Solar interference to GPS downlink signals appears to be a possibility during the lock acquisition phase, but the probability is very low, and is expected to occur only a few times in each solar cycle (around 11 years in duration) and only for a few minutes on each occasion.

To examine link margins, consider the following figures for a typical small handheld GPS navigator. With an antenna gain (G) of one (0 dBi), the typical GPS power at the receiver input is around -160 dBW. A typical system noise temperature Ts of 350 degrees K (this includes ambient noise plus receiver noise). gives an equivalent input noise power over a 1 MHz bandwidth of -143 dBm (GPS C/A signals are spread over 2 MHz but most of the power is concentrated within a 1 MHz bandwidth). The signal to noise ratio (SNR) at the receiver input is thus about -17 dB. However, during initial acquisition of the GPS, the correlation loop reduces the bandwidth to about 1 KHz providing a 30 dB increase in SNR. During the acquisition phase the SNR is therefore about +13 dB. Once tracking the signal, the bandwidth is further reduced to about 50Hz which provides an additional increase of 13 dB, giving a total tracking SNR of +26 dB.

The background solar flux at L-band varies from 50 to 150 SFU (solar flux unit) over the solar cycle. This is much too low to cause interference to GPS signals. However, during certain explosive events on the sun, bursts of radio energy are produced which vastly exceed this value. The maximum burst component Sb recorded at L-band is around 100,000 SFU. The decrease in SNR produced by a solar noise burst is given by equation (1).

Change in SNR (dB) = 10 log ₁₀ [1.0 + ( Sb G c² ) / ( 4 pi f² k Ts) ] (1)

where: Sb is the maximum burst in SFU, G is the antenna gain, c is the speed of light, f is the frequency (1575 MHz), k is the Boltzmann constant; and Ts is the system noise temperature.

The table below lists the SNR decrease (in dB) for GPS receivers with different antenna gain and system noise temperature combinations. For the receiver example given above an 8 to 9 dB reduction can be expected for this magnitude solar burst. This is likely to make signal acquisition very difficult if not impossible, but is unlikely to cause signal loss when loop lock has been established. However, the link margin will be reduced, and this, in combination with other effects such as hi-g maneuvers or ionospheric scintillations may affect system performance. Such large solar bursts are rare. In solar cycle 20 (1965-1976) one such burst of 165,000 SFU occurred on 29 April 1973. Total burst duration was 40 minutes although the time above 100,000 SFU was much shorter. In December 2006 radio noise bursts exceeding 100,000 SFU and maybe as high as 1,000,000 SFU were recorded at the IPS/USAF Learmonth Solar Observatory (see below).

Reference: John A Kennewell, "Solar Radio Interference to Satellite Downlinks", Proceedings ICAP 89, Sixth International Conference on Antennas and Propagation, vol 2, pp334-9 (IEE London 1989).

   Antenna                 System Noise Temperature (K)
   Gain(dB)     50   100   150   200   250   300   350   400   450   500

     0        16.3  13.4  11.7  10.6   9.7   9.0   8.4   7.9   7.5   7.1
     1        17.3  14.4  12.7  11.5  10.6   9.9   9.3   8.8   8.4   8.0
     2        18.3  15.3  13.6  12.4  11.5  10.8  10.2   9.7   9.2   8.8
     3        19.3  16.3  14.6  13.4  12.5  11.7  11.1  10.6  10.1   9.7
     4        20.3  17.3  15.6  14.4  13.4  12.7  12.0  11.5  11.0  10.6
     5        21.2  18.3  16.5  15.3  14.4  13.6  13.0  12.4  12.0  11.5
     6        22.2  19.3  17.5  16.3  15.4  14.6  13.9  13.4  12.9  12.5
     7        23.2  20.2  18.5  17.3  16.3  15.6  14.9  14.3  13.9  13.4
     8        24.2  21.2  19.5  18.3  17.3  16.5  15.9  15.3  14.8  14.4
     9        25.2  22.2  20.5  19.2  18.3  17.5  16.9  16.3  15.8  15.3
    10        26.2  23.2  21.5  20.2  19.3  18.5  17.8  17.3  16.8  16.3
    11        27.2  24.2  22.5  21.2  20.3  19.5  18.8  18.3  17.7  17.3
    12        28.2  25.2  23.5  22.2  21.3  20.5  19.8  19.2  18.7  18.3
    13        29.2  26.2  24.5  23.2  22.3  21.5  20.8  20.2  19.7  19.3
    14        30.2  27.2  25.5  24.2  23.2  22.5  21.8  21.2  20.7  20.3
    15        31.2  28.2  26.5  25.2  24.2  23.5  22.8  22.2  21.7  21.2
    16        32.2  29.2  27.5  26.2  25.2  24.5  23.8  23.2  22.7  22.2
    17        33.2  30.2  28.5  27.2  26.2  25.4  24.8  24.2  23.7  23.2
    18        34.2  31.2  29.4  28.2  27.2  26.4  25.8  25.2  24.7  24.2
    19        35.2  32.2  30.4  29.2  28.2  27.4  26.8  26.2  25.7  25.2
    20        36.2  33.2  31.4  30.2  29.2  28.4  27.8  27.2  26.7  26.2
    21        37.2  34.2  32.4  31.2  30.2  29.4  28.8  28.2  27.7  27.2
    22        38.2  35.2  33.4  32.2  31.2  30.4  29.8  29.2  28.7  28.2
    23        39.2  36.2  34.4  33.2  32.2  31.4  30.8  30.2  29.7  29.2
    24        40.2  37.2  35.4  34.2  33.2  32.4  31.8  31.2  30.7  30.2
    25        41.2  38.2  36.4  35.2  34.2  33.4  32.8  32.2  31.7  31.2
    26        42.2  39.2  37.4  36.2  35.2  34.4  33.8  33.2  32.7  32.2
    27        43.2  40.2  38.4  37.2  36.2  35.4  34.8  34.2  33.7  33.2
    28        44.2  41.2  39.4  38.2  37.2  36.4  35.8  35.2  34.7  34.2
    29        45.2  42.2  40.4  39.2  38.2  37.4  36.8  36.2  35.7  35.2
    30        46.2  43.2  41.4  40.2  39.2  38.4  37.8  37.2  36.7  36.2

Material Prepared by John Kennewell and Andrew McDonald. © Copyright IPS - Radio and Space Services.
Comments or suggestions can be directed to education@ips.gov.au