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| SUB-COMMITTEE ON |
COMSAR 7/11 |
| RADIOCOMMUNICATIONS AND |
2002/10/11 |
| SEARCH AND RESCUE |
Original: ENGLISH |
| 7th session |
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| Agenda item 11 |
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DEVELOPMENTS IN MARITIME RADIOCOMMUNICATION SYSTEMS AND TECHNOLOGY
Revision of the performance standards for Search and Rescue Radar Transponder (SART)
Submitted by Japan
| SUMMARY |
| Executive summary: |
This document presents a proposal of revision of the performance
for Search and Radar (SART) based on the results from a recent
experimental study on SART out in Japan. |
| Action to be taken: |
See paragraph 9 |
| Related documents: |
Assembly resolution A.802(19) |
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Background
1 Three years have passed since the Global Maritime Distress and Safety Systems (GMDSS) was enforced as the entirety. During the period, experiences on application of the GMDSS including Search and Rescue Radar Transponder (SART) have been obtained. These experiences, among others, have shown the necessity of reviewing the performance standards for SART for further improvement of effective search and rescue operation.
2 In such a circumstance, Japan investigated the actual usage of SART at maritime distresses, and conducted, during the year of 1999 and 2000, a study "Research and study on performance improvement about SART". A summary of the report of the study is shown in the annex attached to this paper. As the result of the study, it has been recommended to review the existing performance standards for SART in IMO Assembly resolution A.802(19), and to amend it for inclusion of new technology.
Executive summary of the study
3 During the study, an antenna for transmission of SART signal in circular polarization was developed aiming at reducing the size of SART and it has been found that the SART signal of circular polarization appears clearer on marine radar PPI than SART signal of horizontal polarization, the letter of which is required by A.802 (19), through the following experimental measurements and observations:
.1 measurement of receiving power of the SART signals, and observations of them on PPI(Plan Position Indicator), at radars installed on ships, aircraft and a land-based facility, where the SART was positioned at sea (in on-sea trials); and
.2 measurement of receiving power of the SART signals, and observations of them on PPI(Plan Position Indicator), at a radar where the SART and the radar were positioned at a towing tank facility.
4 It was also found that theoretically expected reduction (by 3 dB) of receiving power from SART signal was not observed in case that the polarization of SART signal was circular, and that the SART signal of circular polarization could be found, in a more stable manner, on radar PPI in a longer distance than SART signal of horizontal polarization could be.
5 The following is the summary of the results of the study:
.1 the SART antennas were kept 1 metre high from the sea level, and distance between the SART and the ship, radar of which was used for observation of the SART signal, was altered. SART signal of circular polarization could be found on PPI of radar in the distance of 14 nautical miles (NM), while SART signal of horizontal polarization could be found on PPI in the distance of 11.5 NM, where the transmitting power of the antennas was the same. This means that the antenna of circular polarization is superior to the antenna of horizontal polarization;
.2 it was found, by the measurement of receiving power of SART signals at the land-based radar at the on-sea trial, that the received power of SART signal of circular polarization was clearly brighter than that of horizontal polarization in the distance of the limit of radio-visibility; and
.3 in the measurement by the land-based radar, PPI visibility of SART signal of circular polarization was clearly stronger than that of horizontal polarization.
