Global Positioning System (GPS) receivers receive signals from a constellation of 24 GPS satellites and tapping into this $14 billion resource is free of charge. This system is owned by the United States. Russia owns a similar system, but it is used for military purposes and civilians do not have access to it.
The receivers or plotters can be described as hand-held satellite tracking computer receivers which are extremely accurate for determining three dimensional position fixes (latitude, longitude and altitude). It is essentially a time measurement system in which signals sent from a series of satellites are received and time differences measured. Ranges are measured simultaneously from a minimum of four satellites (for a 3-dimensional fix) and providing that the satellites’ positions are known the receiver’s position can be established. The GPS updates its position fix continually.
When the GPS is moved it provides a host of other information useful to the navigator. Speed, track, distance covered, distance to go, estimated time to go to destination, track to starting point etc. The GPS gives a sense of security to the traveler, and as a pilot it has taken much of the stress out of long distance flying.
Once coded to reduce accuracy with a system called Selective Availability, a GPS with ten satellites can today read with an accuracy of between 15 and five meters, anywhere inside our atmosphere.
Choosing a GPS
Like everything relating to computers, the moment you take your GPS receiver home it will be superseded by a better, smaller and faster model. Not all modern GPSs are alike and a four-wheel driver will demand different things from a GPS than say a pilot or urban GPS user. For a 4×4 adventurer, there are things that are essential, so when selecting a GPS, consider the following:
Important features to look for:
• Ease of use and instructions that are easy to follow
• External power supply
• The ability to drop bread crumbs and record and save a track log
• Connectability (mapping on a PC)
• Software availability and internet support
• Auto-routing is recommended but not essential.
• A map-enabled GPS is highly recommended.
Hand-held GPS recievers
No single product or GPS maker stands out as the very best. Like all technology, changes are swift and new models become old models very quickly.
The agents for Garmin products are Avnic Trading, 011 701 3244. The range is considerably wider than Magellan. Both Garmin and Magellan share a similar map base (It comes from the same organisation) Garmin’s PC software is calledMapSource.
Magellan products are available from Pertec, 011 805 1996 (Johannesburg), 021 419 4450 (Cape Town).
Tom Tom make a wide range of GPS units but most are better suited to the urban motorist than the advanced 4×4 explorer. Tom Tom’s update system and software eclipse that of both Garmin and Magellan and their maps are brilliant.
Like all products in the IT world, things change rapidly and so I suggest a good look at both of these manufacturer’s products.
Using a GPS reciever
Many GPS users make the mistake of switching off their brains when they turn on their GPS. A GPS is not a magical piece of gear that will prevent you getting lost. It is a navigation aid. It assists the brain, not replaces it. I say this because as a map publisher I have been accused of being responsible for someone driving, “600 miles in the wrong direction”. Unbelievably, this person blamed an inaccurate coordinate (out by 30 kms) on a map for his day-long drive to who knows where. GPS receivers are not infallible and nor are maps, be they electronic or paper. Be aware of your position all of the time, or a GPS will get the better of you. However, armed with a GPS, good maps and the willingness to learn, there is nothing quite like the certainty that a GPS can provide during challenging adventures in the bush.
A GPS receiver (depending on features) can accomplish
• Pinpoint your position.
• Give you direction from your position to a given way point.
• Tell you which way to travel to get to a way point. It will give a compass bearing which must be followed. If no compass is available then the trip must be begun by guessing the direction until the GPS reads a position change and corrects the course.
• Calculate a speed over the ground.
• Calculate an average speed between two way points.
• Estimate the time it will take to reach a given way point/s.
• Calculate the distance to a given way point/s.
• Calculate the distance covered.
• Record a path taken to allow the navigator to find the way back to the starting point covering the same path.
• Store way points for instant retrieval, such as favorite fishing spots and secret campsites.
• Record a journey in the finest detail to be played back in real time on a computer screen.
• Converting latitude and longitude measurements from the GPS to a map and vice versa.
• Plot and record positions on a map.
• A compass converts bearings supplied by the GPS into a direction in which to travel.
• Auto-routing means that given a destination a GPS will follow the recorded roads and tell you where to turn etc. Many, but not all GPS maps have the auto-routing feature.
Converting GPS co-ordinates onto a map
Let’s say for example, the co-ordinate (in this case Sylvia Shoal, Mozambique) is your position fix. Your GPS reads: 23° 13’ 15S, 35° 29’ 15E. Translated into English this means: 23 degrees, 13 minutes and 15 seconds south of the equator, line of latitude by 35 degrees, 29 minutes, 15 seconds east of the zero meridian, line of longitude. To pin-point this onto a map do the following:
Make sure the GPS datum is the same as the map that is being used. This is very important. Secondly, the notation must be set to degrees, minutes & seconds (hddd°mm’ss,s) or degrees, minutes & decimals, (hddd°mm.mmm) If in doubt how to set these to begin with, set the datum on WGS84 and the notation on the most commonly used format: hddd°mm’ss,s.
The ideal maps for pin-point navigation are topographical as they are small scale with an accurate grid. Doing this with large scale (e.g. 1: 1000 000) road maps is likely to result in major inaccuracies.
At the extreme top and bottom of the grid there are rulers displaying longitude and along the sides, rulers displaying latitude. Read off and measure the latitude (flat lines) first. Run along this line and locate your co-ordinate and make a mark. In this example 23° 13’ 15S. Then do the same for the latitude – in this example 35° 29’ 15E. The final step is to run lines parallel to the grid from the marks you have made and where the lines intersect indicates your position.