Case Studies
Storm-Watch / Tornado Radar
On average, the United States experiences 100,000 thunderstorms each year, causing about 1,000 tornadoes. The customer is trying to determine what meteorological conditions enable a large rotating thunderstorm (a "supercell") to drop a dangerous tornado-funnel. By determining what conditions must exist for the formation of an especially fierce tornado, researchers hope to develop accurate predictions of when and where such a tornado may touch down, giving the people time to evacuate.
The customer wants to study tornadoes and other severe-storm features with an X-Band polarimetric Doppler radar. This mobile cloud-profiling radar is capable of mapping both the reflectivity and the Doppler velocity-field at a high spatial-resolution of 12 ´ 12 ´ 15 meters at a range of 4 km. The beam-width is 0.18 degrees and the maximum range of the radar is 15 km. The system can resolve radial Doppler velocity with an uncertainty as low as ±79 m/s without ambiguity using polarization-diversity pulse-pairs. The receiver bandwidth can be set to 2, 5, or 10 MHz. Doppler signals give information on cloud motion. Polarimetric capabilities give information on the precipitation type and the rain or snow rates. Typical scan-modes include:
PPI (Plan-Position Indicator, horizontal scanning at near-grazing angles) to image tornadoes.
RHI (Range/ Height Indicator, scanning in elevation) to image the vertical structures of storms.
The data-acquisition requirement is to capture the two returned radar signals, one channel for each orthogonal polarization, with the highest possible vertical resolution. Since the receiver's bandwidth is 2 to 10 MHz, a sampling rate of 10 to 50 MS/s will be sufficient. The radar's Pulse Repeat Frequency (PRF) is 5 kHz. Transmission durations are 0.2 to 2 microseconds with a 95 GHz carrier frequency or a 3 mm wavelength. Transmissions alternate between horizontal and vertical polarizations. The pulses are transmitted once every 200 m s, during which 256 to 1024 samples or "range-gates" on each channel need to be captured for a duration of up to 100 m s.
GAGE'S SOLUTION
Gage's CompuScope 14100 provides the necessary high-resolution, high sampling speed and the fast PCI-bus transfer-speed to meet the requirement.
First we will determine if the application is possible using Single Record Mode. In this mode, each record is captured into the CS14100's on-board acquisition memory and then is rapidly transferred through the PCI bus to PC RAM. As a typical example, acquiring a single record of 1024 samples at 50 MS/s would take about 20 m s. Using the PCI bus-mastered transfer-rates of 100 MB/s, transferring the resulting 1024 * 2 = 2 KB from the card into PC RAM would take about 20 m s per channel. (Note that each 14-bit sample point occupies two Bytes.) Rearming the cards to prepare for the next trigger takes about 30 m s. Therefore, the minimum total required time to handle a pulse is 20 m s + 20 m s ´ 2 channels + 30 m s = 90 m s.
The 90 m s required to handle a captured record is less than half of the 200 m s between signal pulses. It would seem, therefore, that the card is capable of continuously capturing pulses without missing triggers. This is true in a single-tasking Operating System such as MS-DOS, where the CompuScope data capture process has the undivided attention of the CPU. This is not true, however, in multi-tasking MS Windows. Furthermore, Windows is not a Real-Time Operating System (RT O/S). This means that the amount of time during which a given process "sleeps" while Windows services other processes is indeterminate. Consequently, no repetitive capture performance can be guaranteed. However, optimal performance under Windows can be achieved in a PC unburdened by multiple peripherals and competing processes. Gage engineers have determined that within a tight C loop under Windows, PRFs of about half those achievable under DOS can be practically realized under Windows without missed triggers. With a safety factor of more than 2, the customer should be able to continuously capture, usually without missing a trigger.
The customer can easily incorporate a sample program from the CompuScope Software Development Kit (SDK) for C/C++ for Windows into an existing Windows NT application. The sample programs in the SDK are intentionally simplified for easy transfer into any given software application.
The combination of Gage's superior hardware and easy-to-use software products enables the customer to quickly construct a fully-operational system.
PRODUCT RECOMMENDED
CompuScope 14100 - 1M
CompuScope SDK for C/C++ for Windows
Instrument Mainframe IM2020A
This application note is provided "as is" without any warranties of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Gage Applied Technologies further does not warrant the accuracy and completeness of the material contained herein. Gage Applied Technologies may make changes to this material, or to the products described in it, at any time without notice.
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