If the interference comes from space…
Detection of anomalous harmonics in the L1 spectrum
Interfering signals are one of the most well-known nuisance for GNSS receivers. A number of terrestrial systems and devices can generate various types of interference, either intentionally or not, but one would not expect interfering signals to arrive from space. On May 17, researchers of the Navigation Signal Analysis and Simulation (NavSAS) Group at the Politecnico di Torino detected the presence of anomalous spikes in the L1 signal spectrum. The origin of the spikes was identified to be the transmission of non-standard codes from a non-operational GPS satellite (GPS IIF-9, SVN49). In this article, we report on some of the most significant signal observations we performed in an effort to identify and localize the source of the interference and we address the possible impact it could have on GNSS signal processing.
By Fabio Dovis, Nicola Linty, Mattia Berardo, Calogero Cristodaro, Alex Minetto, Lam Nguyen Hong, Marco Pini, Gianluca Falco, Emanuela Falletti, Davide Margaria, Gianluca Marucco, Beatrice Motella, Mario Nicola and Micaela Troglia Gamba
On the afternoon of May 17, 2017, during an outdoor data collection experiment, researchers of the NavSAS Group detected the presence of two spikes in the L1 spectrum, with sufficient power to be clearly visible on a display of the spectrum obtained by processing the raw digital samples at the receiver’s intermediate frequency. The initial check looked for a possible interfering source in the experimental set-up, since it was quite complex and included multiple GNSS receivers, PCs, a video camera and a couple of car batteries. But the likelihood of this source was soon dispelled as the same kind of spectrum was visible on a spectrum analyzer (SA) connected to an active, survey-grade GNSS antenna mounted on the lab roof, as displayed in FIGURE 1. The spectrum is centered at 1575.42 MHz, with the SA set to a frequency span of 5 MHz. Connecting the SA to a different survey-grade antennas on the lab roof, we saw no remarkable differences.
The spikes also appeared on subsequent days, becoming clearly visible at about 13:00 UTC and disappearing at about 19:00 UTC, as illustrated in FIGURE 2. The main lobe of the GPS signal spectrum is visible, along with two spikes, at approximately ±0.5 MHz above and below the L1 carrier frequency. Weaker harmonics are also visible at ±1.5 MHz from the central frequency.
Response from the U.S. Air Force about the anomaly
The 2nd Space Operations Squadron is performing maintenance on a presently non-operational satellite. SVN49 is broadcasting non-standard C/A and non-standard Y codes as described in IS-GPS-200. Space professionals continue to conduct safe and responsible command and control of the constellation to continue to provide accuracy that exceeds established system requirements.
As always, GPS users who experience issues should address them through the appropriate channels: military users should contact DSN 560-2541, commercial 719-567-2541 while civilian users should contact the U.S. Coast Guard Navigation Center at 703-313-5900.
Very Respectfully,
NICHOLAS J. MERCURIO, Capt, USAF
Director, 14th Air Force (Air Forces Strategic)/JFCC SPACE Public Affairs
Exclusion of terrestrial sources
The 24-hour repetition period of the phenomenon, along with the shape of the spectrum, could indicate the presence of a signal anomaly from a GNSS satellite. However, we could not exclude the hypothesis of unintentional interference generated by a nearby terrestrial communication system, since the area is crowded with research labs belonging to the Instituto Superiore Mario Boella and the Department of Electronics and Telecommunications of Politecnico di Torino. Nevertheless, we probed the L1 spectrum in a wider area using a simple setup, consisting of a patch antenna and a narrow-band front end. We analyzed the spectrum at the output of the front-end’s analog-to-digital converter, plotting the results on a smartphone running our software receiver in real time.
