Spectra Precision Survey Pro v3.80 Recon GPS User Manual - Manual (Page 22)

GPS User’s Manual
18
Differential Solutions: Types and Quality
Code Differential
Code differential solutions use the Coarse Acquisition (C/A)
navigation code transmitted on the GPS carrier wave. Because the
wavelength of the code segment is long (300m), code differential is the
least precise differential solution. Accuracies of 1-10 meters are
possible with DGPS using C/A code differential positioning.
Carrier Phase Differential
Highly precise coordinate differences can be measured using pseudo-
range positioning with the carrier signal wave. Because the
wavelength of the carrier wave is only 19 cm, mm level positioning is
possible. When the signal arrives at the antenna, we can measure
the fractional part of the carrier wave. If we can then calculate the
whole number of wavelengths between the SV and the antenna (the
ambiguity), we can add it to the fractional part and multiply by the
length of one cycle to measure a precise range.
Calculating the exact number of wavelengths uses a complicated least
squares process, which is often called ambiguity resolution. The
ambiguity resolution will yield either a float or a fixed solution.
•
Fixed Solution
We know the number of wavelengths will be a whole number.
Techniques are used to constrain the least squares solution to yield a
whole number. If we get an acceptable solution, we say that this
solution is fixed. A fixed solution will generate coordinate differences
precise to about 15-ppm (single frequency) or 5-ppm (dual frequency),
which translates into 15-mm or 5-mm over a 1-km base line.
Several things may prevent you from achieving a fixed solution: bad
multi-path, low number of satellites and bad constellation geometry,
poor radio link for corrections (RTK).
•
Float Solution
If the constraint algorithm does not produce an acceptable fixed
solution, then the ambiguity is allowed to be a decimal (float)
number. A float solution will generate coordinate differences precise
to about 100 to 500-ppm, which translates into 0.1-m to 0.5-m over a
1-km base line.