Institute of Communications and Navigation of DLR is involved in development of many advanced signal processing algorithms for GNSS applications. These algorithms are developed for signal enhancement and interference mitigation at user navigation equipment with a single receiving antenna, which is the common case for mass-market applications, and with an antenna array system for applications with stringent requirements toward service performance and reliability, Safety-of-Life applications for instance. In all these cases, the algorithms should be thoroughly tested under realistic conditions and adjusted for specifics of a target application. This is the place where the software defined radio concept comes in hand.
The basic idea of a software defined radio receiver, or simply a software receiver, is to move the analog-to-digital conversion as close to the antenna as possible in order to reduce the harmful effects occurring in hardware and provide more flexibility for implementing signal processing algorithms, which are realised now in software and operate on digitized signals. The flexibility offered by software receivers makes them especially attractive for the purposes under consideration. For example, the signal flow in a software receiver can be easily adapted to represent specifics of a target application and simulate interfaces between different processing blocks. The digitized signals used for the tests can be collected in representative application environments and therefore deliver better assessment as pure numerical simulations based on theoretical models.
The largest part of algorithm development for GNSS signal processing often takes place in Matlab© environment. Therefore the focus of our investigations in this area is on the development of Matlab© code of GNSS receivers with a single and multiple receiving antennas used for post-processing of recorded digital signals stored.
For recording the GNSS digital signals we use currently GPS L1 front end hardware produced by Nordnav Technologies: R-Series Front-End© with a single antenna and QuadFront-End© with four antennas. For the tests under controlled signal conditions (GNSS signals, multipath echoes and radio interference) the RF front ends can be connected to the dedicated DLR MASTER generator that is capable to simulate signals as seen by a single antenna and an antenna array.
The developed software receivers serve currently as test-beds for GPS/Giove-A signal acquisition algorithms and antenna array techniques for GNSS signal enhancement and interference mitigation.