Laser altimeter for planetary exploration technology demonstrator: the timing system

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The European Space Agency has nominated the laser altimeter as one of the principal devices for planetary research for the next decade. The first mission in view is Mercury with scheduled launch in 2010. The device should be capable to range over the distances 400 to 1000 km and to acquire the information about the probe altitude above the planet surface and about the surface terrain profile with the precision of the order of one meter. The requirements on the device are rather strict: total mass below 5 kilograms, power consumption below 10 Watts. Recently, the Technology Demonstrator of the altimeter is under development at German Air and Space Agency, Institute of Planetary Research, Germany. The altimeter Technology Demonstrator is based on the diode pumped frequency doubled Nd:YAG laser delivering 50 mJ at 532 nm in 3 nanosecond long pulses with the repetition rate of 20 kHz. The solid state echo signal detector in photon counting mode will be used. The optical part of the altimeter is scaled down to simulate the real background count rate scenario and to reduce the energy budget link by a factor of 104 at the same time. The demonstrator should be capable to range objects at distances 0 - 5 kilometers in both night and day time. We are presenting the concept, design and construction of the timing system part of the laser altimeter technology demonstrator, which has been developed at the Czech Technical University in Prague optimized for photon counting altimeter concept. The timing system has interval resolution 0.25 ns, stability and linearity ~0.1 ns, epoch resolution 100 ns and accuracy 1 μs, and programmable range gate.