High Accuracy Measurement of Geostationary Satellite Location Uncertainty using TWSTFT via NMI’s Time and Frequency Laboratories

Abstract

A dense distribution of space debris is present in geostationary orbit (GEO). To avoid collision with debris, maneuvers to change the satellite's position are necessary. The uncertainty of the satellite's location defines the region that must avoid the passage of debris. The larger this uncertainty region of the satellite's location, the greater the number of maneuvers that must be performed. Therefore, a reduced uncertainty region, from a few kilometers to a few meters, means reducing the number of maneuvers, reducing fuel consumption and increasing the satellite's lifetime, revenue and safety. Two-way Satellite Time and Frequency Transfer (TWSTFT) is one of the most accurate methods to compare and synchronize time scales between remote stations connected by a geostationary satellite whose clocks are references as time and frequency standards, most of them contributing to UTC. The characteristics of TWSTFT, such as high accuracy of time measurement, time difference almost in real time, multichannel modems, among others, allow that this technique be used to measure the uncertainty of the GEO satellite's location with high accuracy. The time and frequency laboratories (TF-Labs) that contribute to the UTC have highly accurate timescales and interest in intercomparison with other laboratories using the TWSTFT method. Such laboratories, located in national metrology institutes (NMI) and designated institutes (DI), are ideal candidates for implementing a system for determining the uncertainty of the location of geostationary satellites with high accuracy which will reciprocally provide a network for intercomparison among them. These TF-Labs are located in different countries, separated by large distances, which is the ideal characteristic to increase the accuracy of the GEO satellite location. In 2022, my article (DOI: 10.1109/LATINCOM56090.2022.10000557) developed a study to reduce the location uncertainty of the Brazilian geostationary satellite through TWSTFT links. The stations used were TF-Labs from some NMI in Latin America. The results indicated that it is possible to accentuate the reduction of PDOP (position dilution of precision) parameter and, consequently, reduce the location uncertainty of the geostationary satellite to a value below 4.3 m with the current carrier phase TWSTFT technology. A future evolution would be to use the ‘Binary Offset Carrier’ technique in TWSTFT systems to increase accuracy and reduce bandwidth used in geostationary satellites. A new need in metrology is to adopt the Optical Frequency Standard (OFS) for the redefinition of the second. This depends on remote comparisons between OFS located at different NMI/DI. An intercomparison network using TWSTFT represents a step forward to meet this need, considering two-way system has an expected evolution to reach the same accuracy of OFS. The proposed network could be addressed by BIPM CCTF-WGTWSTFT study group that would develop specific studies for the use of TWSTFT on selected GEO satellites, as described in my article. A proposal would be presented to satellite operators for the joint development of the TWSTFT system with a dual purpose: (1) to increase the revenue (lifetime) and safety of satellites and (2) to implement an intercomparison network between NMI/DI.