SEPTENTRIO PolaRx5TR, Reference Manual

Page: 63 / 512

2 Operation Details
Note on the RTCM v2.x corrections
: the receiver takes the
τ
gd
parameter transmitted by
the GPS satellites into account during the computation of the pseudorange corrections, as
prescribed in v2.2 and v2.3 of the RTCM standard. The RTCM standard version 2.1 is ambigu-
ous in this respect: it does neither prescribe nor discourage the use of
τ
gd
. The receiver can
be configured in both modes using the command
setRTCMv2Compatibility
.
2.4.3 RTK Positioning
Real-Time Kinematic (RTK) is a carrier phase positioning method where the carrier phase
ambiguities are estimated in a kinematic mode.
To work in RTK mode, the receiver requires the reception of RTK messages. Both the RTCM
and the CMR message formats are supported. Multiple-base RTK is not supported: by de-
fault, the receiver selects the nearest base station if more than one base station is available.
In RTK mode, the absolute position is reported in the
PVTCartesian
or
PVTGeodetic
SBF
blocks, and the baseline vector is reported in the
BaseVectorCart
and
BaseVectorGeod
SBF blocks.
2.4.3.1 Integer Ambiguities (RTK-fixed)
The key to high-accuracy carrier phase positioning is the fixing of the carrier phase integer
ambiguities. Under normal circumstances the receiver will compute the integer ambiguities
within several seconds and yield an RTK-fixed solution with centimeter-level accuracy. The
less accurate pseudorange measurements will not be used. As long as no cycle slips or loss-
of-lock events occurs, the carrier phase position is readily available.
RTK with fixed ambiguities is also commonly referred to as phase positioning using ’On-The-
Fly’ (OTF) ambiguity fixing. The RTK positioning engine of the receiver uses the LAMBDA
method
(1)
developed at Delft University, department of Geodesy.
2.4.3.2 Floating Ambiguities (RTK-float)
When data availability is low (e.g. low number of satellites) or when the data are not of suf-
ficient quality (high multipath), the receiver will not fix the carrier phase ambiguities to their
integer value, but will keep them floating. At the start of the RTK-float convergence process,
the position accuracy is equal to that of code-based DGPS. Over the course of several min-
utes the positional accuracy will converge from several decimeters to several centimeters as
the floating ambiguities become more accurate.
2.4.4 Precise Point Positioning
Precise Point Positioning (PPP) provides high accuracy positioning without the need for a
local base station. PPP uses precise satellite orbit and clock corrections computed by a global
(1)
Teunissen, P.J.G., and C.C.J.M. Tiberius (1994) Integer least-squares estimation of the GPS phase ambiguities.
Proceedings of International Symposium on Kinematic Systems in Geodesy, Geomatics and Navigation KIS’94,
Banff, Canada, August 30-September 2, pp. 221-231.
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