FIGURE 3 shows the L1 spectrum observed several kilometers from the NavSAS Lab. The shape of the spectrum is different than that in Figure 1 because of the narrow-band filter of the front end, but again, the presence of the two spikes is clearly visible at ±0.5 MHz from the central frequency, approximately with the same power strength. In addition, during a dynamic data collection experiment, we recognized that the interfering signals disappeared when the western part of the sky was obscured by buildings, as demonstrated in Figure 3. This was further investigated (and confirmed) when we processed the collected set of data in the lab. At that time (May 19), the hypothesis of an interfering signal from space became more plausible.
Meanwhile, the presence of suspicious spikes was confirmed by colleagues at the European Commission Joint Research Centre located in Ispra, Italy, and also from researchers of the Finnish Geodetic Institute in Helsinki, Finland, and by the South African National Space Agency at the station of the South African National Antarctic Expedition IV. These multiple observations definitely excluded the possibility that the interference it could be coming from terrestrial sources or from within the receiving equipment.
Checking the satellites in view during the presence of the spikes in the spectrum (that is, from about 13:00 to about 19:00 UTC) and bearing in mind the periodicity of the event over consecutive days, we excluded the possibility that a Galileo satellite could be the source of interference. It is indeed known that, due to an orbital period of approximately 14 hours for observers on the ground, the constellation geometry repeats only every 10 days.
FIGURE 4 shows the visibility of operational satellites over the full time window of interest for the GPS, Galileo and BeiDou constellations.
Considering the duration of the satellites’ visibility, the search for the source of interference was restricted to SVN71 (PRN26), SVN45 (PRN21) and the C11 BeiDou satellite. However, considering the previous tests, the satellite should have been in the western portion of the sky with respect to our location, and the only operational satellite of this set is SVN71, which we initially identified as the possible source of the interfering signal.
GPS SVN71 (PRN 26) or SVN 49?
The frequency of the harmonics could be measured over time. The first peak at approximately 0.5 MHz above the central frequency was analyzed by post-processing a set of digital samples collected with an Universal Software Radio Peripheral, which was slaved to a 10-MHz rubidium standard and which converted the RF signal to baseband, sampling it at 5 MHz. The frequency was measured exploiting a Welch periodogram, based on a 102,400-point discrete Fourier transform, with rectangular windowing and no window overlaps.
FIGURE 5 (a) shows the trend of the measured frequency versus time, from 12:43 to 18:38 UTC, on May 21. The frequency profile reveals that it is not constant and has a trend similar to the typical Doppler frequency shift of a GPS satellite. FIGURE 5 (b) shows the derivative of the frequency, with a minimum around 16:22 UTC. At that time, we expected to have a null Doppler shift from GPS PRN26, whereas the frequency of the peak was equal to 510.449 kHz. This is the inverse of 1.959056 microseconds, which is close to the inverse of twice the chip length, 2/Rc = 1.955034 microseconds. This indicates that the interfering signal could be a square wave with the same rate as the C/A spreading code.
FIGURE 6 shows the Doppler frequency of PRN26 (blue line), as estimated by the tracking loop of a GNSS software receiver, and compares the Doppler shift to the frequency of the first upper peak (orange line), measured on the spectrum. It is possible to note that the two curves almost overlap, with a significant difference at the beginning and at the end of the observation. Thus, although the frequency of the peak follows the Doppler trend of a GPS satellite, it does not exactly match the Doppler curve of PRN26. This result weakened the hypothesis indicating that PRN26 was the source of the interference.
Furthermore, since it was still possible to acquire and track the L1 C/A-code signal from PRN26, this satellite was unlikely to be the source of the interfering components. Thus, also motivated by the mismatch in the Doppler shift of PRN26 (as previously highlighted in Figure 6), we started to think that the source of the interference could be another satellite broadcasting a GPS-like signal.
The search then focused on potential sources of interference coming from a non-operational satellite. The non-operational GPS satellite SVN49, launched on March 24, 2009 (also known as NAVSTAR 63 with NORAD ID 34661), has an orbit similar to that of SVN71 (see FIGURE 7). The previous remarks, let us guess that the transmission of a non-standard code (NSC) from such a satellite was the origin of the problem in the L1 spectrum. Such a case, could be similar to what has been previously reported in by Zhu et al. [1,2] when discussing the effects of the transmission of an NSC on Nov. 28, 2006.
