ELECTRONICS AND GPS ERRORS
All your different electronic systems should run on their own individual circuits, so if one develops a fault you do not lose them all.
GPS – Errors
GPS contains errors and therefore your position should always be checked via other methods.
Errors that occur within the GPS unit
- Systematic: Some errors are caused by the changing satellite geometry. This is called Dilution of Precision (DOP). For a three-dimensional fix, you need four satellites. Ideally, one of the satellites should be directly overhead, with the other three separated by 120 degrees of azimuth. An arrangement other than this ideal situation will cause DOP inaccuracy. A good value of HDOP (Horizontal DOP) is between two and four, values in excess of this size are of poor accuracy – you can check the values on the GPS.
- Environmental errors: Moisture and salt in the air, poor weather conditions or lightning.
- Ionospheric delay: Signals from satellites bend on entering the ionosphere (this is also known as refraction) and their speed varies. (The ionosphere is the outer region of the Earth’s atmosphere.)
- Multipath error: A satellite signal may be reflected off the water, or off metal objects on board your vessel. This may cause the GPS receiver antenna to receive the signal by two or more paths.
- Receiver errors: Mismatch of satellite signals.
- Selective Availability: SA is the military’s ability to scramble the GPS readings. At the time of writing, the USA is not scrambling the signal (the SA was turned off in 2000). DGPS (Differential GPS) was developed to reduce SA.
Common errors in using the GPS
- Offset: The main position fixing error occurs when using the GPS in combination with older charts. The correct offset from the chart needs to be applied to the GPS reading.
- Ensure your GPS settings match the spheroid of your chart. The GPS is usually operated on a spheroid called World Geodetic Spheroid 84 (WGS84). In the past, charts have been based on various spheroids, e.g., Australian Geodetic Datum 1966 (AGD66). You must check your chart to find out whether it is based on WGS84 or AGD66 (for example) and set your GPS accordingly.
- Magnetic or True: You can select Magnetic or True settings for bearings and courses. Ensure you have selected your preferred setting before completing these calculations. (See ‘True to Compass or Compass to True’ under Charts, earlier in this section.)
- Measurements: Are you reading statute miles or nautical miles for distance? Check your unit is set to nautical miles.
- Measurements: Are you reading knots for speed? Ensure your GPS is set correctly.
- Measurements: To reduce errors, check your GPS is set to degrees, minutes and decimals of a minute instead of seconds – to match your charts.
- Location: Follow the manufacturer’s instructions when mounting your GPS. Obstructions can affect the signal and the GPS unit can affect the ship’s compass.
- Input Errors: Double check the co-ordinates that have been inputted by you into the GPS. Ask someone else to read and check the numbers too.
GPS – Tracking Function
This function provides a track that, providing you have safely entered a harbour or port, you can simply follow the track back out and know that you are in safe water. This is a great advantage if you find yourself on a lee shore in the middle of the night.
Our charts are in degrees, minutes and decimals of a minute and our GPS is set the same. To reduce errors, set your GPS to match your charts.