Nautical Studies

about Marine Navigation & Technology

Shall a navigator rely on GPS fixes for anchoring in port?

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The errors in fixing a position with GPS may be grouped into three types. There are errors in the transmission, errors with the propagation and errors in reception. As far as ship stations are concerned, the errors related to the transmission and its propagation can be considered as common errors affecting every station within a small area. It can be corrected using differential technique. Errors in reception are differed from one station to another. The error is local to each station and will not be corrected by differential corrections.

The error in GPS position fixing onboard a ship is a compounded one. It consisted of errors related to the transmission, propagation as well as reception. From the following photo, GPS positions were marked on screen for a number of ships as reported by their Automatic Identification Systems. Some of these positions were very close to the echo of realted vessels. Some other was rather far away from its echo. It demonstrated that GPS positioning errors were different on every ship.

A radar screen picture was used to illustrate the problem in using GPS for anchoring within confined water. It was taken onboard a containership while approaching an anchorage in Hong Kong. On the southern part of the screen was the
northern coast of Lamma Island. Hong Kong Island was along the eastern part of the screen.

An assigned anchor position was given and plotted in paper chart to obtain range to landmark readings. The position was then marked by variable range markers and GPS referenced mark on a radar. The anchor position should be 0.92 nm off the northern coast of Lamma Island and 1.5 nm off the coast of Hong Kong Island. The square marker with the letter “i” in the middle was the anchor position using GPS as a reference. The length between the bridge of the ship and its bow was about 0.1 nm.

It could be agreed that the ship was proceeding along a course towards its assigned anchor position, though, it was not proceeding towards the GPS referenced marker. Apparently  the discrepancy between the positions fixed by ranges and GPS was more than 100 metres.

The proper uses of modern technology in assisting navigation help to improve safety. However, mariners should be cautious with the limitations of each system. GPS is handy. It could be very helpful in more open waters. In a port like Hong Kong, required accuracy of anchoring position could very often down to tenth of metres. The use of GPS fixes in anchoring may be inappropriate. Try again with basic techniques.

Written by Bruce Chun

October 9, 2012 at 12:30

How to setup raster scan radars (with automatic tune control)?

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Make sure that nobody is working near the scanner and it is free from obstruction before power on a radar.

  1. Power on and switch the radar to Standby;
  2. adjust Brilliance for preferred viewing brightness;
  3. turn both manual Anti-sea Clutters and Anti-rain Clutters to minimum;
  4. set Gain to minimum;
  5. switch off all automatic signal suppressing and video enhancing sub-systems;
  6. switch radar on at medium range when it is ready;
  7. switch on automatic Tune;
  8. increase Gain until some background noises are seen all over the display;
  9. switch to appropriated operation range;
  10. select preferred display orientation and mode (ensure heading and speed inputs are corrected if True Motion Mode is used);
  11. adjust Anti-rain Clutters if echoes caused by rain are affecting the identification targets seriously;
  12. adjust Anti-sea Clutters if echoes caused by sea waves are affecting the identification of targets seriously.

Try not to use automatic suppressing sub-system if the use of manual anti-clutters control could achieve an usable display. Use automatic enhancing sub-system only if it is really necessary.

Written by Bruce Chun

April 15, 2011 at 15:54

Speed Derived from GPS Fixes

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As discussed in a previous note on Global Positioning System (GPS), it argues that the accuracy of position fixing could be limited. The accuracy of derived information such as course over ground (COG) and speed over ground (SOG) could be even worst.

A velocity of one meter per second can be translated into a speed of about two knots. As GPS produces fixes every second, any difference of one meter between subsequent fixes could result a speed of two knots. A one meter eastward and another meter westward will appear as a course change of 180 degrees. Similar to many other navigation systems, GPS receivers attempt to minimize the effect of these random errors by averaging its readings over a longer period of time. The downside of the technique is the delay in reflecting changes.

At open sea while a vessel is sailing over a steady course, the delay will not cause any practical problem. Indeed, any differences caused by the averaging technique, or damping, is not of any significant.

However, it is a very different story in port or within a harbour. Course and speed of vessels are changing constantly. Thus, the readings from any system deploying damping in reducing the effects of random errors are likely wrong.

In the radar screen on the left, the observer’s ship was making a turn to starboard rounding an island. The radar was set to present true motion picture in which fixed objects such as land were stabilised with GPS inputs. Echo trails after moving targets should be showing path of those targets in true motion. Fixed objects should not have any echo trail.

As shown, there were clearly echo trails built up for fixed objects such as land and bridges. In other words, the corrections used to calculate motion were wrong. Amongst the others, it was caused by damping. It was causing errors in both course and speed of the observer’s ship. The echo trails of the fixed objects in the picture were reflecting an error of its GPS derived speed between two to three knots.

Written by Bruce Chun

October 3, 2010 at 22:58

Posted in Nav Aids, Navigation

How well position fixes from GPS help in close waters?

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There were six targets moving around the ownship in the radar picture on the left-handed side. All six targets were transmitting navigational information with their Automatic Identification System (AIS). Amongst the others, AIS of each target was sending positions fixed by their Global Positioning System (GPS).

The width of a lane in this section of the East Lamma Traffic Separation Scheme (TSS) was a bit more than 500 meters. Accordingly, the differences between a radar target and its AIS reported position in the image was ranging from 40 meters to 200 meters, some 0.2 to 1.1 cables.

