A Nd:YAG Laser Altimeter at 1064 nm for use with low-earth orbiting spacecraft has been designed. The Altimeter has a 17 mJ pulse energy with 7 ns pulse width and is nadir pointing with abeam divergence of 2 mrad. The radiation backscattered from the ocean surface, soil/vegetation and ice is received by a 305 mm diameter f/3.0 Cassegrain telescope with an avalanche photodiode receiver. The elapsed time between the transmitted pulse and the received pulse is converted into the altitude of the spacecraft. Considerations of atmospheric attenuation due to cirrus clouds and aerosols/gases are presented for best- and worst-case conditions. The backscatter coefficients for various cases are also considered and discussed. The signal-to-noise ratios are calculated from all these conditions. The system can give height resolution of 5 metres at 400 km altitude. This data can be used in precision image processing of remotely-sensed data and improvement of the orbital and attitude parameters of the satellite.