User's Manual Part 1

98 OEMV Family Installation and Operation User Manual Rev 5B
Chapter 5 Positioning Modes of Operation
ROVER TRACKING LOSS
If less than 4 satellites are maintained, then the RTK filter can not produce a position. When this
occurs, the BESTPOS and PSRPOS logs will be generated with differential (if pseudorange
differential messages are transmitted with RTK messages) or single point pseudorange solutions if
possible.
DIFFERENTIAL LINK BREAKDOWN
1. Provided the system is in steady state
, and the loss of observation data is for less than 60
seconds, the Low-Latency RTK positions will degrade according to the divergence of the
base observation extrapolation filters. This causes a decrease in accuracy of about an order of
magnitude per 10 seconds without a base station observation, and this degradation is
reflected in the standard deviations of the low latency logs. Once the data link has been re-
established, the accuracy will return to normal after several samples have been received.
2. If the loss of differential corrections lasts longer than 60 seconds
, the RTK filter is reset and
all ambiguity and base model information is lost. The timeout threshold for RTK differential
corrections is 60 seconds, but for Type 1 pseudorange corrections, the default timeout is 300
seconds. Therefore, when the RTK can no longer function because of this timeout, the
pseudorange filter can produce differential positions for an additional 240 seconds by default
(provided pseudorange differential messages were transmitted along with the RTK
messages) before the system reverts to single point positioning. Furthermore, once the link is
re-established, the pseudorange filter produces an immediate differential position while the
RTK filter takes several additional seconds to generate its positions. The base model must be
healthy before solutions are logged to the low latency logs, so there is a delay in the use of
real time carrier positioning to the user once the link has been re-established. The RTK logs,
such as MATCHEDPOSA/B, use matched observations only (no extrapolated observations).
These matched observations will be available after three base observations are received, but
will typically have about 1.5 seconds latency associated with them, although longer latencies
may occur with some slower data links.
3. The RTK system is based on a time-matched double difference observation filter. This means
that observations at the rover site have to be buffered while the base station observation is
encoded, transmitted, and decoded. Only 8 seconds of rover observations are saved, so the
base station observation transmission process has to take less than 8 seconds if any time
matches are to be made. In addition, only rover observations on even second boundaries are
retained, so base station observations must also be sent on even seconds if time matches are
to be made.