Nautical Studies

about Marine Navigation & Technology

Clean Sweep or Sweep Clean! (掃清功能)

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What are the differences between these radar pictures?

On the left hand side is a screen capture of radar with a function called “Clean Sweep” switched on. On the right is another screen captured soon after the left hand one being taken. The function “Clean Sweep” was switched off when the second screen was captured. There was no other settings has been changed.

These photos were taken when the vessel passing the Western Fairway in Hong Kong. Towards the north and north-east of the radar screen are Tsing Yi Island and the Kowloon Peninsular. Hong Kong Island is laid on the south-east part of the radar picture. Another Island named Lamma is laid right ahead of the vessel that these pictures were taken. There are two buoys close to the Lamma Island marking a shallow area called Lamma Patch. There is another buoy marking the centre line a Traffic Separate Scheme leading to the Western Fairway. These buoys were marked with symbols on the radar by its mapping system.

As shown on the radar image on the left, there were very little sea clutters with the function “CLEAN SWP” switched on. The number of returned echoes was also fewer. The buoys marking Lamma Patch and mid-channel were not detected. A number of possible anchored ships at the anchorage north of Lamma Island were missing. Actually, targets were missing everywhere. A large portion of the Kowloon Peninsular and Hong Kong Island were also suppressed.

The returned echoes of the unfiltered screen were bigger and clearer as shown by the picture on the right. At least, those islands and landmasses can be properly detected. There are more echoes for the navigator to determine if they are genuine targets such as anchored ships and buoys. Actually, the sea clutters were not sufficient to cause any real problem to navigation even without the filtering function. Is the use of these automatic filtering functions such as the “Clean Sweep” real needed?

尋找到兩個畫面不同的地方嗎?

在雷達上先後捕捉了兩個畫面,在左邊是一個開啟了“Clean Sweep”功能的畫面,在右邊是另一個沒有開啟“Clean Sweep”功能的畫面。除了“Clean Sweep”功能外,並沒有更改其他的設定。

這些畫面是通過在香港的西航道時捕捉的,雷達屏幕上的北部和東北部是青衣島和九龍半島,香港島就在雷達圖像的東南部,南丫島就在船隻的前方遠處。於接近南丫島有兩個標誌著稱淺水區域的浮標,另外還有一個浮標標誌著通往西航道的分隔航道的中線,在雷達上有符號標記著這些浮標的位置。

從左側的雷達圖像中可以看見,在開啟“Clean Sweep”功能的情況下,祇有非常小量的海浪雜波。然而,其他的回波的數量也較少,標記南丫排及分隔航道的浮標亦未能檢出,有可能在南丫島以北錨地停泊的船舶亦失了踪,其他失踪的目標亦隨處可見,大半的九龍半島及香港島也被抑壓而未能顯示。

在右側的圖像中,未經過濾的回波顯得更大和更清晰,至少,那些島嶼和陸地可以正確地檢測得到,還有很多回波可以讓大家確定它們是否真正的目標,例如錨地停泊的船隻和浮標等。其實,畫面上的海浪雜波並不足以造成任何實際問題,即使沒有使用過濾功能亦不會對航行做成影響。有真正的需要使用這些自動過濾功能嗎?

Written by Bruce Chun

June 2, 2010 at 20:21

Posted in Radar Operation

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