GPS receivers can receive time information accurate to nanoseconds that can
be used for timing; forecast ephemeris for forecasting the approximate position
of satellites in the next few months; broadcast ephemeris for calculating
satellite coordinates required for positioning , with an accuracy of several
meters to tens of meters (different for each satellite and changes at any time);
and GPS system information, such as satellite status, etc.
The distance from the satellite to the receiver can be obtained by the
measurement of the GPS receiver's code. Because it contains the error of the
receiver's satellite clock and the error of atmospheric propagation, it is
called pseudorange. The pseudorange measured for the 0A code is called the UA
code pseudorange, with an accuracy of about 20 meters, and the pseudorange
measured for the P code is called the P code pseudorange, with an accuracy of
about 2 meters.
The GPS receiver decodes the received satellite signal or uses other
techniques to recover the carrier after removing the information modulated on
the carrier. Strictly speaking, the carrier phase should be called the carrier
beat phase, which is the difference between the carrier phase of the received
satellite signal affected by the Doppler shift and the phase of the signal
generated by the local oscillation of the receiver. Generally, the measurement
is made at the epoch time determined by the receiver clock and the tracking of
the satellite signal is maintained, and the phase change value can be recorded,
but the initial phase value of the receiver and the satellite oscillator at the
beginning of the observation is unknown. The phase integer of the initial epoch
is also unknown, that is, the integer ambiguity, which can only be solved as a
parameter in data processing. The accuracy of the phase observations is as high
as millimeters, but the premise is to solve the ambiguity of the whole circle,
so the phase observations can only be used when there is a relative positioning
and there is a period of continuous observations, and the positioning accuracy
better than the meter level can only be achieved. Phase observations can be
used.
According to the positioning method, GPS positioning is divided into
single-point positioning and relative positioning (differential positioning).
Single-point positioning is a method of determining the position of a receiver
based on the observation data of a receiver. It can only use pseudo-range
observations and can be used for rough navigation and positioning of vehicles
and ships. Relative positioning (differential positioning) is a method of
determining the relative position between observation points based on the
observation data of two or more receivers. Relative positioning using phase
observations.
GPS observations include errors such as satellite and receiver clock
errors, atmospheric propagation delays, and multipath effects, and are also
affected by satellite broadcast ephemeris errors during positioning
calculations. Cancellation or weakening, so the positioning accuracy will be
greatly improved. The dual-frequency receiver can offset the main part of the
ionospheric error in the atmosphere according to the observations of the two
frequencies. ), a dual-frequency receiver should be used.
During positioning and observation, if the receiver moves relative to the
surface of the earth, it is called dynamic positioning. Pseudo-range
differential positioning with meter-level accuracy, or centimeter-level phase
differential positioning (RTK) for measuring stakeouts, etc., real-time
differential positioning requires a data link to transmit the observation data
of two or more stations together in real time for calculation. During
positioning observation, if the receiver is stationary relative to the earth's
surface, it is called static positioning. When conducting control network
observation, this method is generally used to observe simultaneously by several
receivers, which can maximize the use of GPS. Positioning accuracy, receivers
dedicated to this purpose are called geodetic receivers and are the best
performing class of receivers. At present, GPS has been able to meet the
accuracy requirements of crustal deformation observation, and the perennial
observation stations of IGS have been able to form a millimeter-level global
coordinate frame.
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