Characteristics of GPS Surveying and GPS Navigation Some characteristics of GPS Satellite Surveying are: The points being coordinated are stationary. GPS data are collected over some "observation session". Relative positioning modes of operation, and hence high accuracies.
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Some characteristics of GPS Satellite Surveying are:
The points being coordinated are stationary.
GPS data are collected over some "observation session".
Relative positioning modes of operation, and hence high accuracies.
The measurements are made on the L-band carrier wave, hence requiring special instrumentation and software.
Mostly associated with the traditional surveying and mapping functions.
Some characteristics of GPS Satellite Navigation are:
The points being coordinated are generally in motion.
GPS is collected for an "instant", and the solution is obtained in real-time.
Absolute and relative positioning modes of operation, of comparatively low accuracy.
The measurements are typically made on the PRN codes, and requires the processing of pseudo-range data.
Mostly associated with defining safe passage of ships and aircraft.GPS Methods
any contamination by systematic errors is of greater concern.
Appropriate processing techniques must t be used.
primary drawback is its range "ambiguity".
In GPS surveying the major biases are accounted for in the following ways:
Differencing data collected simultaneously from two or more GPS receivers,
to several GPS satellites, between satellites and between receivers.
This eliminates, or significantly reduces, most of the biases.
All position results are therefore expressed relative to (fixed) datum stations.
The "ambiguity" bias is often estimated, though a weaker solution can be obtained from the appropriate triple-differenced observable (section 6.3).distinctions in data processing (minimise the effect of biases in the measurements)
Pseudo-range data is relatively "noisy
the significant biases are accounted for
In the point positioning mode,
satellite clock error is ignored, (smaller than the measurement noise)
Receiver clock error is estimated in real-time through redundant measurements,
all data is contaminated by the same bias.
In the relative positioning mode, all satellite and propagation biases are significantly reduced.
In SURVEYING mode
the receivers are stationary,
data is collected over some observation period
permits the ambiguities to be reliably estimated strong solution obtained.
There are alternative means of estimating ambiguities that permit real-time kinematic baseline determination to be carried out as well.
Cartesian components can be easily transformed at the result presentation stage
GPS phase processing result in relative 3-D coordinates.
3-D baseline components that are computed
a minimum of one station held fixed in a GPS adjustment
GPS pseudo-range solutions are single point
3-D coordinates are independently determined for each receiver
not as accurately determined
often used to provide apriori values for the subsequent phase solutions.THE NATURE OF GPS SOLUTIONSmost distinctive feature of GPS surveying:determination of 3-D coordinates
All results relate to the GPS antenna phase centres
must be corrected to the groundmark by applying the antenna height and eccentric station offsets.
All results are, nominally, referred to the WGS84 reference system.
fractional phase (measured as an angle in the range 0° to 360°, where 360° corresponds to
about 19cm for the L1 phase and 24cm for the L2 phase
CR is the current reading on a zero-crossing "counter“
only registers the number of whole cycles since lock-on when the counter had an initial value of CRo (usually zero).
in square brackets is an integer.
The additional electronics to count the whole cycles since lock-on is the identifying characteristic of GPS "surveying" receivers.
spread spectrum signal is received at the GPS antenna,
the signal power is below the background noise the ranging code modulations are removed
the satellite signal collapses into the original very narrow carrier frequency band
signal power is again boosted well above the background noise
the broadcast message can then be extracted.
The incoming and receiver-generated sine waves are continuously aligned within a "phase-lock loop"
ultimate aim to obtain an ambiguity-fixed solution
Triple- and double-differenced data solutions have different strengths and weaknesses
Triple-, double- (ambiguity-free) and double-differenced (ambiguity-fixed) solutions represent a hierarchy of processing strategies that are generally applied in sequence: first the triple-differenced solution,
through to the double-differenced solutions,
phase data collected beyond the minimum necessary to ensure an ambiguity-fixed solution is obtained has almost no influence on the final results
sequential transition from an 100% ambiguity-free solution to an 100% ambiguity-fixed solution
REOCCUPATION GPS SURVEYING TECHNIQUESCentimetre positioning accuracy with two occupations per site, each for a short static observation period (few minutes) ...
STOP & GO" GPS SURVEYING TECHNIQUESCentimetre accuracy positioning during very short static observation periods (<1minute) ... receiver moves carefully from point to point
An additional requirement is that the stationary reference receiver must continue to track all the satellites being tracked by the roving receiver. The accuracy attainable is about the same as for the "rapid static" technique. As with the"reoccupation" technique, the receiver must have the ability to handle data files from several different sites. The software then has to sort out the recorded data for the different sites, and to differentiate the "kinematic" or "go" data (not of interest) from the "static" or "stop" data (of interest). It can be implemented in real-time if a communications link is provided to transmit the "carrier-range" data from the reference receiver to theroving receiver(s).
One particular negative characteristics of this technique is the requirement that phase lock must be maintained by the roving receiver as it moves from site to site. This requires special hardware mounts on vehicles if the survey is carried out over a large area.
static positioning (the "stop" part),
kinematic movement of the antenna (the "go" part).
ambiguities must be determined by some initialisation process
all positioning takes place with carrier-range observations (that is, within region B of prior figure above).
There are several methods of ambiguity resolution,
including standard static baseline determination,
observing a known baseline,
"antenna swap" method,
determination of ambiguities "on-the-fly" (that is, as the antenna moves).
Such ambiguity resolution (or initialisation) takes place at the start of the survey (before moving to the first point to be surveyed), and at any time loss-of-lock occurs.