Progress Toward One-Nanosecond Two-Way Time Transfer Accuracy Using Ku-Band Geostationary Satellites

Abstract

The National Bureau of Standards (NBS) Boulder Laho- ratory is assembling a system for time transfer with a projected accu- racy of 1 ns to Incations that can access a Ku-band geostationary com- munications satellite in common view with Boulder. The system includes a 6.1-m diameter antenna and satellite earth station at Boul- der, two transportable earth stations, each with a 1.8-m dish, and mo- dems designed for two-way timing. The elements of the system are de- scribed and a method of using them to achieve high-accuracy time transfer is presented. A satellite simulator has been used in measure- ments of the stability of the loop delay for different configurations. The day-to-day variations exhibit peak deviations of less than 1 ns and offer encouragement that similar stability will he found for the ground seg- ments in two-way time transfer configurations. Allan-variance stability plots are presented for sample times of 1 S to several days at various carrier-to-noise (C/N,) density ratios. The results are compared to theoretical limits obtained from the model of phase jitter given for the spread spectrum modem. The 1.8-m earth stations have full duplex capability to permit signal turnaround at a remote location while using a geostationary satellite in common view with the NBS 6.1-m earth station. The use of one portable earth station to calibrate the critical differential delays between two other earth stations is discussed. The calibration offers a method of converting the previously demonstrated nanosecond and suhnanosecond time transfer precision to nanosecond time transfer accuracy between widely separated locations on Earth.