TECHNIQUES IN RELATIVE RTK GNSS POSITIONING
SIMON LIGHTBODY, GARY CHISHOLM, TRIMBLE MARINE GROUP
OVERVIEW
This paper discusses a Precise Real-Time Kinematic
(RTK) GNSS positioning technique called Moving
Base RTK developed by Trimble. In conventional RTK
positioning, the reference station must remain
stationary at a known location, while only the rover
receiver can move (see Figure 1 and Figure 2). With
Moving Base RTK, both the reference and rover
receivers can move while calculating a centimeter
accurate 3D vector between them (see Figure 3).
Figure 1: Autonomous GPS. Five meter accuracy.
Moving Base RTK is ideal for applications where the
precise relative offsets and closing velocities of two or
more moving vessels are required, for example, when a
shuttle tanker is approaching an FPSO, FSRU, or an
SPM. This is required for safety while docking and
alongside as well as saving time in such operations. It is
also ideal for dual-antenna use on a single vessel to
determine real-time true heading and an attitude vector.
In its simplest form, the Moving Base RTK solution
provides absolute vessel positioning, that is,
Figure 2: Conventional RTK. 3D precision positioning.
‘real-world’ positions that are accurate to autonomous
GPS level (approximately 5 m or 16 ft) but relative
positioning between each vessel accurate to a
centimeter. Enhanced Moving Base RTK enables either
shore or satellite broadcast DGPS or shore-based RTK
corrections to be included in the solution. Absolute
‘real-world’ positions are then improved to sub-meter
(for DGPS) or centimeter levels (for RTK). Regardless
of whether or not Enhanced Moving Base RTK is used,
the relative positioning between each vessel remains at
the centimeter level.
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Figure 3: Moving Base RTK. Precise 3D relative
positioning. No base station is used.
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MOVING BASE RTK
Moving Base RTK is implemented in the Trimble SPS
modular receivers, such as the SPS361, SPS461,
Precise tracking of the relative separation of two or
more moving vessels is also possible using at least two
SPS modular receivers.
SPS852 GNSS, and the SPS552H GNSS Heading
Add-on receiver. Moving Base RTK frees the
restriction of the required proximity to the fixed
reference station. Relative precise positioning is now
possible deep offshore.
Figure 6: SPS461 for vessel heading
Figure 7 shows the SPS461 receiver for vessel heading
as a function of antenna separation.
Figure 4: The SPS461 GPS dual-antenna receiver
Moving Base RTK is also used in the dual-antenna
SPSx61 GPS receiver and the SPS852/SPS552H GNSS
receiver pair to determine vessel orientation (see
Figure 5 and Figure 6).
Figure 5: The paired SPS852 and SPS552H GNSS
receivers for position, attitude, and heading on a vessel.
The receivers can be split and used on separate vessels.
Figure 7: Heading accuracy on a vessel using SPS461
With Moving Base RTK, the reference receiver can
now be mobile and broadcast precise GNSS signal
corrections (called Moving Base CMR), through the
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radio data link. The rover receiver then computes its
acting as the moving base reference station when
relative position. The resultant vector solution is
getting corrections to a fixed reference station.
accurate to centimeter-level, while the absolute location
Figure 9 shows a shore-based (fixed) reference station
of the reference to rover vector is only accurate to
sending GPS corrections (RTCM, CMR+™, or CMRx
autonomous GPS levels, approximately 5 meters
message types) to the moving reference station (the
(16 ft).
SPS852 receiver) on the SPM. The moving reference
Figure 8 shows the Moving Base RTK technique as
station receives corrections from the shore-based
applied to a Single Point Mooring (SPM) installation.
reference station and generates position solutions.
In this case, the receiver on the SPM transmits
corrections to the vessel receiver so vector A is
computed. The heading of the vessel (vector B) is also
determined by the same method.
