Spectra Precision Survey Pro v4.5 Ranger User Manual User Manual

Page 200

User’s Manual – GPS Mode

190

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, you can measure
the fractional part of the carrier wave. If you can then calculate the

whole number of wavelengths between the SV and the antenna (the

ambiguity), you 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

You 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 you get an acceptable solution, you 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.