Club Navig8

FAQ: About GPS


1. What is GPS? GPS, or Global Positioning System is a constellation of 27 satellites (24 active satellites and 3 back up satellites) that orbit the earth twice every 24 hours, on a very precise trajectory at an altitude of approximately 20200km. Whilst orbiting they transmit a signal down to Earth, which GPS receivers pick up and by means of triangulation calculate the users exact position, which can then be displayed on an electronic map. GPS was developed by the United States Department of Defense in 1973 to assist soldiers, military vehicles, planes, and ships in accurately determining their locations world-wide. In 1983 President Reagan opened up GPS for civilian application due to the shooting down of Korean Air Flight 007 by Soviet fighter planes.

2. Is there a charge to use the GPS satellites? The United States government permits worldwide, continuous access to GPS signals, free of charge

3. How many satellites does a GPS receiver need to detect a signal from in order to Navig8 ? GPS receivers work on line of sight with the satellites orbiting the earth and need to be outdoors with a view of the sky in order to operate. A secure signal from a minimum of three satellites is needed to determine the latitude and longitude positions of a GPS device and to continue to track the movement of the device. With a signal from three satellites a GPS device will be able to display a 2D electronic map, but with a signal from four or more satellites the GPS device will be able to display a 3D view. Once the device’s position has been determined, the GPS unit can calculate other information, such as speed, expected time of arrival, trip distance, distance to destination and altitude.

4. How long does it take to establish a GPS fix? When you turn the GPS receiver on for the first time it can take up to 5 minutes to establish a GPS fix. After the first connection, when the GPS receiver is switched on, it will look for the last satellites it connected too. If you have changed your location i.e. flown down to Cape Town from Johannesburg, the GPS receiver will have to search the horizon for new satellites and establish a new connection and position and this could lead to an extended delay in picking up a GPS signal.

5. How Accurate is the Navig8 GPS and what can affect a GPS signal? Navig8 GPS receivers are accurate to within 15 meters, although they can be affected by atmospheric factors and other sources of error: Signal Path – The GPS signal is reflected off of objects such as buildings, this increases the time taken for the signal to travel to the GPS receiver, thereby causing errors on the GPS receiver Ionosphere and troposphere delays – The GPS signal from the satellites slows down as it passes through the Ionosphere and Troposphere. To counter this, the GPS receiver has software built-in that allows it to correct, to a degree, the error caused by atmospheric delays. Satellite Positions – The more spread out over the horizon the satellites are, the better the accuracy of the signal being measured by the GPS receiver. The accuracy can be affected by satellites being in a straight line or bunched together Ephemeris errors – This is the difference between the satellites actual position and its expected position Number of satellites – The more satellites that a GPS receiver can see and obtain a fix from, the more accurate the GPS reading will be. A GPS needs a fix from a minimum of 3 satellites to display a 2D image and a minimum of 4 satellites to display a 3D images (4 satellites allows for an accurate positioning.) Selective Availability (SA) – The US Department of Defense turned off SA on the 1st of May 2000. SA was the intentional degrading of the accuracy of the civilian GPS signal in order to prevent hostile parties from using this information. Whilst SA remains off, it can be activated at anytime by the US Department of Defense.