Reasons for the superiority of circular polarization of SART signal
6 The reason why the SART signal of circular polarization is superior to that of horizontal polarization should be considered as follows:
.1 according to the equation of transmission of radar beam, received wave is the mixture of direct transmission and reflected transmission at sea surface, in both ways from radar antenna to a SART (irradiation) and from SARTs to radar antenna (reflection). The phase of the wave changes by pi radian (180 degrees) at the sea surface reflection. The difference of distance of wave passes between the direct transmission and the transmission reflected at sea surface causes the divergence of the phase at the mixture of both wave at receiving point (radar antenna and at SART). This divergence results in weakening the power of the mixture which decreases along the distance between the radar and the SART in showing the maximum and the minimum. The phenomenon has been known we11;
.2 it is also known well that the degree of the phase change at sea surface reflection depends on the polarization of wave which goes into the reflection point. The wave from radar, which polarization is horizontal, changes its phase at sea surface reflection by pi radian. Then its return signal from the SART shows clear maximum and minimum of power at the receiver of the radar;
.3 the SART signal of circular polarization changes continuously the polarization plane in 360 degrees. Therefore, the phase change at the sea surface is not constant. This results in the fact that the compensation happened at mixture of direct wave and reflected wave is not constant and the radar can receive the maximum power of the mixture. Then, this fact leads the extension of the maximum visible distance of SART of circular polarization; and
.4 the SART antenna is always rolling at sea. This movement affects the polarization plane of the SART signal. The SART signal of circular polarization would have much probability to be caught by radar antenna which polarization of horizontal than SART signal of horizontal polarization would have.
All the reasons mentioned above show the superiority of circular polarization for SART signal, and these have been proved by the sea trials.
Proposal of amendment to resolution A.802(19)
7 Japan believes that the SART signal of circular polarization will enhance the search and rescue capability by SART. Therefore, Japan proposes to amend the third sentence of paragraph 2.5 of Annex to the Assembly resolution A.802 (19) as follows. This amendment has no effect of existing radars or existing standards of radar.
Proposed amendment:
Insert "and/or any polarization which is received effectively by search radars" after "Horizontal polarization" in the third sentence of paragraph 2.5 of Annex to the Assembly resolution A.802(19).
So, the whole sentence should read:
"Horizontal polarization and/or any polarization which is received effectively by search radars should be used for transmission and reception."
8 This amendment will allow the use of circular polarization which shall contribute the enhance of maritime safety by the following merits:
.1 it will allow a development of a new type of SART which needs smaller power in keeping its capability of radio-communication at sea;
.2 the signal from such SART can be observed stably by 9GHz radar in further distance than existing SART can be;
.3 it will allow more flexible design of SART by not limiting the polarization in horizontal. Such SART may be of lighter weight and smaller and easy to install into liferafts and lifeboats. It will be also possible to develop a smaller SART which can be fitted in lifejackets and float-free VDR; and
.4 there is no need to change the existing radars and the existing standards for radars.
Action requested of the Sub-Committee
9 The Sub-Committee is invited to consider the proposal described in paragraph 7 of this document, which will allow the use of SART signal of circular polarization and will allow people enjoying many substantial merits for enhancement of marine safety as described in paragraph 8 above, and take action as appropriate.
ANNEX
Abstract of the Investigation on performance of Circular Polarization SART from Research and Study on Performance Improvement about SART
This study was conducted by Japan Ship-Machinery Quality Control Association in cooperation with Japan Coast Guard and other related organizations.
Foreword
Visibility of SART signal on radar PPI is greatly affected by, among others, the condition of sea surface. Capability of response of SART to radar is affected by change of directivity of antenna which is floating on sea wave and by interruption of radio transmission caused by the sea wave. The polarization of SART has been defined as horizontal in the performance standard of IMO A.802(19), because the polarization of 9GHz radar is also defined as horizontal. However, SART antenna will be in rolling and pitching motion at sea, and then the polarization plane will nor be always horizontal. This results in the loss of received power at radar antenna and then in reduction of detectable distance and probability of detection of the SART signal. The loss of received power due to the change of polarization plane would be avoided by using circular polarization. But, a fundamental loss (by 3 dB) shall happen when radar antenna of horizontal polarization receives SART signals of circular polarization. This loss may affect the visibility of SART signal of circular polarization on radar screen.
Therefore, a series of trials were conducted in order to investigate the performance of a SART which antenna polarization was circular.
Figure-1 The structure of the helical circular polarization antenna |
Figure-2 Directivity of the SART with circular polarization antenna Azimuth pattern in horizontal plane
Gain in vertical polarization: -1.5 to +0.5 dBi
Gain in horizontal polarization: 0 to +4.0 dBi
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