Setup and Validation of a 6-element Wide-band GNSS Antenna Array using a WFS OTA Testbed
J. Rossouw van der Merwe, Alexander Rügamer, Alexander Popugaev, Christopher Schirmer, and Wolfgang Felber, Fraunhofer IIS, Germany
In this paper we present a wideband six element antenna array global navigation satellite system (GNSS) receiver. The design of the antennas, the array and multi-channel receiver was done in-house. The antennas have a wideband operational range to cover all applicable L-Band GNSS signals. However for this paper the focus will only be to analyze the performance for GPS L1 C/A. Further the elements have with high polarization purity to suppress any odd-reflected multipath components. The antenna array is designed to be compact yet to have low coupling between the elements, such that coupling cannot significantly limit the beamforming.
The array includes a port for a calibration signal, which can be used to calibrate the magnitude and phase offsets of the radio frequency (RF) components, receiver front end, and the digital down converters (DDCs). A clock and time synchronized six-channel receiver with a sample rate of 10.125 MHz, analog bandwidth of 8 MHz and an ADC word-length of 8 bit per channel is used. This allows sufficient dynamic range for digital beamforming without the creation of unwanted artifacts or saturation of the receiver.
Beamform-on-receive is done digitally for each satellite before the tracking stage of the receiver. The tracking and PVT results with and without beamforming will be presented in the paper. Two beamforming methods will be evaluated in this paper. The first method estimates the phase and offset of the receiver using the calibration signal, and then calculates the beamsteering coefficients based upon the array orientation and AOA of the satellite of interest. The second method uses the correlation of pseudo-random noise (PRN) code of each satellite with the received signals to estimate the beamsteer coefficients, thereby circumventing the need for a calibration signal. The second method may estimate the coefficients based upon multipath components; however the polarization purity of the antennas limits this occurring for single reflected multi-path signals.
The testing and evaluation of the array processing and the performance is evaluated using a 3D Over-The-Air (OTA) Wave Field Synthesis (WFS) testing facility. This facility allows the controlled generation of multiple signals from different AOAs in an anechoic indoor test range. This allows thorough and reliable testing of equipment in 3D space, thereby reducing development time and cost. An outdoor test of the GNSS receiver will also be done to verify the performance from the OTA WFS.
The proposed usage of the system is to improve the navigation capability in obstructed environments, such as forests . In such environments it is often impossible to acquire and track a sufficient number of satellites for a PVT solution. With the added beamforming to improve signal strength and quality, and to suppress multipath components; it is possible to have a sufficient number of satellites for a reliable and successful PVT.