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Reporting back to base

The largest GPS base station network used so far in the UK has been set up on the Channel Tunnel Rail Link. Richard Bennett reports

The Channel Tunnel Rail Link is a long, thin and very complex site. Section one involves six contracts, five contractors, and numerous subcontractors. By any standards this means a massive amount of survey data to be collected and processed.

Using optical survey on a ribbon development such as this can present problems with data misfits, particularly when a number of contractors are involved. But GPS avoids these ambiguities, according to Chris Kelly, project manager for client Rail Link Engineering (RLE), supplying a common reference, allowing all contractors to dovetail in.

Once the final alignment of the route had been agreed, a differential GPS network was set up to cover the whole length, with seven fixed real time kinematic base stations - the largest in the UK so far. The system was supplied by Tr im b l e .

Each base station receives GPS satellite signals, post processes the positional error and broadcasts it by radio link to any roving receiver within range. Location can be obtained within three seconds, 24 hours a day and in all weathers.

The accuracies produced - typically +10mm in plan and + 20mm in level - are acceptable for setting out.

For the CTRL the base station network was homogenised, so contractors avoided the potential for radio frequency conflict setting up stations of their own. Contractors simply purchased their own approved roving GPS receivers. Each base station was sited to optimise coverage of the radio signal.

UK regulations restrict signal strength to 0.5MW which means a range of 16km.

For the first time on a major UK road and rail project, CTRL design was done digitally using Microstation, with the Inroads and Inrail modules from Intergraph. This presented difficulties, as the contractors were more familiar with MOSS (now Infosoft), and problems were encountered in transferring data between site and the design office.

A new system developed by Trimble allows site geometry (rather than coordinates) to be taken straight from the design package and used for setting out.

This is particulary helpful where obstructions are encountered. But typical design tools presume a flat earth model. So plans, alignments and levels had to be transformed to ellipsoidal GPS co-ordinates GPS is also playing a role in earthmoving operations on the CTRL. GPS receivers are mounted on the dozer blades, and after loading a digital design into the computer system, grade work can be guided by a driver display and an array of cab- mounted light bars. 'In the old days, surveyors had to bang in stakes at 20m centres, ' says Campbell Brooke of Trimble. 'GPS removes the need for stakes, and the driver's productivity increases, with less rework and less overcut.'

The system allows blade motions and hence productivity to be monitored and centrally logged via a GSM data link. This high tech update on the time and motion study has encountered some resistance from dozer drivers. 'There have been drivers complaining about big brother and turning them off, ' says Brooke, 'but generally people are getting used to them, and time savings for earthmoving can add up to 5% in the long term.'

Satellite signal

The GPS system consists of a constellation of 24 satellites which orbit the earth at an altitude of 12,000 miles. The satellites travel in one of six different orbits, each one circling the earth roughly twice a day.

A GPS unit computes its position based on radio signals received from different satellites. Using the time signal, the GPS receiver calculates the distance to each satellite, based on the travel time and the speed of the signal, and determines the receiver's position on earth.

Delays are introduced as the signal travels through the atmosphere, such as bouncing off reflective objects. Another error is selective availability, introduced by the US government to reduce positioning accuracy. To correct these, a Real Time Kinetic base station receiver is set up at a precisely known location, and surveys conducted with a roving receiver. When data from the two receivers is combined differentially, the position of the roving receiver can be determined to centimetre level accuracy.

Capital connection

The Channel Tunnel Rail Link will provide a high speed connection to the Channel Tunnel.

The line is being constructed in two sections. Work started on on site in October 1998 and the first section of line will open in October 2003, with trains terminating at Waterloo.

Tenders have gone out for Section 2 which will take the line into a new terminal, north of the Thames at St Pancras and provide links to the rest of the national rail network. This is due for completion in December 2006 when travel time from London to Paris will be 2 hours 20 minutes.

The consortium London & Continental Railways holds the government franchise to own and operate the CTRL and the own and operate Eurostar. Shareholders are Bechtel, SBS Warburg Dillion Read, National Express Group, Systra (as a subsidiary of SNCF), Ove Arup & Partners and Sir William Halcrow & Partners).

Rail Link Engineering is a consortium of the engineering partners in LCR; Bechtel, Systra, Arup and Halcrow.

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