Radar Parameters

The radar is an air surveillance radar, ARSR-3, which has a sweep period of 12 sec. This was calculated by searching for the saturated pulses in a contiguous data samples.

The acquired data, when plotted near the direct pulse arrival time, look as shown in figure 1. The pulse saturation values are -88 and +127. The negative saturation values are not -127 because the receiver RF amplifiers saturated at lower negative voltage.

A typical, unsaturated pulse shape is shown in figure 2. This pulse shape is distorted due to long distance of travel from the radar to the GBT. Also seen in figure 2 are the multiple echoes from nearby features, such as mountain peaks.

Figure 1: Radar pulses just after the radar passed over GBT.

Figure 2: A typical pulse shape. This is the pulse shape as received at the GBT. The intrinsic pulse width is 2 $\mu$ sec.

The radar pulse period, measured from the data to within  $\approx$ $\pm$0.01 $\mu$sec, is 2929.11 $\mu$sec, which corresponds to a frequency of 340.40 Hz. However, the radar pulse pattern is not uniformly spaced. There is an 8-pulse sequence which repeats itself with a period equal to 8 times the above time period. Within each 8-pulse sequence, there are fixed delays of [0.00, 395.59, 0.18, 297.77, 98.96, 197.75, 98.62, 296.41]  $\mu$sec, which are presumably used to resolve ambiguities beyond the radar round-trip pulse travel distance given by the nominal pulse repetition period. This 8-pulse sequence was found by finding the pulse arrival times near the time when the radar beam pointed at the GBT and then plotting the difference between the arrival times and the expected times based upon the nominal pulse frequency. The timing data were fine-tuned by getting a match with the pulses in the next sweep of the radar.