More on Mystery Signals, Concerns Maybe Hurting GPS, GNSS

Editor’s Note: Here is a follow-on Linkedin post to a item we posted a couple days ago:

We have highlighted in our previous post how, in these days it is clearly visible the presence of “spikes” in the GNSS spectrum when SVN-49 is in view. You can read here our previous post.

The harmonic nature of the interference let us guess that we are facing the transmission of a Non-Standard C/A code (NSC), as described in the GPS SIS ICD. Navstar 63- SVN-49 was originally launched as a standard replenishment satellite on 24/03/2009, and later intended to be used for experimentation on L5, officially stating that no impact on L1 and L2 was expected. However, previous anomalies, coming from such a satellite were already detected.

Transmission of the NSC for testing purposes is foreseen in the GPS ICD. However, in this case the choice of the “spreading” sequence, which is which is actually not spreading the transmitted power, let the total power of the signal to be concentrated on few spectral components, thus originating interference of “continuos wave” kind.

According to the NANU 2001701, SVN-49 was broadcasting PRN-04 since the beginning of the year, but NANU 2017042 announced that PRN-04 was allocated to SVN-38 starting from May, 18th.

This switch actually matches the dates when we started to see the spikes, since probably the SVN-49 started that day to use the “square wave” for the spreading.

Using a technique of average and sum over the code period considering a time window during which the Doppler on the signal is negligible, as in [1], the “spreading” code has been extracted, confirming the square wave hypothesis.

Implementing the square wave local code, it has been possible to successfully acquire and track the NSC.

Despite of the nice exercise of processing the signal from an unhealthy satellite, we are still concerned about the effect of having such harmonic components affecting the GNSS bandwidth.

We are carefully assessing how much this affects or might worsen the quality of the GNSS signals, thus worsening the performance of GPS and of the other GNSS systems.

This work is being performed by the NavSAS group of Politecnico di Torino and by the Navigation Technologies research area, of Istituto Superiore Mario Boella (ISMB)