The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use the GPS signal or GPS receivers.
How it works:
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.
A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude). Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
How accurate is GPS?
Today’s GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Garmin’s 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin GPS receivers are accurate to within 15 meters on average.
The GPS satellite system:
The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly 7,000 miles an hour.
GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there’s no solar power. Small rocket boosters on each satellite keep them flying in the correct path.
Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):
- The first GPS satellite was launched in 1978.
- A full constellation of 24 satellites was achieved in 1994.
- Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
- A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
- Transmitter power is only 50 watts or less.
Some Technical Details:
GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.
A GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your Garmin GPS unit’s satellite page, as it identifies which satellites it’s receiving.
Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.
The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.
I was recently the recipient of a Garmin Nuvi 200 as a present from my wife. Having tried the unit out I am disappointed with two aspects of what I have found to date bearing in mind that the maps are advertised as being updated twice annually. First of all the maps do not include a number of streets in Fourways (not so recently built) and when saing the coordinates of my brother’s home the unit battles to find it. The complex that he stays in (Lonehill Village) is not listed.
The second frustrating issues is that I live on a small farm that borders on the N14 highway. I have saved the coordinates as HOME on my favourites list but the unit insists on plotting me in the middle of the highway. Although the tar and gravel roads that lead to our farm are clearly indicated on the unit, it gets very confused when approaching the roads and advises me to take the highway onramp presumably because it has plotted me on the highway. I am told to make a -turn when I take the road leadin to our home and on the last part of the journey advised of recalculating on numerous occasions until I enter our driveway at which point I am advised smartly that I have reached my destination.
Is there a way to rectify these problems? Aside from this there are a number of roads around the Muldersdrift area that are clearly marked with street names – some dating back many years – that are merely recorded as “driving on road”. Any view why this is so?
Hi Roy
I would suggest giving us (012 – 361 8300) or the Garmin SA Technical Helpdesk a call (011 – 251 9999 ext.1) to assist with this matter.
Also check out the Garmap Forum for a better understanding on how the map software works.