POLISH MARITIME DGPS REFERENCE STATIONS COVERAGE AFTER THE IMPLEMENTATION OF NEW FREQUENCY NET PRELIMINARY RESULTS. Cezary Specht Institute of Navigation and Hydrography of Naval University in Gdynia ABSTRACT According to IALA recommendations and ITU-R approval, a new frequency plan for DGPS reference stations in The European Maritime Area was initiated on 19 th of September 2001. Radio broadcasting systems of two Polish DGPS reference stations: Rozewie and Dziwnow has been retuned to new assigned frequencies. In result it caused a necessity to modify their signal coverages based on former measuring campaigns followed after the transmitting antennas were modernized (Rozewie in 1998 and Dziwnow in 2001). The article presents some results of DGPS signal strength simulation and actual measurements, as well as resulted, common DGPS RS coverage of both stations. Presented calculations were done on a base of the specialized software. For signal attenuation determination, a South Baltic Sea vector chart of the conductivity data was used. In result, the new values for Effective Radiated Power and nominal ranges were established. 1. SYSTEM ARCHITECTURE. DGPS reference stations are being used to broadcast differential GPS corrections to marine users. The corrections can increase GPS accuracy up to 2 m. (p=0.95) and allows to fulfill all navigation requirements in coastal navigation and hydrographic surveying. Idea of Polish DGPS system was started by Polish Hydrographic Office in 1993. In december of 1994 technical equipment consisted of the
two DGPS reference stations (MX-9112) with local monitoring and Control station was bought in Magnavox. With installation (in 1995) Naval Academy in Gdynia started investigation project for determining the role of DGPS in Polish Radionavigation Plan. The researches focus on coverage, availability, accuracy and integrity of system. National DGPS system consists of two separated subsystems controlled by control stations located in Gdynia and Szczecin. Architecture of the system has been separated on two parts because Polish coastline is being legislative divided between three independent Maritime Offices. Fig. 1. Architecture of the DGPS Reference Stations in Rozewie and Dziwnów. In the last years Polish DGPS system was modernized. In 1998 the old T type antenna (0.4 ERP) of DGPS reference station Rozewie was replaced by a new more efficiency system and later (in 2001) the same thing was done on the second reference station Dziwnow. 2. NEW FREQUENCY NET. According to IALA recommendations and ITU-R approval, a new frequency plan for DGPS reference stations in The European Maritime Area was initiated on 19 th of September 2001. The new frequency division was steered for better reception DGPS signals during the nights (sky-wave effect) and decrease interference between nearby transmitters. Calculations were done in University of Wales. Poland received 3
frequencies for DGPS signals, two of them for existing stations and the third one for planned station in Jaroslawiec (Tab. 1). Tab. 1. IALA Frequency Plan for the European Maritime Area in the Band 283.5-315 khz, (Revised March/May 2001) lp. Ch. Freq. NAME Typ Range No [khz] Function Lat Long [km] 1 0 283,5 DZIWNOW DGP 54N01 14E44 150 2 44 305,5 HEL MB 54N36 18E49 19 3 23 295,0 JAROSLAWIE C DGP 54N33 16E33 90 4 25 296,0 MIELEC NDB 50N19 21E32 90 5 35 301,0 ROZEWIE DGP 54N50 018E20 150 6 33 300,0 SWINOUJSCIE MB 53N55 14E17 90 7 4 285,5 SZCZECIN NDB 53N34 14E56 10 8 9 288,0 SZYMANY NDB 53N26 20E55 10 Freq = Frequency NDB = Non-Directional Beacon MB - Marine Beacon DGP = Differential GNSS Lat = Latitude Long = Longitude Frequency re-tuning caused necessity measurement verification of the system coverage. 3. COVERAGE OF THE POLISH DGPS REFERENCE STATIONS Coverage of DGPS system can be analyzed as a product of two coverages : GPS system and reference station. Coverage of GPS - which is by definition global but can be alliteratively wieved as a percentage of time over specified time interval that a sufficient number of satellites are above specified mask angle and provide an acceptable position solution geometry at any point on or near the Earth. Coverage of DGPS Reference station - is a 3 dimensional area with respect to reference station position in which the telemetry transmission can be received with specified level of reliability. This level can be defined by minimum usable signal to noise ratio which have to be no lower than 7 db corresponds to BER=10-3. Coverage of DGPS reference station is a function of: Atmospheric noise level Field strength - Groundwave
