Repeater Technology Ensures GPS Testing Is An Inside Job

Published in Electronics Weekly 30 October 2012

Most people have experienced the frustration of not being able to receive a mobile phone signal while indoors; it’s even more irritating when there’s a perfectly usable signal just outside the door.

The situation is usually caused by the structure of the building; the signals just can’t penetrate through the walls. Ironically it’s often modern insulating materials such as foil-backed insulation that prevent the ingress of signal and makes it necessary to go outside in the cold to make a call.

Nowadays of course mobile devices aren’t just phones; they perform a vast range of other functions, some of which rely on the reception of GPS satellite signals which are usually only available outside the building.

Mobile phones represent just one industry sector in which GPS technology has embedded itself; the range of equipment that needs to provide its operator/owner with accurate location information is ever-expanding.

Consequently the need to manufacture, test, calibrate, service and operate GPS products in an indoor environment is also growing; and so the challenge of bringing the outdoor GPS signal inside the building grows with it.

There is a relatively simple solution; an enabling technology called a GPS repeater, also referred to as a re-radiator.

First – let’s define a GPS repeater system: a GPS repeater operates by receiving GPS satellite signals outside the building and re-radiating them inside the building or covered space.
There is an outdoor antenna to pick up the signals from your rooftop, and a coaxial cable that carries the signal inside the building to a small, mains-powered, repeater unit. The repeater unit has a built-in antenna that can cover a radius of up to 20 metres. Multiple repeaters can be used to provide coverage over a larger area by using a coaxial signal splitter attached to the bottom of the feeder cable.

If a smaller area of coverage is required, for example when the need is to provide coverage within a specific area of a workshop or laboratory, a gain control can effectively reduce the coverage to a few square metres.

Note that all repeaters in a network will transmit the same coordinates, i.e. the position of the outdoor antenna and not the position of the repeater unit itself.

GPS Repeater Applications

Consider the following applications and see if any other situations where indoor GPS signal would be useful (or even crucial) spring to mind.

• Fire & Rescue Stations: A repeater system installed in a fire station ensures that satellite navigation equipment in rescue vehicles is “locked on” to the GPS satellite service at all times while indoors. This means that when they exit the station “on a shout” the satnav devices don’t have to wait maybe three or four minutes to acquire the satellite signals – possibly qualifies as a “crucial” situation.

• Aircraft Servicing Hangars: Indoor GPS signal allows on-board satellite navigation equipment to be tested indoors without having to push the aircraft outside in order to pick up a signal. Maybe not crucial in the life-or-death sense, but would save a lot of time and effort if aircraft can stay inside until fully tested and ready to go.

• Personal Communication Devices: Similar to the Fire and Rescue application; when police officers go inside a station they are effectively “off the grid” as far as the command and control centre is concerned. Their radio devices these days are far more than walkie-talkies; they have GPS chips in them that provide location coordinates that are relayed constantly back to the control room. A GPS repeater in a police station means that the command and control staff has a fuller picture of where their resources (officers) are. Arguably a crucial situation in certain circumstances.

• Computer/electronic test facilities: So many systems contain embedded GPS chips these days that, similar to the aircraft test example, each system has to be taken outside to be tested. If GPS signal is available inside the workshop, each device can be fully tested without having to leave the comfort and safety of the workshop. Removes the need to run multiple outdoor antennas and feeder cables, or rely on pushing an antenna out of an open window on the end of a long pole! Useful as opposed to crucial; but still easy to see the benefits.

Current legislation

Thanks largely to some “rogue” installations of repeaters that caused disruption at airports in Germany, Canada and the USA, Ofcom concluded that there is a risk of a GPS signal repeater causing interference with other systems in the vicinity.

Initially the use of GPS repeaters in the UK was prohibited under the Wireless Telegraphy Act and there hadn’t been, until recently, any prescribed parameters in place to limit and regulate any potential interference.

Ofcom’s proposal sought to limit the output power and signal gain to levels which are considered to reduce the potential for any interference. As a result a “light license regime” was introduced in July 2012. The reason for the implementation of such a regime is essentially for Ofcom to keep records of the whereabouts of GPS repeater equipment so that incidents of wireless interference could be investigated.