Figure 9: Example of Enhanced Moving Base RTK for
SPM docking
The moving reference station can be operated in Low
Latency 1 or Synchronized modes. Moving Base CMR
message data is output to the rover receiver at 1, 5, or
Figure 8: Example of Moving Base RTK for SPM docking
10 Hz.
ENHANCED MOVING BASE RTK
The rover receiver accepts Moving Base CMR message
data from the moving reference station (SPM) and
Although the Moving Base RTK technique provides
generates a precise 3D vector solution (vector A).
centimeter-level vector components between the
moving reference station and the rover receiver, the
absolute coordinates of the reference station and rover
receiver are generally only known to 5 meters (16 ft).
The SPS modular receiver can perform DGPS,
Location RTK, or Precision RTK positioning while also
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The Low Latency positioning mode delivers 20 Hz position fixes with around 20
millisecond latency at a precision that is only slightly less accurate than
Synchronized RTK positioning. Based on the predictability of the reference station
phase data.
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Both vector A and B are available aboard the vessel to
An SPS461 dual-antenna GPS receiver is installed on
provide the user with closing velocity, distance, and
each vessel. On each vessel, one antenna is designated
vessel heading. When the roving reference station is
as the position antenna and the other as the vector
differentially corrected, the absolute location of the
antenna. Both antennas are mobile.
moving base reference station and the rover are
calculated to sub-meter levels or even Precise RTK
levels.
CHAINED MOVING BASE RTK
One of the vessels is designated the base vessel; the
SP461 receiver on this vessel becomes the Moving
Base RTK base station and outputs Moving Base CMR
messages to an external radio to the other vessel that is
designated as the rover vessel. To conserve radio
The moving Base RTK mode can chain together
frequencies, the SPS461 receiver on the base vessel
multiple moving reference receivers. See Figure 10.
(the Moving Base RTK base station) simultaneously
Consider the case of a lightering operation where a
transmits its position and heading.
shuttle tanker is approaching a Very Large Crude
Carrier (VLCC). In this example, the heading position
Wireless Ethernet can replace radios enabling the
and closing velocities of both vessels are required on
position and heading of all vessels to be available to
the shuttle, which is coming alongside the VLCC. See
each other. Select the best radio option based on
Figure 10.
required range and the number of vessels.
Thus, the base vessel transmits to the rover vessel, its
position (optional), heading, and Moving Base RTK
messages, enabling the rover vessel to display the
relative positions and current headings of both vessels.
You can then use this real-time information for:
•
Closing velocity bow and stern
•
Closing distance bow and stern
•
Relative orientation
•
Relative heave for mooring line adjustment
Figure 10: Example of the Chained Moving Base RTK
technique using two SPS461 GPS receivers for a
lightering installation
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CONCLUSION
The history of Dynamic Positioning (DP) over the last
25 years has seen equipment like Artemis range/bearing
systems installed on platforms, laser-based self-tracking
total stations, and the use of submeter-based relative
and absolute DGPS systems.
The Moving Baseline RTK GNSS system continues the
trend of innovation in this area. The advantages of
Moving Baseline RTK are:
•
Provides relative position, heading, and velocity
to centimeter-level accuracy
•
No line-of-sight issues like range and/or bearing
•
One base station serves multiple vessels that are
inbound
•
The combinations of relative and absolute
positioning make it suitable to the offshore
industry requirements
•
Provides relative heave to centimeter accuracy
•
With shore-based corrections:
o
It provides absolute position up to
centimeter-level accuracy
o
The option to compute the current Tide
value.
Trimble Marine Division, 10355 Westmoor Drive, Suite #100, Westminster, Colorado 80021, USA
© 2010, Trimble Navigation Limited. All rights reserved. Trimble, and the Globe & Triangle logo are trademarks of Trimble Navigation Limited, registered in the United States and in other
countries. CMR+ is a trademark of Trimble Navigation Limited. All other trademarks are the property of their respective owners.
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