GNSS Receiver Development: Hardware Receiver

GNSS Hardware Receiver

A dual frequency (L1 C/A and L2 civil signal) real-time kinematic (RTK) receiver is currently developed at the Institute of Geodesy and Navigation in collaboration with FhG IIS and IfEN GmbH. This investigation is supported within the scope of the research project FKZ: 50NA0003 in contract with DLR.

The development includes a large number of paper studies to investigate existing RTK receiver designs, new boundary conditions due to the Galileo system and due to GPS modernization. Also new possibilities to integrate the Internet, low cost INS and GIS into the RTK receiver have been explored. An overview of all work packages in this project can be found here (German only).

The most exiting part of the project is the development of a breadboard receiver using a wideband L1 CA/L2CS frontend. Signal processing is performed by a FPGA card plugged into a PC and by a software correlator. RTK positions are calculated by fixing the integer ambiguities using the LAMBDA method. The positioning software runs under the Windows Operating System and has interfaces to the FPGA and the Software Correlator.

By using this dual correlator approach, software and hardware correlation techniques can be directly compared (click to enlarge).

The positioning performance of a L1 C/A/L2 CS receiver has been assessed in detail and published in

The arrival of the new L2-CS will improve the current RTK receivers performance. As it was expected, the improvement is more significant when the dual frequency receiver L1/L2-CS is compared with the single frequency L1 receiver and supposing full constellation in both cases.

But also an important point to emphasize is the evolution of the receiver performance with the number of available satellites with L2-CS. With only a few satellites broadcasting L2-CS the receiver has already better performance than when only L1 is received.

Single frequency receivers can be upgraded to a cheap and simple dual frequency L1/L2-CS receiver and as it was analyzed, it will be translated in a better receiver performance, similar to the more complex dual frequency receivers L1/L2-P(Y). The improvement will be specially important in short baselines, where the single epoch success-rate will be increased to values near 100% almost all the time and it will allow a more reliable and precise RTK positioning. Also the time-to-ambiguity fix will be improved, allowing also a faster ambiguity solution.

If the results for the medium baseline are analyzed, it can be observed that also in this case the single epoch success-rate increases a lot, but the mean success-rate achieved when using L2-CS (about 40%) is also not a guarantee for the correct integer estimation. If not single epochs are taken, the improvement in time-to-ambiguity fix for medium baselines is also important, thus when the only signal available is the L1 signal an average time to ambiguity fix of about five minutes is needed, while when also L2-CS is available this time is reduced to about 10 seconds.

For long baselines the most useful improvement is also the average time necessary to ambiguity fix, which in the case of using a dual L1/L2-CS will be of about only one minute. The results also show an improvement performance when using L2-CS instead of L2-P(Y) in dual frequency receivers. Maybe the improvement when using L2-CS seems to be not so significant as in the single frequency receiver case, but the advantage of the new dual frequency receivers can be the less complex techniques needed.