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Friday, July 5, 2013

GPS Repeaters - tracking and alerting inside man-made structures

GPS repeaters have been used for several years to provide GPS tracking signals inside of man-made structures such as buildings, parking structures, and tunnels.  Due to the signal attenuation caused by construction materials or natural structures, the satellite based Global Positioning System (GPS) can lose significant power indoors affecting the coverage for receivers. In addition, multiple reflections at some surfaces (usually metallic) can cause multi-path propagation which can result in additional location related errors or 'null spots'.  Repeater systems can help by providing strong GPS signals to areas that would not normally be capable of receiving the necessary signals for location tracking.

The GPS repeater system operates by using an antenna with a good view of the sky cabled to a repeater, usually inside the structure, which then re-radiates the selected signals inside of the building or structure using a separate antenna.  For complicated internal structures, multiple repeaters may be needed.

For vehicle tracking systems, GPS repeaters are typically used with vehicles or assets that are stored indoors - such as fire trucks, construction equipment, and ambulances.  The use of the repeater is two fold: the repeater system allows the GPS tracking systems on the vehicles to stay locked with a relatively strong signal while inside the building, so that after leaving the building the GPS will not be required to re-acquire the GPS signal, which can take several minutes for a moving vehicle - resulting in a potential loss of location data.
Secondly, this prevents a weak GPS signal, caused by the building or structure, from causing invalid position reports to occur from the tracking devices on the vehicles as the GPS/GNSS constellation changes above the devices.

This same repeater system can be used to help track hand-held GPS devices such as smartphones inside of large indoor structures, such as malls - though the  placement of the repeaters requires that a radio propagation coverage analysis assessment be conducted in order to make sure there is ubiquitous coverage inside the structure under a variety of potentially changing conditions.  

This repeater system can also be used to help track important assets inside the structure, even if they only leave the structure infrequently - and combined with wireless systems using software that provide geofencing capabilities, an alarm can be sent out when the device leaves the indoor perimeter of the location it is being stored at.

Keep in mind that the size of the geofence should include some margin for error caused by GPS drift, which can occur even with the repeater generating a relatively strong signal; so that false alarms are not generated by changes in the satellite constellation that provide less than optimum GPS signal coverage.

Since GPS repeaters are a transmitting device, they are covered by local regulatory agencies such as the FCC in the US; and if used must be done so as to comply with local regulations.