Marine Glossary

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GPS

GPS is a modern initialism for Global Positioning System, which is a worldwide, all-weather US satellite navigation system that provides continuous position accuracy to within some 10 meters. An extensive study from 2014 found that globally the open-sky accuracy of cellphone GPS was ±5 m—open sky means you see all satellites above 5º in all directions.

A Small hand held battery operated units cost as low as 100 dollars and provide full navigation functions for some 10 to 20 hours of operation (many days of actual navigation) on a few AA cells or on rechargeable internal battery. Most modern phones and some tablets also include a GPS receiver. Small stand alone receivers are available for computer connection via USB or Bluetooth.

Graphic display of vessel track and other options are included with many units. Improved accuracy is available in some areas with differential GPS (DGPS), which combines satellite signals with precise reference signals from land stations.

The true value of DGPS, however, is not so much the extra accuracy itself, but the more accurate course and speed data that the unit can provide once it has more accurate position data. Recall that the GPS itself is providing only precision values of position and time. All other parameters are derived from these, such as COG, SOG, XTE and VMC. In fact without differential, GPS is not particularly good for quick speed and course. As a test, just sit and watch your GPS speed (SOG) for some time as you are tied up at the dock. It will vary up to a knot or so. Hence we need to use some level of averaging to make this a useful number, which can be done by setting various options in the instrument or in the nav program in use.

GPS also provides a continual access to accurate UTC as discussed under universal time. Whenever the unit is in contact with a satellite, you can be confident that the GMT (UTC) it shows is correct. Some nav programs like qtVlm let you use the UTC from a live GPS signal to set the computer clock. This is especially valuable for long offshore voyages.

Elevation from GPS is relatively poor without DGPS from a WAAS satellite.

The latest in the status of GPS and DGPS is found online at the USCG NavCenter.

See also GPS.gov as an excellent source of related information.