GNSS Synthetic Aperture Processing with Artificial Antenna Motion

IFEN is working since a few years on developing a new GNSS signal processing scheme to significantly improve the GNSS tracking reliability and accuracy. It is based on the principles known from synthetic aperture radar. Like in a multi-antenna phased array receiver, GNSS signals from different spatial locations are combined coherently allowing forming an optimized synthetic antenna gain pattern. Thereby, multipath signals can be rejected and the line-of-sight signal received signal power is maximized. This is beneficial in forests and other degraded environments. Two synthetic antennas are currently manufactured by a subcontractor: one rotating antenna, and one antenna moving up and down. The rotating antenna targets forest or indoor surveying work, the up/down antenna suppresses extremely well ground multipath. The antennas are operated together with IFEN’s multi-frequency multi-GNSS software receiver SX-NSR, who retrieves the position of the antenna and forms the synthetic aperture. Results from real-world trials will be documented in this paper. The idea of synthetic aperture processing is realized as a coherent summation of correlation values for each satellite over the so-called beamforming interval. Each correlation value is multiplied with a phase factor. For example the phase factor can be chosen to compensate the relative antenna motion over the beam-forming interval and the resulting sum of the multiplied correlation values represents a coherent correlation value maximizing the line-of-sight signal power. Another option is to blank (null) a signal from a certain direction; e.g. ground-multipath signals at low elevation angles can be suppressed. The simultaneous estimation of receiver clock parameters is part of this process. After forming the correlator values over the beamforming interval, code pseudoranges and carrier phases are extracted and used in a conventional way. That is, they are written into RINEX files and standard geodetic software can be used to evaluate them. The GNSS signal processing removes the known part of the movement from the observations and the observations are like from a static antenna; static positioning algorithms including carrier phase ambiguity fixing can be applied. Two main difficulties arise in the synthetic aperture processing. First, the clock jitter during the beam-forming interval must be precisely known. It can either be estimated based on data from all signals or a stable oscillator is used. In one of our setups, a modern OCXO with an Allan variance of 0.5e-13@1 s is used. Second, the precise relative motion of the antenna during the beamforming interval must be known. In our application, the antenna is moved along a known trajectory. IFEN develops in cooperation with an mechanical engineering company Blickwinkel a vertical and a rotational antenna displacement unit. The vertical unit moves an antenna element vertically up and down by up to 1.2 m with a frequency of 0.5 Hz and aims at ground multipath mitigation. The rotational unit moves the antenna element horizontally along a circular trajectory with an radius of up to 0.5 m and a rotation rate of 30 rpm and targets forestry and indoor surveying applications. An important point is the mechanical realization of the antenna movement. This has to be done in a cost efficient and reliable way. Lubrication free actuators are used together with magnetic displacement sensors. The sensors are synchronized to the NavPort-4 frontend with < 1 ms accuracy. The movement of the vertical antenna is triggered by full GPS seconds. The rotational antenna uses slip rings to connect the antenna elements to the receiver. The rotational antenna can also be used to map the received signal power as a function of elevation and azimuth angles. This is beneficial for researchers, if e.g. one is interested, which object cause multipath in an indoor environment. For this purpose the rotational antenna can be equipped with an LHCP and RHCP antenna on both ends of the rotating bar. The rotational antenna is mounted on a geodetic tripod, the vertical antenna has an external mark suitable for connecting it to other points via terrestrial surveying. Both antennas are tested in a multipath test-bed by the Austrian Academy of Sciences / Institute of Space Research, where a large copper plate with variable inclination is used to generate strong multipath. In addition some ground multipath is present. The RINEX data is analyzed by code-minus-carrier plots to identify code multipath, by C/N0 plots to identify multipath fading and by WaSoft/Multipath to identify carrier multipath. The results of the synthetic aperture processing are compared against conventional processing schemes and against commercial receivers/antennas from Ashtech.