10jan19:327 hires azimuth swings
Processing the data
Dynamic spectra of the azimuth swings
Average/PeakHold spectra for each az swing:
The azimuth dependence of all the birdies
birdies with no az dependence (coming from
birdies with common az dependence
look for ionosonde transmissions in 327
Previous 327 azswings:
It's been almost 3 years since a hi res azimuth
swing has been done with the 327 MHz receiver to check for rfi. Two
azswings (cw then ccw) were done on 10jan19 starting around
15:20. The weather was clear during the swings (no lightning).
- 327 MHz receiver. 750MHz narrow band IF
- azswings 270 to 630 degs and back at .4 degs/sec (probably
should do it at .35 deg/sec next time). Dome at za=18 deg.
- mock spectrometers:
- 7 spectrometers cover 300 to 350 MHz.
- Spectrometer cfr, 304,311,318,325,332,339,346 MHz
- + 5 MHz added in iflo menu, -5 MHz added in mock digital
mixer.. to move DC spikes out of band
- 8 MHz bandwidth, 8192 channels; 976 hz resolution, 1
second sampling (about 1 sec)
- .1 sec sampling
- The alfa motor controller was turned off during the azimuth
- The dome cameras were turned off toward the end of the 1st
Processing the data: (top)
- For each azimuth swing a bandpass was constructed from the 993
spectra. A fit to a bandpass was needed rather than an average
since some rfi are constant and would remain in the bandpass
average. The bandpass was created by:
- Compute a median bandpass from the 993 spectra.
- Fit a 20th order harmonic and a linear polynomial to this
median bandpass using robfit_polyfft(). This iterates the fit
throwing out outliers.
- Divide each of the 993 spectra by this fitted bandpass. This
should remove most of the bandpass.
- For each spectra compute the median total power and subtract
it. This should remove the source contribution.
- PolA and polB were then averaged.
Dynamic spectra of the azimuth
Dynamic spectra were made for each of the
azimuth swings showing spectral density plotted against azimuth and
frequency. The plots were aligned so that azimuth always increased
bottom to top (so the swings from 270 to -90 were flipped). At
the bottom of each image is a peak hold spectra (maximum in each
frequency channel) for the entire spin.
- The horizontal green dashed line in the CW spin is when the
dome cameras were turned off.
- You can see the birdies go away
- Some of the birdies have an azimuth dependence. This means
that they are coming from outside the dome
- A number of these came from 3rd harmonics of 102.9,104.1,
107.3, and intermods between 102.9 and 104.1 (these towers are
located within 100 meters of each other). (more info
- Other birdies have no azimuth dependence. This tell us that
they are probably coming from inside the gregorian dome.
The first plots show the
average and peak hold for each azimuth swing (.ps) (.pdf)
The second set of plots has the
azimuth dependence of 143 different rfi frequencies (.ps
..12Mbytes) (.pdf 4.7Mbytes)
- all 7 bands were combined into a single spectra (976 Hz
- The top two frames show the average of each azimuth swing:
- black is CW azswing, red is the CCW swing.
- This average over 993 seconds.
- Topframe: vertical scale 0 to 10*Tsys
- 2nd frame: vertical scale 0 to 1*Tsys
- The bottom two frames are a peak hold for each swing
- frame 3: vertical scale 0 to 10*Tsys
- frame 4: verical scal 0 to 1*Tsys.
The third set of plots shows rfi
that has little azimuth dependence (.ps) (.pdf)
- I went through and selected 143 different birdies.
- they had to stick up at least .2*Tsys (in 976 hz channels, 1
- I left out some of the wider rfi (3rd harmonics of fm
stations and their intermods)
- This list is not complete..i know i skipped some of the rfi
- The black trace is the CW swing, red is the CCW swing
- Where the black and red traces overlay each other in azimuth
shows that the rfi is stable in time and coming from a
particular azimuth direction (outside of the dome).
- rfi that does not change much with azimuth is probably
coming from inside the dome.
- To compute rfi pwr vs azimuth i:
- for each 1 sec spec i took the max value within a few
channels of the peak value for the az swing.
- This list is not complete. I have probably left out some
- Page 1: over plot the 9 rfi frequencies vs azimuth
- I normalized each frequency to its max value for the swing
(to keep them all on the same page)
- I then added a display offset to each spectra (so they
wouldn't sit on top of each other
- Each line is then labeled with its frequency.
- Page 2: plot the average power over the azswing for each rfi
- This shows how strong each rfi was
- The actual values are labeled with
- the strongest rfis were:
- 329.314 18.6*Tsys .. dome camera (only in cw
- 321.124 4.3*Tstys.. dome
camera (only in cw spin)
.25*Tsys.. this is a harmonics of little star inside the
turret control box
Rfi with common azimuth
The 4th set of plots shows rfi that has a common azimuth direction:
Combs seen in the 327 band:
Average spectra versus freq with
combs flagged (.ps) (.pdf)
- The average spectra for the cw and ccw spins were plotted in 4
frames (12.5 MHz/frame).
