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PRIMUS
r
880
Digital
W
eather
Radar
System
A28-1146-102-00
Radar Facts
5-42
Turbulence Detection Theory
The PRIMUS
â
880 Digital Weather Radar uses a turbulence detection
technique called Pulse Pair Processing (PPP). The PPP technique
used in the new PRIMUS
â
880 Digital Weather Radar is adapted from
the proven technique used in the earlier PRIMUS
â
Weather Radars.
In theturbulence detection mode ofoperation, the PRIMUS
â
880Digital
Weather Radar transmits about 1400 pulses per second with a power
of 10 kW. The pulse pair processor compares the returns from
successive pulses to determine the presence of turbulence (i.e.,
the return from pulse one is compared to the return from pulse two,
pulse two’s return is compared to pulse three’s, and so on). Since the
processor is comparing the returns from two subsequent pulses (a
pair), it was given the name pulse pair processor.
To perform the comparison, the radar first divides the selected range
into 128 equal parts with each part called a range bin. The radar
compares the return data in each range bin for the first pulse with the
return data in each range bin for the second pulse. For example, the
data returned from pulse one in range bin 34 is compared to the data
returned from pulse two in range bin 34. This process continues
throughout the entire area covered by the radar (all range bins) and a
turbulence decision is made for each range bin. When turbulence is
detected in any bin, the color of that bin is made white.
The return data being compared is the total return vector (TRV). TRV
is the vector sum of the return from each raindrop contained within the
range bin. In other words, the first pulse TRV of range bin 34 is
compared to the TRV for pulse twoin range bin 34. A total return vector
is shown in figure 5-30.
In the simplified example of figure 5-30, the range bin contains
five raindrops of equal size that are at slightly different ranges. The
amplitudes ofthe returns from theraindrops (vector length)are identical
because all the drops are equal in size, but the phase (vector rotation)
of the individual returns varies because of the variation in the range of
the raindrops. The radar does not see the individual returns, rather it
sees the total return vector which is the vector sum of the returns from
all the individual raindrops. In reality, the range bin could contain
thousands and thousands of raindrops which means thata longer chain
of vectors are summed, but the result is still one total return vector.