The main activity of Emitel is to provide broadcast infrastructure for terrestrial radio and television, but they also provide GMDSS radio-communication services for Polish maritime administration. Therefore Emitel decided to build a VDES test installation that would allow them to learn the basics of the VDES system and gather some practical skills. The goal was not to verify the theoretical assumptions of the VDES system, but to carry out basic testing of its physical layer in a real marine environment.
After studying the available solutions for implementing VDES, the company acquired a pair of VDES 1000 modules from us at CML Microcircuits.
The Office of Electronic Communications, an administration body responsible for managing frequency resources, allocated them channel 86 (157.325 MHz ship / 161.925 MHz land) of 25 kHz bandwidth for testing purposes.
The VDES 1000 modules were installed in housings equipped with 24 V DC / 100 W power supplies, fans for forced air cooling and additional bandpass filters.
Tests were conducted in sequence in two areas: the Szczecin Bay and the Gulf of Gdansk. Two coast stations installations were prepared in their radio communication sites located close to the test areas:
1. Mysliborz: located at 14E 18’ 08” / 53N 38’ 40” (WGS-84), tower at 5 m ASL, antenna installed at 66 m AGL. An AV 1523-3 antenna by Aerial was used, facing 90° azimuth. The maximum ERP power was about 11 W.
2. Gdansk: located at 18E 36’ 06” / 54N 22’ 00” (WGS-84), tower at 83 m ASL, antenna installed at 68 m AGL. Same type AV 1523-3 antenna installed at 45 ° azimuth. The maximum ERP power was also about 11 W.
On the vessels used during the tests, a Procom GP 160-5/8 type antenna was installed at about 7 m above sea level. The maximum ERP power was about 9 W.
Here are photographs of the vessels showing VDES antenna installation:
Galaktyka used for tests in Szczecin Bay
Sztorm used in the Gulf of Gdansk
Assuming (ITU-R M.2092) receiver sensitivity at -102 dBm (for 25 kHz channel and MSC-5 modulation scheme and coding), we estimated that the required minimum field strength should be at least 18 dBμV/m. Forecasts of field strength distribution from coast stations were prepared according to the 1546 method. Based on these distributions we expected communication would be possible:
- Throughout the Szczecin Bay, except for the southern part of the waterway close to the port of Szczecin
- Throughout the entire Gulf of Gdansk
To verify the coverage the VDES1000 ping function was used which gives regular real time information on the received signal level (RRSI), SNR and mobile station coordinates. The log file is available in KML so the actual results are easily compared to the estimates.
The tests were an opportunity for us to get acquainted with the basics of VDES system and gave the possibility to verify the coverage forecasts in practice. We also gained both practical and theoretical knowledge.
Estimated field strength distribution in Szczecin Bay
Estimated field strength distribution in Gulf of Gdansk
VDES reported sensitivity throughout the bay of Gdansk
VDES reported sensitivity travelling north from Szczecin
VDES reported sensitivity travelling South toward Szczecin