Several incidents reported by CHIRP related to the discrepancy due to settings of the GPS – specifically the smoothing.
In this regard, CHIRP provides a simple explanation about GPS smoothing and good practices to follow.
What is GPS smoothing?
The accuracy of a GPS position obtained onboard a vessel can be affected by the quality of satellite signals received at the GPS receiver’s antenna.
In addition, the accuracy may be affected by satellite geometry, satellite system errors, and other factors such as signal blockage, atmospheric conditions, and receiver design features/quality.
The shipborne marine GPS receiver computes the ship’s position several times per minute.
Each of these positions is affected by these errors to varying degrees and thus, if all were plotted, the track might appear erratic.
-The shipboard navigator should be aware that LAT/LONG smoothing and COG/SOG smoothing parameters can be adjusted by the user to adjust the level of responsiveness of the shipborne marine GPS receiver. The ship’s GPS User Manual should be consulted on how to adjust the GPS smoothing setting. The smoothing setting of shipborne marine GPS receivers should be correctly adjusted according to the expected dynamics of the vessel.
- If the vessel is navigating near harbour entrances, harbour approaches and coastal waters, where the passage plan requires the vessel to frequently change course and/or speed, the GPS smoothing settings should be set low so that small changes in the vessel’s course/speed over the ground (COG/SOG) and position (LAT/LONG) are tracked closely by the GPS.
- If the vessel is navigating in ocean waters, where the passage plan requires the vessel to maintain a steady course and speed for relatively long periods of time, the GPS smoothing settings should be set around mid-range so that scattered positions obtained by the GPS receiver are smoothed to obtain a stable course/speed over the ground output (COG/SOG) and a steadily changing position (LAT/LONG).
-Failure to appropriately adjust the GPS smoothing settings, as above, can lead to situations where there is a significant offset (reported to be as high as 160 metres) between the displayed position on the ECDIS and the ship’s actual position derived from visual observations (bearings/distances) or from radar overlay.
-Whenever a ship’s passage plan is being prepared, choice of position fixing methods to be used for each leg of the passage must take into consideration that prevailing IMO performance standards for a marine GPS receiver (without differential GPS capability) require, in ocean waters, that the system should provide positional information with an error not greater than 100 metres with a probability of 95%. Such a position fixing system should not be chosen as the primary position fixing system near harbour entrances, harbour approaches and coastal waters, where the latest IMO standards prescribe that the positional information error should not be greater than 10 metres with a probability of 95%.
-Where the ECDIS on board provides a radar overlay facility it must be used at every available opportunity to verify the GPS position. If there is a discrepancy between the radar overlay picture and ECDIS display it must be recognized as an indication of an underlying inaccuracy in the GPS position received. This is also the reason why good seamanship suggests that we should cross-check the GPS position against the radar by means of LOPs and visual bearings during coastal navigation.
-The shipboard navigator must not abandon the basic tenets of navigating by sight and sound. Where available, maintain visual verification of the vessel’s position to ensure situational awareness and safe navigation.
From the above, it can be determined that there are two distinct issues:
- the accurate knowledge of where the vessel is, and
- the adjustment of GPS smoothing to compensate for inaccuracies.
CHIRP Maritime has repeatedly highlighted the importance of traditional navigation and keeping a good lookout. It is imperative that critical sections of every passage are carefully planned and executed. Transits, wheel over positions, turning curves, visual bearings supplemented by radar ranges, and parallel indexing are just a few of the tools available to the navigator.
In today's electronic age, it seems that far too much reliance is being placed upon GPS derived positions displayed on ECDIS with reluctance, in some cases, to use other available and reliable means of navigation to verify the ship’s position, CHIRP highlights.
This being the case, it is imperative that the information being displayed is correct.