- black is the clockwise spin, red is the ccw spin,
- Page 1: 1MHz comb
- the blue vertical lines are placed every 1 MHz.
- An rfi birdie is seen at each 1MHz.
- a 2 and 1MHz comb was seen back on 24jul02 coming from the
430tx power meter in the dome. Check to see if this is really
powered off (more
- Page 2: 1 MHz comb
- top shift each comb element to DC and then overplot
(with a display offset).
- The comb stays aligned with 1 MHz comb spacing.
- Some of the elements have other rfi on top of the 1MHz
- bottom: plot peak value for each comb element.
- 325 MHz is the strongest element (.3*Tsys)
- 328 MHz is the 2nd strongest
- 307,and 320 are larger, but they have other rfi in the
- page 3: comb from encoders
- same plots as page 1 except that blue flags are now at the
- comb =.8192MHz . used 323.581 as one of the comb
- Page 4: verify that .8192MHz is the correct comb
- Use 323.581 MHz as the comb reference freq
- Shift each comb element down to DC.
- Top: use .8189 MHz as the comb spacing. the birdies move to
- Center: use .8192 MHz as the comb spacing.. things
- Bottom: use .8195 MHz as the comb spacing. birdies
drift to the left.
ionosonde transmitting (top)
The ionosonde was transmitting on 15 minute
intervals during the 327 Mhz azimuth swings.
A third clockwise swing was running when the 16:00 ionosonde
ran. The ionosonde transmissions are:
The azimuth was moving clockwise at .4 deg/sec.
- use a 7 baud barker code of 20usec baud (140 usec rf pulse)
- use a 25 ms ipp.
- output 8 ipps at one frequency, then step to the next
- go from 1 Mhz to 20Mhz
- steps start around 10Khz and go to 200 Khz at the top end
- the transmision is then
- 140 usec rf pulse every 25 ms 8 times at 1 freq: total
- step in frequency every 200 ms
- total transmission time 60 secs + any slop when changing
- 16:00 ionosonde started transmitting
- 16:02 ionosonde probably done
- so the azimuth swung through the direction of ionosonde during
How fast was the ionosonde frequency stepping
at 325 MHz? If via harmonics
Depending how the harmonics are created, there could also be other
- tx = 1 MHz, step .01 Mhz, harmonic = 325, freqStep at 325Mhz
= 3.25 Mhz.
- Steps every .2 secs
- In 1 sec it would step 16.25 Mhz
- tx=20 Mhz, step .2 Mhz, harmonic=16.25, freqStep at 325 =
- steps every .2 secs
- in 1 sec it would step 16.25 Mhz
A dynamic spectra was
made for the time around 16:00 (.png)
- All 7 freq bands were combined.
- The data was smoothed and then decimated to 30 Khz (from
- The y axis was smoothed by 4 to fit on the screen. This gave a
time resolution of .2*4 - .8 sec.
Looking at 10 seconds after 16:00 there seems to be something
from 309 to 322 Mhz, but the spacing is less than a Mhz.
To Get a better idea whether the ionosonde causes trouble in the
dome receivers, we should probably dump data fast, and look for
the 140 usec rf pulse repeating itself at the 25 ms ipp.
- The average and peak hold spectra were plotted
- Over 143 frequency birdies had Levels greater than .2
- 9 frequencies had no azimuth dependence. Most of these came
from the cameras in the dome.
- There are two sets of rfi that have:
- birdies with az 126-165 deg and a peak 180 deg below that.
- birdies with peaks in -90 to 10 deg with no birdie at + 180
- The table lists the source of some of the known birdies:
|325,328 MHz strongest
|339.909 little start clock harmonic
|308.7 = 102.9*3 HQ
312.3 = 104.1*3 radio redentor
319.5 = 106.5*3
321.9 = 107.3*3
|ac 80MHz harmonic
- No obvious signal was seen from the ionosonde in the 327 band
- But we probably need to redo the experiment sittting still
with fast sampling.
- Things to do
- the 329.314 Mhz dome camera birdie is 20*Tsys in a 1Khz rbw.
We need to check whether this is coming from a camera, the
multiplexor, or a bad cable.
- Find the 1 Mhz comb. It may not be the the 430tx power meter
since this comb is on top of 1 Mhz to better then 450 Hz. The
430tx pwr meter was offset by about 3 khz (and drifted).
- We need to do a better job with the encoder birdies. Could
we have a damaged encoder cable running from the encoder to
the vertex shelter?