Ionosphere Effects at the Nanosecond Level Observed in Common-View Time Transfer

Abstract

The single-channel “common-view” GPS time-transfer technique is frequently used to minimize the inaccuracy caused by satellite clock errors, including Selective Availability (SA). Even when carefully conducted, these measurements are subject to receiver noise, satellite ephemeris errors, multipath effects, and variable propagation delays. Many of these errors can be reduced if continuous measurements are made on all of the satellites that are in common view. This paper reports results of using this method with multichannel, modular, L1 C/A-code time receivers over a 4,000 km east-west baseline. These results show measurement noise at a level of about 1 ns for averaging times of between 100 s and 2 days without postprocessing. In addition, significant diurnal effects at the level of up to 15 ns peak-to-peak are often observed. These effects appear to be due to differences between the actual ionospheric delays and the predictions of the GPS built-in model. Techniques for mitigating these errors are discussed.