Airborne Radar Specifications

The research flight speed of the P-3 aircraft is approximately 130 m/s. At this flight speed the scientific requirement for samples every 300-500 m dictates an antenna rotation rate of approximately 24 RPM. This resolution also dictates at least one integration period (dwell time) be completed every degree of rotation which gives dwell times the order of 8 msec. Since the phenomena to be studied has a time to independence of 3-7 msec., only two or three independent samples can be taken in a dwell time with a simple radar pulse. To meet the velocity accuracy of 1 m/s requires about 10 independent samples. A complex waveform is therefore necessary to produce the required radial velocity measurement accuracy. The waveform chosen was a 5 step, stepped chirp in the transmitted frequency. Physically, a stepped chirp waveform consists of a pulse of RF energy, within which are sub-pulses or "chips" which are coded in some way. In the ELDORA system these chips are distinguished by discrete shifts in transmit frequency. The frequency shifts enable the received signals to be processed individually, thus improving the sampling statistics of the radar measurements. The resulting transmitted waveform thus consists of the composite pulses which are repeated each pulse repetition time. The frequencies of the chips must be selected close enough together such that the unambiguous velocity and the beam squint angle for each frequency are all close to the same value, but far enough apart that the information from each frequency is independent and can be detected in only one receiver channel. The following table summarizes the various engineering characteristics of the ELDORA/ASTRAIA radar.

Table 1.2.1 - ELDORA Characteristics

Characteristic Spec.
Number of Radars 2 (fore and aft)
Wavelength 3.2 cm
Transmit Frequency 9.3 - 9.8 GHz
Beamwidth (circular) 1.8°
Antenna Gain 38.7 dB
Polarization (00 elevation) horizontal
First Sidelobe Power -35 dB
Beam Tilt Angle (fore and aft) ± 15-19°
Antenna Rotation Rate 5-144 °/s
Dwell Time 7-50 ms
Rotational Sampling Rate 0.75-2.00°
Peak Transmitted Power 35-40 kw
Receiver Bandwidth 0.5 - 4.0 MHz
Receiver Temperature (at antenna) < 600°K
Pulse Repetition Frequency 2000-5000 Hz
Minimum Detectable Signal (at 10 km) -12 dBZ
Unambiguous Range 20 - 90 km
Unambiguous Velocity (single PRT) ± 13 - 20 m/s
Unambiguous Velocity (dual PRT) ± 80 - 110 m/s
Number of Frequencies 4
Pulse Chip Length 0.25 - 3.00 µs
Range Averaging 1-4 gates
Total Cell Length 37.5 - 1200 m
Along Track Sweep Spacing 0.3 - 1.0 km