The position calculated by a GPS receiver requires the position of the satellite and the measured delay of the received signal at the current instant. Accuracy is dependent on position and signal delay.
When entering the delay, the receiver compares a series of bits (binary unit) received from the satellite with an internal version using (a correlation engine hardwired into a specialized chip, based on the Gronemeyer '216 patent).[4][5] When comparing the limits of the series, electronics can set the difference to 1% of a BIT time, or approximately 10 nanoseconds per C/A code. Since then, GPS signals propagate at the speed of light, which represents an error of 3 meters. This is the minimum possible error using only the GPS C/A signal.
Position accuracy is improved with a P(Y) signal. Assuming the same accuracy of 1% BIT time, the P(Y) signal (high frequency) results in an accuracy of plus or minus 30 centimeters. Errors in electronics are one of several reasons that impair accuracy (see table).
The accuracy of even standard (non-military) GPS receivers can also be improved through software and real-time techniques. This has been tested on a global navigation satellite system (GNSS) such as NAVSTAR-GPS. The proposal was based on the development of a precision relative positioning system equipped with low-cost receivers. The contribution was made through the development of a methodology and techniques for the processing of information that comes from recipients.[6].
Factors Affecting Data Quality:.
Satellite Errors.
It refers to the errors that affect the quality of the results obtained in a GPS measurement.
Orbital errors (ephemerides): Because satellites do not follow a normal Keplerian orbit due to perturbations, better orbit estimators are required, which implies a process that is hampered by insufficient knowledge of the forces acting on the satellites. These errors affect the determination of the satellite's position at a given instant with respect to a selected reference system. To reduce the error, instead of using the ephemeris captured in the receiver, precise ephemeris calculated by the IGS and NASA days after the measurement are used.
Clock errors: They refer to variations in the time system of the satellite clock, produced by the drift of the oscillators and those caused by the action of relativistic effects. These errors lead to a differential between the time system of the satellite and the GPS system, which will not be constant for all satellites but varies from one to another, because the standard frequency of the satellite oscillators has defined values for each satellite.
Geometric configuration errors: uncertainties in positioning are a consequence of distance errors associated with the geometries of the satellites used, four or more. The effect of geometry is expressed by the parameters of the so-called Geometric Dilution of Precision (GDOP), which considers the three parameters of three-dimensional position and time. The GDOP value is a composite measure that reflects the influence of the satellite constellation on the combined accuracy of a station's time and position estimates.
For this purpose, the following are considered: PDOP: Precision dilution for the position. HDOP: Dilution of precision for position. VDOP: Vertical Precision Dilution. TDOP: Dilution precision for time.
Errors coming from the propagation medium..
Ionospheric refraction errors: At the GPS frequency, the range of the error due to refraction in the ionosphere goes from 50 meters (maximum, at noon, a satellite near the horizon) to 1 meter (minimum, at night, a satellite at the zenith). Because ionospheric refraction depends on frequency, the effect is estimated by comparing measurements made at two different frequencies (L1=1575.42 MHz. and L2=1227.60 MHz.). Using two stations, one with known coordinates. We can correct timing errors. The travel time delay in the ionosphere depends on the electron density along the signal path and the signal frequency. An influential source on the density of electrons is the solar density and the Earth's magnetic field. Therefore the ionospheric refraction depends on the time and location of measurement.
Tropospheric refraction errors: Tropospheric refraction produces errors between 2 meters (satellite at zenith) and 25 meters (satellite at 5° elevation). Tropospheric refraction is independent of frequency, therefore a two-frequency measurement cannot determine the effect but this error can be compensated for using tropospheric models.
Multipath: It is the phenomenon in which the signal arrives through two or more different paths. The difference in path lengths causes interference of the signals when they are received. Multipath is usually noticed when measuring near reflective surfaces, to minimize its effects an antenna capable of making discriminations against signals arriving from different directions is used.
Errors in reception..
These errors depend on both the measurement mode and the type of receiver used.
Noise: As the standard deviation of noise in the measurement is proportional to the wavelength in the code. Noise in carrier phase measurements conditions the amount of data and tracking time required to achieve a certain level of precision, making continuous tracking and measurements crucial to ensure said precision.
Antenna phase center: This can change depending on the azimuth elevation angle (figure 15). The apparent electrical phase center of the GPS antenna is the precise navigation point for relative work. If the antenna phase center error is common for all points during the measurement, they cancel. In relative measurements, all the antennas in the network are used aligned in the same direction (usually magnetic north) so that the movement of the phase center of the antenna is common and cancels with a first approximation.[7].