Author(s):
Scott McLaughlin -
DeTect Inc.
At Applied Technologies, we have designed, tested, and installed a new type of radar wind-profiler system in response to the need for a system capable of on-site operational support for an aerostat system.
An aerostat – a large, tethered airship used for carrying aloft surveillance radars or other electronics – normally flies between 3,500 and 5,000 meters above the ground. Our new wind-profiler system is a modern, pulse Doppler radar system that operates at 449 MHz. It is unique in that the antenna uses a Yagi element array and can point anywhere within a cone above the radar up to 25 degrees off the vertical axis. By continuously and actively steering the antenna beam, we can avoid pointing the main beam at the aerostat, thus preventing any significant illumination of the aerostat payload, greatly reducing interference.
Other important system features include a digital intermediate-frequency (IF) receiver, advanced signal detection (ASD) algorithms, and complete health and status monitoring. With the digital IF, the receiver is greatly simplified and highly reliable. The ASD software utilizes multiple peak-picking and identification routines and time-height continuity analysis to screen out radio-frequency interference (RFI), such as birds and other non-atmospheric backscatter signatures. The ASD routines also allow for shorter averaging times and higher data update rates than traditional processing.
Using LabVIEW software, we were able to complete the application in only 18 months with only two programmers, and with a significant number of intuitive graphics for both diagnostics and normal data acquisition. We can view all facets of subsystem control, communications, and data flow in real time. We can also set up most displays to accumulate data for 24 hours or more and to play back data, so we can easily see any trends in atmospheric data or system monitoring. With long raw time series, we can achieve fast Fourier transform (FFT) lengths of 64k or more (used to help filter interference such as RFI). The many varied features available in LabVIEW – from communications to easy, low-level debugging to comprehensive user interface displays – let us write a very sophisticated control and display program in a relatively short time.
The data system software we developed using LabVIEW is composed of two main programs. The first is the dwell module, which controls the radar hardware and performs initial data processing. The second is the advanced signal detection (ASD) module, which is a new implementation of published techniques developed by National Oceanic and Atmospheric Administration researchers over the last several years. The dwell module acquires the data and sends clipped spectra to the ASD module for every acquisition cycle. The ASD module then performs multipeak picking using time-height continuity analysis with built-in automatic quality control (QC) to reduce outliers and erroneous data due to birds, radio frequency interference, and other factors.
Data quality is extremely important to aerostat operations, as is minimizing the total averaging time (preferably to under 10 minutes) and maximizing the update rate (every two minutes, at least). Using the ASD module, we can achieve short averaging and continuously acquire rapidly updated data on wind and turbulence, alerting operators of possible meteorological threats.
With the unique full-beam steering feature, the radar uses aerostat position information and correspondingly moves the main beam away. The dwell software accomplishes this automatically and slowly, so that the time-height QC algorithms can continue to track the wind spectra signal. Fortunately, the aerostat also moves slowly, so very little data is lost during normal flight operations. In addition to the aerostat avoidance through the use of beam steering, the system also uses a programmable sensitivity time control (STC) that attenuates the receiver’s front end and tracks the aerostat radial distance to further prevent saturation of the wind profiler receiver, particularly when the aerostat is near the ground. The STC also prevents receiver saturation in the first few gates caused by close-in ground clutter.
The radar hardware monitor (RHM) is composed of a simple and reliable microprocessor and various interface boards for sampling the DC power, RF power, and digital status bits. The RHM fully monitors the radar system to detect equipment failures, and points the antenna via commands received from the dwell software. It runs independently of the dwell computer, but provides status data to the dwell software for review by the operators. The data is displayed in real time in the dwell software and on an LCD on the RHM chassis. Values outside of specification are recorded, and the radar can be shut down if a faulty subsystem is detected.
The system was designed for ease of maintenance, with most maintenance connections (and fuses) on the front of the rack-mounted equipment, allowing for quick value checks with a volt meter or oscilloscope without having to remove equipment from the rack or make connections on the back, where the software cannot be seen or controlled. Surge suppression is also built into virtually all external interfaces.
The new software makes extensive use of graphics displays to show the data in its various states, to help the operators determine the validity of the data and the health of the radar system. Using LabVIEW, we were able to design the ATI profiler displays for easy “quick looks” by the aerostat flight directors, and build in several new features to aid the operators, such as histogram-like wind plotting.
Designed to address the needs of an operational aerostat site, these new design features also facilitate more traditional uses, such as research and weather forecasting. These new capabilities do not significantly affect the relative price of the wind-profiler system – the new system features the same data height capturability – yet they enhance the overall reliability, ease of use, ease of maintenance, and data quality.
Author Information:
Scott McLaughlin
DeTect Inc.
1820 Delaware Place, Unit D
Longmont, CO 80501
Tel: 303-848-8090
scott.mclaughlin@detect-inc.com
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