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Intro

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The blanking tests.

• For the blanking intervals, +=North of Ao, -=South of AO.
• The minutes between tests column shows that there was not the same time interval between each test. This is why some of the airplane echoes jumped differently between some adjacent tests.
• nb is narrow band (5 Mhz bandwidth filter). wb is wide band (500 Mhz bandwidth filter).
• The A/D levels on the noise (col 4) remained constant except for the 2 tests (test3 and test6) where we inserted a 10 db bad in the 1382nb signal path before the square law detector.

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Data taking setup:

• The punta salinas radar transmitted on channel 18. This uses the frequency bands:1366.615-1367.765 and 1381.515-1382.765. We chose channel 18 to be away from the other radars.
• During these tests the aerostat radar was off. The FAA radar (1330,1350) and the remy radar (1270,1290) were transmitting.
• The az, gregorian dome  was parked at az=270, za=18 degrees.
• The lband wide receiver was used in linear polarization mode.
• Three separate bands of data were recorded. Each was square law detected with a 20 usec time constant and then sampled at 10 usecs using a 12 bit a/d converter.
• A 5 Mhz band at 1382 Mhz. This measured the power from the 1382 band of the punta salinas radar. For tests 3 and 6 an extra 10 db pad was placed in front of the square law detector so the a/d would not clip.
• A 5 Mhz band at 1400 Mhz. This measured the compression of the system caused by radars.
• A 500 Mhz band centered at 1400 Mhz. This allowed us to identify different radars by their ipps.
• About 200 seconds of data was taken during each test. This is about 16  12 second rotations of the radar.

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Making the plots:

• The start of the southern blanking period was used to register the plot. The data was split into separate rotation periods. It looks like the rotation period is actually 12.019 seconds. This gave 1.2019e6 samples per radar rotation.
• The rotation phase of the radar was computed. I defined 0 phase to be the time when the radar pointed at the observatory. The observatory is at az=257 degrees when measured from the radar itself (with a clockwise from above rotation).

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The 1382 Mhz total power versus rotation angle as we change the blanking:

a peak hold over 100 samples (1 millisecond) was done for each test. This was to decrease the data so it could be plotted. The pulse lengths for the radar are 51.2 and 409 useconds.

The data was plotted as total power versus rotation phase (ao based). Each rotation was offset from the previous for display purposes. The vertical access is linear in power. In most cases the A/D converted saturated during part of the rotation phase.

Each line of the plot is a separate test. At each phase of the radar a peak hold was taken over the N rotations for this test. Some of the things you can see in this plot are:

• The AO blanking is easily seen. The start of the blanking remains constant since most tests started with blanking at -45 Degrees (south) of AO. The right edge of the blanking changes depending on the blanking interval of the test.
• There is a second blanking region centered at 210 degrees from AO zero. This blanking is at the airport. The larger signal about the airport blanking region is coming from radar reflections from planes above the airport getting back to AO.
• There are two large peaks that drift. One to positive phase and one to negative phase. The apparent motion of these peaks between tests varies because there was a different time interval between the start of each test. Both of these birdies saturated the digitizer. The birdies were:
• Starting at phase=105 degrees and moving to higher phase. It disappears in the fifth test. It was present until an azimuth of 180 degrees.
• Starting at phase=55 and moving toward lower phase. It disappears on the 7th test. It was last seen at a phase of 10 degrees.
• Online we thought that these large peaks were back lobes of the radar. After looking at the summary plot it is clear that these are reflections from airplanes. The birdie with increasing phase was moving west to east. It probably disappeared when it landed at either isla grande or the isla verde  airport. The second birdie  was a plane moving east to west. It disappeared due north of the observatory (0 degrees) when we went back to 40 degree blanking. The power in these peaks is much larger than the radar far out sidelobes. See test 3: +30 blanking with 10db pad. These two peaks stick well above any non blanked sidelobes.
• The phase less than 0 (south of AO) shows sidelobes at (-50, - 65, and -80). The negative blanking was never less than -45 degrees.
• The positive phase (north of AO) shows sidelobes at 40, 62, and 80 degrees. Some of the closer in sidelobes are swamped by the reflection from the plane.

This is the standard blanking used by punta salinas. The peaks at 55 degrees and 105 degrees are airplanes.

The peaks at 45 and 125 degrees are the planes. On rotation 14 the signal from the plane at 45 degrees increased. The angle must have changed to give some spectral reflection.

This is the same blanking as test2. We added a 10 db pad before the square law detector. The peaks at 42 and 130 are airplanes. The 15th rotation shows a large increase in the power from the airplane at 130 degrees. It must have turned so that we were getting a spectral reflection between the radar and us.

• The airplane at small angle has moved  inside the blanking region so we don't see it. Most of the close in sidelobes must be now coming from the beam rather than the airplane.
• The 2nd airplane is at 165 to 175 degrees. Its strength drops dramatically after the 15th rotation. It goes from very strong to barely noticeable in 12 seconds. The plane must have dropped low enough to not reflection much back in our direction. It looks like it was still flying on rotation 22 as it neared the isla verde airport.

