Specifications

44 Z-Family Technical Reference Manual
Fast RTK
In this mode the remote receiver’s update rate is selectable up to 10Hz, and is
independent of the rate at which it receives type 18, 19, 20, 21, or DBN messages.
Use the command $PASHS,NME,PER to control the update rate. The latency of
position is less than 50 milliseconds. Typical accuracy (1
σ
horizontal) in centimeters
is equal to the base-remote data latency in seconds, for data latency of up to 10
seconds. For base-remote data latency of greater than 10 seconds and less than 30
seconds, the accuracy degrades up to 2 meters. In Fast RTK mode the receiver will
always provide the best possible position solution at the data rate selected by the user.
If the accuracy degrades for any reason (such as cycle slips, lost radio link, etc), this
will be reflected in the RRE message. By contrast, the Synchronized RTK position
only provides positions when a fresh set of base station data has been received.
Fast RTK should be used when you need position updates at regular intervals, (such
as in machine control). Synchronized RTK should be used when you can afford to
wait a few epochs for the highest available accuracy (such as in surveying).
During Fast RTK mode the receiver runs synchronized RTK mode in the background
at the same rate that it receives base station data. If the receiver detects a cycle slip, it
fixes the cycle slip at the next synchronized epoch (typically within one second).
If you are using Fast RTK, monitor position accuracy with RRE message.
Example: Fast RTK running at 10Hz. Type 18 & 19 message updates at 1Hz.
Cycle slip occurs at time 12:00:00.1 (100 milliseconds past noon). There
will be 9 epochs of Fast RTK positions with an error of a few decimeters,
each will have a corresponding RRE message showing that there is an error.
Then, at 12:00:01.0 (1 second past noon) the cycle slip will be repaired.
Position Latency
Base data latency, discussed above, is the delay between when a base station
measures the GPS signals and when the remote receiver receives the RTCM or DBN
messages. Position latency is the delay between when the remote receiver measures
the GPS signals and when the position is available at the serial port. In other words,
position latency is the delay in providing the user’s actual position to the user.
Position latency is typically less than 50 milliseconds, it varies with the number of
satellites in view.
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