Knowing the exact location of a highly valuable consignment of prescription drugs, for example, would surely make any logistics professional sleep better at night. A tracking device can be fitted to almost anything, in an open or a covert way.
While there are many, many available options and variations on a theme, there are essentially two types of trackers – passive and active.
Passive trackers use GPS location information to record their position (and possibly environmental data such as temperature and humidity) as it makes its journey. The data is logged within the tracking unit itself and is stored in internal memory or on a memory card and can be downloaded at a later date for analysis on a computer.
Active trackers provide real-time location and environmental information to a central tracking portal. This data can be viewed by the owner of the cargo and/or the tracking company employed to carry out the monitoring on behalf of its clients. When the shipment is delayed, for example by stormy weather at sea or by leaves on the railway line, the tracking data can be used to update clients with an accurate timeline.
So this isn’t exactly breaking news to those who have been in the asset tracking industry for some time. Even in the retail sector a lot of courier firms provide a link to an online tracking portal so that one can follow the progress of a parcel as it makes its way to the delivery address.
So, how does a GPS tracking device determine its position when it is under cover?
For example when it’s inside a warehouse, the hold of a ship, a transport depot, a train station, or an aircraft hangar. The issue is the same for all indoor locations where a container may spend a period of time during its journey. If there is no sky view the GPS signal can’t penetrate to the interior of the building
But that’s OK surely – the last time it had a live signal was just before it went into the building, so it must still be in there, right? Not necessarily.
When the GPS tracking device loses sight of the sky due to the blocking effect of the building structure, it will continue to report its location periodically as it was just before it entered the building.
Not only would the tracking device be reporting stale, out-of-date location data, there may be a long acquisition delay (possibly several minutes) upon exit as the GPS receiver regains a positional fix.
Additionally, when exiting the building, the tracker may have to download the entire ephemeris database from scratch because the satellites orbiting above will have moved on to a new location.
Using a GPS repeater
A GPS repeater relays live GPS signal from the outside of the building to the inside. This ensures that sky view is available at all times and the GPS tracker will always be reporting accurate location information to the tracking portal.
An antenna is placed on the roof and a coaxial feeder cable transports the signal to an indoor GPS repeater unit. The repeater re-radiates the signal inside the facility, providing live sky view to all GPS receivers in the vicinity.
End to end visibility of cargo
It’s reasonably easy to solve a potentially serious issue. A GPS repeater kit comes with all that is required to provide signal in any indoor space.
For larger, more complex situations, a custom-designed system is created to fit the exact requirements based largely on the size of the building.