth determines the resolution in the range (cross-track) direction. Higher bandwidth means finer resolution in this dimension. The length of the radar antenna determines the resolution in the azimuth (along-track) direction of the image: the longer the antenna, the finer the resolution in this dimension.

Synthetic Aperture Radar (SAR) refers to a technique used to synthesize a very long antenna by combining signals (echoes) received by the radar as it moves along its flight track. Aperture means the opening used to collect the reflected energy that is used to form an image. In the case of a camera, this would be the shutter opening; for radar it is the antenna. A synthetic aperture is constructed by moving a real aperture or antenna through a series of positions along the flight track.

As the radar moves, a pulse is transmitted at each position; the return echoes pass through the receiver and are recorded in an 'echo store.' Because the radar is moving relative to the ground, the returned echoes are Doppler-shifted (negatively as the radar approaches a target; positively as it moves away). Comparing the Doppler-shifted frequencies to a reference frequency allows many returned signals to be "focused" on a single point, effectively increasing the length of the antenna that is imaging that particular point. This focusing operation, commonly known as SAR processing, is now done digitally on fast computer systems. The trick in SAR processing is to correctly match the variation in Doppler frequency for each point in the image: this requires very precise knowledge of the relative motion between the platform and the imaged objects (which is the cause of the Doppler variation in the first place).

Synthetic aperture radar is now a mature technique used to generate radar images in which fine detail can be resolved. SARs provide unique capabilities as an imaging tool. Because they provide their own illumination (the radar pulses), they can image at any time of day or night, regardless of sun illumination. And because the radar wavelengths are much longer than those of visible or infrared light, SARs can also "see" through cloudy and dusty conditions that visible and infrared instruments cannot.

Radar images are composed of many dots, or picture elements. Each pixel (picture element) in the radar image represents the radar backscatter for that area on the ground: darker areas in the image represent low backscatter, brighter areas represent high backscatter. Bright features mean that a large fraction of the radar energy was reflected back to the radar, while dark features imply that very little energy was reflected.

Backscatter for a target area at a particular wavelength will vary for a variety of conditions: size of the scatterers in the target area, moisture content of the target area, polarization of the pulses, and observation angles. Backscatter will also differ when different wavelengths are used. Backscatter is also sensitive to the target's electrical properties, including water content. Wetter objects will appear bright, and drier targets will appear dark. The exception to this is a smooth body of water, which will act as a flat surface and reflect incoming pulses away from a target; these bodies will appear dark.

Backscatter will also vary depending on the use of different polarization. Some SARs can transmit pulses in either horizontal (H) or vertical (V) polarization and receive in either H or V, with the resultant combinations of HH (Horizontal transmit, Horizontal receive), VV, HV, or VH. Additionally, some SARs can measure the phase of the incoming pulse (one wavelength = 2pi in phase) and therefore measure the phase difference (in degrees) in the return of the HH and VV signals.

Track angle will affect backscatter from very linear features: urban areas, fences, rows of crops, ocean waves, fault lines. The angle of the radar wave at the Earth's surface (called the incidence angle) will also cause a variation in the backscatter: low incidence angles (perpendicular to the surface) will result in high backscatter; backscatter will decrease with increasing incidence angles.

A space-based imaging radar can see through clouds, and utilization of synthetic aperture radar (SAR) techniques can potentially provide images with a resolution that approaches that of photographic reconnaissance satellites. A project to develop such a satellite was initiated in late 1976 by then-Director of Central Intelligence George Bush. This effort led to the successful test of the Indigo prototype imaging radar satellite in January 1982. Although the decision to proceed with an operational system was very controversial, development of the Lacrosse system was approved in 1983.

The distinguishing features of the design of the Lacrosse satellite include a very large radar antenna, and solar panels to provide electrical power for the radar transmitter. Reportedly, the solar arrays have a wingspan of almost 50 meters, which suggests that the power available to the radar could be in the range of 10 to 20 kilowatts, as much as ten times greater than that of any previously flown space-based radar.

It is difficult to assess the resolution that could be achieved by this radar in the absence of more detailed design information, but in principle the resolution might be expected to be better than one meter. While this is far short of the 10 centimeter resolution achievable with photographic means, it would certainly be adequate for the identification and tracking of major military units such as tanks or missile transporter vehicles. However, this high resolution would come at the expense of broad coverage, and would be achievable over an area of only a few tens of kilometers square. Thus the Lacrosse probably utilizes a variety of radar scanning modes, some providing high resolution images of small areas, and other modes offering lower resolution images of areas several hundred kilometers square. The processing of this data would require extensive computational power, requiring the transmission to ground stations of potentially several hundred mega-bits of data per second.

Lacrosse 1 (1988-106B 19671) was launched on 2 December 1988 by the Space Shuttle. The spacecraft entered an orbit with an inclination of 57 degrees, with an perigee of 680 kilometers and an apogee of 690 kilometers, and had not maneuvered significantly since launch.

Lacrosse 2 (199- A) was launched from Vandenberg AFB CA on a Titan-4 on 08 March 1991

Lacrosse 3 (199- A) was launched from Vandenberg AFB CA on a Titan-4 in the Fall of 1997, replacing Lacrosse 1.

Resources

What is Imaging Radar ?
by Tony Freeman, Jet Propulsion Laboratory An imaging radar works very like a flash camera in that it provides its own light to illuminate an area on the ground and take a snapshot picture, but at radio wavelengths. The length of the radar antenna determines the resolution in the azimuth (along-track) direction of the image: the longer the antenna, the finer the resolution in this dimension. Synthetic Aperture Radar (SAR) refers to a technique used to synthesize a very long antenna by combining signals (echoes) received by the radar as it moves along its flight track. Aperture means the opening used to collect the reflected energy that is used to form an image. In the case of a camera, this would be the shutter opening; for radar it is the antenna. A synthetic aperture is constructed by moving a real aperture or antenna through a series of positions along the flight track.

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Maintained by Steven Aftergood
Created by John Pike
Updated Thursday, August 17, 2000 7:38:09 AM