- Skywave Required signal to noise ratio Atmospheric noise level is generated by lightning and dominates other natural radio noise sources below 20 MHz. It can be reasonably estimated for a given location, season, time of day, bandwidth and frequency by using the data from NTIA and CCIR reports. These rapport s use sequence of 24 world maps which give contours of equal noise level and corresponds to one of 24 season blocks it means: 4 seasons of year and 6 time blocks of the day. Each map has a curve that allows translation of the plotted value of noise power to any frequency in 283.5-325 khz. Fig. 2. Atmospheric noise level in dbu not exceed 95 % of the time in summer between 4 to 8 p.m. on the South Baltic region. The groundwave field strength of the propagated DGPS signals can be calculated using the curves of measured data in CCIR report 717-2 or other methods. (Millington, Eckersley). The field strength depends on: Radiated power, distance between transmitter and receiver, conductivity of the ground. Vertical component of groudwave electric field amplitude as a function of distance can be calculated by:
where : E E 0 electric field strength [ V/m], P Effective Radiated Power [kw], d distance [km], F attenuation coefficient. 0 5 3 10 P F d Millington s method, used in model, operates on discretized path from the transmitter to the receiver (DGPS). Value F for mixed path (different conductivity) was calculated from based on formula: F where: F( d F d F d F d 1, 1) ( 2, 2) ( 2, ) F( 1, d) (, ) F(, d ) 2 1 F( 1,d 1 ) attenuation coefficient for single path with: 1 - conductivity and d 1 - length. For single path, F can be modeled by: 1 2 F 2 0. 3xd 2 x 0. 6x 2 d d and: where: x d distance value, x d d ' ( 1) ( 60 ) ' 2 2 ( ) ( 60 ) d distance between transmitter and receiver in m. ' relative permittivity, - wave length in m. 2 2 Each beacon is characterized by the nominal range which is a distance at which its field strength falls to 34 dbu. Groundwave field strength of the Polish DGPS reference stations was calculated by formulas for a mixed paths. The skywave signal arrives from the sky returns to the Earth and interferes with groundwave to product skywave interference. It depends on the state of the ionospheric D- region, which is specified by local electron density and collision freaquency profiles.
Skywave signals are a function of: time (day, night, season), latitude. Skywave signals wasn t analyzed in this model. 4. TRIALS AND RESULTS. After the antennas modernisation the new coverage of the Polish DGPS reference stations have to be establish. The long-term signal strength measurements were done in the Gdynia city 40 km far from DGPS reference station Rozewie. Measurement receiver Leica 9212 was used for recording and measuring electric field signal strength level during one week campaign (01.09-8.09.2001). The mean signal strength value for this campaign was 51 dbu. Similar campaign were done for the second Reference station Dziwnów. 56 54 52 SS [dbu} 50 48 46 44 1 60 119 178 237 296 355 414 473 532 591 650 709 768 827 time [min] 886 945 1004 1063 1122 1181 1240 1299 1358 1417 11.09 07.09 day of week 07.09 08.09 09.09 10.09 11.09 12.09 12.09 Fig. 3. Signal strength level as a function time. One week measurements for Polish DGPS reference station Rozewie. The results-preliminary coverages of Polish DGPS Reference Station Rozewie and Dziwnow after the implementation new IALA frequency net are presented on fig.: 4 and 5.
Fig. 4. Coverage of DGPS reference station Rozewie. Fig. 5. Coverage of DGPS reference station Dziwnow.
CONCLUSIONS 1. According to IALA regulations two Polish DGPS Reference Station Rozewie and Dziwnow were retune on the new frequencies. 2. In 1998 and 2000 antennas of the station were changed for more efficient systems. In caused significant growth of true ranges. 3. Preliminary results shown that nominal ranges of the Polish DGPS reference stations Rozewie and Dziwnow covers all Polish Exclusive Economic Zone. 4. It is necessary to mention that DGPS RS Dziwnow changed it s boud rate to 200 bps without any legitimate reasons. It only cause decreasing of the true range. BIBLIOGRAPHY Enge P., Levin P., Hansen A., 1993, Coverage of DGPS/Radiobeacons, Navigation, vol. 49, no 4. Enge P., Ruane M., Langlais D., 1988, Coverage of Radio Beacon-Based Differential GPS Network, Navigation vo. 34 no 4. Last D., Poppe D., 1997, Coverage Prediction Model for Radio Beacon Differential Satellite Navigation Systems, Navigation vol. 43 no 4. Mitsiopoulos J., Morris P., 1995, USCG DGPS Navigation Service Skywave and Topographical Analysis, Springfield.