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TECHNIQUES IN RELATIVE RTK GNSS POSITIONING
SIMON LIGHTBODY, GARY CHISHOLM, TRIMBLE MARINE GROUP
www.trimble.com 2
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).
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,
‘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.
Figure 1: Autonomous GPS. Five meter accuracy.
Figure 2: Conventional RTK. 3D precision positioning.
Figure 3: Moving Base RTK. Precise 3D relative positioning. No base station is used.
www.trimble.com 3
MOVING BASE RTK
Moving Base RTK is implemented in the Trimble SPS
modular receivers, such as the SPS361, SPS461,
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 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.
Precise tracking of the relative separation of two or
more moving vessels is also possible using at least two
SPS modular receivers.
Figure 6: SPS461 for vessel heading
Figure 7 shows the SPS461 receiver for vessel heading
as a function of antenna separation.
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
www.trimble.com 4
radio data link. The rover receiver then computes its
relative position. The resultant vector solution is
accurate to centimeter-level, while the absolute location
of the reference to rover vector is only accurate to
autonomous GPS levels, approximately 5 meters
(16 ft).
Figure 8 shows the Moving Base RTK technique as
applied to a Single Point Mooring (SPM) installation.
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 8: Example of Moving Base RTK for SPM docking
ENHANCED MOVING BASE RTK
Although the Moving Base RTK technique provides
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
acting as the moving base reference station when
getting corrections to a fixed reference station.
Figure 9 shows a shore-based (fixed) reference station
sending GPS corrections (RTCM, CMR+™, or CMRx
message types) to the moving reference station (the
SPS852 receiver) on the SPM. The moving reference
station receives corrections from the shore-based
reference station and generates position solutions.
Figure 9: Example of Enhanced Moving Base RTK for SPM docking
The moving reference station can be operated in Low
Latency1 or Synchronized modes. Moving Base CMR
message data is output to the rover receiver at 1, 5, or
10 Hz.
The rover receiver accepts Moving Base CMR message
data from the moving reference station (SPM) and
generates a precise 3D vector solution (vector A).
1 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.
www.trimble.com 5
Both vector A and B are available aboard the vessel to
provide the user with closing velocity, distance, and
vessel heading. When the roving reference station is
differentially corrected, the absolute location of the
moving base reference station and the rover are
calculated to sub-meter levels or even Precise RTK
levels.
CHAINED MOVING BASE RTK
The moving Base RTK mode can chain together
multiple moving reference receivers. See Figure 10.
Consider the case of a lightering operation where a
shuttle tanker is approaching a Very Large Crude
Carrier (VLCC). In this example, the heading position
and closing velocities of both vessels are required on
the shuttle, which is coming alongside the VLCC. See
Figure 10.
Figure 10: Example of the Chained Moving Base RTK technique using two SPS461 GPS receivers for a lightering installation
An SPS461 dual-antenna GPS receiver is installed on
each vessel. On each vessel, one antenna is designated
as the position antenna and the other as the vector
antenna. Both antennas are mobile.
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
frequencies, the SPS461 receiver on the base vessel
(the Moving Base RTK base station) simultaneously
transmits its position and heading.
Wireless Ethernet can replace radios enabling the
position and heading of all vessels to be available to
each other. Select the best radio option based on
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
www.trimble.com 6
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.