Transmission of NSCs for testing purposes is foreseen in the GPS Interface Specification, IS-GPS-200 [3]. GPS satellites can switch off regular broadcasts of the C/A code and the P/Y code and transmit a non-standard C/A code and non-standard Y code. Such operation is intended to protect users from receiving and utilizing erroneous satellite signals in case of unhealthy conditions on the spacecraft. Strictly speaking, this case cannot be formally considered as an “anomaly,” because the transmission of non-standard codes is documented in the IS-GPS-200. Therefore, the transmission of an NSC can be considered a normal operation in itself, even though it may reflect a problem with the transmitting satellite.
However, in this case the choice of the spreading sequence, which is likely a square wave, allowed the total power of the signal to be concentrated in just a few spectral components, thus originating continuous-wave-like in-band signals.
The distribution of the harmonics, the main components of which are at ±500 kHz, and the presence of the odd harmonics only, matches the case recalled by Zhu et al. [1,2], of a transmission of an NSC modulated as a binary-phase-shift-keying (BPSK) sequence with alternating logical 0s and 1s, transmitted at the C/A code chipping rate (Rc=1.023 megachips per second). The spectrum of this “square wave” with period 
used as a spreading signal is in fact know to be
(1)
where δ is the Dirac-δ function. Zhu et al. [1,2] considered this specific case of a “non-standard code” to be especially remarkable, because it can affect the L1 spectrum, introducing multiple harmonic components similar to those previously illustrated in Figure 1 and Figure 3 (a).
The hypothesis of the BPSK with Rc=1.023 megachips per second spreading signal has been verified by simulation. Figure 8. shows how the tested case of a received signal from SVN49 with a C/N0=55 dB-Hz best matches the measured spectrum when SVN49 is at its maximum elevation angle and the power of the received signal is the strongest.
However, it has to be remarked that according to Zhu et al. [1,2], the NSC is designed to have negligible effect on tracking other healthy GPS satellite signals. Nonetheless, their analyses showed that an NSC transmission (as occurred on Nov. 28, 2006) can have a non-negligible impact in the performance on user equipment. In detail, when a GPS satellite is switched to NSC mode, a receiver immediately loses its capability to track that satellite signal. This is not the case with SVN49 as it is currently declared non-operational. However, due to the modified code sequence, an even worse effect is possible. In fact, the NSC introduces irregular components at a sustained level in the GPS signal spectrum.
As a final confirmation of the transmission of the NSC from SVN49, we have used the technique of averaging and summing over the code period as described by Mitelman [6]. Considering a time window during which the Doppler shift of the signal is negligible, we have extracted the spreading code, confirming the square wave hypothesis (see FIGURE 9).
According to the Notice Advisory to Navstar Users (NANU) 2001701, SVN49 was broadcasting standard signals as PRN04 (although set unhealthy) since the beginning of the year, but NANU 2017042 announced that PRN04 was to be re-allocated to SVN38 starting from May 18. This switch actually matches the dates when we started to see the spikes in the spectrum, since, probably, the SVN49 started that day to use the “square wave” for the spreading.
Implementing the square wave local code, it has been possible to successfully acquire and track the NSC, as shown in FIGURE 10.
The real-time software receiver N-Gene, documented by Molino et al. [5],has been forced to acquire and track in real time the signal coming from SVN49. FIGURE 11 shows a screenshot of the N-Gene graphical interface while processing this signal.
The receiver was able to perform the decoding of the navigation message transmitted by SVN49, which exhibits a regular format, even if marked with an unhealthy flag (see FIGURE 12).
