User's Manual Part 1

Positioning Modes of Operation Chapter 5
OEMV Family Installation and Operation User Manual Rev 5B 93
which is affected primarily by the speed of the differential data link. The MATCHEDPOS log contains
the matched RTK solution and can be generated for each processed set of base station observations.
The RTKDATA log provides additional information about the matched RTK solution and shows a
verification flag in the "rtk info" field. It is recommended that you check this verification flag,
especially in severe environments.
The Low-Latency RTK position and velocity are computed from the latest local observations and
extrapolated base station observations. This supplies a valid RTK position with the lowest latency
possible at the expense of some accuracy. The degradation in accuracy is reflected in the standard
deviation and is summarized in Section 5.3.2, Position Solutions starting on Page 82. The amount of
time that the base station observations are extrapolated is provided in the "differential lag" field of the
position log. The Low-Latency RTK system extrapolates for 60 seconds. The RTKPOS log contains
the Low-Latency RTK position when valid and an "invalid" status when a low-latency RTK solution
could not be computed. The BESTPOS log contains the low-latency RTK position when it is valid,
and superior to the pseudorange-based position. Otherwise, it contains the pseudorange-based
position. Similarly, RTKVEL and BESTVEL contains the low-latency RTK velocity.
RT-20 solutions always use floating L1 ambiguities. When valid L2 measurements are available, RT-2
solutions have other solution types that depend on convergence time, baseline length, number of
satellites, satellite geometry and the level of ionospheric activity detected.
RT-2 Performance
RT-2 software, in both static and kinematic modes, provides accuracies of 1 cm +1 ppm RMS for
baselines from 0 to 40 km. A plot of convergence versus baseline length is shown in Figure 30 on
Page 94 for typical
multipath, ionospheric, tropospheric, and ephemeris errors, where typical is
described as follows:
• A typical multipath environment would provide no carrier-phase double-difference
multipath errors greater than 2 cm or pseudorange double-difference multipath
errors greater than 2 m on satellites at 11° elevation or greater. For environments
where there is greater multipath, please consult NovAtel Customer Service.
• Typical unmodeled ionospheric, tropospheric and ephemeris errors must be within
2σ of their average values, at a given elevation angle and baseline length. It is
assumed that the tropospheric correction is computed with standard atmospheric
parameters. All performance specifications are based on a PDOP < 2 and
continuous tracking of at least 5 satellites (6 preferred) at elevations of at least 11.5°
on both L1 and L2.
 Refer to the GPGST usage box, like this one, in the OEMV Firmware Reference
Manual for a definition of RMS and other statistics.
RTKPOS or BESTPOS logs contain some error due to predictions from base station observations. The
expected error of a RTKPOS or BESTPOS log will be that of the corresponding MATCHEDPOS log
plus the appropriate error from Table 19.
There are no data delays for a matched log and therefore no need to add an additional error factor.