From Past to Present: Investigating Retrofitted GPS Antennas for Multi-GNSS Functionality

Abstract

This paper investigates the re-use of highly precise equipment through the retrofitting of historical GPS choke ring antennas to accommodate Multi-GNSS systems. Specifically, it focuses on the former "reference" antenna of the International GNSS Service (IGS), the AOAD/M_T NONE antenna in a JPL 2d choke ring design, supported by the the GeoForschungsZentrum Potsdam (GFZ). The Institut für Erdmessung (IfE), a calibration institution certified by the International GNSS Service (IGS), had two different types of such antennas available: one delivered with the original Low Noise Amplifier (LNA) and retrofitted with the updated LNA, and another already equipped with the updated LNA. The IfE performed calibrations with both the original and retrofitted LNAs, providing legacy GPS L1/L2 calibrations and Multi-GNSS calibrations for broader variety of signals and frequencies. These calibrations are based on the robot calibration approach with a robot arm on a short baseline with common clock to reduce all external error sources. In this study, we present the different performance of the antenna with the retrofitted LNA based on Signal-to-Noise decrease functions, signal and frequency comparisons, variations in carrier phase, and analysis of code phase (group delays). Furthermore, we have performed an analysis on the basis of the pattern domain of PCC (Phase Center Corrections) and GDV (Group Delay Variations), as well as on the parameter domain, including positioning and other parameters such as clock corrections. Comprehensive performance analysis has been carried out to evaluate the effectiveness of such retrofitted antennas to also support Multi-GNSS systems. Additionally, we apply newly developed scalar measures for comparisons and performance analysis, providing a robust framework for assessing the suitability of retrofitted antennas for modern navigation needs. The findings shed light on the practical implications and technical considerations involved in adapting historical GPS choke ring antennas, emphasising responsible equipment re-use and performance optimisation in today's technological landscape.