Design and Deployment of a Portable, Pencil-Beam, Pulsed, 3-cm Doppler Radar (2024)

a. Mobile platform

The DOW (Figs. 4–6) is mounted on a modified medium-sized General Motors Chevrolet light duty panel truck (5.4-m length) provided by NSSL. Hydraulic outriggers allow for leveling and stability of the 4900-kg system. The maximum height of the antenna dish when pointed rearward for transport is 3.38 m (11′ 1") above ground level (AGL) permitting travel on most roads and allowing easy loading onto cargo aircraft for rapid deployment to remote locations. When pointing vertically, the feedhorn extends to 3.68 m (12′ 1") AGL, which is lower than most highway overpasses. Deployment costs are very low, mainly consisting of gasoline and operator salaries. The radar has scanned to collect data while mobile in headwinds of 40 m s⁻¹, and it is estimated that the vehicle can sustain winds and continue data collection in winds exceeding 50 m s⁻¹. A gasoline-fueled generator with a capacity of 9 kVA (208 V, three phase) provides power to all systems. The truck and generator proved to be somewhat underpowered. Future mobile radars, now in design, will use larger, more powerful vehicles and generators.

b. Transmitter

The DOW incorporates a surplus transmitter from the NCAR CP-2 radar (Keeler et al. 1989) that operates at 9.375 GHz. This transmitter has been modified to operate at a higher pulse repetition frequency (PRF), 2000 Hz, resulting in a Nyquist interval of 32 m s⁻¹ (velocity folds at ±16 m s⁻¹, ±48 m s⁻¹, ±80 m s⁻¹), which is suitable, although not optimal, for tornado studies. The pulse length has been shortened to slightly less than 0.5 μs, both to reduce the duty cycle and to permit finer-scale range measurements (75 m). Peak power output is 45 kW. Use of this relatively high power transmitter enables the DOW to achieve sensitivities very similar to that of the pre-1994 CP-2 radar. The radar characteristics, similar in several categories to the NCAR CP-2 radar, are summarized in Table 1. The transmitter was of the noncoherent magnetron type, requiring software phase locking, accomplished digitally, in order to produce Doppler velocities. Future mobile systems, under construction, use higher power transmitters and higher frequency and staggered PRFs.

c. Antenna, pedestal, and scanning

A military surplus MP61 antenna with a diameter of 1.83 m (6′) was attached to a SCR-584 pedestal system (similar pedestals are used in the NCAR CP-3, CP-4, and ELDORA test bed radars), resulting in a beamwidth of 1.2°. Azimuth and elevation drive motors were capable of scanning up to 60° s⁻¹ but were limited to 30° s⁻¹ by the antenna controller. Since the first application of this radar was in severe weather, the rugged nature of this military pedestal system was desirable, despite the difficulties caused by its weight—estimated in excess of 1000 kg—and archaic design.

A 1.2° beamwidth provided beamwidths of 64 m at a range of 3 km. This, combined with the 500-ns pulse width described above, resulted in resolution volumes of 64 m × 64 m × 75 m, 310000 m³. This is less than 0.25% of that typically achievable with stationary radars and 4% that of airborne systems at typical ranges to tornadoes, thus providing a 25 to 400 times improvement in volumetric resolution. The narrow beamwidth and the ability to choose sites with good visibility permitted data to be retrieved to within 100 m of the ground in tornadoes.

The antenna controller for the DOW radar was implemented with a 486-class PC and a custom interface designed for utilization of the SCR-584 pedestal (Fig. 7). Antenna position is sensed by synchrotransducers that provide three phase signals to the interface. Two synchro-to-digital converters (SDCs) convert the three phase signals to weighted binary values. The binary values are converted to binary-coded decimal (BCD) using lookup tables for display. Binary position information is provided to the parallel input card in the PC.

The PC’s internal 50-ms clock is used to sample the input antenna angular position values. The antenna angular position, antenna direction, and antenna rotation rate are calculated and sent to the data acquisition computer and the control program. Corrections are calculated with a predictive algorithm, and torque-limiting and antihunt features were implemented. Adjustments are output to the digital-to-analog converters (DAC) on the analog output card in the PC then to the isolation amplifiers in the interface. Analog drive circuits in the interface were designed to provide isolation, bias offset, and current drive for the field windings of the amplidynes. The original SCR-584 pedestal electronics system that was based on vacuum tube technology was not stable enough for this application.

Both PPI sector and RHI-type volume scans were possible providing scan capability comparable to typical research radars. PPI sector scans were configured to allow for volume update times of as little as 50 s. In the future, with the use of a planned additional DOW system, dual-Doppler retrievals at 50-s intervals will be possible.

A novel type of vertical wind profile can be obtained with the DOW. With the antenna pointed vertically, the truck can travel down a level highway and obtain traveling vertical wind profiles (Fig. 8). If the truck travels at 30 m s⁻¹ and transmits with a PRF of 2000 Hz, pulses will be separated by only 15 mm, a small fraction of the beamwidth, maintaining pulse-to-pulse volume coherence. Radar beams can be integrated over many pulses, incorporating more independent samples without loss of resolution, in order to increase sensitivity. Using this method, two-dimensional slices through cold and warm frontal boundaries, gust fronts, microburst outflows, and other phenomena can be retrieved.

