Float And Fixed Solutions - ashtech GG Surveyor Reference Manual

Float and Fixed Solutions

When the receiver is in RTK mode the crucial difference from Differential mode is
that it uses the carrier phase measurement to generate the range measurements to
centimeter accuracy. The receiver can measure the fractional part of the carrier phase
to centimeter accuracy, it derives the integer number of full carrier phase wavelengths
by processing both the carrier and code phase measurements. This process of deriving
the integer numbers is known as integer ambiguity resolution or carrier phase
initialization. This carrier phase initialization is only necessary following power-on,
or after the receiver has lost lock on the satellites (e.g. after passing under a bridge).
The receiver performs carrier phase initialization automatically. The receiver does not
have to be stationary while initializing. Once the receiver is initialized it will provide
centimeter-level accuracy, while moving, in real time. The time for carrier phase
initialization is a few seconds up to several minutes, depending on baseline length,
number of satellites in view, and required reliability; these are discussed in the next
section.
During the carrier phase initialization the receiver is said to be in "float" mode, once
initialization is complete the receiver is said to be in "fixed" mode. This terminology
derives from computer terminology: floating-point numbers (real numbers) and fixed
numbers (integers).
When in float mode the accuracy will range from Differential accuracy (1m) down to
sub-decimeter. The longer the receiver has been in float mode the higher the
accuracy. The convergence to 20 cm accuracy takes approximately five minutes,
convergence to 10 cm accuracy requires approximately ten minutes. Convergence
time is a function of baseline length and number of satellites in view. When the
receiver fixes integers, accuracy makes a quantum change to centimeter level.
The POS and GGA messages have fields which indicate whether the receiver is in
float or fixed mode.
Carrier Phase Initialization
The time required for carrier phase initialization is a function of base-remote baseline
length, number of satellites in view, satellite geometry, and required reliability. With
a large number of satellites in view (≥14), initialization time can be as low as a few
seconds. With fewer satellites in view, the receiver takes as long as necessary to
guarantee the required reliability.
Reliability
The process of carrier phase initialization has a non-zero probability of error. If an
error is made the receiver will fix the integers to the wrong value. This will result in
floating point accuracy (typically between 10cm and 1m). After an error in fixing
integers the receiver automatically detects and corrects the error when the satellite
60 GG Surveyor GPS+GLONASS Reference Manual