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Using GPS indoors

Published: 
18 August, 2005

Charles Curry explores the issues of taking GPS to places where GPS has never been before!

Applications using the Global Positioning System (GPS) constellation of satellites for location, navigation, timing,telematics, security and safety amongst other activities are increasing dramatically.

Consequently the need to manufacture, test,service, calibrate and operate professional GPS products in an indoor environment for these applications is becoming more frequent. Since the GPS signal comes from a constellation of satellites continuously orbiting the Earth it is very weak when it reaches the ground; as a direct result, most

GPS receivers will fail to pick up signals strong enough to be usable indoors or in certain types of "shadowing" e.g. tall buildings (the urban canyon effect).

This article examines the problem from a number of different application perspectives and illustrates how GPS reradiating technology is coming to the aid of many new applications from Formula 1 pits to secure money transport.

What is GPS?

The Global Position System is a constellation of 24 satellites that the US Government has placed in orbit at 11,000 nautical miles above

the Earth in 6 orbital planes with 4 satellites in each plane. Initially conceived as a military navigation system, GPS usage in the civilian sector has increased dramatically in recent years, particularly since Selective Availability was switched off in 2000 which enhanced the accuracy available form the non-military signal.The satellites take 12 hours to completeone orbit of the earth.They are in orbits that allow world-wide coverage so that at any one time at least four satellites are visible to a GPS receiver with a clear view of the sky.

When the GPS satellites transmit their signals back to Earth the public signal (known as L1) is broadcast at 1575.42 MHz at a power level which is far too weak to be received through walls, roofs and even dense foliage. In fact with some GPS receivers even just covering the antenna with a hand will inhibit its ability to receive the signal.

A GPS receiver decodes the signal from the visible satellites and can then accurately work out its position to within a few metres using a triangulation method. If there are enough satellites in range it can also work out its altitude.The reason that the receiver may not see enough satellites is because trees or tall buildings may be blocking the signal from one or more satellites.

What is GPS Re-radiating?

Since the GPS signal is too weak to penetrate buildings, in order to use a GPS receiver indoors the signal has to be received outside and then amplified and rebroadcast inside. The receiving antenna has to be placed where it has the best possible view of the sky, for example on a pole higher than roof height. This will enable it to see as many satellites as possible.The signal broadcast into the building will contain the position information relating to the external antenna location. The amplifier in the GPS re-radiating kit provides a signal that is capable of being broadcast at a level higher than the minimum level required by GPS receivers thus ensuring excellent coverage in buildings.

Typical Applications of GPS Indoor Reradiating Technology Formula One Pits - GPS now has an important function to play in monitoring speed and position of Formula 1 racing cars.

At least one Formula One Racing Team now uses GPS re-radiating technology to test the GPS functionality inside the pits where a clear view of the sky is obstructed by the building.

The solution is portable and easily redeployable and has now been used at several F1 circuits throughout the world including Silverstone and Monza. On-board GPS telematics equipment needs to be tested whilst the cars are being prepared and tuned rather than outside where practise laps and racing are the priority.

Vehicle Assembly Plants - At least two major UK car plants have now adopted GPS reradiating to assist final vehicle assembly and test. This takes the GPS signal via a roof antenna and then re-transmits it inside the plant to enable the in-car GPS navigation systems to be tested on the assembly line. Normally during manufacture the GPS navigation system would be compromised because of not having a direct line of sight to the GPS satellite network. This means that while still on the production line the in-car GPS has been tested without the need to wait until the vehicle can be driven outside the production facility – a major time saving impact and therefore a clear business case for investment in the equipment.

Aircraft Maintenance Hangers - All navigation systems fitted into aircraft have to be tested on a regular basis. Where GPS is fitted, even if it is only a back-up, it too has to be calibrated in order to ensure full functionality. When a plane is serviced or stored in a hangar, in order to provide a functional test on the GPS a similar system to the car assembly plant can be implemented. However in order to calibrate a GPS system fitted into a plane the engineer needs to show at what point the system will cease operating.

An adaptation of the re-radiating technology provides a hood that fits over the plane’s GPS antenna. The re-radiated GPS signal is fed directly into the hood.A variable attenuator is fitted in-line with the hood enabling the engineer to decrease the signal level. This enables an accurate measurement to be made of exactly when the GPS will go "out of lock" and provides a repeatable test of its operating tolerance.

The GPS signal can be re-radiated using amplifiers to anywhere in a car plant, workshop or hangar so that even quite largeindoor spaces or shadowed areas can receive the signal. Smart Cash Protection Vehicles - In-building re-radiating is not the only type of GPS

application to make use of this technological innovation. Smart cash protection systems embed GPS trackers into an intelligent cash case. However this cash case has to know where it is the instant it is taken out of the armoured delivery vehicle. By retransmitting the GPS signal into the vehicle, the cash case is primed and knows where it is as soon as it is taken out. This is particularly important if a geofence locking mechanism is utilised otherwise the cash case would not be able to be opened.

An extension of this in-vehicle application could be applied to military vehicles and aircraft – for example inside a Hercules for parachute jumps.

GPS Manufacture and Maintenance Facilities Many GPS application test labs have multiple antennas on the roof or rely on pushing an antenna out of an open window on the end of a long pole! Using a combination of GPS accessories such as amplifiers, splitters and reradiators, a single roof antenna can deliver the GPS signal deep down into a building to enable GPS manufacturing and maintenance activity to proceed with ease. The GPS signal can even be transposed onto a fibre driver system to deliver it into electrically isolated environments. Activities and products benefiting from this use of GPS re-radiating include public sector vehicles such as ambulances, fire engines and police cars, and military products such as sonar buoys.

Site Survey and Professional Installation Whilst GPS re-radiating technology is relatively simple to install indoors, some GPS products can get confused by signal reflections. It is therefore important to carefully survey and plan an installation. Also,since the outdoor GPS antenna installation may require roof or tower work, relevant health and safety or insurance issues need to be addressed. A successful installation needs to be well planned, professionally installed and fully calibrated for reception in all locations within the user environment, then backed up with a maintenance contract and the provision of information or advice regarding the health of the GPS signal to ensure that reception issues can be accurately pinpointed to the local ground equipment rather than the GPS satellites themselves.

Conclusion

GPS re-radiating technology has revolutionised the way that many GPS applications can be maintained and operated. It has enabled new applications to emerge and is a very cost effective solution which eases the problem of delivering the weak GPS signal deep into a building or complex industrial facility.

About the Article’s Author

Charles Curry is Managing Director of Chronos Technology Ltd. Charles graduated in Electronics from Liverpool University in 1973, and started his career at GEC Hirst Research Centre, progressing to Racal Instruments where he was responsible for sales of test equipment including specialist frequency and time products. He founded Chronos, a leading system integrator for synchronisation and timing products in the UK telecom industry, in 1986. Charles has been involved with professional GPS products in navigation and timing applications since the early 80’s. Charles can be contacted on charles.curry@chronos.co.uk. For further information on Chronos GPS capabilities – please visit our websites www.chronos.co.uk and www.gps-world.biz.






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