First Field Experience with L5 Signals: DME Interference Reality Check

This paper summarizes experimental results of the behavior of a multi-frequency receiver with pulse blanking in the presence of DME/TACAN interference on L5 using GPS-L5 signals transmitted by the SVN 49 GPS satellite launched at the end of March 2009. In our interference tests we used a multi-frequency GPS/Galileo/GLONASS/Compass receiver. This receiver has two down-converters for the L1/Inmarsat and L2/L5 bands. Due to the presence of pulsed DME/TACAN interference in the L5 band, the pulse blanking is implemented in the digital processing of the L2/L5 band. The operation of the pulse blanking circuit is analyzed with the use of a simple statistical model for amplitudes of the interference signal. It is shown that in the presence of intensive interference, the wrong choice of the pulseblanking threshold may lead to the instability of the control loop which maintains the optimal value of the magnitude bit duty cycle. The intensity of DME/TACAN interference increases with the number of aircraft in the area, so we tested the effectiveness of pulse blanking in the vicinity of several DME beacons at two large metropolitan airports. It is shown that pulse blanking works and is effective, but the interference was still not sufficiently intensive to significantly degrade the receiver performance even if pulse blanking were not used. It is well known that DME/ TACAN interference is a lot more intensive in the air, so future tests with airborne receivers are likely to provide examples when pulse blanking is indispensable. The paper also contains a general analysis of multipath performance and signal power of L5 signals. In accordance with expectations, multipath noise on L5/E5a is lower than on L1CA/L2C. It is shown that a well- known signal anomaly of SVN 49 is also affecting the L5 signal, which shows a pronounced elevation-dependent component in code multipath.