Benefits of a quality GPS installation 

The Global Navigation Satellite System (GPS, GLONASS, Galileo and BeiDou) signals used by our everyday devices such as Satnavs, smartphones, connected cars and fitness trackers are extremely weak, originating in medium earth orbit over 20,000 km away. Therefore, if you need to get GPS or a GNSS signal indoors then a repeater system is required.

A repeater system consists of an active antenna installed outside to receive live GNSS, and a repeater amplifier and passive antenna assembly installed indoors to re-broadcast the signal. It sounds simple in principle but incorrectly installing such a system leads to many complications which can result in a repeater that operates inefficiently or cause a fault to develop within the receiver.

Firstly, the location of the active antenna outside the building will be the static position reported anywhere indoors. For most users, this is not too complicated as a known location with a live signal is sufficient for their application, but if the active antenna installation is done incorrectly, then the repeated signal will reflect this. For example, installing the active antenna with a limited view of the sky will cut down on the number of visible satellites receivers can see indoors. Similarly, installing the active antenna too close to a surface that can reflect RF (Radio Frequency) signal such as a metal clad roof or wall limits sky view and can cause what is known as multipath. Multipath occurs when the signal from a satellite bounces from the building surface, delaying the signal time-of-arrival and lowering the accuracy calculated by the receiver. Lightning protection should always be considered for any external antenna installation; should a lightning strike hit the antenna pole, excess energy will then be sent safely to ground instead of being directed into the building where it could cause significant damage or harm.

The cabling between the active antenna and the repeater is also very important. Coaxial cabling has an RF loss, and the longer the cable the higher this loss is until eventually the amplifier cannot sufficiently boost the signal thereby preventing the re-broadcast of the repeated signal to the indoor receivers. Different cable types have different losses, so the correct cable must be used to carefully balance the repeater solution taking into consideration cable length and any amplifiers or splitters in the system.

Most importantly, and most potentially damaging to other users of GNSS, is the placement of the repeater assemblies indoors. As the repeated signals are of the static location of the roof antenna, receiving them whilst also in a live-sky environment confuses the receiver and can lead to large errors, for example, in a second the receiver could think it is on its actual course outside, then pick up the repeated signal and suddenly jump to the roof of the nearby building, then back again. If this happens to a vehicle being tracked, this could lead to alarms and failsafe procedures activating.

This scenario is especially prevalent when it comes to buildings with large vehicle doors, such as first-responder stations where a poorly installed repeater signal could interfere with users on the road, or, aircraft hangars where the sudden jump of the location being reported by aircraft on the runway could lead to serious alarms being raised in a control tower.

Rectifying a faulty GPS repeater installation

Chronos was contacted by a customer who had a GNSS repeating system from another vendor installed inside their aircraft maintenance hangar.The system was not providing any useable signals and when inspected by Chronos experts, several key faults were found with the design.

The active antenna was installed half-way up the metal clad exterior of the hangar, with no visibility of the south of the sky, and with potential multipath errors. This made reliable and accurate testing of the aircraft avionics impossible, as the receiver was not able to lock on to sufficient satellites to bring a signal inside the hangar.

The lone repeater unit inside the hangar was also insufficient to cover the entire building as it did not have enough range to reach the far side and was installed far too low to reliably pass any signal over any aircraft parked in its path. The dedicated team of Chronos installation engineers successfully surveyed the hangar, identifying the shortcomings in the existing systems and designed a rebuild from the ground up (or roof down…).

Firstly, a new active antenna was installed at the top of the building, using a long reach cherry picker, and a larger coaxial cable brought down to the existing cable entry from the old system so as not to drill further unnecessary holes in the building cladding. Once the cable was inside, a lightning arrestor was installed and bonded to a nearby building girder providing a safe path to the ground.  As a single repeater was proven to be insufficient to cover the entire hangar, multiple repeaters were installed, all connected to the same active antenna through an amplified splitter. The customer now has a complete tailor-made fully operational system, with reliable and strong signal throughout the entire hangar, enabling continuous operations without any delay. They no longer need to waste precious time and resource wheeling the aircraft outside or moving other aircraft to test the avionics but can simply test the aircraft wherever it is indoors.