What is a GPS / GNSS Repeater?

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- everything RF

Jul 9, 2020

A GPS / GNSS Repeater is a device that is used to transmit GPS signals from a satellite to places where they normally would not reach. So for example, if a GNSS Receiver is located in the basement of a building or inside a house with thick concrete walls it may not be able to receive signals from the GNSS satellite constellation. In this case, a GNSS Repeater can be placed outside the building to receive the signal from the satellites and then retransmit the signal indoors resulting in consistent and uninterrupted access to GNSS signals underground or in hard-to-reach areas.

A GPS Repeater system usually consists of an active GPS Antenna (Antenna, Low Noise Amplifier and Filter), that is mounted on the roof of a building to pick up signals from the satellite. The signal is then amplified, filtered, and transmitted through a coaxial line/fiber line to a repeater located inside the building. The repeater unit then re-transmits the GPS signal indoors via a passive antenna. This is a basic configuration of a GPS repeater. In case the same signal needs to be transmitted to multiple floors of a building, this can be done by using a signal splitter and then connecting each split signal to a repeater located on a different floor.

The environmental conditions are very important for deploying a GNSS repeater system in a building. As this RF system works with a very low power level, it is very important to install the receiving antenna and the repeater unit at planned locations to avoid unnecessary interference.

Using a GPS Repeater system allows numerous GPS devices, whether inside or outside of a building, to access and lock on to the same signal at all times. This is very important for emergency service workers, first responders, and military personnel who need instant location accuracy. With the help of GPS repeaters, rescue vehicles no longer have to wait to acquire signals upon exiting a building, allowing a constant state of readiness. Repeaters are also useful in computer and electronic testing facilities, aircraft servicing hangars, where on-board satellite navigation equipment is tested indoors, and in all areas or locations where signal strength may be an issue.

Some Applications of a GPS Repeaters

GPS Repeaters in Aircraft Hangars

The structure of an aircraft hangar can block the ingress of GPS signals, making it impossible to test onboard navigation equipment without pushing the aircraft outside where it has a view of the sky. A GPS repeater system overcomes this physical barrier by transferring the outdoor signals to the interior of the hangar, thereby allowing all maintenance work to be carried out indoors.

GPS Repeaters in Police Stations

When officers are inside a police station their hand-portable radio (Sepura, Motorola, etc) will not be able to receive a GPS signal due to the blocking effect of the building structure. The effect of this is that the officer’s location will continually be reported to the command center as the last time the radio device was receiving a live signal from the satellites, i.e. just before entering the building. When the officer leaves the station it can often take several minutes to re-acquire a GPS fix. During the period between leaving the building and re-acquiring a satellite fix the officer’s radio would still be transmitting incorrect location data to the command and control center. This ensures that the response times are not impacted due to the lack of navigation information.

GPS Repeaters for Tunnels

Accurate positioning and timing information may be a requirement for some GPS-enabled equipment underground. If there is a clear view of the sky at ground level, a GPS antenna can be installed to receive data from the satellites. From here a coaxial cable is installed to carry the signal to the underground location, where it is required. Line amplifiers are used to overcome the attenuation losses caused by long cable run.

The example shown above illustrates an installation in the Crossrail tunnel underneath the streets of Central London. Large-bore mains water pipes are continuously monitored for leaks and the telemetry equipment requires accurate timing signals from a receiving antenna at street level.

GPS Repeaters for Bus Stations

Modern passenger busses use GPS satellite navigation systems to determine their exact location at all times. When the busses are in underground stations / terminals, they can’t receive a live GPS signal. As a result, Real-Time Information (RTI) systems are not updated and passengers receive inaccurate timetable information.

The on-board system transmits its location data to a central computer. In turn, the computer communicates the information to passengers via an array of interfaces. However, when a bus enters an underground station the GPS receiver loses sight of the satellites orbiting overhead.

It continues to transmit location data to the central computer via its mobile communications system. However, this is no longer real-time. The location information now comprises of the coordinates of the place where it last had a sky-view, just before it entered the underground bus station.

In effect, the system transmits old location information when the bus leaves the terminal until it receives live GPS signals again. There are two additional issues that may affect the time it takes for a bus to be fully online and sending real-time information again.

If it has been underground for say an hour, the satellites that the bus was receiving a signal from have moved on to another section of their orbit. The GPS receiver would have to download location information of the satellites that are now visible which can take up to 15 minutes, especially in urban canyons. The bus is out of sync with the satellite.