DESIGN AND OPERATIONAL ASPECTS OF A
REDUCED-APERTURE 449 MHZ WIND PROFILING RADAR
Herb Winston John Neuschaefer Vaisala Incorporated Boulder, Colorado, 80301, USA
Scott McLaughlin Daniel Wolfe
National Oceanic and Atmospheric Administration Environmental Technology Laboratory
Boulder, Colorado, 80305, USA
Introduction
The United States Air Force operates a network of tethered “lighter-than-air” aircraft (aerostats) along the southern U.S. border. Having volumes of up to 625,000 cubic feet and supporting flight altitudes up to 15,000 feet, each Tethered Aerostat Radar System (TARS) carries aloft a surveillance radar that is used to observe aircraft entering U.S.
airspace (Figure 1). Like any aircraft, aerostats can only operate safely within certain operational constraints. Wind is often the most important factor determining when it is unsafe to fly.
Over ten years ago the National Oceanic and Atmospheric Administration’s (NOAA) Environmental Technology Laboratory (ETL) began working with the TARS operators to study whether the use of a radar wind profiler (RWP) could improve both total time aloft and overall safety to the Aerostat and ground crew. A 404 MHz RWP (Figure 2) using a Yagi antenna array was temporarily installed at a TARS site in southern Arizona (Moran et. al., 1989). The RWP collected wind profiles every half hour and sent them directly to the TARS site where they were used to assess atmospheric flight conditions. The project was considered a success.
Funding to install a permanent RWP became available in late 1999. The Air Force asked NOAA ETL to act as system integrator; to design, specify, purchase, and assemble a complete RWP with advanced signal processing and remote
data display and control capability. Figure 1. Aerostat system deployed by the US Air Force
System Description
Several options were evaluated to determine the best overall hardware, software, and location for a RWP demonstration program. For cost considerations, the Air Force wanted to use as many commercial-off-the-shelf (COTS) parts as possible. The requirement for high data quality and a 449 MHz frequency meant that new parts and software would be required. There was also interest in placing the RWP as close to the aerostat as possible. This led NOAA to a “hybrid” RWP design consisting of commercially available parts augmented with NOAA-designed monitoring hardware and ETL’s advanced Signal Processing Software (SPS). It was also determined that the best location for the RWP demonstration was approximately 3 km from the TARS site at Ft. Huachuca, Arizona, USA.
Unlike the 404 MHz tropospheric wind profilers operated in NOAA’s Wind Profiler Network and similar international instruments, the 8 km altitude requirement established by the Air Force allowed for a smaller antenna array aperture and lower transmit power. This “reduced- aperture” system operates at 449 MHz and utilizes two orthogonal arrays of twelve, 18-element coaxial-collinear (Co-Co) antennas. A 2 kW solid-state amplifier drives a 36 square meter antenna array which has a row-to-row phasing of 60 degrees implemented using delay cables and RF switches to change
Figure 2. NOAA ETL 404 MHz RWP with Yagi antenna array installed for testing in support of TARS in 1989
array axis and phasing. An air-conditioned 2.4 by 3 meter shelter is located directly adjacent to the antenna, which houses the computers, receiver, amplifier and the beam steering unit. COTS components integrated into the system are from the Vaisala Inc. LAP® wind profiler product line. These include a Radar Processor Unit consisting of a PC with control and signal processing boards, Receiver/Modulator Unit, Interface Unit, Co-Co antennas, and a Beam Steering Unit. The radar processor runs Vaisala LAP®-XM control and signal processing software. Most of the items were selected because they were commercially available and are compatible with NOAA’s SPS software. The deployed antenna and system electronics are shown in Figures 3 and 4, respectively.
Figure 3. 449 MHz Reduced-Aperture RWP antenna, TARS site in Ft. Huachuca, AZ, USA.
A specialized Hardware Monitor system was developed by NOAA ETL to support the maintenance and operation of the RWP. The monitoring is mostly performed by the use of an A/D board located in the radar processor PC. One unique item developed to support the radar was a 6-way power divider that utilizes an RF divide/combine technique allowing the use of individual RF power reflect loads. Each load's individual temperature is directly monitored by the Hardware Monitor. This monitoring technique allows noninvasive detection of failed antenna components.
NOAA Advanced Signal Processing
Figure 4. 449 MHz RWP system electronics, Ft. Huachuca, AZ, USA.
The NOAA/ETL Signal Processing Software (SPS) is responsible for generating meteorological products from RWP averaged-Doppler spectra. It differs substantially from the traditional signal processing system, in which signal processing is linear and sequential throughout and where one signal per Doppler spectrum is detected and reported. Fundamental to the NOAA signal processing is a recognition that, even with attempts to suppress possible contamination from ground clutter, RFI, spurious signals, noise, etc., RWP averaged-Doppler spectra may contain multiple spectral peaks. The SPS signal processing removes the constraint of being restricted to a single data channel while addressing the possibility of multiple signals in a single spectrum. Multiple data channels (at different ranges, at different times, and on different antenna beams) are analyzed from the spectra level up to the meteorological parameter calculations to determine which signals are wind- induced, even in the presence of different kinds of contamination.
The SPS software runs on a separate PC from the radar processor and is collocated at the radar site. The SPS consists of four individual signal processing modules, each processing a different part of the radar data stream beginning with the averaged spectra level and ending with meteorological data products. All of the data for each level is managed by complementary modules providing specialized high-speed database. An additional routine handles spectral data ingest into the database from the radar processor computer (Wolfe, et. al., 2001).
The SPS uses time-height continuity, opposing beam continuity, and other parameters to aid in objectively determining which peak (if there is more than one) is most likely to have originated from clear-air radar back scatter. After the winds have been calculated, additional quality control software is run to check the resulting wind profiles again for time height continuity. A “confidence” parameter is calculated and carried along with the data at each level. This parameter can be set to limit the presentation of the data at differing confidence levels. The wind data is sent directly to the TARS site for real-time presentation on a PC. Data is updated every five minutes utilizing a 15-minute sliding window. The data is also ingested by a specialized software suite (provided by a third party) that analyzes and displays stresses placed on the aerostat by the measured wind profiles.
Wind Data
Figure 5. shows an example of data processed using the standard LAP®-XM consensus method (top panel) and NOAA signal processing system (bottom panel). Differences in the data density are due to a fifteen-minute averaging with a five-minute sliding window used in producing the SPS figure and a thirty-minute block consensus averaging for the standard method. Though it would be possible to produce a fifteen-minute consensus average, it would not provide a
fair comparison; A fifteen-minute consensus would limit the amount of data used, while the SPS method provides for a larger quality control window and spatial cross beam checks in quality controlling the data. Agreement between the two processing methods is quite good. Validation (not shown) with balloon and 915 MHz profiler data both located ~7 miles NNW of the TARS site shows general agreement despite the spatial separation, which is compounded by the complexity of the topography including the Huachuca Mountains rising to more than 2000m directly west of the profilers.
Figure 5. Sample data from a Reduced-Aperture 449 MHz Radar Wind Profiler collected from 0600 May 20, 2001 to 0600 hours, May 21, 2001, Ft. Huachuca, AZ, USA. Top panel shows thirty-minute consensus averages produced from LAP®-XM software. Lower panel shows fifteen-minute time averages with a five-minute update cycle produced from NOAA/ETL’s Signal Processing System (SPS).
Future Plans
