Az, za dependence of radar signals in dome

april 2003

The azimuth, zenith dependence of various radars was measured on 27mar03 during the day.

radar	freq	rotation	bandwidth	duty cycle (1 freq) (approximate)	notes
FAA	1350	12 secs	200 Khz	5usec/2500=.2%
remy	1290	12 secs	200 Khz	5usec/2781=.2%
aerostat	1256	11.6 secs	1.6 Mhz	160/3200=5%	only on for 1 azswing
fps117	multi	12 secs	1.6 Mhz	51/1500 = 3.4% 401/2500=16%	freq hopping. The duty cycles are only for the number of pulses until the next hop.

Taking the data:

Azimuth swings were done at .2 degrees/sec between az=270 and az=630. After each swing the zenith angle was stepped by 2.5 degrees. The measurements covered za's of 2, 4.5, 7, 9.5, 12, 14.5, 17, and 19.5 degrees za alternating the azimuth direction. Data was taken with the correlator sampling at 1 second. 25 Mhz. 1024 channel, 9 level spectra were taken centered at :1405,1350,1290, and 1258 Mhz.

Reducing the data:

    All of these are pulsed radars with rotation rates of about 12 seconds. The strength of the radar changes as it rotates around. The 1350 and 1290 radars are at peaks when they point at us. The aerostat and punta salinas radars blank their signal when they point at us (but the time right before/after the blanking is usually the strongest).
    The first plots show the max and average of the spectra for each azimuth swing. This shows the general shape of the pulses.
    The 2nd and 3rd set of plots are made at the resolution of the sampled data (25 Khz) and a second set averaging the radar pulse over its transmitted bandwidth. Sets of 12 seconds (1 rotation) were taken at a time and the peak value (for both bandwidths) divided by the median system temperature for the az swing were plotted versus azimuth angle. This gave an azimuth resolution of 12 secs *.2 deg/sec or 2.4 degrees in azimuth.

A few comments on what exactly has been measured.

We are not able to measure the actual power of the radar pulse with the 1 second integrations of the correlator. The radar data has been way under sampled. The duty cycle of the FAA radar (1350 only) is .2%. That means the integrated pulse is smaller by a factor of 1/.002= 500 than what it really is. The radars are probably also saturating the system when the pulse is on and pointing at us. A very strong pulse may appear as a weak pulse if it has driven the system so far into saturation that you end up getting negative gain.

FAA 1350 Mhz radar plots.

The FAA radar is a dual frequency (1330,1350) Mhz radar with a pulse width of 5 usecs. The two pulses are separated by about 6 usecs. The radar itself is situated east of us.

Fig 1 plots the max (top) and average (bottom) spectra for each azimuth swing. The units are kelvins. 20*n kelvins has been added to the nth spectra for display purposes.
Fig 2-3 plot the peak value in each 12 second section using 25 Khz resolution.
Fig 3-4 plot the peak value in each 12 second section averaging the data over 225 Khz.

The wider band shows a small 3 azimuth variation. The peaks change with za. They also do not line up with similar 3az term with the 1290 remy data. Since the data was taken at the same time, it is probably not something like the sun (unless the peaks are frequency dependent).

REMY 1290Mhz radar plots.

This radar transmits a single frequency 5 usec pulse. It is located west of us at the end of the runway at remy airport (but it is used for monitoring drug smuggling).

Fig 1 plots the max (top) and average (bottom) spectra for each azimuth swing. The units are Kelvins. 20*n kelvins has been added to the nth spectra for display purposes.
Fig 2-3 plot the peak value in each 12 second section using 25 Khz resolution.
Fig 3-4 plot the peak value in each 12 second section averaging the data over 225 Khz.

Aerostat 1256 Mhz radar plots.

This radar is on a tethered balloon about 5000 feet (???) up over lajas (southwest of us). It has a dual and quad frequency mode. It blanks its transmitted power +/- 20 degrees about our direction. When the weather gets bad it has to be lowered to the ground. It was transmitting during the za=2 and part of the za = 4.5 azimuth swings.

Fig 1 plots the max (top) and average (bottom) spectra for each azimuth swing. The units are Kelvins. 20*n kelvins has been added to the nth spectra for display purposes. The aerostat is only on for the first and part of the 2nd azimuth swing. The punta salinas radar at 1256-1257 Mhz was on for the entire time.
Fig 2-3 plot the peak value in each 12 second section using 25 Khz resolution (za=2 only)

za dependence of 1350,1290 radars.

The plot shows the zenith angle dependence of the 1350 and 1290 radar. The data was smoothed in frequency to 225 khz and the averaged over each azimuth swing. The dotted lines show tsys vs za (with no radars). There is a small increase with za and both radars show as jump up at za=19.5.

Discussion:

The 1350 Mhz radar narrow band peak values are larger at high zenith angle for azimuths around 310 degrees. They get up to 10 times the system temperature. This is when the dome is located about 180 degrees from the direction of the radar (so the beam on the sky is in the direction of the radar).
The 1290 Mhz remy radar does not show very much peaked azimuth dependence.
The aerostat radar at za=2 is 7 times Tsys. This is twice as strong as the 1350 and 1290 radar. It's to be expected since the aerostat is looking down on the dish. We need to take some data at higher za when it is transmitting.
Both the 1350 and 1290 radar average levels go from 4*Tsys to 5.5 * tsys when moving from za=2 to za = 19.5
The smoothed data for both radars show a 3 azimuth term. It's phase changes with zenith angle, and it is not the same phase for the two radars (whose data was taken simultaneously). The difference must be do to the direction of the radar or the frequency difference.

processing: x101/030327/dordr.pro

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