C-SPAD Detector

The Single Photon Avalanche Detector (SPAD) is the main device used to detect the few photons that make the round trip from the laser system to the satellite and back. As its name implies, it is capable of detecting a single photon; however, our extensive tests have shown that if more than a single photon reaches the SPAD 'at the same time' significant bias in the range measurement can result. This result lead to the development, at the Czech Technical University and the Technical University of Graz, Austria, of a detector incorporating an automatic time-walk compensation unit. The compensated detector is known as a C-SPAD, and has been in operation in several SLR stations, including Herstmonceux, for several years. We have continued to experiment with the device and our current philosophy is to continue, despite the very stable characteristics of the C-SPAD, to work strictly at a single-photon level of return. Theoretical and experimental considerations lead us to the conclusion that significant and essentially un-measurable range bias can result when high levels of return are obtained from extended satellite reflector arrays, which include those of all the principle geodetic satellites.

A recent paper detailing our C-SPAD test results can be read here