The ELDORA (for ELectra DOppler RAdar) radar is an airborne, dual beam, meteorological research radar that was developed jointly at the National Center for Atmospheric Research (NCAR) in the USA and the Centre de Recherches en Physique de L'Environnement Terrestre et Planetaire (CRPE) in France. This radar is called the Analyse Stereoscopic par Radar Aeroporte sur Electra (ASTRAIA) by CRPE. The first field project for this radar was in the Solomon Islands during January and February of 1993.
The radar mounts on a Lockheed P-3 aircraft, operated by the Naval Research Lab (NRL). Its two antennas extend back from the tail of the aircraft and spin about the longitudinal axis of the aircraft. One antenna points slightly ahead of the aircraft and one slightly aft. As the aircraft translates the antennas through space the radar traces two conical helixes through the atmosphere, essentially observing all of the atmosphere with two separate looks within 50-100 kilometers of the aircraft at resolutions finer than 400 meters.
These two separate looks yield two wind vectors at each location in the atmosphere. Post acquisition analysis software is then used to combine these two wind vectors and, using a set of assumed initial conditions and applying the conservation of momentum and mass, produce a three dimensional "picture" of the atmosphere. This picture can then be sliced through any axis to produce two dimensional plots of the atmosphere.
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.
| 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 |
The ELDORA radar system consists of five major functional blocks: the RF signal generator/receiver unit, the high power amplifiers, the signal processor, the antenna/rotodome system, and the radar control equipment. Since the radar system consists of two separate fore- and aft-pointing radars much of the hardware contains two identical modules. Only the basic signal generation equipment and the radar control equipment does not contain duplicate modules.
The entire system is synchronized to a high-precision 10 MHz crystal oscillator which serves the clock and stable reference for the radar. This frequency is used as the basis for synthesis of all frequencies needed by the rest of the system. The 60 MHz intermediate frequency (IF) is generated by the RF signal generation hardware and is used by the receivers, the digital IF signal processors and the master timing module. The timing of the transmitted waveform and the spacing of the range gates are synchronized to the 60 MHz signal.
The master timing module, located within the digital signal processing equipment, generates the 7 optical pulse trains that initiate the major transmit activities of the radar. One of these trains consists of a pulse that occurs each pulse repetition time (PRT) and enables the high power amplifier to transmit if it is fed an RF input. This pulse is often referred to as the video pulse or the pulse repetition frequency (PRF) pulse. Five other pulse trains occur synchronously with the video pulse to turn on the five chips to be transmitted - each chip being a slightly different transmit frequency. The last pulse train turns on a test pulse that is injected into the receiver for calibration purposes.
The RF signal generator continuously generates all four RF frequencies for both the fore and aft radars. For each radar the five frequencies are fed to a five-input, single-output switch that is controlled by the five chip pulse trains generated by the master timing module. The output of the switch is amplified and then fed to the high-power amplifiers in order to generate the transmitted signal.
The received signal consists of the total reflected energy from all transmitted frequencies. The received signals are amplified by a single wide-band, RF low-noise amplifier (LNA), and then fed to a splitter which distributes the received signals to each channels mixer. There, the received signals are mixed with the appropriate frequencies to create separate IF signals, one for each transmitted frequency. Separate digital IF signal processors then digitize and mix the IF for each frequency down to baseband. Finally, the data from the different frequencies are processed in separate auto-covariance processors, and the results combined to provide the desired output measurements.
The ELDORA output variables include radar reflectivity, radial mean velocity, spectral width, and normalized coherent power. These data products are passed to the data recording and display system over a high-speed parallel link. The fore and aft digital signal processors both sit on this high-speed link, thus enabling the data system to receive all data from both the fore and the aft radars, to tag it with aircraft position information and to record and display the data.