Citation
Abstract
Foremost among issues affecting the potential use of astrometric techniques to locate and track laser-carrying spacecraft is the apparent brightness (detected energy or photon flux) of a laser relative to reference stars. Broadband detectors offer improved sensitivity to stars (broadband “‘blackbody”’ sources), but not to lasers. The important and fundamental step of calibrating laser and star brightnesses according to detector spectral sensitivity is carried out here for four representative kinds of broadband detectors, located above and below the atmosphere. Stars are identified according to parameters traditionally used by astronomers: temperature (spectral class, or color) and apparent brightness at visible wavelengths. These are translated into an energy and photon flux for each kind of broadband detector and are then compared with the corresponding flux from a laser. The comparisons are also given as “magnitude correction factors,” ie., in astronomers’ traditional units of 4.0 dB. Astrometrists typically characterize the sensitivity of their instruments in terms of the precision with which they can make a relative measurement of position and the minimum brightness needed to achieve that precision. (In an ideal instrument, limited only by 1//N photon statistical fluctuations, these two descriptions would coincide.) Given this information and the instrument’s spectral sensitivity, one can use the calculations described in this report to infer the detectability of a laser and the precision with which it could be located and tracked. Results show, for example, that while a star and a laser may have comparable brightnesses at visible wavelengths, to a broadband detector the star might appear brighter than the laser by anywhere from 2 to 17 dB, depending on the star’s spectral class and the detector spectral sensitivity. Since the limiting brightness quoted for a particular instrument is typically firm to within 4.0 dB (one magnitude), judging a laser’s detectability by its visible brightness alone could lead to serious underestimation of the requirements on its effective radiated power. The laser brightness corrections given here solve this problem.
Details
- Volume
- 42-93
- Published
- May 15, 1988
- Pages
- 111–130
- File Size
- 784.0 KB