GPS L3 band occupancy Nov13
Project a2811 reported excessive gps L3
interference during their observations 24Nov13 - 30Nov13.
Their setup was:
To search for the occurence of GPS L3 interference:
- lband wide receiver
- interim correlator with 4 sub correlators:
- sbc 1,2 : 25 Mhz polA,polB each with 2048 channels (12.2Khz
- sbc 3,4: 6 Mhz polA,B coverage center of sbc 1,2
- 1 second integrations
- 180 second on , off position switching followed by a
10 sec cal on,off.
The plots show the value of the
1381.05 Mhz channel for the data taken 24Nov13 through 30Nov13
- find all a2811 files for Nov13: 271
- 271 files
- each file had 4 scans:
- 180 secs on source
- 180 secs off source
- 10 secs cal on
- 10 secs cal off
- Only keep the 180 secs on, or off sources (542
- find all scans that include 1381.05 Mhz
- for each scan
- extract the channel closest to 1381.05 Mhz (25 Khz after
- normalize each 180 numbers for a scan with the median value.
- The 218 scans were split into 4 sets (to see them a bit
- The x axis holds the 1 second data samples of the 1381.05 Mhz
- The y axis is in units of Tsys (since i divided by the median
- each scan of 180 seconds is plotted in a different color
- I defined GPS L3 as anything 2% away from Tsys.
- The dashed horizontal lines show this clipping level.
- This algorithm overestimates the amount of gps L3 rfi if the
rfi is on for > 50% and < 100% of the 180 seconds (the
median will use the gpsl3 value if it is on > 50% of the
- There were 32240 1 second samples.
- There were 1720 secs with gpsL3 rfi
- GPS L3 was on : 1720/32240. = 4.38% of the time
The zone of
avoidance project (a2754) has also been running during Nov13. Their
To search for GPS L3 interference:
- ALFA receiver
- mock spectrometer covering 1250 to 1520 Mhz (spectral
resolution of 21Khz).
- The frequency bands are fixed (they don't change scan to
scan like a2811).
- 1 second samples
The plots show the 1381 Mhz power vs
time for the a2754 data during Nov13 (.ps) (.pdf)
- find all of the A2754 ALFA beam 0 files (590)
- each file had 197 1 second spectra
- that last 17 secs of each file had 8 secs cal on, 8
secs cal off.
- for each rec (second) of each file compute:
- the average power [1380.29 to 1382.559] Mhz
- Compute a baseline using
- .5 Mhz average center at +/- 2 Mhz from 1381.
- Divide the average 1381 Power the the average of the two
- This does a better job a estimating the gps l3 rfi since
it doesn't rely on the median value over the entire scan.
- This wasn't used with the a2811 data since the 1381
Mhz band occasionally fell at the very edge of their
25 Mhz band.
- Top: 1381 power vs date.
- The + show the 1 second samples
- The y axis is Tsys (after removing 1).
- Middle,Bottom frames: 1381 Power vs 1 second samples
- I've plotted each days data with a separate color
- the dashed horizontal threshold (1.5% of tsys) was used
define gps l3 rfi.
- There were 115594 1 second samples
- There were 3936 samples with gps l3 rfi
- So GPS L3 was on for 3.41 % of the time.
GPS L3 occurrence Nov 13
- The a2754 measurement is probably more accurate because is
didn't use a median bandpass for each scan
Processing: x101/131129/ gpsl3_a2811.pro