700-SDI-RADAR-300WL-Man Rev 2 11 Apr Mar 2022
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Part# 21371
measurements even in environments with very small SNR so the required surface roughness of
1mm is usually enough for precise measurements.
When selecting the location for the surface velocity sensor additional care must be taken to avoid
reflected power away from the radar (red arrow) to hit moving objects (gray cloud) on the side of the
water channel as this can cause additional inbound reflection to the radar and can significantly
affect measurement accuracy. Installations where pedestrians, cars or other objects are moving in
front of the sensor closer than 75 m should be avoided as it is proven in practice that it can cause
problems.
2.7
SIGNAL STRENGTH
Good signal-to-noise ratio (SNR) is the most important parameter of the radar signal that provides
accurate and stable surface velocity measurements. When more radar energy is reflected back from
the water surface to the radar sensor, the overall signal strength is higher. When less energy is
reflected back, as it is when the water surface roughness is lower, the signal strength is lower. If the
amount of noise present in the signal remains the same, when the surface roughness is lower, SNR
will drop. To improve SNR internally, the radar uses low-noise programmable gain amplifier (PGA). If
the strength of reflected signal is ow, the radar will increase gain level on PGA. If the strength of
reflected signal is higher, gain level will be automatically reduced.
The best indication of good signal strength is the PGA value in the radar status report messages.
This value is automatically changed with the AGC (automatic gain control) algorithm in the radar.
Minimal possible gain is 1 and maximal possible gain is 200. The best measurement results are
obtained when the PGA gain level is between 5 and 100; a PGA gain lower than 5 means that the
reflected signal is very strong and it can oversaturate the receiver, which could result in reduced
accuracy. Gain 200 should be avoided as it is usually an indication of very low reflections from the
water surface.
