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8 RTK Mapping
High accuracy GPS receivers measure the phase difference between
the carrier waves. The wavelengths of both the L1 and L2 waves are
known, so satellite ranges can be determined by adding the phase
difference of the L1 and L2 signals to the total number of waves that
occur between each satellite and the antenna.
Determining the fractional component of the signals is relatively
simple. However, determining the full number of carrier wave cycles
between the antenna and the satellite is more complex. The
determination of the integer number of waves is referred to as the
integer ambiguity search or, more simply, search. For Real-Time
Kinematic operations that require centimeter-level accuracy, the
integer is determined during a process called initialization.
The initialization process for RTK techniques is, in essence, solving
ambiguities that exist between the base and roving receivers and the
satellites they are observing. The resultant solution of these
ambiguities is referred to as either being Fixed or Float as defined in
Table 8.1.
Table 8.1 Fixed versus Float solutions
Fixed solution Float solution
A fixed solution is obtained when the integer
values of the wave lengths are able to be
computed with confidence. A fixed solution
achieves centimeter levels of precision.
8.2.3 RTK Correction Messages
During an RTK data collection session, the base station broadcasts
correction messages using the radio modem. There are two types of
message that can be broadcast: CMR and RTCM 2.1. These messages
are configured in the Asset Surveyor software using Configuration /
Communication options.
190 TSC1 Asset Surveyor Operation Manual
A float solution is obtained when a definitive
integer value for the wave lengths cannot be
computed with confidence. Float solutions result
from unmodeled atmospheric, orbital and other
errors. A float solution is not as precise as a
fixed solution.