Measuring Instruments
The 94-GHz radar:
The radar used within the FRM4Radar project is a 94-GHz Frequency Modulated Continuous Wave (FMCW) Radar. The FMCW cloud radar is a bistatic system that transmits continuously a linearly frequency modulated chirp around a central frequency of 94-GHz repeated at a variable pulse repetition rate (PRF), creating a saw tooth frequency pattern. The chirp frequency is used to detect the range of the targets and the I/Q time series and a Doppler spectra estimator is used to retrieve the Doppler moments (radar reflectivity, mean Doppler velocity, spectrum width) and to also record the radar Doppler spectrum. and a second Fourier Transform is used to estimate the I/Q times series at each range gate. The high duty cycle of the system (100%) allows the use of a solid-state transmitter the 94-GHz radar also has relative low transmit power with low peak power, thus, minimizing the overall cost of the system and its operational and maintenance costs.
In the FRM4Radar project the 94-GHz radars point vertically and additionally equipped with a passive radiometric channel at 89-GHz that is sensible to cloud liquid water. Thus, the liquid water path (LWP) can be retrieved using the 89-GHz and by using the same antenna for the passive and active received signal a perfect beam overlap is accomplished.
Due to this additional feature, the CloudNet (see also: http://devcloudnet.fmi.fi/) algorithm can be operated in synergy with a ceilometer, only (usual instrumental synergy; cloud radar, microwave radiometer, and ceilometer). The CloudNet algorithm is a cloud target categorization algorithm and be used to validate the EarthCARE radar products against.
More information can be found here.
Parameter : Specification – Remark
Frequency: | 94 GHz ± 100 MHz – corresponds to wavelength of 3.19 mm; |
IF Range: | 350 kHz to 3 MHz |
Transmitter Power: | ~ 2 W – solid state amplifier |
Antenna gain: | 51.6 dB |
Beam Width: | 0.48° FWHM |
Polarisation: | V |
System Noise Figure: | 3 dB |
Dynamic Range: | -54 dBZe to +20 DBZe at 1km height, |
Ranging: | 50 m to 18 km |
Calibration: | Transmitter power monitoring, Receiver Dicke switch (for radar and DD channels), |
A/D Sampling Rate: | 8.2 MHz |
Profile Sampling Rate: | 0.2 s to 30 s |
Vertical Resolution: | 1 m to 100 m – (user selectable) |
Doppler Resolution: | ± 1.5 cm/s |
Doppler Range: | ± 18 m/s max |
Chirp Variations: | 4 typical – 10 possible, re-programmable |
Passive-Channels: | 89 GHz – for integral liquid water (LWP) detection (2 GHz BW) |
Data Products: | Reflectivity, |
Data Formats | proprietary binary, netCDF (conformity with CF convention), ASCII (only moment profiles) |
Rain / Snow Mitigation System: | Super blower for rec. / transm. radome (2000 m³/h each) – Optional heater modules (2 kW to 4 kW) |
Weight: | Radar main body: 100 kg, Table: 130 kg, Air conductors: 25 kg |
Power Consumption: | 220 V AC, 50-60 Hz, Radar: 400 W, Blowers: 1000 W |
Manufacturer: |
Reference:
- N. Küchler et al, 2017 – A W-Band Radar–Radiometer System for Accurate and Continuous Monitoring of Clouds and Precipitation; Journal of Atmospheric and Oceanic Technology, doi: https://doi.org/10.1175/JTECH-D-17-0019.1
Ceilometer:
A ceilometer measures the cloud base height, i.e. the ceiling in aviation. It is based on the light detecting and ranging (LIDAR) principle, i.e. it sends laser pulses and measures the backscattered light. From travel time the distance of the back-scattering object can be determined. From strength and attenuation of the backscattered signal the backscatter coefficient is determined. The instrument identifies cloud bases by the strong increase of the backscatter coefficient. Some ceilometer instruments also detect aerosol layer heights and vertical visibility. Usual operating range is up to 15 kilometers.