GPS: Global Positioning System
Discover Magazine recently published an article about the current state and potential future state of the GPS system (the article can be found here). From the article:
The U.S. government has announced increasing concern over the quality of its Global Positioning System (GPS), which could begin to deteriorate as early as next year, resulting in regular blackouts and failures – or even dishing out inaccurate directions to millions of people worldwide [The Guardian]. The possibility that new satellites would not be launched in time was announced in late April, but the warning was stepped up this week in a government statement that recognized cost over-runs of defence department space programmes [Nature] as part of the problem.
Basically, the article explains that the existing GPS system could deteriorate in the near future if old functioning GPS satellites in space go out of commission without fresh replacements. So, with this buzz about the GPS system which many people in the world today use in their everyday activities (sometimes without knowing it too), how does GPS work anyway? Here is a very high-level basic concept of how GPS works.
Artist's rendition of a GPS satellite in orbit.
There are currently 31 functional GPS satellites orbiting the Earth (although 24 satellites is the bare minimum for GPS to work at any location on Earth). Each GPS satellite periodically broadcasts “messages,” which GPS receivers read and interpret. These messages that GPS satellites broadcast contain three pieces of information:
- Time: the (standardized) time that the message was sent from the GPS satellite
- Ephemeris: the orbital information of the GPS satellite (position, etc.)
- Almanac: general information about all functional GPS satellites in orbit
The time and ephemeris are key pieces of information that GPS receivers use in order to determine precise location of the receiver. A GPS receiver can calculate its distance to a GPS satellite by figuring out how long it took the message to travel from satellite to receiver (transit time), then using that with the known speed of light to calculate distance. Theoretically, a receiver is able to calculate its precise location if it knows the distance to three different GPS satellites, since there are three dimensions to a position in space. However, in practice four GPS satellites are used to calculate the precise location of a receiver, because a fourth satellite can be used to correct potential errors in the GPS receiver’s clock. There are economic reasons for this necessary correction. In order to make GPS devices affordable to the general public, GPS receivers are equipped with less-expensive adequately-functioning clocks (that are used to calculate transit time of the broadcasted messages) rather than super-expensive uber-accurate clocks. These less-than-perfect receiver clocks can cause large errors in calculating distances to GPS satellites if there is even a small error in the transit time of the GPS message due to the large magnitude of the speed of light.
Now, the number of GPS satellites in space affects the speed and accuracy of GPS algorithms. If a GPS receiver can track more GPS satellites than the minimum four satellites, then it can calculate its position faster and more accurately. There is a higher probability of tracking more satellites if there are more satellites in orbit. If there are fewer satellites in orbit (like if old satellites are put out of commission without a new replacement), then that can lead to less accurate GPS calculations from receivers. Don’t panic yet! It does not mean that the GPS system will suddenly stop working – rather, it will lose its robustness if the number of functioning GPS satellites in space were to decrease, which the article in Discovery Magazine explains.
[Image from Wikipedia]
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