Recently published test range images provide an interesting illustration of U.S. capabilities to image satellites.
By combining range and Doppler measurements, wide-bandwidth radars can produce photograph-like images of satellites. The range resolution of a radar is limited to about c/2β, where c is the speed of light and β is the radar’s bandwidth, although for radar processing reasons this limit is not always achieved. For example, for β = 1 GHz, such as is used by the X-Band Haystack Long-Range Imaging Radar (LRIR) (and the U.S. X-Band missile defense radars), this formula gives a range resolution of 15 cm, although the Haystack LRIR reportedly only achieves a resolution of 25 cm. A small cross-range resolution can then achieved by observing a target as it rotates relative to the radar. This rotational motion can be either due to the satellite’s own spin or simply due its orbital motion relative to the radar. Typically a rotation of a few degrees is required to get a cross-range resolution equal to the range resolution.
All U.S. radar images of real satellites are officially classified (although two apparently from HAVE STARE have been published on the internet).
However, the recently published 2011 Lincoln Laboratory Annual Report contains two images of a satellite model at a test range, corresponding to the resolutions of the Millimeter Wave (MMW) radar at Kwajalein before and after its upgrade from 2 GHz bandwidth (12 cm resolution) to 4 GHz Bandwidth (6 cm resolution). The previous year’s Annual Report contained similar images for the Haystack LRIR at 1 GHz (25 cm resolution) and after its planned upgrade to 8 GHz (~ 3 cm resolution). I have combined these to give a progression of improving resolution:
Here is a picture of the model used for the Haystack measurements (from the 1010 Annual Report):
It is not clear that the same model is used for both the Haystack , as its relative dimensons appear different in the two sets of images (although this could be a viewing angle effect). Nevertheless it provides an intersting illustration of the rapidly improving U.S. capabilities in this area (although the last two images don’t look very different to me).