TECHNIQUES IN RELATIVE RTK GNSS POSITIONING

SIMON LIGHTBODY, GARY CHISHOLM, TRIMBLE MARINE GROUP

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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.

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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

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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.

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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

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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.