Optical time transfer for future disaggregated small satellite navigation systems

Wednesday July 9, 2014 - h 11:00 am
Meeting room, via Sommarive, 9 - 38123 Trento

• John W. Conklin, University of Florida

Abstract

Precision time-keeping is a critical requirement of any satellite navigation system, including GPS. Even the most stable space qualified atomic clocks drift over time to the point where they can significantly degrade navigation precision. Periodic re-synchronization of these clocks with respect to terrestrial time standards is therefore required. Time transfer through Earth’s atmosphere using optical frequencies offers improved accuracy due to reduced time delay uncertainties relative to radio frequencies. Borrowing some technologies from previous optical time transfer demonstrations, we have developed a new compact, low-power Optical Precision Time-transfer Instrument (OPTI) that will simplify the process of correcting for atomic clock drift on spacecraft. The operation of OPTI will be demonstrated on a low Earth orbiting 3U CubeSat in the 2017 time frame as part of the Air Force’s University Nanosatellite Program.

In addition to improving the precision and simplifying the operation of satellite navigation systems, OPTI and its CubeSat demonstration mission CHOMPTT (CubeSat Handling of Multisystem Precision Time Transfer) will aide in the realization of disaggregated satellite navigation systems in the future. A disaggregated navigation system is one in which the precision timing function is separated from the high gain pseudorange transmission component. A small number of timing satellites (1-3) would be placed in low Earth orbit and transmit timing updates to a larger

constellation of broadcast satellites in higher orbits. These timing satellites could also provide precision time to any other space asset that required it, including communications satellites and fundamental physics and astrophysics missions.

The concept of operations for the CHOMPTT mission is as follows. A satellite laser ranging facility on the ground will transmit nanosecond laser pulses to the CHOMPTT CubeSat. These pulses are timed with the atomic clock on the ground and are detected by an avalanche photodetector on OPTI. An event timer records the arrival time with respect to the on-board clock with an accuracy of ~100 ps. At the same time, a retroreflector returns the transmitted beam back to the ground. By comparing the transmitted and received times on the ground and the arrival time of the pulses at the CubeSat, the time difference between the ground and space clocks can be measured. The optical link will then be used to transmit the timing information from the ground to the CubeSat so that the CubeSat’s atomic clock offset can be corrected by OPTI in real time. This talk will present the mission architecture and the design and laboratory testing of the optical precision time-transfer instrument.