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Satellite systems

GPS for construction

The Global Positioning System (GPS) has revolutionised the way we establish our position on Earth and brought about a degree of accuracy that was unimaginable 20 years ago. It is primarily a military navigation system which uses 24 satellites orbiting at an altitude of 20,000km.

However, permission from the US government to allow the positional data of the satellites to be freely available to civilian users has led to a host of usage and established GPS throughout the world.

Accuracy varies with the systems available, from a 10m position fix which is the most accurate available with the normal GPS signal, through submetre accuracy with the receiver set to pick up broadcast correction data, to millimetre accuracy using differential (two receiver) techniques.

Construction requires the most accurate receivers so we need a minimum of two receivers working with differential techniques further enhanced by a telemetric link so that the position can be established in real time. The procedure is referred to as 'real time kinematic' (RTK) and although manufacturers state centimetre accuracy for this, there are many examples where millimetre accuracy has been obtained.

What do we need in construction?

Of course, 10m is not good enough for construction, hence the differential RTK approach. With this one receiver is stationary (the base station) and a second receiver moves around picking up the detail or setting out the points (the rover).

The receivers are linked by radio so that the relative position between the two receivers can be calculated immediately and this relative position is going to be more accurate than the absolute position of either receiver.

In addition, the receivers do not just use the broadcast code - they use the carrier wave frequencies, L1 and L2, and by using measurement techniques not dissimilar to those used by EDM are able to get down to millimetre accuracy. There are further benefits in using both frequencies so a system capable of tracking satellites on both the L1 and the L2 frequencies is the one to use in construction.

Twelve satellites tracked are the norm but nine satellites should be perfectly adequate. Some systems utilise the Russian GLONASS satellites in addition to the GPS satellites, which increases the number of satellites available to around 40. GLONASS gives more coverage than GPS at the higher latitudes so there is a bonus with those systems that use it Although on-board computerisation and data storage has increased dramatically over the years, RTK systems use external computer/controllers to store and process the design data and display the satellite information etc.

These controllers are normally customised third party hand held computers with a large graphic display and store the data on PCMCIA cards. While this may be sufficient for RTK work it is false economy if you do not also get the post-processing software.

With this you can compute and adjust your GPS observations to obtain the highest accuracy which you will need for control point coordination and structural work.

There are lots of other useful features which will make for more efficient use of the system.

The expert Mike Fort has been in the industry for more than 40 years and for the last 10 he has been a senior lecturer in surveying and setting out at the CITB's National Construction College. He is a past president of the Institution of Civil Engineering Surveyors and was chairman of the ICE committee that produced the design and practice guide on The management of setting out in construction.

DSNP

Instrument: Scorpio 6502 SK & MK

No. Satellites tracked (L1): 12 + 4 GNSS Channels

No. Satellites tracked (L2): 12

Real Time Horizontal Accuracy: 10mm

Real Time Vertical Accuracy: 20mm

Real Time Range in the UK: 20km

Mike Fort says: 'DSNP systems are first class and are linked to the Global Navigation Satellite System that enables differential corrections to be received from the geo-stationary satellites EGNOS in Europe and WAAS in the US.

Topcon Positioning Systems (JAVAD)

Instrument: Legacy E and Odyssey

No. Satellites tracked (L1):20 - Assignable, can include

No. Satellites tracked (L2): 20 - GLONASS, EGNOS & WAAS

Real Time Horizontal Accuracy: 10mm

Real Time Vertical Accuracy: 17mm

Real Time Range in the UK: 10km UHF radio, 30km GSM phone

Mike Fort says: 'JAVAD is a recently established company formed by GPS pioneer Dr Javad Ashjaee and has just been taken over by Topcon. Its receivers include the most advanced GPS technology, a lot of which has been patented by JAVAD and they have created a lot of interest at exhibitions throughout the world.

