Nautical Studies

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Different Courses and Speeds of Targets (不同的航向及航速)

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There were two targets being shown on the captured radar screen. AIS information from both targets were displayed graphically as triangular marks with two vectors. A solid vector was used to represent the ship’s heading. Another dotted vector was showing its course made good. The targets were also showing a ninety seconds trail in true motion.

As well, the south-east moving target near the left side of the picture was tracked with ARPA. Tracked information was shown graphically with a circular symbol and another vector.

As shown with the AIS symbol, the heading of the north-west moving target at the right upper corner was changing. Yet, its made good course was still lagging behind. The course made good calculator in the GPS receiver was taking longer time to reflect changes made by a vessel.

The heading and the made good course of the vessel south-east bound were similar. The made good course was appeared to be the same as its heading. However, the ARPA tracked course and speed of the target was still lagging behind. As indicated by the target’s ninety seconds echo trail, the south-east moving target should had it course changed for more than one and a half minute ago. Yet, the ARPA tracking system was still indicating a vector closer to its previous course. It was more than one and a half minute delay.

Due to the limited accuracy of radar detection itself, ARPA system need time to smooth its calculated outputs of tracked targets. Or else, the vector of targets could be rather fluctuated and difficult to be used. The time needed to produce a stable output is not related to processing speed. The time needed is related to the number of scans. Advance in processing power is not going to shorten the time needed. It still needs a larger number of scans for the tracker to work properly and each scan takes two to three seconds.

In summary, ARPA should work well on a slow changing ship with steady targets. ARPA will become less reliable if it is used on ships that make drastic changes in course or speed. ARPA need time to reflect changes of tracked targets. Tracked information will be reliable only if the target has been keeping its course or speed. Apparently, ARPA may not be that useful in area such as port and harbour where ships might keep changing their courses and speeds.

在雷達顯示屏幕上有兩個目標,來自兩個目標 的AIS信息分別以兩個三角形的標誌及矢量線顯示,實線表示船舶的航向,虛線表示船舶的實際移動方向,各目標亦顯示了九十秒的真航跡。

靠近照片左側向東南移動的目標同時亦被ARPA追踪,追踪資料以圓形符號及另一條矢量線顯示。

如圖中的AIS符號所顯示 ,在右上角向西北方移動的目標正在轉向,它的實際移動方向矢量則仍是落後於航向的矢量,GPS接收機正需要更長的時間作計算以反映船隻作出的改變。

向東南方的船隻的航向矢量似乎跟它的實際移動方向矢量是一致的,然而,ARPA追踪所得的航向及航速矢量仍然是落後於AIS的輸出。正如那目標的九十秒航跡所顯示,向東南移動的目標應該在一分半鐘前已改變了的航向它,然而,ARPA跟踪系統仍然顯示早前航向的矢量,超過一分半鐘的延遲。

由於雷達本身精確度的限制,ARPA系統需要好一段時間來產生穩定的目標資料,否則,目標的向量會變得相當波動,難以應用。產生較穩定輸出所需的時間與處理速度是沒有當然關係,需要的時間是與掃描的次數有關。先進的處理能力是不會縮短所需的時間,跟踪系統仍然需要一定數量的掃描才能夠正常工作,而在海事雷達的應用上,單一次掃描就需要2至3秒。

在一般的船舶上以ARPA追踪穩定的目標,應該是可靠的,快速的變化將令ARPA變得不可靠。ARPA亦需要時間來反映被追踪目標的變化,追踪資料僅在目標保持或速度時才可靠。在海港中,船隻可能會不時改變的航向和速度,ARPA可能並不是想像中的那麼有用。

Written by Bruce Chun

June 11, 2010 at 18:36

Posted in Nav Aids

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