Using Pulse Position Modulation in SLR stations to transmit data to satellites

Satellite Laser Ranging (SLR) Stations determine distances to satellites by measuring the time-of-flight of short (10 ps to 100 ps) laser pulses, reflected from corner cube reflectors (CCR) on satellites. The laser repetition rates vary from 10 Hz to 2 kHz. At the Graz 2 kHz SLR station, we upgraded the software to modulate the - usually constant - interval between laser pulses; using such a Pulse Position Modulation (PPM) scheme, we successfully transmitted text files via a 4288 m distant CCR back to a Multi-Pixel Photon Counting (MPPC) module in our receiver telescope. With such a setup at any SLR station, and a suitable detector plus simple time tagging electronics at Low Earth Orbiting (LEO; <; 1000 km) satellites, it is possible for any kHz SLR station to transmit data to satellites with a rate of up to 2 kBytes/s - even during standard SLR tracking. As this technique is easy to implement and does not affect routine kHz SLR tracking, it can be applied to upload data to satellites, using the more than 30 available SLR stations around the world, and with higher data rates than some of the conventional microwave uplinks.