Global Positioning Systems Demystified - Tech Details
(Page 3 of 5 )
Tech Details
Now that you've understood the underlying process we can look further and get a little bit technical. I am sure that you've wondered, "What's in the signal?" Kudos to human curiosity. So, let's answer that question before we move on to popular brands of receivers and discuss their reliability, precision and accuracy.
The GPS satellites transmit two radio wave signals. These low power radio signals are split into two categories: L1 and L2, respectively. In our case the L1 frequency is used at 1575.42 MHz on the UHF band. L1 represents the signal that's used for civilian GPS.
This signal can travel through clouds, plastic, acrylic and glass, but not through substantial solid objects such as metals (e.g., buildings), mountains, et cetera. This signal contains three valuable pieces of data. All of these three are crucial to the fullest functionality of the GPS.
The first is pseudo-random code and it stands for the identification (ID) that represents the source satellite. The second bit of information is ephemeris data; it contains the precise locations of the GPS satellites. This is used by the receiver unit to match via the ID with the source satellite. The third is called almanac data; it transmits the date and time of the source satellite and also its current condition.
The Navigation Message (NM) is broadcast at 50 bits per second and contains the almanac and ephemeris data. The almanac transaction takes up to 12.5 minutes, while the ephemeris takes 30 seconds. Alongside the NM, the satellites broadcast the Coarse/Acquisition (C/A) and Precise (P) codes. C/A is free for civilian usage, while the latter is strictly for military measurements. The C/A contains the pseudo-random code.
All of the 24 satellites are located universally on 6 circular orbit planes with 55 degrees of tilt evaluated to the equator. The additional 6 satellites are placed in a non-uniform manner. Their purpose is to increase the localization precision, enhance the availability, and ultimately serve as backup satellites in case of main satellite failure.
Each of the satellites are maintained and verified on a continuous basis from local monitoring centers like Hawaii, Colorado Springs, Ascension Island and Kwajalein. In case of data inconsistency, the internal clock and other specifications of the malfunctioning satellite are fixed. It is crucial to keep the satellites synchronized with atomic clocks.
Common Error Sources
One hundred percent precision is impossible. The minimum error rate is approximately three meters if only C/A codes are used. If the receiver has the access to additional P codes then this error rate is lowered to around 30 centimeters. This is critical for military applications.
Most common sources of errors are the following: ionosphere and troposphere delays (the signal slows down but this speed loss is predicted by the receiver), signal multi-path distortion (the signal gets reflected by mountains, buildings and such, therefore the distance between the satellite and receiver is extended), orbital or ephemeris errors (the wrong location is sent by the satellite), geographical location (the receiver should be able to have access to multiple GPS satellites -- the more, the better), clock errors (the internal clock in the receiver is incorrect), satellite geometry (when some of the satellites are shading each other, thus, resulting in poor geometry), etc.
Next: User GPS Devices >>
More Opinions Articles
More By Barzan "Tony" Antal
/ 20 
