Geolocation Indoors, Underground and in other GPS-Denied Areas
Posted on Tue, Jul 20, 2010
by Carole Teolis
An increasing number of technologies are cropping up to deliver location where GPS cannot - for example, inside natural or man-made structures. Most indoor geolocation approaches depend on some level of pre-installed infrastructure and accuracy varies widely.
Many solutions available to augment GPS leverage signals of opportunity from surrounding infrastructure such as cell towers, Wi-Fi, TV signals or location information from “ad hoc” infrastructure (e.g., RFID tags), and the accuracy of the results varies widely based on infrastructure location and availability.
Wireless cell site IDs can deliver accuracy in the hundreds or thousands of meters, and triangulation techniques that leverage signal strength from multiple adjacent cell towers enable accuracy of 50 meters at best. WiFi location applications (e.g., such as that from Skyhook Wireless) use databases of known WiFi access points and observed signal strength to identify proximity to observed access points in order to determine an approximate location. Accuracy is dependent on the number of access points in the area, obstacles (buildings, walls, etc.) but location returns are sufficient for many consumer applications that require an approximate location (e.g., 10s to 100’s of meters accuracy) without elevation data.
Inside structures such as hospitals, factories, and warehouses, tracking applications often use the WiFi techniques outlined above as well as relatively short-range asset tagging and ultra wideband approaches. RFID and other tag-based solutions rely on a pre-installed network of fixed scanners throughout a building or indoor structure. Accuracy of these systems is determined in part by the density of scanners, proximity of assets to scanners, and environmental obstacles (e.g., walls). UWB location systems measure the time difference in the arrival of signals from multiple high bandwidth radio sources. These systems are generally rely on pre-installed readers and tags that operate at relatively short ranges. All of these technologies are useful for in-building tracking of assets or people when ownership of the area in which people are being tracked is tightly controlled and the area in which tracking is required is well defined (e.g., a hospital or factory).
What happens in the case where you cannot rely on such infrastructure?
Today, personnel operating indoors, underground, or in other GPS-denied areas are often secured using simple no-motion (e.g., PASS) devices which produce audible alarms. They are often accounted for using “roll calls” via voice radio. Locating personnel accurately – without infrastructure – requires the intelligent integration of many types of available navigation sensor data including GPS (where available) and also ranging radios, compasses, gyroscopes, accelerometers, and altimeters. To avoid incorrect position estimates, algorithms must intelligently assess the quality of sensor data and eliminate or de-emphasize data from sensors providing inaccurate information (e.g., compass data affected by surrounding environments or metal structures).
A segment of the overall location market, GPS-denied location is the least served by viable commercial solutions today. Practically, users want to know where their personnel are – at a minimum which floor and which quadrant of the building - and whether they are safe. And they want to do this without adversely impacting critical, time sensitive operations by requiring slow manual setup or incremental tasks for personnel in what are typically emergency and dangerous situations. This is the market TRX Systems is working to serve.