Monday, 8 April 2013

How to move GPS signals inside and why bother?

If you have ever used a GPS-enabled device, you will have encountered the scenario that location-based services using GPS tend not to work inside buildings. For the individual user that might not pose too much of a problem, but there are certain environments where having a GPS signal indoors would be extremely helpful.

Examples include the following (though this is by no means an exhaustive list):
  • Electronic manufacturing and test laboratories: perform all GPS receiver tasks indoors.
  •  Any equipment that has a GPS receiver within that needs to be built, tested, calibrated or repaired at certain stages of its life-cycle. It would be helpful if the manufacturing, testing, calibration, repair and operation of GPS-enabled equipment could be carried out indoors, instead of stepping outside or hanging by fingernails out of the nearest window! It has been known for each bench, area or department within a facility to have its own outdoor GPS antenna and feeder cable.
  • Fire stations: on-board command and control equipment maintain contact with satellites while safely locked up in the station, so there is no need for an agonising wait for satellite acquisition upon exiting the station during a callout.
  • Retail outlets: provide GPS signal in the store so that technology can be demonstrated with live satellite signals - think of the array of satnav systems in your local electrical store that display a message that says "signal lost 5 days, 24 hours, 12 minutes and 8 seconds ago".
  • Vehicle manufacturing plants: on-board GPS systems can be tested indoors - no need to sit outside waiting for the satnav systems to acquire a satellite fix.
  • Aircraft servicing hangars: it may be inconvenient to take a smartphone outside to test the GPS functionality, but it's even harder to push an Airbus jet out of the hangar.
Thanks to recent changes in licensing policy in the UK and the new “light licensing” approach, GPS repeaters can now be deployed to address in-building GPS requirements and as such, the list of applications for GPS repeater technology is not quite endless, but it is ever-increasing.

A repeater offers an easier, more convenient, less time-consuming way to work with a wide range of GPS-enabled systems, but what does a GPS repeater system consist of?

In general terms, a repeater is a device that relays GPS signals to any indoor location that isn't normally reachable and will typically include the following components:

  • Outdoor GPS antenna
  • Antenna mounting bracket
  • Coaxial feeder cable
  • Indoor GPS repeater unit
  • AC/DC power adapter
The outdoor antenna should be installed in a location where it has a good view of the sky, preferably at or above roof level, in order to pick up signals from as many satellites as possible. The Outdoor antenna can vary – from medium gain to high gain units – take a look at the GPS-TMG antenna range as examples of quality GPS antennas for fixed applications. These GPS antennas are designed for precision performance and are used not only in GPS repeater systems, but also in timing and synchronisation applications.
The received satellite signal travels down the coaxial cable and into the repeater unit, which in turn boosts the signal and transmits it to the indoor space

In terms of coaxial cable to be used, this would depend on the gain of the outdoor antenna and the length of cable required from the outdoor antenna to the repeater. Typical examples of coaxial cable for these applications include BWL195 and BWL400 low loss RF cables. The cables can be pre-terminated and customised for each application. For extremely long cable runs (underground installations, mines etc.), then the coaxial cable can be replaced by Fibre Optics due to the extremely low latency of Fibre. For more information on GPS repeaters using Fibre links, please contact Phil at FaltchGPS.

The indoor repeater unit has an internal re-radiating antenna that emits signals in a beam approximately 140 degrees wide, giving a coverage radius of up to 18 metres. A good example of fully approved GPS repeater can be found here.

If one repeater unit isn't enough to cover the required area, extra repeaters connected together by a signal splitter can be connected together to increase the effective coverage area up to a nominal 60M by 60M (depending on the shape and layout of the space).

Overall, the major benefit of this technology is that a GPS signal can now be accessed where previously it couldn’t be, making applications more efficient and more reliable for organisations ranging from electronics manufacturers to emergency services. Using high gain outdoor GPS antennas, quality low loss cables and GPS repeater technology, many organisations can now take advantage of cost-effective solutions to solve GPS voids indoors.

For more information GPS antenna products, feel free to take a look at the offering from SAS. For fully custom GPS repeater solutions, please visit Faltech GPS.

A big thank you to Phil at FaltechGPS  for assistance with the content on this post.