Proceedings of the 16th International Workshop on Laser Ranging 332 Transmitter Point-Ahead using Dual Risley Prisms: Theory and Experiment John Degnan (1), Jan McGarry (2), Thomas Zagwodzki (2), Thomas Varghese (3) (1) Sigma Space Corporation (2) NASA Goddard Space Flight Center (3) Cybioms John.Degnan@sigmaspace.com /Fax +01-301-577-9466 Abstract For maximum detection efficiency and solar noise rejection, eyesafe photon-counting Satellite Laser Ranging (SLR) systems rely on narrow transmit beams and receiver FOV‘s. Because of the finite velocity of light, the transmit and receive FOV centers are angularly separated by up to 11 arcseconds in SLR and by several tens of arcseconds in interplanetary laser transponders or communications systems. We have successfully implemented and tested a dual Risley prism beam steerer for ―Transmitter Point Ahead‖ (TPA) compensation in NASA‘s Next Generation Satellite Laser Ranging (NGSLR) System Introduction Conventional multiphoton SLR systems typically employ coaligned transmitters and receivers. The combination of high pulse energies, large transmitter beam divergences and even larger receiver fields-of-view ensure that a sufficient number of photons are reflected off the target and into the receiver to exceed the multiphotoelectron detection threshold. The latter is set sufficiently high (3 to 4 pe) to minimize false alarms under high solar background conditions. In contrast, eyesafe photon-counting systems operating at 532 nm in daylight must operate with over 3 orders of magnitude lower pulse energies, tight transmitter beam divergences to concentrate more of the transmitted light onto the satellite, and narrow receiver fields-of-view to reduce the solar background and improve the contrast between signal and solar noise. As a consequence, the transmit and receive FOV‘s may no longer overlap if the point -ahead angle is sufficiently large. Thus, NASA‘s Next Generation Satellite Laser Ranging System (NGSLR, formerly known as SLR2000) is designed to point the receive telescope where the satellite was at the time the photons were reflected and independently point the transmitter ahead to where the satellite will be when the subsequent pulse arrives at the satellite (see Figure 1). Overview of NGSLR Transceiver The overall design and operation of the NGSLR transceiver has been described in prior workshops [Degnan, 2004], but a short overview is needed here to comprehend the nature of the Transmitter Point-Ahead‖ (TPA) issue. Figure 2 provides a schematic of the NGSLR transceiver optical bench. The transmitter is input to a computer-controlled 5-element Special Optics beam expander which controls the final beam divergence while maintaining a constant beam spot size at the telescope exit aperture (for eye safety). This is followed by a Matched Dual Risley Prism Pair, which implements the TPA feature, and a passive T/R switch