# Experimental Validation of a GNSS Receiver Antenna Absolute Field Calibration System
## 1 Introduction  
Achieving high-accuracy Global Navigation Satellite System (GNSS) positioning, essential for various fundamental applications such as the realization of geodetic reference frames [1], monitoring and geodynamics [2], large-scale engineering projects [3], and national Continuously Operating Reference Station (CORS) networks, as well as for the determination of consistent GNSS products including satellite orbit solutions, clock corrections, atmospheric parameters, and Earth orientation parameters [4, 5], requires precise GNSS antenna absolute phase center correction (PCC) determination as a crucial precondition. In simplified terms, PCCs are a direct consequence of the antenna's designproperties and electromagnetic characteristics [6]. The geometric location of GNSS signal reception within the antenna, or the so-called receiver antenna phase center (PC), varies based on the direction and frequency of the incoming signal, therefore causing delays and advances in carrier-phase observations and corresponding azimuth-, elevation-, and frequency-dependent range errors [7; 8; 9; 10; 11]. This measurement error source is solved through a special procedure known as GNSS receiver antenna calibration, within which PCC grids (models) for the entire antenna upper hemisphere and per frequency are determined. Calibration results, i.e., PCC values, are stored in a separate file, in line with the IGS ANTenna EXchange (ANTEX) file format [12], to be applied in GNSS data processing algorithms. It is also essential to clarify that two distinct types of antenna calibration models exist: _individual_ and _type-mean_[13]. An individual receiver antenna PCC model is derived from one or more calibrations of a specific receiver antenna (defined by its unique serial number). This model applies only to that specific antenna. In contrast, a type-mean PCC model is created by averaging calibration results from multiple antennas of equal type but different manufacturer serial numbers. Such a type-mean model is thus representative of that entire antenna type. The IGS publishes its official ANTEX file [14], which includes type-mean multi-GNSS multi-frequency PCC models for almost all geodetic-grade antenna types. The file is regularly updated, and, at the time of this research, the latest official version is from GPS Week number 2335 (igs20_2335.atx). The topic of antenna calibration has been researched for nearly 30 years since the early beginnings of Global Positioning System (GPS) positioning [15]. Today, the _absolute calibration_ approach represents the state-of-the-art standard in antenna calibration methodologies. Two mutually independent absolute calibration methods have been developed and are in use: _chamber_ and _robot_. The chamber calibration method [16; 17] is a research laboratory technique in which the receiver antenna undergoing calibration is set up within an anechoic chamber alongside an artificial signal source. In contrast, the robot calibration method [7; 8; 11; 17; 18; 19; 20], also referred to as the absolute field method, is an in situ technique in which the receiver antenna undergoing calibration is set up at the open-sky calibration field alongside a reference antenna in a short-baseline setup. During the calibration, raw carrier-phase observations are registered. This method is the de facto standard for GNSS receiver antenna calibrations. This claim is supported by the fact that out of 430 receiver antenna type-mean PCC models contained in the current official IGS ANTEX file (igs20_2335.atx) [14], 331 PCC models are the result of absolute field, i.e., robot, calibrations by different calibration institutions. Given the importance of this field of research for geodesy as well as many other related scientific and professional disciplines, several institutions and working groups worldwide have developed, or are developing, their own GNSS receiver antenna robotic calibration systems.