TDC: Wind Profiler Database: Profiler Parameters Pertinent to the Recorded Doppler Velocity Spectrum
Even though a profiler is technically a Doppler radar, the collection and processing of profiler observations is fundamentally different than high power scanning weather radars. A profiler transmits thousands of pulses of energy over an approximate 30 second dwell time to retrieve one spectrum of Doppler velocity. Compared to the few pulses used in scanning radar techniques, the dwell time of the profiler observations is substantial. This section covers some fundamental signal processing techniques inherent in profilers that the user of the profiler data should understand.
Transmitted Pulse Width or Radar Resolution Volume Pulse Length
The profiler transmits a pulse of energy of a finite pulse length. The duration of this pulse determines the pulse length of the radar resolution volume. For TOGA COARE, the pulse length was either 700 or 3300 nanoseconds, which corresponds to 105 and 495 meter pulse lengths, respectively. The different pulse lengths are used to identify the different sensitivities used during TOGA COARE. These two modes are also referred to as the 100 and 500 meter modes.
Receiver Sampling Time or Range Gate Spacing
After the profiler transmits the pulse of energy, the transmit circuitry is disabled and the receive circuitry is enabled. The profiler then samples the received signals (the real and quadrature channels) at a uniform rate. The time between samples determines the distance between range gates. The distance between range gates was 105 meters for the 100 meter mode, and 255 meters for the 500 meter mode.
It should be noted that the 500 meter mode has a radar pulse length of 495 meters and range gate spacing of 255 meters. This means that the atmosphere is over sampled and adjacent observations are not independent. In contrast, the 100 meter mode has independent observations due to the 105 meter pulse length and 105 meter range gate spacing.
The altitude recorded in the TOGA COARE data sets is the height above mean sea level (MSL) at the middle of the range gate.
Number of Coherent Integrations (NCI)
Coherent integration is the process of averaging consecutive samples and using this average in subsequent processing. Coherent integration is essentially a digital filtering process used by profilers. Coherent integration does not increase the signal-to-noise per unit bandwidth in the signal band, but it simply filters out much of the wideband noise. This digital filter is called the Time Domain Averaging filter (TDA filter). One side effect of using coherent integration is the decreased power return at frequencies different than zero Doppler shift. This decreased power follows the sinc function with a transfer function with unity at zero Doppler velocity and the first null located at +/- 2 VNyquist velocities. This TDA filter will be discussed in more detail when the equivalent reflectivity factor is determined from the Doppler Spectra.
Number of Spectral Points (NPTS)
After NCI consecutive samples are averaged together, this one averaged datum constitutes one value in a time series of observations. The next NCI consecutive samples are averaged. This process is repeated until there are NPTS values in the time series. These NPTS values are converted from the time domain into the frequency domain using a Fast Fourier Transform (FFT). The number of points in the time domain determines the number of points in the frequency domain.
Number Fast Fourier Transform (NFFT)
In order to improve the signal-to-noise ratio, several FFT's are averaged together to form the average Doppler Spectrum. The noise variance decreases with increased number of spectral averages. This Doppler spectrum is recorded and saved for future analysis. The Number of FFT's averaged together is labeled NFFT. In the process of determining this one Doppler Spectrum, there are a total of NCI*NPTS*NFFT transmitted pulses.
Inter-Pulse Period or Unambiguous Range
The Inter-Pulse Period (IPP) is the time between transmitted pulses. This time can be converted into distance and represents the unambiguous range. The profiler determines the scattering characteristics at the true range expressed with: range + k*(Unambiguous range), where k is an integer. The inter-pulse periods were set such that the resolved scattering was from ranges less than the unambiguous range.