GPS phone tracking errors explained
The global positioning system(GPS), is a space based global navigation satellite system that provides location and time information anytime, anywhere on or near the earth.GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the earth. Each satellite continually transmits messages that include time of message transmission, precise orbital information and general system health and rough orbits of all GPS satellites.
GPS error sources and analysis:
GPS errors are affected by geometric dilution of precision and depend on signal arrival time errors, numerical errors, atmospherics effects, ephemeris errors, multipath errors and other effects
One of the significant sources of error is the GPS receiver’s clock. As an error of one microsecond corresponds to an error of 3oometers, it’s important to have an extremely expensive and accurate clock for working of GPS receiver.
Measures for improving GPS efficiency:
Integrating external information into calculation process can materially improve efficiency. Such augmentation systems are generally named or described based on the arrival of information. Additional error information such as clock drift, ephemera or ionospheric delay) are transmitted by some systems, prior errors are characterized by others and additional navigation or vehicle information is provided by few others.
Precise monitoring and measurement of GPS signals in additional or alternate ways can improve accuracy of GPS systems.
Unpredictable delay through the ionosphere accounts for the remaining largest error. Errors remain in spite of ionospheric parameters broadcasted by spacecraft. This forms one of the reasons for GPS spacecraft transmitting IN L1 and L2 frequencies. Ionospheric delay is a well defined function of frequency and the total electron content9TEC) along the path, so TEC is determined by the measurement of arrival time distance between the two frequencies which thereby determines the ionospheric delay at each frequency. Currently, decoding by military receivers is slow and is currently available only on specialized equipment. In future, additional civilian codes are expected to be transmitted on L2 and L5 frequencies. Then, all users will be able to perform dual frequency measurements and directly compute ionospheric delay errors.
Time keeping and leap seconds:
The atomic clocks are set to GPS time while most of the clocks are synchronized to coordinated universal time. The time difference is not corrected to match the rotation of the earth, so it does not contain leap seconds or other corrections that are added periodically to coordinated universal time. In the year 1980, GPS time was set to match coordinated universal time. After that, there has been deviation due to lack of corrections meaning that GPS time remains at a constant offset with international atomic time. To keep on board clocks synchronized with ground clocks, periodic corrections are performed.
The accuracy of GS time is about 14 nanoseconds.
The GPS date is expressed as a week number and a second into week number as opposed to the yea, month and day format of Gregorian calendar. This concern is addressed by usage of a 13 – bit field that repeats every 8,192 weeks (157 years) thus lasting until the year 2137.