Exploiting new signals for localization and navigation is a current trend in research e.g. to improve the accuracy and availability of localization in urban environments. Therefore, the receiver needs to know the signal structure (so-called PRN codes and modulation) of the satellite signals. Since for non-GNSS satellite missions, this information is not available and needs to be estimated using a high-gain antenna.
Therefore, within an R&D project at the Institute DIGITAL, a Concept Demonstrator (CD) of a 40-element antenna array system was developed, based on low-cost software-defined-radio (SDR) platforms and commercial off-the-shelf (COTS) components. This approach compared to a dish antenna has the big advantage of having a high-gain omnidirectional antenna being able to track all satellites in view.
The so-called “Antenna Platform” was built consisting of an array of 40 identical commercial low-cost GNSS antennas connected to 20 dual-channel low-cost SDRs and 20 low-cost PCs for data storage. All the loosely synchronized data are collected in a single database and jointly synchronised and processed by a software tool especially designed to recover unknown code chip sequences running on a high-end PC with a 32-core CPU. The system was designed in a way that the chip error rate is sufficiently low for the encrypted GNSS services of GPS, BeiDou and consequently also for Galileo – provided the satellites are above a certain minimum elevation. For increasing the achievable gain of low-standing satellites, a tilting mechanism for the antenna array was implemented. Finally, an existing GNSS software receiver was upgraded in order to implement the hybrid position, velocity and time solution by combining open signals together with the blindly tracked signals from the antenna array. Therefore, a “Receiver Platform” consisting of 4 commercial low-cost antennas connected to two low-cost dual-channel SDRs and one mass-market GNSS receiver was built.
Algorithms and software were verified first with signals from a GNSS simulator. In the following, numerous experiments were executed to prove the the performance with the signals in-space – in particular the GPS M-Code. The full system was verified by cross-checking against signals acquired by a 2.4 m steerable dish antenna proving that the finally obtained tracking results (C/N0, code/carrier pseudorange, …) match the expectations.
Finally, encrypted M-Code signals (using the blindly estimated chip sequences from our system) as potential unknown L-band signals were used for localization.
This array enables a huge range of R&D activities like enabling localization with signals of opportunities, signal quality monitoring, direction finding, beam forming, controlled reception pattern antennas. Possible future LEO navigation satellite systems broadcasting in L1 band can also be handled by the system. For the exploitation of different frequency bands, only the antennas and LNAs will need to be replaced. The results indicate that also for those other satellite systems reliable chip estimation can be performed and those signals can be acquired and tracked, e.g. for deeper understanding of higher order BOC modulation schemes