GPS asset tracking was first introduced around 25 years ago and, to the general public, it is the most familiar type of asset tracking solution today. It is used for cell phones and digital mapping systems, as well as for tracking vehicles in the trucking industry and other transportation sectors.
While GPS does a good job of pinpointing the location of people and vehicles, however, there are many applications where it is ineffective or cost-prohibitive.
Limitations Of GPS Asset Tracking Systems
To understand why the technology that works so well for locating people and vehicles isn’t ideal for tracking shipping containers, hospital equipment, or materials on construction sites, it’s helpful to know how GPS works.
The Global Positioning System, or GPS, utilizes a series of satellites orbiting the Earth to send signals to GPS devices. The number of satellites may vary at any given time, as new ones are launched and existing ones are decommissioned. A minimum of 24 operational satellites are required for full coverage of the globe. As of August 2018, there were31 GPS satellites in operation.
Each satellite in the GPS “constellation” orbits at a height of 12,550 miles above the Earth’s surface, circling the planet twice a day in one of six orbits. As they orbit, GPS satellites broadcast radio signals providing their locations, status, and the precise time.GPS devices on the ground receive these radio signals, noting their exact time of arrival, and use them to calculate the distance from each satellite in range.Once a GPS device knows its distance from at least four satellites, it can use geometry to determine its location on Earth in three dimensions. The GPS device reports its current location to a GPS portal with data transmissions at set intervals, delivered via the cellular network.
The obvious limitation to GPS accuracy is familiar to anyone who owns a satellite TV dish: In order to get an accurate signal from the satellite, the dish must have a clear view of the sky. Buildings, tree branches, or even heavy cloud cover can interrupt the signal. The same is true for GPS asset tracking systems. For accuracy, they depend upon a view of the sky that is free from obstructions. This means any asset located inside a building, truck, or container can’t be effectively tracked via GPS, because the view of the sky is obstructed.
Urban location tracking can also prove challenging for GPS accuracy. GPS signals are subject to multipath distortion in areas with tall buildings, as satellite signals are bounced off buildings before reaching devices. Positioning algorithms assume a straight line path for the signal between the satellite and the receiving device. When the signal bounces off one or more objects before reaching the device, it takes a fraction of a second longer to receive, resulting in an inaccurate calculation of position. The degree of inaccuracy varies depending on the magnitude of the multipath distortion.
GPS receievers are relatively power-hungry for applications that require real-time location tracking. Most “personal” GPS tracking devices only last days or weeks at most. There are lower power positioning systems that can be used when battery life is a concern, but these only operate in defined areas.
GPS Asset Tracking Hardware Is Costly
In addition to the challenges posed by physical barriers, GPS tracking is subject to cost barriers as well. GPS asset tracking hardware is expensive, sometimes costing hundreds of dollars per tag. And because GPS receivers are always “listening” for satellite signals to calculate position, they use a lot of power, requiring frequent battery recharging. In addition, tracking the location of assets via cellular networks incurs ongoing monthly data use costs.
For these reasons, GPS tracking is most useful for tracking expensive assets such as cars, freight trucks, farm equipment, and construction equipment, all of which operate outdoors and can receive unobstructed signals from GPS satellites. Other types of assets, such as shipping containers, hospital equipment, or manufacturing and construction materials, can be tracked more effectively—and economically—using an RTLS (real-time location system) solution.