The AIS position of each target and the overlaying chart were plotted on the radar display in reference to the position of ownship. Any error in the plot was compounded with those of the ownship and the target.

The bigger echoes on the left and the lower edges of the image were larger containerships anchored heading towards east. The smaller echo near the left was a smaller containership. Similar to ownship, these vessels have their superstructure and the navigation bridge located at the after part of the ship. Yet, their AIS seem to have their positions fixed towards the bow of the ship, well away from the navigation bridge.

There were two other fast moving objects running along opposite courses at the lower part of the image. These were high-speed crafts from the same company running a particular ferry service. Vessels were similar. Yet, their position fixes seem to have different errors even in close proximity.

The error of GPS fixes on different vessels could be very difference. In addition to those global errors, there were somethings local to each vessel affecting their GPS independently. In a shipboard environment, the accuracy of GPS may not be as good as expected. Even the use of differential corrections may not help in getting the accuracy down to tenth or even hundredth of meters. In summary, GPS in harbours should only be used with extreme cautions.

Written by Bruce Chun

October 3, 2010 at 10:44

Posted in Nav Aids, Navigation

Head on Situation

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An argument on using sidelights in defining head-on situation was put forwarded in court. It was argued that both sidelights of a target involved in the risk of collision must be seen to effect a head-on situation. The argument was later supported by an oversea maritime expert. Another renowned author also seems to express similar argument in his book about COLREG as follows:

“… Rule 14 is apparently not intended to apply to cases in which, from a vessel which is ahead or nearly ahead, one sidelight can be seen, but the other is obscured. (Cockcroft & Lameijer 1996, p99)”

It could be confusing. It could be difficult to understand if one will look into the text of the rule as well as its application at sea.

Rule 14(a) stated the actions required when ships “meeting on reciprocal or nearly courses” and involved in a risk of collision. Rule 14(b) follows with examples that a head-on “situation shall be deemed to exist”. Rule 14(a) defines head-on situations by the meeting courses and the risk of collision. The text of Rule 14(b) described certain “shall be ” situations but does not seem to exclude any other encounters meeting on reciprocal or nearly courses and involved in a risk of collision. The rule has not exclude any other possible situation such as those have only one sidelight or even none could be seen.

The argument went on and suggest that Rule 14(c) will not apply at all as there will not be any doubt about a situation if only one sidelight can be seen. Thus, it will never be a heading-on situation if not both sidelights were seen at night. It can only be a crossing, Rule 15, situation if it is the case.

As shown in the radar picture on the left with true motion trails displayed, a south bound ship detected three approaching targets on her port side. These approaching targets were met on reciprocal or nearly reciprocal courses.

The north bound targets were going to pass the south bound ship with a distance ranging from less than half a cable to about two cables, about 90 metres to 350 metres. To most mariners, these targets were involved in a head-on situation with the south bound vessel if risk of collision between them was considered.

The second picture shows the visual scene when the radar picture was captured. Actually, the first radar screen is an enlarged portion of the radar screen in the second picture.

It can be seen that none of the three approaching targets was showing both sidelights. Should these approaching targets be considered as ships involved in a head-on situation? Could they be crossing targets if there was risk of collision?

Each of the vessels “shall alter her course to starboard so that each shall pass on the port side of the other” if it is a head-on situation (COLREG Rule 14). Otherwise, “the vessel which has the other on her own starboard side shall keep out of the way” as stipulated in Rule 15 if it is a crossing situation. The problem is that there is nobody with the other on her own starboard side. There is no stand-on vessel either as there was not any vessel required to keep out of the way as stated in Rule 17(a)i.

Apparently, these encounters might not be simple. Is the rule in collision avoidance at sea so complicated? There might be the problem of the rules. Perhaps, it is the problem in reading or interpreting the rules.

Written by Bruce Chun

June 24, 2010 at 01:58

Posted in COLREG

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。


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。




Written by Bruce Chun

June 11, 2010 at 18:36

Posted in Nav Aids

Can a buoy help in setting up a radar properly? (可以利用導航標調教雷達嗎?)

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Navigation buoys may be considered as an object to tell if a radar system is properly setup. It has been suggested that it can be considered properly setup and tuned if buoys can be seen on-screen. Is it a reliable mean?

The reflective quality of a radar object depends on numerous factors such as material, size and shape. Amongst the others, they affect the echoed strength and returned direction of radar pulses.

Navigational marks are normally made with steel. In the contrary, those sampan and fishing boats in the picture are constructed with fibre-glass and wood. The reflective quality of these sampan and fishing boats are far inferior to those of steel navigational buoys. The echoes strength of these sampan and fishing boats should be much weaker than those of buoys.

As illustrated, a navigational buoy on the left is not a small object in comparison to sampan and fishing boats. Buoys are taller than most of the small boats in the photo. Apparently, the navigational buoy should have greater chance in reflecting detectable signals.

It may be argued that a can or cone shaped buoy is a poor radar object due to its rounded structure. Except the base that provides buoyancy, a buoy is normally made out of steel frame. Very often, radar reflector is fitted on the top. It appears to be a conspicuous radar object.

In fact, navigational buoys are designed to be picked up with eyes as well as by radar at a reasonable distance. Navigational marks are rather good radar objects in comparison to small boats. Apparently, the detection of navigational buoys will not assure the signals echoed from small objects such as sampan can be detected.







Written by Bruce Chun

June 10, 2010 at 11:25

Posted in Radar Operation


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