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1. SART of circular polarization
The SART used in this study has a helical antenna (see Figure-1) which radiates a radio wave in circular polarization. Figure-2 shows the directivity of the helical antenna in the horizontal plane against horizontal and vertical polarization. The SART was used in a series of tank tests and on-sea trials. In addition, an existing SART which antenna polarization is horizontal was used for a comparison purpose.
2. Observation results on visibility of SART signal at tank tests
Measurement of received power of SART signal and observation of visibility of the signal on radar screen were carried out in various water-wave conditions at a test tank of Tokyo University of Merchant Marine. The results are as follows.
.1 A strong interference caused by the signal reflected at the water surface was observed for SART signal of horizontal polarization where the water-wave height is low. In this case, interruption of SART signal was also observed sometimes.
.2 Any strong interference caused by the signal reflected at the water surface was observed for SART signal of circular polarization where the water-wave height is low. In this case, interruption of SART signal was not observed.
.3 There is no significant difference on observation between SART signal of horizontal polarization and circular polarization when water-wave height is rather high.
These results lead us to a conclusion that the circular polarization is superior to the horizontal polarization for SART.
3. Observation results on visibility of SART signal at on-sea trial
On-sea trials were conducted on December 14th 2000 in co-operation of ships and aircrafts of Maritime Authorities of Japan in Sagami Bay to observe visibility of the SART signal on the radars on board the ships and the aircrafts. In the meantime, the received power of the SART signal was measured by a land-based radar. The following results have been obtained.
3.1 Observation by aircraft radar
The SART signal of circular polarization was observed stably on PPI of the aircraft radars. The maximum visible distance of SART signal of circular polarization was 37 NM, while that of horizontal polarization was 30 NM. This confirms the superiority of circular polarization.
3.2 Observation by marine radar
Table-1 shows the results of the observation of the SART signals on marine radars equipped on vessels. The antenna height of both SARTs was 1 m from the sea level. The maximum visible distance of SART signal of circular polarization was 14 NM, while that of horizontal polarization was 11.5 NM. These results confirm the superiority of circular polarization.
| Table-1 Comparison of visibility
of SART signals of circular polarization and horizontal polarization |
| Distance(NM) |
Results of observation of SART signal
of horizontal polarization |
Results of observation of SART signal
of circular polarization |
| 14.0 |
Could not observed |
Observed with steady gaze |
| 12.8 |
Could not observed |
Observed but weak |
| 12.1 |
Could not observed |
Observed almost clearly |
| 11.5 |
Faintly observed |
Observed clearly |
| 10.5 |
Slightly observed |
Observed clearly |
| 9.4 |
Slightly observed |
Observed clearly |
| 9.1 |
Observed, but sometimes slightly |
Observed clearly |
| 8.6 |
Observed clearly |
Observed clearly |
| 7.5 |
Observed clearly |
Observed clearly |
| 5.0 |
Observed clearly (Escorting vessel could
be seen.) |
Observed clearly (Escorting vessel could
be seen.) |
| 1.8 |
SART signal on PPI started becoming circular. |
Observed clearly (Escorting vessel could
be seen.) |
| 1.5 |
SART signal on PPI was totally circular. |
SART signal on PPI started becoming circular. |
| 0.9 |
SART signal on PPI was totally circular. |
SART signal on PPI was totally circular. |
| 0.4 |
SART signal on PPI was totally circular. |
SART signal on PPI was totally circular. |
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3.3 Observation and measurement by land-based radar
A 9GHz radar of research purpose owned by Tokyo University of Merchant Marine installed on a vehicle was used to observe and measure the SART signal of circular polarization. The radar was set at a shore. The SART was moved on sea by an escorting small vessel. The antenna height of the SART was kept 1m from the sea level. The distance between the radar and the SART was changed. The received power of the SART was measured by the radar.
Figure-3 shows the calculation of received power of SART signal of horizontal polarization and measured results of received power of SART signal of circular polarization. The calculation was based on the assumption that reflectivity of SART signal at sea surface is 1, and phase change at the reflection is pi radian.
The measured value of received power of SART signal of circular polarization was, in any case, stronger than the calculated value of SART signal of horizontal polarization (see Figure-3). The black circle marks are the measured value of SART signal of circular polarization. The measured values always appear above the calculated curve for "SART to radar".
Reason of this measurement results can be considered as follows. 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, even though there is a fundamental loss (by 3 dB) shall happen when radar antenna of horizontal polarization receives SART signals of circular polarization.
Appearance of SART signal of circular polarization on the radar PPI was stronger and clearer than that of horizontal polarization.
| （拡大画面：89KB） |
Figure 3.
The radar's received power of the SART signal. (Curves show the calculated values in case
of the horizontal polarization antenna SART
according to the radar transponder equation,
and dots show the observed values in case of
the circular polarization antenna SART.)
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4. Consideration
4.1 Consideration on comparison between circular and horizontal polarization
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 SARTs). 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 well.
.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.
.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.
4.2 Consideration of possible amendment to the SART standard
In order to allow the use of circular polarization antenna for SART, the following amendment to the third sentence of paragraph 2.5 of Annex to the Assembly resolution A.802(19) has been derived. This amendment has no effect on 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."
4.3 Merit of SART of circular polarization
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
.4 There is no need to change the existing radars and the existing standards for radars.
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