By Peter Steigenberger, André Hauschild, Steffen Thoelert and Richard B. Langley
Between Feb. 7, 05:02 UTC and Feb. 8, 12:30 UTC, 2017, all seven operational GPS Block IIR-M satellites were consecutively subject to short periods of unavailability. These official outage periods, when the satellite signals were set unhealthy and deemed unusable, were announced ahead of time through Notice Advisories to Navstar Users (NANUs). An overview of the outage periods and the corresponding NANUs for each satellite identified by their pseudorandom noise code (PRN) assignment and space vehicle number (SVN) is provided in TABLE 1.
An analysis of the measured signal-to-noise-density ratio (C/N0) from several tracking stations of the International GNSS Service (IGS) indicates that the satellites’ transmit powers were increased during the outage periods. The effect is visible in the plots in FIGURES 1 and 2, which show C/N0 of the L1 C/A-code over time for satellite passes on the three consecutive days Feb. 6, 7 and 8, 2017.
Figure 1 shows the results for PRN 17 as tracked by a Septentrio PolaRx4TR receiver (USN8) located in Washington, DC. The pass on the outage day Feb. 7 is plotted in blue. Obviously, the receiver is configured to not track unhealthy satellites, since no observations are available during the outage period. However, a clear increase in the C/N0 is visible from about 50.5 dB-Hz before the outage to approximately 52 dB-Hz after the outage. The C/N0 level of the day before is similar to the level prior to the outage. The C/N0 level on the following day is very similar to the C/N0 after the outage, which indicates that the satellite continues to transmit with an increased power.
The plot in Figure 2 shows the same analysis, this time for PRN 05 and for a Leica GR10 receiver (KOUG) located in Kourou, French Guiana. This receiver continues to track the satellite during the unhealthy period. The distinct step in C/N0 is clearly visible shortly after the satellite is set unhealthy. Also, this satellite continues to transmit with increased power during the pass on the following day. The same observations as in Figure 1 and Figure 2 can also be made for all other Block IIR-M satellites and other receivers.
The difference between the measured C/N0 before and after the unhealthy period is typically 1–2 dB-Hz depending on the receiver and the satellite (see TABLE 2). On average, a power increase of 1.5 dB with a scatter of ±0.25 dB among the various satellites is suggested by the measured data.
Furthermore, it may be noted that different receivers respond with a different change in C/N0 for a given change in transmit power. At the average 1.5 dB power increment, C/N0 changes between 1 dB and 2 dB are reported by the different types of receivers. This indicates manufacturer-specific algorithms for C/N0 estimation that impact the use of measured C/N0 as a reliable indicator of received signal power strength.
It is interesting to notice in this context that NANU 2017005 issued Jan. 19, 2017, states that “The 2d Space Operations Squadron (2 SOPS) periodically conducts configuration changes on GPS satellites to assess current capabilities, validate future capabilities and ensure continued interoperability.”
Furthermore, the Civil GPS Service Interface Committee Executive Secretariat released the following statement on Jan. 25, 2017: “Beginning 25 January 2017, Air Force Space Command (AFSPC) will conduct a limited duration test implementing an increase of the L1 C/A power level on the GPS Block IIR-M and IIF satellites (19 vehicles).”
However, no maintenance has been announced so far for any of the Block IIF satellites, and no obvious increase in the measured C/N0 could be found for these satellites. A repeated analysis for the Block IIR-M satellites on Feb. 22, 2017, confirmed that the L1 C/A-code power levels were still at their increased levels.
Measurements with the German Aerospace Center’s (DLR’s) 30-meter-diameter high-gain antenna at Weilheim, Germany, have been recorded to independently confirm the GPS Block IIR-M transmit power increase of the L1 C/A-code. FIGURE 3 shows the L1 spectral flux density for March 4, 2017 (blue line), and a previous measurement taken on Dec. 7, 2015 (red line). The sharp peak in the middle of the spectrum represents the C/A-code. A clear increase of the power in the measurement of March 2017 compared to Dec. 2015 is visible. Further analysis of the high-gain antenna data yields a power increase of about 2 dB.
However, the M-code flux density with main lobes near 1565 and 1585 MHz is reduced in March 2017 compared to Dec. 2015, whereas the P(Y)-code signal strength remains essentially unaltered. The total transmit power in the L1 frequency band is the same for both time periods. Therefore, the analysis reveals a redistribution of transmit power from M-code to C/A-code for the Block IIR-M satellite PRN 29 (SVN 57).
Authors Peter Steigenberger, André Hauschild and Steffen Thoelert are from the German Aerospace Center (DLR).
Richard B. Langley is from the University of New Brunswick and authors the monthly Innovation column for GPS World magazine.