Page 8 Micro Rain Radar MRR-2 METEK
2012-02-29 Valid for MRR Service Version 6.0.0.3 / Firmware 6.00
3Measuring Principle
The Micro Rain Radar MRR-2 retrieves quantitative rain rates, drop size
distributions, radar reflectivity, fall velocity of hydro meteors and other rain
parameters simultaneously on vertical profiles up to several kilometers above
the radar.
It operates with electromagnetic radiation at a frequency of 24,230 GHz with
a modulation of 0.5 15 MHz according to the height resolution (e.g. 300 m
10 m). The radiation is transmitted vertically into the atmosphere where a
small portion is scattered back to the antenna from rain drops or other forms
of precipitation.
Due to the falling velocity of the rain drops on the antenna there is a
frequency deviation between the transmitted and the received signal
(Doppler frequency). This frequency is a measure for the falling velocity of
the rain drops. Since drops with different diameters have different falling
velocities the backscattered signal consists of a distribution of different
Doppler frequencies. The spectral analysis of the received signal yields a
power spectrum which is spread over a range of frequency lines
corresponding to the Doppler frequencies of the signal.
The RADAR Control and Processing Device (RCPD) determines this power
spectrum with a high time resolution (10 per second) and sends mean power
spectra every 10 s to the connected MRR-PC where the reflectivity spectrum
is calculated considering the calibration parameters of the RADAR module.
Using known relations between fall velocity, rain drop size and scattering
cross section the drop spectrum (or drop size distribution) is derived. The
integration over the entire drop size distribution, considering further
correction terms, followed by further averaging over 10 to 3600 seconds,
results in rain rate and liquid water content.
The output signal of the RADAR is transmitted continuously (CW mode), a
linearly decreasing saw tooth modulation of the transmit signal (FM mode)
makes it possible to perform profile measurements with selectable range
resolution.
The RADAR antenna is an offset parabolic dish with vertical beam
orientation. This antenna design allows rainwater to drain without building
ponds. In order to avoid disturbances from snow which could cover the
antenna dish, optional antenna heating is offered.