Sokkia

Instrument: Radian

No. Satellites tracked (L1): 12

No. Satellites tracked (L2): 12

Real Time Horizontal Accuracy: 10mm

Real Time Vertical Accuracy: 20mm

Real Time Range in the UK: 5km

Mike Fort says: 'Sokkia has a good reputation for reliability and I have no doubt that its GPS system will perform well.'

Trimble

Instrument: 4700 GPS

Total Station No. Satellites tracked (L1): 12

No. Satellites tracked (L2): 12

Real Time Horizontal Accuracy: 10mm

Real Time Vertical Accuracy: 20mm

Real Time Range in the UK: 10km

Instrument: 4800 GPS

Total Station No. Satellites tracked (L1): 9

No. Satellites tracked (L2): 9

Real Time Horizontal Accuracy: 10mm

Real Time Vertical Accuracy: 20mm

Real Time Range in the UK: 10km

Mike Fort says: 'Trimble is arguably the leader in GPS systems and it was the first to introduce a RTK system back in 1994. The 4800 has everything integrated into the antenna while the 4700 has the more traditional modular arrangement. Both systems have been very successful and are used extensively in construction. '

Leica

Instrument: SR 530

No. Satellites tracked (L1): 12

No. Satellites tracked (L2): 12

Real Time Horizontal Accuracy: 5mm

Real Time Vertical Accuracy: 10mm

Real Time Range in the UK: 20km Radio, 40km GSM phone

Mike Fort says: 'Leica has a lot of experience with GPS receivers and its systems are improving all the time. The field computer will run the SKI software so postprocessing on the PC is optional. Training and support is second to none.'

Topcon

Instrument: GP-DX1

No. Satellites tracked (L1): 9

No. Satellites tracked (L2): 9

Real Time Horizontal Accuracy: 10mm

Real Time Vertical Accuracy: 20mm

Real Time Range in the UK: 10km

Mike Fort says: 'Topcon has extensive experience in GPS and has been very innovative in machine control. Its systems are widely used in construction.

However, as Topcon has just taken over JAVAD Position Systems, it is likely that it will use JAVAD receivers in the future.'

Spectra Precision

Instrument: Geodimeter GPS

No. Satellites tracked (L1): 12

No. Satellites tracked (L2): 12

Real Time Horizontal Accuracy: 10-20mm

Real Time Vertical Accuracy:10-30mm

Real Time Range in the UK: 10km

Instrument: Elta GPS

No. Satellites tracked (L1): 12

No. Satellites tracked (L2): 12

Real Time Horizontal Accuracy: 10-20mm

Real Time Vertical Accuracy: 10-30mm

Real Time Range in the UK: 10km

Mike Fort says: 'Trimble has only recently taken over Spectra Precision and the systems shown are those that were marketed before the takeover. Both systems are part of an integrated suite including software and total stations.'

Hire or buy?

RTK GPS systems are still comparatively expensive and are undergoing rapid development.

They are more suited to linear projects such as railways, roadworks and pipelines where an investment in GPS can quickly pay for itself in increased productivity and accuracy. So buy if you have these types of projects. However, GPS has so many uses in construction that any project will benefit from its use and increase the technical proficiency of that company. On these jobs hiring would be the best option.

Recommendation

Always undergo training before you use GPS. All manufacturers offer training, so check on this and its cost beofe you decide on a system. With takeovers and partnerships, the GPS industry is in a state of flux at the moment but it should settle down in the near future and I cannot see any further surprises. As it stands I am impressed by the Leica, Trimble and DSNP systems and having had experience of Leica I would tend towards it.

However, all of the systems listed should perform well and I am particularly impressed by the JAVAD system Topcon Positioning Systems would be at the top of my list if I were in the market for a GPS system.

Cost

The price of RTK systems has come down over the last few years but not by much. To get milimetre accuracy calls for some very sophisticated software programming and equally sophisticated hardware. So single receivers cost about £10,000 and complete RTK systems nearer £30,000.

Manufacturers point out that there are so many options available for RTK systems that it is difficult to give a typical cost.

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