METEK
Micro Rain Radar MRR-2
6
10.2010 Valid for MRR Service Version ≥ 5.2.0.9
3
Measuring Principle
The Micro Rain Radar MRR-2 retrieves quantitative rain rates, drop size dis-
tributions, radar reflectivity, fall velocity of hydro meteors and other rain pa-
rameters 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 antenna there is a frequency devi-
ation 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 backscat-
tered 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 frequencies lines corresponding to the Doppler fre-
quencies of the signal.
The RCPD determines this power spectrum with a high time resolution (25
per second) and sends mean power spectra every 10 s to the connected
control and data acquisition system, where the reflectivity spectrum is calcu-
lated considering the calibration parameters of the RADAR module. Using
known relations between fall velocity, rain drop size and scattering cross sec-
tion 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 orienta-
tion. 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.
