In 2008, there were 33 satellites in orbit making up the constellation of satellites required for the GPS network to operate. Out of 33 satellites, 31 were actually being used. Initially 24 satellites were required for the initial constellation. These satellites provide basic information on timing, the status of the network in times of orbital positions. But how does a GPS work and what is GPS data?
GPS Data is in three forms
Almanac: The almanac provides coarse time information along with the status information about the satellites.
Time: Basic clocking information is provided for the general public we can acquire the coarse acquisition signal which is freely available to the public and for military applications as a Precise Code or P(Y) code which is encrypted. This prevents a signal from being spoofed which could cause problems obviously in military applications.
Ephemeris: Along with the Almanac, Ephemeris information is also provided by the satellites. This word is actually derived from Greek which just simply means daily. This information provides orbital information that allows the receiver to calculate the position of the satellite very accurately. This information is transmitted down to the hand-held receivers relatively slowly it can take up to twelve and a half minutes for all this information to be loaded into receiver as it is transmitted at a relatively slow speed of only 50 bits per second.
How does a GPS work
Well basically if you need to determine your position on the surface of the earth you need to have at least three satellites in view. Each one of the satellite is going to provide the basic information required to determine your position on the surface of the earth. To actually determine your position accurately you’re going to need four Spheres. The fourth sphere in this case is actually the surface of the earth itself. Each one the satellite is going to radiate out a signal. The GPS system will actually know where the satellite is supposed to be and it will measure the pseudo random test pattern generated by the satellite.
This pseudo random test pattern will also be generated by the GPS system itself and over a period of time the GPS system will attempt to acquire synchronization between the signal coming from the satellite and the basic pseudo random test pattern it’s generating in your hand. So every millisecond the pseudo random test pattern is transmitted from the satellite and over the coarse of a few minutes the GPS system can very accurately triangulate its position on the surface of the earth, if you are an aircraft its altitude as well.
The receiver does this by delaying the start of the pseudo random bit pattern to sync with the satellite. The amount of delay is used to calculate the distance to the satellite. It’s basically simple triangulation which determines the position on the surface of the earth. This takes a look at the time of course because the GPS system you have in your hand or in your car is not going to have a strengthen signal one clock which is a type of clock provided by the satellite. So it actually has to basically home in and start synchronization with the clocks on the satellite before can actually detect its current position on the surface of the earth.
One other application to the satellite system is also detect missile launches and explosions on the surface of the earth. So if anybody detonate very much explosion anywhere on the surface of the earth these satellite are going to be the first devices to take up that. One of the unusual things about the GPS network its unique technology on that worked with which involves time travel. You may think I’m joking but Einstein’s general relativity and special relativity both come into play.
Einstein predicted that a clock in a large gravitational well will tend to run to slow the speed. So by the time we take clock put up into orbit into its 20,000 kilometers slot that clock is going to speed up by about 45 micro second a day. Now remember the clock is also in orbit its whizzing around the earth that 3.87 kilometers per second. This is relatively high speed and due to special relativity the clock is now going to slow down. The clocks going to slow down actually by about 7 micro second a day. So the net effect is going to be an offset with 38 microseconds that is the clock in orbit at 20,000 kilometers rotating about 3.87 kilometers per second is going to be running 38.7 microseconds faster than its counterpart on the surface of the earth. To offset this, the clock is deliberately slow down to compensate for this affect. So as you can see time travel actually does occur and it’s actually has to be compensated for in the GPS network.
There are other systems somewhat GPS commonly known as Global Navigation Satellite Systems (GNSS). GLONASS– the Russian system is one of the best known systems it’s been in operation since 1995 actually work started on it a lot earlier than that in 1976 and various implementation phases took place in between 1976 to 1995. In 1995 there was officially completed with 21 operational satellites and three spheres. In 2009, 19 of the satellites were still operational. However three of them were on the going maintenance. In May 2007, the President Vladimir Peyton signed a decree allowing civilian access to the GLONASS system and even a quick tease dog KONI with the GPS tracking device, so hopefully KONI won’t get lost.
In Europe, there were working on a system called Galileo Positioning System named after the Galileo Galilee, the famous Italian astronomer. The budget for the system is approximately five billion dollars. Both of the system’s GLONASS and Galileo Positioning System are actually compatible with equipments. The Chinese are also working on their system called COMPASS. They expect 30 satellites in MEO and GEO orbits that are Medium Earth Orbits and Geo Stationary Orbits. The system will have very similar capabilities to GALILEO and GLONASS and of course the GPS system that we’re all familiar with today.
The original GPS network was developed for military applications for navigation of aircraft, ships, military vehicles, rockets, missiles, etc. But today there are many civilian applications as well. For example you may find that you have a GPS enabled cell phones or navigation systems in your cars. These systems can be used for many other applications as well not only finding a specific destination but for example if you’re looking for a restaurant the system knows exactly where you are and will give you all the restaurants within the 200 meter radius.
To Test GPS Systems
Now to test out these GPS systems, in the past you may literally have to go on a road trip. For example you install the navigational system in a car and go off on a trip to see how accurate system is. But now days we got many applications to test the navigational system by sitting at one place. One of the technologies is SPIRENT. We are able to emulate the constellation of satellites and basically send you on a simulated road trip. We can fool a navigational system into thinking it’s really on the road not only that we can introduce deliberate areas into the system as well.
This is how GPS works. After all these work, now you are easily able to use GPS in your mobile phones by just tapping your finger.