GPS L3 interference jul16
Project a2048 uses the alfa receiver in drift
scan mode. It normally takes 1200 1 second records/scan, and then
moves on to repeat the drift over the same source.
I took the data from jul16 to check on how often
gps L3 (1381.05 MHz) was appearing in the data.
Processing the data:
- There were 71 scans (92069 1 second spectra).
- Only data from alfa beam 0 was used.
- For each scan +/- 2 MHz about 1381.05 MHz (164 channels, 24
Khz resolution) was extracted and then used for the following
- PolA and polB were averaged together.
- For each 1 second average spectrum:
- .75 MHz on each edge of the 4MHz band was
averaged and then used to normalize the spectra to Tsys. This
got rid of celestial sources.
- Total power for each 1 second was then computed using +/- .3
MHz around 1381.05
- 1. was subtracted from each total pwr sample.
- Any total power points above .02 Tsys were taken to be rfi
from gps L3
The plots show the processing
steps (.pdf) (2.9 Mbytes)
- Top frame: Frequency limits used to compute total power about
- 2000 1 sec spectra were over plotted showing the gps L3 rfi
- The dashed red lines are the frequency limits used to
compute the total power around the L3 line.
- Bottom frame: total power samples for the month.
- each + is the +/- .3 MHz total power computed about 1381.05.
The units are Tsys (i've removed 1.)
- there are 92000 samples (the plot clips at .05 Tsys)
- The red line is the threshold used to define gps L3 rfi.
- There were 8346 points above the line (not all shown)
- The threshold ignores a number of L3 points (eg around
- 8346/92000 is about 9% usage.
Images and histograms of the month:
There were 78 scans for the month. 76 of these were
1200 seconds long. For each 1200 secs scan an image was made
covering 1200 secs, +/- 2 MHz about the gps L3 frequency
A mosaic was made of 76 1200
- 76 1200 scans were available (two shorter scans were
ignored). This is 91200 1 sec spectra.
- Each frame is 1200 seconds (vertical) by 4 MHz (horizontal)
- The bright lines in the central portion of a frame comes from
- The scan number (on each title line) has Ydddnnnnn. Where Y is
6 for 2016, ddd is dayno of the year (ast), and nnnnn is a
number that increments within a day. This shows which scans were
taken on the same day.
- 51 of the 76 1200 second scans had gps
- Of the 91200 1 sec spectra, 8346 had L3 interference 9%
of the time. This used the .02 threshold above... this
underestimated the rfi by maybe 100.
Bar chart of the rfi by
- The vertical scale is the samples within in a scan
- the horizontal axis is the day number of jul16 when the scan
- The colors show good and bad data
- red: these samples had gps L3 interference
- green: samples with no gps L3 interference.
- I included the 2 shorter scans in the bar chart.
- 76 1200 second scans were taken with the ALFA receiver by
A2048 during july 2016.
- Beam 0 data was used to look for gps L3 interference.
- 91200 total 1 second spectra were used.
- 8346 1 sec spectra had gps L3 interference (using the .02 Tsys
threshold on the total power)
- L3 interference was seen 9% of the time.
- 51 of the 76 scans had L3 interference 67%.
- There were no scheduled gps L3 tests during the month of jul
- there are a number of 200 sec blocks of rfi..
- i should probably go through and see if there is a particular
satellite that is causing more than average rfi.
- (but that will take some more work...)