The peak at 10 degree may be the first airplane (or it could be a sidelobe of the radar). The 2nd airplane is gone.

The peak at 10 degrees is still there so it is probably from the beam and not from the airplane. This is the same setup as test 5 with an added 10db pad before the square law detector.

This is 5 degrees less blanking than we normally use. There are no airplane reflections in this data set (other than the airport). The sidelobe at 40 degrees is being cut down pretty well except for rotation 10.

This has the radar completely blanked the half of its rotation period that points at the observatory. The only thing we see are the planes flying close to the san juan airport. The one spike on the second trace around -60 degrees is probably some other rfi.

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The system compression by the radar:

• Three different instances were looked at:
1. Compression caused by decreasing the punta salinas blanking to +10 degrees (test 5)
2. Compression caused by plane reflections at az=45 degrees (test 2 rotation 14).
3. Compression caused by plane reflections at az=130 degrees (test 3 rotation 15).
• The 5 Mhz band about 1400 Mhz was plotted in red. When it went negative, the system compressed.
• The radar band was over plotted in black: either the 5 Mhz band at 1382 (punta salinas) or the 500 Mhz band about 1400 (all radars). This let you line when the compression occurred with the various radars.

• Fig1 : Test 5  punta salinas blanking of +10 deg north. The 9 12 second rotations were reduced to 1 rotation. For the radar bands  a peak hold over the 9 rotations was used. For the red compression band a min hold was used. The data was then reduced to 1 millisecond resolution using the same peak/min holds.
• Top: The black line is the 1382 Mhz band. The peak at az=11 is from the sidelobe of the punta salinas radar. The red line is the 5 Mhz wide 1400 Mhz band (it was scaled up for display). There is no dip in the red line at az=11. You do see a dip at az=0 degrees.
• Center. The 500 Mhz band centered at 1400 Mhz. This contains all of the radars. The red dip at az=0 is being caused by a radar at az=0.
• Bottom. Plotting the spacing between the ipps of the radar at az=0 shows that the ipp is about 2750 useconds. This is the ipp of the remy radar. Looks like the remy radar was pointing at AO the same time that the punta salinas radar was.
• Fig 2: Test 2,3 compression from reflections from planes. These plots are single 12 second rotations at 10 usec resolution. The red lines are the 5 Mhz band at 1400 Mhz. The blank lines are the 1382 Mhz band (punta salinas).
• Top: Test 2 rotation 14, strongest plane reflection at az=45 degrees. The red line does not show any compression during this reflection.
• Center: Test 3 rotation 15, the  strongest reflection from the plane at az=130. There is a small compression at az=131.76
• Bottom: blowup showing the compression (blue dotted line). The width of the pulse causing the compression is about .015 degrees. This is about 500 useconds for a 12 second rotation period. The compressing pulse is one of the wide (410 usec pulses) of the punta salinas radar.
• Fig 3: Test 3 compression by other radars.
• Top: test 3 rotation 15. The black line is the 500 Mhz band centered at 1400 Mhz. You can see 3 spikes in this rotation: az=0 remy radar, az=60  faa radar, az=130 plane reflection from punta salinas radar.
• Center: blowup around remy radar at az=0. You can see the red lines going negative on many of the remy radar ipps.
• Bottom: blowup around faa radar at az=60. Very few of the red lines go negative on the faa radar pulses.

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Summary:

• The radar has a rotation period of 12.019 seconds.
• The punta salinas echoes from planes are many times stronger than the sidelobes of the radar (at least the sidelobes beyond 10 degrees). For test3 rotation 15, the power from the plane at az=130 degrees was 15 db higher than the sidelobe at 60 degrees.
• The strength of the echoes from planes can change a lot (5-10db) in a 12 second rotation period. This is probably do to the  geometry between the radar, plane, and AO changing.
• The sidelobes that we were able to see (no plane reflections present ) occurred at:
• North of AO: 10 degrees, 40 degrees, and 60 degrees.
• South of AO: -50, -65, and -80 degrees.
• Changing the positive blanking interval changes which side lobes we see:
• +45 degrees is the current blanking. The +40 degree sidelobe is not seen.
• +40 degree blanking leaves the +40 degree sidelobe smaller than the +60 degree sidelobe.
• +30 degree blanking. The plane was present during these measurements so we can't tell the relative strengths of the +40 and +60 degree sidelobes.
• +20 degree blanking has  the +40 degree sidelobe larger than the +60 degree sidelobe.
• +10 degree blanking. A large sidelobe around az=12 degrees appears. It is much larger than the 40 or 60 degree sidelobe.
• The strongest signals that we receive from the punta salinas radar are echoes from planes (not the direct path from the radar. This is true down to +10 degree blanking.
• We see no compression from the punta salinas radar from its sidelobes down to +10deg blanking. This statement is true for the gregorian position of az=270, za=18. There are other az,za's of the telescope that would have stronger radar returns.
• We do see small compression from the 410 usec pulse of the punta salinas radar when it reflects off of a plane (test 3 rotation 15, az=130).
• The system was also being compressed by the remy radar. We saw no compression from the faa radar for this az,za.
• It is important to remember that these results will vary as we move the telescope az,za.