What is GPS?

The GPS used is a system of 24 navigational satellites placed into orbit by the United States Department of Defense. The satellites orbit the Earth twice a day, continually transmitting signals to the Earth’s surface.
The first ever GPS satellite started orbiting in 1978 and by 1994 there were 24 satellites launched, the satellites are in use for 10 years before being replaced. All satellites are powered by solar energy with a battery backup during periods of solar eclipse.

Image courtesy of Garmin

 

Although originally launched for military use, the system was extended for civilian use in the 1980s. There are two systems currently in use Wide Area Augmentation System (WAAS) and European Geostationary Navigation Overlay Service (EGNOS) The WAAS system was developed for use in precision flight approaches by correct GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors. EGNOS (European Geostationary Navigation Overlay Service) has been developed by the European Space Agency to provide a comprehensive coverage of Europe for GPS users. This means that European users no longer have to rely on American satellites and instead have a dedicated service to provide more accurate signals with increased availability.

How does it work?
GPS receivers use the range of satellites signals to triangulate the user’s position on the Earth’s surface. The receiver transmits a signal, and then compares the time between the signal being transmitted and subsequently received, thus calculating the distance between the receiver and a specific satellite. Once the position is ascertained, the GPS receiver will display information on geographic location and altitude. With more than one position, other information can be provided, for example speed and bearing.
The receiver requires 3 satellite fixes for a 2D position (longitude and latitude) and a minimum of 4 fixes for a 3D position (latitude, longitude and altitude)

 

Navigation
It is essential to use a GPS unit with a map, as the information from both items will aid navigation. Receivers are multifunctional units that have various utilities; one important function includes the creation of waypoints. GPS units can be used to aid with navigation by storing important waypoints or locations that are to be revisited or navigated to in the future.
The receiver can then direct the user to the waypoint using a compass bearing represented by a digital compass and displaying the distance to the point. Some units are also able to offer a 3D representation of the route. However it is important to use this with a map which may redirect the user to a safer route to get to the intended waypoint, or to have created various waypoints which take into account topographical and morphological differences in the terrain. Connected waypoints are known as a route. The GPS unit will still continuously update the compass and other useful information such as grid reference, speed, distance covered, distance to go and time.
Some units are also able to create a track logs that will record the journey, this allows the user to follow their journey back exactly on a reverse route.

 

Use of GPS in recording
GPS is an incredibly useful tool for recording; it offers the user the ability to accurately pinpoint a location to a 10 digit grid reference. This allows future recorders or determiners to then revisit the location and confirm the record or chart the flora or fauna over time. All GPS grid references are entered and stored with every record on Recorder and create a valuable entry for the species. They are of therefore great use in surveys which require a number of visits to a site. For further details on grid references, click here.

 

Limitations of GPS
GPS receivers need to be used in an area where there are few obstructions above to get a good locational fix from the satellites. For example the reception can be blocked by mountains, trees, infrastructure and even bodies. It is not affected by bad weather or any cloud cover.
Receivers are electrical and therefore require batteries or a charge, they can also be affected by accidents like other general electrical items, and you may wish to ensure you have a waterproof cover in case of heavy rain or if the GPS is dropped in a puddle.
Not all GPS units offer maps and, if they do, these maps are small and may lack the detail that regular ordinance survey maps have. The general rule is the better the maps, the more expensive the unit.
Much of the GPS data in terms of waypoints is based on straight line projections so estimation of time, speed and distance may be misleading if the route requires extra navigation. Direct navigation to a waypoint, without the use of a route, also gives no indication of obstacles such as rivers or land with ni public access.
Finally as some GPS satellites are part of the American system it is always possible that they may turn the satellites off, therefore the new European EGNOS project should offer European users a more reliable and accurate service.

 

For further information about GPS, you can visit the Garmin website.