Impact on receiver signal processing
It is well known that the spectrum of GNSS signals is basically a line spectrum in the frequency domain, which is susceptible to interference (see, for example, the book edited by Davis [4]).
Interference with harmonic components such as those generated by the use of a square wave could strongly impact a GNSS receiver in the acquisition and tracking blocks because the interference power is dispersed over the whole search space by the correlation with the local code, compromising the acquisition accuracy and impacting other functional blocks. The impact of interference spectral lines strongly depends on their location within the frequency band. This is due to the almost periodic nature of the GNSS signals. In fact, the spectrum of a GNSS signal has components spaced at multiples of the inverse of the code period (for example, 1 kHz for GPS C/A code) with different power allocated to each component depending on the shape of the code spectrum. The effect is larger in case of matching of the interference spectral components with the ones of the GNSS signal. Furthermore, in the present case, the strongest harmonics are close to the L1 carrier frequency and are not mitigated by the front-end filter since they fall within its narrow bandwidth.
As opposed to the case discussed by Zhu et al. [1,2] when GPS was virtually the only code-division-multiple-access system occupying the bandwidth around L1, the overall GNSS scenario has changed a lot recently. Galileo and BeiDou are also present, and the signals of the Galileo system, due to the different structure and code periods, have spectral lines spaced at 0.25 kHz. The frequency modulation of the interfering signal due to the variable Doppler shift is then even more likely to hit some of the spectral components of these signals.
We are performing further investigations are being performed to assess the impact of the interfering signal from SVN49 on Galileo-based high accuracy applications.
Acknowledgments
The NavSAS Group thanks Dr. Matteo Paonni (EC Joint Research Centre) for the support given in the analysis of the L1 signal spectrum and Dr. Laura Ruotsalainen (Finnish Geospatial Institute) and Danielle Taljaard (South African National Space Agency), who performed the data collection in Antarctica.
Bios
Fabio Dovis, Nicola Linty, Mattia Berardo, Calogero Cristodaro, Alex Minetto and Lam Nguyen Hong are with the Navigation Signal Analysis and Simulation (NavSAS) Group, Politecnico di Torino, Torino, Italy.
Marco Pini, Gianluca Falco, Emanuela Falletti, Davide Margaria, Gianluca Marucco, Beatrice Motella, Mario Nicola and Micaela Troglia Gamba are with the Navigation Technologies Research Area of Istituto Superiore Mario Boella, Torino.
References
[1] “GNSS Watch Dog: A GPS Anomalous Event Monitor” by Z. Zhu, S. Gunawardena, M. Uijt de Haag, F. van Graas and M. Braasch in Inside GNSS, Vol. 3, No. 7, Fall 2008, pp. 18–28.
[2] “Satellite Anomaly and Interference Detection Using the GPS Anomalous Event Monitor” by Z. Zhu, S. Gunawardena, M. Uijt de Haag and F. van Graas in Proceedings of the 63rd Annual Meeting of The Institute of Navigation, Cambridge, Massachusetts, April 23–25, 2007, pp. 389–396.
[3] Navstar GPS Space Segment / Navigation User Interfaces, Interface Specification, IS-GPS-200 Revision H including Interface Revision Notices 1–3, Global Positioning Systems Directorate, Systems Engineering and Integration, Los Angles, California, Dec. 2015.
[4] GNSS Interference Threats and Countermeasures by F. Dovis (ed.) published by Artech House, Norwood, Massachusetts, 2015.
[5] “N-Gene GNSS Software Receiver for Acquisition and Tracking Algorithms Validation” by A. Molino, M. Nicola, M. Pini and M. Fantino in Proceedings of EUSIPCO 2009, the 17th European Signal Processing Conference, Glasgow, Scotland, Aug. 24–28, 2009, pp. 2171-2175.
[6] Signal Quality Monitoring for GPS Augmentation Systems by A.M. Mitelman. Ph.D. dissertation, Stanford University, Stanford, California, Dec. 2004.
