VehiCal – GNSS Antenna Calibration for Cars

High accuracy positioning of road vehicles is a prerequisite for many emerging technologies, including vehicle to everything communications (V2X), advanced driver-assistance systems (ADAS), and autonomous driving. The required positioning accuracies for these applications are typically specified in a range from 0.1 to 1 meter. To reliably achieve these accuracy levels, often a fusion of augmented GNSS and other sensors is performed. While a lot of research attention is directed to the fusion algorithms and the GNSS augmentation technologies, another critical aspect that affects the positioning accuracy is often overlooked: The vehicle’s GNSS antenna. In traditional high accuracy GNSS applications, the antenna carrier phase center variations (PCV) and code group delay variations (GDV) that describe the error of the GNSS measurement depending on the direction of the incoming satellite signal are calibrated for the different antenna models and corrected in the positioning engine. The antennas are calibrated either in an anechoic chamber or in a robot field calibration setup. To calibrate vehicle antennas, it is not sufficient to calibrate the antenna element only, as the car itself has a significant impact on the PCV and GDV. The near-field of a GNSS antenna that actively influences its radiation characteristics extends up to 2 wavelengths around the antenna element, which is 38 to 51 cm for GNSS signals. Additionally, parts of the car that are outside of the near-field region can influence the PCV and GDV via multipath and diffraction effects due to reflections or insertion of surface waves on the chassis of the car. Therefore, for the most accurate results, the whole car needs to be calibrated. The traditional calibration facilities cannot easily be used for this purpose as the chambers and robots are not designed for the size and weight of cars. Here, we present results from VehiCal, a novel GNSS antenna calibration facility that was designed specifically for vehicle calibration. VehiCal uses a rotating platform with a diameter of 4.5m that enables quick and controlled azimuth changes to receive measurements from all upward directions relative to the car. The position of the platform and therefore also the position of the rigidly mounted car is always known to within 1 mm relative to a nearby GNSS reference station. Multipath effects of the platform itself are suppressed by an RF absorbing coating and are further reduced by positioning the car in different heights and orientations. First results indicate that the generated PCV and GDV calibration patterns are reproducible and significant. For example, a “shark fin” type antenna on the roof of a mid-size car shows repeatable GDV of more than 1 m and a strong azimuthal variation for a large range of incident angles of the GNSS signals. The vehicle positioning impact of antenna-induced measurement errors for GNSS observations is highlighted in this paper for a set of scenarios, representing light, medium, and heavily obstructed sky views. We show that the PCV/GDV induced positioning error can reach meter-level and that measures need to be taken to correct it, especially when GNSS positioning is used within safety-critical applications. Vehicle GNSS antenna calibration is a novel and efficient method to achieve this, especially when using high-quality antennas is not an option due to appearance or cost reasons that often dominate vehicle design. With VehiCal, we demonstrate that a one-time calibration of a certain car type is sufficient to significantly improve the positioning accuracy of the whole fleet of this model. This technology, therefore, supplies one more and maybe the last missing component on the path to widespread adoption of high accuracy GNSS positioning in road vehicles.