It is also possible to conduct pseudo-multiple-Doppler retrievals using an extension of the method employed by research aircraft (Hildebrand and Mueller 1985), by scanning quasi-steady-state phenomena first from one viewpoint, then traveling and scanning from other vantages separated by 30°–150° as shown in Fig. 9. If the range to a phenomena is approximately 3000 m, then the DOW would have to travel roughly 4300 m between perpendicular observations. At 30 m s⁻¹, this would require roughly 140 s, which is comparable to what is achieved with aircraft and suitable for many quasi-steady phenomena. Data may be collected continuously at intermediate locations in order to provide many observations from many vantage points, thus permitting highly overdetermined multiple-Doppler solutions to be calculated.

d. Receiver and data acquisition

The receiver design was typical for a research weather radar (Fig. 10) but used a local oscillator that was voltage controlled by software in the data acquisition system in order to track the varying frequency of the magnetron output. The receiver had approximately 60 dB of gain and a 65-dB dynamic range.

Data acquisition was accomplished on a PC-based card, the PIRAQ (PC Integrated Acquisition), developed at NCAR. The PIRAQ is a long format ISA bus card and is compatible with standard desktop PCs. This card, along with its PC host, compose a complete, inexpensive, high-performance radar data acquisition system. The system takes as input the radar IF signal and provides as output a multiparameter polar display (Doppler velocity, reflectivity, spectral width, normalized coherent power, or any calculated quantity), EXABYTE or CD-R tape data recording, magnetron AFC, and system timing.

The PIRAQ card [Fig. 11 (schematic)] provides several of the subsystems required for radar signal processing and control. A digital IF scheme (Randall 1991) is used to provide the I and Q baseband conversion. In this scheme, the 60-MHz IF input signal is downconverted to a second IF of 4 MHz that is sampled at 16 MHz with a resolution of 12 bits. These samples then pass through a pair of accumulators that implement two programmable decimating FIR filters, one each for I and Q matched to the typical rectangular radar pulse.

The digital IF scheme provides several advantages over its analog counterpart. Because it derives I and Q outputs digitally, it inherently provides phase and amplitude balance that are limited by the system noise floor. As a result, image rejection is typically greater than 60 dB. Another advantage is that DC drift and other low-frequency interference are essentially eliminated because of the characteristics of the digitally matched filter. In addition, the digital filter acts to reduce the quantization noise and thereby increases the system dynamic range. For the case where the filter is programmed to match a 1-μs pulse, this can result in a 10-dB increase to 70 dB of dynamic range.

The PIRAQ card contains a flexible timing generator. The timer sets the number of range gates to be sampled and the range gate spacing. There are three possible trigger modes: internal trigger, external trigger, and external sync. The internal trigger mode allows the PIRAQ to generate its own PRF, which is then available as an output to control external devices. The external trigger mode slaves the PIRAQ to an external PRF via an external trigger input connector. A variable delay can be programmed in this mode to offset the start of timing with respect to the external trigger. The external sync mode can be used to synchronize the internal triggers to an external event. All of these settings are programmed through I/O ports on the ISA bus. In all of these modes, the timing is referenced to an oscillator that can be phase locked to an external master oscillator. [This is particularly useful in bistatic multiple-Doppler applications; Wurman (1994).] In the DOW system, the external trigger mode is used to lock to a special ISA bus timing generator card that is capable of generating staggered pulse timing. There is a special mode of the timing generator that is used for magnetron radars. In this mode the raw analog/digital (A/D) samples from the first gate are latched into the FIFO (first in, first out) along with the normal I and Q data. When the first gate is aligned in time with the transmit pulse, these raw A/D samples can be used to implement a digital discriminator to be used as the error signal in an AFC loop (Li et al. 1994; Nutten et al. 1979).

Digital I’s and Q’s are fed into a FIFO register on the PIRAQ card. The FIFO’s half-full flag is used to interrupt the onboard TMS320C40 digital signal processor chip operating at 50 MHz. The DSP chip has 128000 words of memory on its local bus and 16000 words of true dual-port memory on its global bus. The other side of the dual-port memory is mapped into the ISA bus memory space of the PC host and used as program and transfer data by the system.

The DOW data acquisition system uses a PIRAQ, a PRF generator, and a digital/analog (D/A) card to perform all the functions necessary to retrieve Doppler from its magnetron transmitter. The PRF generator card creates the base PRF. This card can generate a staggered pulse repetition time (PRT) for a planned upgrade. The output pulse triggers the transmitter and the PIRAQ card. The PIRAQ delay is adjusted so that the first gate samples the transmit pulse so its phase can be determined. The receiver is specifically designed such that when it saturates it does not overload the PIRAQ front end. Also, the 16-MHz raw samples are used to adjust the D/A card voltage output. This voltage steers the receiver X-band local oscillator to maintain an approximately 60-MHz IF center frequency. Each I and Q is normalized with the phase offset as measured from the transmit pulse. This results in a set of pseudo-coherent I’s and Q’s from which Doppler data can be retrieved. The PIRAQ DSP performs an optional clutter filter algorithm, the digital discriminator algorithm, and the standard pulse-pair algorithm. The first and second moments are generated and the results placed in the dual-port memory. In addition, a flag is set in dual-port memory. The PC host recognizes this flag and further processes the moments to arrive at the final products: velocity, power, spectral width, normalized coherent power, reflectivity, and coherent reflectivity. The PC uses a lookup-table-based display algorithm to efficiently display each parameter in standard PPI or RHI format. The PC records the pulse pair moments to either disk, CD-R or EXABYTE tape, in real time. During the 1995 deployment, the system was capable of processing approximately 33 beams per second with 285 gates per beam (approximately one-half of this when using a clutter filter). Data acquisition was limited to roughly 20 beams per second by the rate at which the PC could display and archive data. The incorporation of PCs using Pentium 133-MHz chips has eliminated this constraint. Sampling rates can be adjusted in the range of 0.25–10 μs in 0.25-μs steps. The complete time series of I’s and Q’s was collected at one adjustable range. PC resident software allows adjustments during operations of most DSP parameters, including clutter filtering, gate length, integration time, and full spectrum sampling range, with almost no loss of data.

Table 1.

DOW radar characteristics.