The Global Positioning System (GPS) network most commonly used is called Navstar and is paid for and operated by the US Department of Defence (DoD). This Global Navigation Satellite System (GNSS) is currently the only fully operational system but Russia has GLONASS, China has COMPASS and the EU has GALILEO each at varying stages of development or testing.
The Satellite Network
The GPS satellites transmit signals to a GPS receiver. These receivers passively receive satellite signals; they do not transmit and require an unobstructed view of the sky, so they can only be used effectively outdoors. . GPS operations depend on a very accurate time reference, which is provided by atomic clocks on board the satellites.
The Navstar GPS Constellation
Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are further away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.
There are at least 24 operational GPS satellites at all times plus a number of spares. The satellites, operated by the US DoD, orbit with a period of 12 hours (two orbits per day) at a height of about 11,500 miles traveling at 9,000mph (3.9km/s or 14,000kph). Ground stations are used to precisely track each satellite's orbit.
Here is an interesting comparison. The GPS signals are transmitted at a power equivalent to a 50 watt domestic light bulb. Those signal have to pass through space and our atmosphere before reaching your satnav after a journey of 11,500 miles. Compare that with a TV signal, transmitted from a large tower 10 - 20 miles away at most, at a power level of 5-10,000 watts. And compare the size of your TV's roof mounted antenna with that of your GPS, often hidden inside the case itself. A wonder then that it works as well as it does and when the occasional hiccup occurs you will at least understand the reasons why.
How Position is Determined
A GPS receiver "knows" the location of the satellites because that information is included in the transmitted Ephemerisdata (see below). By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centred at the satellite. It then determines the sizes of several spheres, one for each satellite and therefore knows the receiver is located where these spheres intersect.
GPS Accuracy
The accuracy of a position determined with GPS depends on the type of receiver. Most consumer GPS units have an accuracy of about +/-10m. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby. Observations made by the stationary receiver are used to correct positions recorded by the roving units, producing an accuracy greater than 1 meter.
How Is The Signal Timed?
All GPS satellites have several atomic clocks. The signal that is sent out is a random sequence, each part of which is different from every other, called pseudo-random code. This random sequence is repeated continuously. All GPS receivers know this sequence and repeat it internally. Therefore, satellites and the receivers must be in synch. The receiver picks up the satellite's transmission and compares the incoming signal to its own internal signal. By comparing how much the satellite signal is lagging, the travel time becomes known.
What does the signal consist of?
GPS satellites transmit two radio signals. These are designated as L1 and L2. A Civilian GPS uses the L1 signal frequency (1575.42 MHz) in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass, plastic etc but will not travel through solid objects such as buildings and mountains.
The GPS signal contains three different bits of information — a pseudo random code, almanac data and ephemeris data.
The pseudo random code is simply an I. D. code that identifies which satellite is transmitting information. You can often view this number on your GPS unit's satellite information page, the number attached to each signal bar identifies which satellites it's receiving a signal from.
Almanac data is data that describes the orbital courses of the satellites. Every satellite will broadcast almanac data for EVERY satellite. Your GPS receiver uses this data to determine which satellites it expects to see in the local sky. It can then determine which satellites it should track. With Almanac data the receiver can concentrate on those satellites it can see and forget about those that would be over the horizon and out of view. Almanac data is not precise and can be valid for many months.
Ephemeris data is data that tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite will broadcast its OWN ephemeris data showing the orbital information for that satellite only. Because ephemeris data is very precise orbital and clock correction data necessary for precise positioning, its validity is much shorter. It is broadcast in three six second blocks repeated every 30 seconds. The data is considered valid for up to 4 hours but different manufacturers consider it valid for different periods with some treating it as stale after only 2 hours.
To compute a PVT (position velocity time) solution the receiver will look for satellites based on where it 'thinks' it is roughly located and the almanac if current. If it finds one or more of the satellites it expects to see it will lock onto that satellite and begin downloading ephemeris data. Once data from three satellites has been received an accurate positional fix is calculated.
If you are moving whilst trying to obtain a fix this process may take much longer than it would if you were stationary. Your receiver must complete reception of ephemeris data without error, this data is transmitted in three packets. Should any one packet not be received completely without error then it must start over again. Clearly doing this whilst moving leads to much higher error rates and longer fix times. Considerably less than a second of interruption is enough to mean the receiver will have to wait for the next transmission.
