Appendix A. Basic Radar
Meteorology
A.1 Introduction to Radar Meteorology
A weather radar sends pulses of microwaves and measures how much is reflected by targets
in the atmosphere. Targets are mainly raindrops, hail, or snow.
Microwaves travel at the speed of light. By measuring the time when the reflection comes
back to antenna, we can count how far the target is.
When a microwave pulse meets raindrops, a very small fraction of the pulse is reflected
(scattered), and the most of the waves go on. This happens because typically, the raindrop
size is 1 mm (0.04 in) and they are at the distance of 10 cm (3.94 in) from each other. This
means we can see "rain behind the rain". This also means a radar must be very sensitive.
We can move the antenna up and down (elevation) and round in horizontal plane (azimuth).
Typically, in a measurement task, we take a low elevation and measure in all azimuths, then
increase the elevation a little, and measure in all azimuths again, and so on, so we get data
from all directions at dierent ranges. This pack of data is called a polar volume, "polar"
because it is measured in polar coordinates with the antenna in the center.
The radar alternates between sending and receiving at pulse repetition frequency (PRF).
That is typically between 100 and 1000 times a second. When the antenna is moving in
azimuth direction all the time, each pulse is sent in a slightly dierent direction. In signal
processing, we process the data from same distance from dierent pulses. These are called
samples. Slices in distance which are processed are called bins. In the following image, the
value of each bin is processed from four samples. The distance between samples is
determined by antenna speed and PRF. To get reasonable values, usually 32 ... 128 samples
are processed together.
Figure 100 Range Bin Geometry
IRIS Product and Display User Guide M211319EN-H
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