Marián Jurašek, Ján Kaňák, Ľuboslav Okon, Ladislav Méri Slovak Hydrometeorological Institute (SHMU), Jeséniova 17, 833 15 Bratislava, Slovakia

e-mails: firstname.lastname@shmu.sk

Calibration & Monitoring of Slovak Weather Radar Network

Weather Radar Calibration & Monitoring workshop, 18th - 20th October 2017

Introduction of Slovak Weather Radar Network

Future plans • to find appropriate methods for the ZDR calibration and start to use them operationally

• to start to perform earlier checks of the TR limiter degradation. The maintenance manual recommends to perform yearly checks of the limiter attenuation with the first check after 5 years from installation. However many radar operators experience that signs of the limiter degradation start to appear earlier, differently for the H and V channels, resulting in too high ZDR values close to radar. Therefore we plan to start with the first check earlier and include a procedure for checking the transient response of the limiter to an injected RF pulse

• next? … we hope to find a few inspirations for our new calibration and monitoring procedures at this workshop

Basic facts: • network consists of 4 identical C-band dual-pol radars Selex METEOR 735 CDP (Fig. 1): Malý Javorník (replaced in 2015) Kojšovská hoľa (replaced in 2015) Španí laz (new site since 2016) Kubínska hoľa (new site since 2016) • central processing system at headquarters comprises of two identical HPE ProLiant DL360 Gen9 interchangeable servers: one for controlling operation and data processing of Slovak radars, another for processing data from 9 foreign radars (3 from Poland, 4 from Hungary, 2 from Czech Republic) • radar sites are equipped with: UPS ensuring at least 1 hour radar operation, diesel-generator (at Španí laz and Kubínska hoľa with GSM module sending SMS in case of electricity outages or problems), primary and backup broadband internet connection, air-conditioner; new unmanned sites Španí laz and Kubínska hoľa are also equipped with automatic fire suppression system and intrusion alarm system • technical specifications of the radars are listed in the table on the right Current operational settings: • operating frequency: Malý Javorník 5605 MHz, Kojšovská hoľa 5645 MHz, Španí laz 5615 MHz, Kubínska hoľa 5630 MHz • 5 minutes full volume scans • volume scan duration cca 280 s • maximum range 240 km • pulse length 0.83 μs • dual-pol mode • dual PRF mode 4:5, Nyquist velocity min. ± 40.1 m/s • 12 elevations: 0°-0.5°-1°-1.5°-2°-2.7°-3.4°-4.4°-7°-11.4°-18.3°-26.7° • measured moments: 8-bit ZH,V, TH,V, VH,V, WH,V, ZDR, KDP, ρHV, SNRH,V, SQI, CCOR; 16-bit uΦDP, ΦDP • angle synchronization 1° • antenna rotation speed 18°/s až 24°/s • ground clutter filtering using GIP (frequency domain) filter • Interference filter in signal processor enabled in Safe mode • Z also filtered by SQI to remove wi-fi • dual-pol precipitation attenuation correction is applied first on the data followed by filtering of non-meteorological echoes using Nexrad dual-pol pre-classification method • ZH, TH and VH volumes are encoded to ODIM HDF5 format and distributed to ODC What we do today

Transmitter Frequency band C (5430 - 5800 MHz) Peak power 400 kW Power supply ~220V-240V/50Hz; 3 phase;

nominal input power 8 kW Type magnetron Operating conditions Temperature: 0 – 50°C

Humidity: 10 – 90% Pulse length 0.5 – 0.83 – 2.0 – 3.3 μs PRF 200 – 2400 Hz Dual PRF 2:3 – 3:4 – 4:5 Dual polarization simultaneous Waveguide filters Band pass filter, harmonic filter Receiver Type Digital GDRX 5 Number of channels 4 (2 per each polarization) Resolution 16 bit MDS ≤-120 dBm Dynamic range ≥130 dB Noise number ≤2.5 dB Max. number of bins per channel ≥8000

Intermediate frequency 60 MHz Sampling frequency 180 MHz Antenna Type Parabolic with feedhorn Diameter 4.3 m Gain 45 dB (nom.) Beam width <1.0° (-3dB) Polarization Horizontal & vertical Side lobes 28 dB (min.) below main lobe Cross-polarization -32dB Radome Material Quasi-random sandwich fiber

glass panels Diameter 6.5 m One-way loss in dry conditions ≤ 0.3 dB

Colour Biela Max. wind ≥200 km/h Accessories ICAO obstruction light,

dehumidifier

Fig. 2: Rainbow5 visualization and monitoring tools (RainDART and RainVIEW)

Reflectivity cross-comparison study Since start of operation, all our current radars have been showing signs of overestimation of measured reflectivity. This overestimation can be often well observed in our composites including Slovak radar data and data from surrounding countries and it is also indicated by unrealistic (too high) radar precipitation estimates. In order to confirm our observations and estimate magnitude of the potential overestimation, we performed a statistical comparison of corrected horizontal reflectivity from Slovak radars with available data from neighboring radars in Czech Republic, Hungary and Poland. The comparison was performed on volumes in ODIM HDF5 format from a one month period. For each individual bin in the Slovak radar volume the closest bin in the foreign radar volume was found and a scatterplot from the corresponding ZH values was created. The compared bins had to meet the following criteria:

