Timing System for the Laser Altimeter for Planetary Exploration Technology Demonstrator

Abstract

We are presenting the design, construction and tests of the timing system for the Laser Altimeter technology demonstrator. Altimeter Timing System (ATS) is a universal timing system for laser ranging in ground-ground, air-ground and ground-satellite experiments. It is dedicated to measure precise time interval with sub-nanosecond resolution. The device for advanced range gating is included. The unit is interfaced to a host personal computer via a serial data link for control, two-way data transfer and diagnostics. The entire ATS has been designed and constructed on the basis of the Portable Calibration Standard (PCS) for satellite laser ranging, which has been developed in our labs within the last ten years. To reduce the complexity, costs, weight and power, considering the modest timing resolution requirements, the sub-nanoseconds instead of picoseconds resolution of the time intervals, the timing part of the original device has been replaced by the Mini Counter. The overall design philosophy, the operational control software, the epoch timing, the range gate generation have been preserved along with the concept of the host computer software package for data acquisition, control and data analysis including the communication protocol, data and command formats etc. The use of well-tested concept of both the HW and SW enabled to shorten the design, construction and testing phase of the final device down to several weeks. The ATS consists of the Mini Counter module, the epoch timing and range gate generator module, the control processing unit, the input/output circuits and of the power supplies. The entire control logic hardware including the epoch timing and range gate generator and the input/output board logic is based on the FPGA (ispGAL) programmable logical arrays. There is a significant array capacity still available for future functional extensions and device upgrades, the arrays are field programmable. This fact ensures the maximum device flexibility and upgradability. The main parameters are: resolution 0.25 ns, linearity and stability better than 0.1 ns and 0.1 ns per K and per hour resp. The laser fire epoch resolution is 100 ns, the range gate is programmable in 40 ns steps. The device is small (2 kg), low power, it is capable to operate 3 hours on eight pieces of AA batteries.