GPS Stuff

GPS Frequently Asked Questions

Some GPS Q & A's

What is GPS?

GPS is an acronym for Global Positioning System.

It was created by the US Department of Defense and uses a constellation of satellites to transmit position fixing signals. Users can receive these signals with a hand-held GPS unit and accurately determine their position on the surface of the earth.


What is a map datum?

A map datum is a reference surface which is defined mathematically and approximates the shape of the earth in particular areas. At different areas across the world, different map datums were (and some still are) used due to the differences in the earth's general surface shape at different places. Specific map datums are more applicable to particular areas or regions than others.

It is important to set your GPS unit up to use the same map datum as the maps that you use for navigation. Most GPS units have a menu of supported map datums. Once the correct datum is selected, the GPS should display grid references which you can use to read your position directly off a topographic map.

Older Australian maps use either the AGD66 or AGD84 datum. These two datums differ by only about 5 metres. NSW, Victoria, Tasmania, and the NT use AGD66. SA, Qld, and WA use AGD84.

New Australian maps use GDA94 (Geocentric Datum of Australia). This new datum was introduced to bring our maps in line with the World Grid System (WGS84) so that they are more easily used with GPS units. (If your GPS does not support GDA94, use WGS84 since this is virtually identical).

AGD66/AGD84 positions and GDA94 positions differ by about 200 metres. A program converting between these datums is available on my software page. See also my page on grid references and datums.

New Zealand uses its own map datum, NZMG. This is not compatible with the Australian map datums. However, New Zealand has recently introduced a geocentric datum similar to Australia's new datum called NZGD2000.


What is the orbital altitude of GPS satellites?

Regular GPS satellites orbit at an altitude of 10,988 nautical miles (12,645 statute miles).

WAAS & EGNOS satellites have a much higher orbits above the equator.


Are GPS satellites in geostationary orbits?


Regular GPS satellites circle Earth every 12 hours (twice per day). However, a few special satellites that broadcast correction data for WAAS & EGNOS are in geostationary orbits above the equator.


What is WAAS?

WAAS is an acronym for Wide Area Augmentation System. It's an enhancement to GPS in North America that can allow specially equipped GPS receivers to more accurately calculate their position.

WAAS uses a network of ground-based stations that compare their precisely known location with locations calculated from GPS satellite signals. Any differences found can be used to create correction data that's broadcast from WAAS satellites.

WASS can help correct for error caused by distortion of GPS signals as they pass through the ionosphere as well as clock and orbital variations associated with individual GPS satellites.

Originally designed for use by aircraft during flight approaches, it's now included as a feature in most consumer GPS models.

Similar WADGPS systems that cover other parts of the world include EGNOS (Europe) and MSAS (Japan & Eastern Asia). WAAS is not available in Australia?


What is Selective Availability (or SA) ?

When GPS was first introduced the civilian signals were intentionally degraded to reduce their usefulness to users other than the US military. This was called Selective Availability or SA. SA meant that the public could only receive GPS signals with an accuracy of about 100 metres.

On 1st May 2000, SA was turned off. GPS users could then receive signals with an accuracy of about 10 metres. The US is committed to leaving SA turned off, but now has the ability to turn GPS signals off on a regional basis when its national security is threatened.


What are waypoints and routes?

A waypoint is just a position stored in the GPS receiver's memory. The receiver can calculate the distance and direction (and time-to-go) to the waypoint, and, if interfaced to an autopilot, will direct the autopilot to steer the boat to the waypoint.

A route is a series of waypoints. When navigating a route, the GPS will automatically change the destination waypoint to the next waypoint on the list as it reaches each waypoint. The GPS receiver or autopilot normally sounds an alarm, and requires an acknowledgment, before making any course change.



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