• Only bins of comparable size were compared: maximum allowed difference of cross-section area: 5 % • Maximum allowed 3D distance between compared bins: 200 m • Only bins without any orographic blockage of antenna main lobe (neither partial) were compared

An SHMU radar dataset with or without dual-pol precipitation attenuation correction was used in the comparison based on whether the attenuation correction has been applied also on the foreign data or not. The results confirmed that our radars generally overestimate the reflectivity against all of the foreign radars. The obtained scatterplots (see example in Fig. 6) indicate existence of an offset in dBZ for all Slovak radars ranging from 3 to 9 dB. The linearity of the scatterplots according to their shape seems to be good although slope of the linear fit reached in some cases less than 0.9 and was always less than 1.0 (probably due to outliers). The biggest overestimation exhibits radar Kojsovska hola followed by Maly Javornik, Kubinska hola and Spani laz (this order was confirmed also by a cross-comparison among Slovak radars). Investigation of the reflectivity overestimation is still ongoing in cooperation with Selex.

Regular procedures: • 24/7 monitoring of basic radars status and products generation by internal (SHMU) staff using Rainbow5 software

tools (Fig. 2)

• automatic noise level sampling by means of built-in Zero Check procedure performed every hour (Fig. 3)

• regular technical maintenance performed by internal technicians in accordance with Selex manual including TX and RX calibration every 6 months; the RX calibration comprises of calibration of the receiver channel difference between high and low sensitive channels and the Single point calibration determining the relationship between a power level at reference point (RF frontend input) and the power detected by the signal processor; in addition to prescribed RX calibrations, the linearity and continuity of transfer characteristics of high and low sensitive receiver channels are regularly checked using external signal generator

• check of detailed radars status using Ravis maintenance software and RainLOG tool from Rainbow5 (not all warnings and errors are shown in the RainVIEW tool)

Fig. 3: Example of noise levels in high (red) and low sensitivity (green) receiver channel sampled hourly by the Zero Check procedure

Fig. 4: Antenna patterns obtained using Solar Raster Scan procedure

in Ravis

Offline procedures:

• Sun antenna pointing and RX gain check/calibration by means of built-in Solar Raster Scan procedure in Ravis (see Fig. 4)

• cross-comparison of measured reflectivity values with surrounding foreign radars (see dedicated paragraph on the right side)

• offline detection of interfering RLAN devices using wi-fi router connected to the radar waveguide output; the used router (Mikrotik RB-711 with enabled Super channels) allows to scan with 500 kHz step and thus detect devices operated on non-standard channels; detected devices are reported to the NRA

Fig. 1: Position of radars in Slovak network. Black – original radar sites, blue – new radar sites.

Malý Javorník Kubínska hoľa Španí laz Kojšovská hoľa

Fig. 6: Example of scatterplots of ZH values measured by Slovak and neighboring foreign radars from July 2017

Malý Javorník

Pogányvár

Malý Javorník

Pogányvár

Skalky

Kubínska hoľa

Kubínska hoľa

Skalky

Rzeszów

Kojšovská hoľa

Kojšovská hoľa

Rzeszów

A 0 00:19:3B:01:4F:E5 EMIDO 5GHz-A/N 20MHz 5580 -78 -108 30

RW 4 24:A4:3C:E4:2E:D5 Jelka.Rita-PD 5GHz-A/N 20MHz 5580 -85 -108 23 Jel.Rit-PD.Kra 2.9.31

APR 5 DC:9F:DB:7E:43:ED WIFIMV 5GHz-A/N 20MHz 5585 -89 -114 25 NanoBridge M5 2.9.31

AR 1 24:A4:3C:F4:0D:76 Neded5NTi 5GHz-A/N 20MHz 5600 -78 -116 38 Neded5NTi 2.9.31

ARW 2 00:0C:42:2B:F2:70 SGnet03_5G 5GHz-A/N 20MHz 5620 -86 -117 31 SGnet 03 Ne 2.9.31

APRW 3 DC:9F:DB:30:54:64 VUTOCUKR 5GHz-A/N 20MHz 5620 -84 -117 33 Titanium CKRVR 2.9.31

Fig. 5: RLAN interference in uncorrected reflectivity image from radar Malý Javorník (left) and detected co-channel devices detected by Scanner tool

of the Webfig utility (right)

OPERA Solar hits detection and analysis:

• we use the Solar hits monitoring provided by OPERA (see Fig. 7) that are based on Sun signals detected in operational ODIM HDF5 volumes to check antenna pointing accuracy of our radars; it is not currently possible to use these reports to check the RX gain offset by comparison of the solar flux measured by radars with DRAO values due to missing metadata in the volumes

Fig. 7: Solar hits at radar Španí laz from September 2017 showing azimuth offset of cca 0.7°