Location is the core of any AVL system. This guide covers the major primary location sources, as well as common methods used for augmenting their accuracy.
Primary Location Technologies:
GPS / GNSS
The Geographic Positioning System is a series of 24-32 satellites in asynchronous orbit around the earth. While GPS has become a household name for any location technology, I will use the term in its original context. Each satellite broadcasts a clock signal. By comparing the signals received, the GPS unit can establish its location.
Unaugmented GPS has two major faults:
1. Signals are weak and subject to “multipath” (bouncing around off buildings, trees, etc).
2. The asynchronous orbit of the satellites means that the number and angle of satellites in the sky will not be constant. Position quality at specific location will vary with time.
Modern receivers and antennas are designed to reduce the impact of these faults. The current generation of mid-range technology is very sensitive, and can acquire even indoors. There are also issues wheb which leads to the axiom:
The quality of a GPS unit is determined not only by what it accepts, but what it rejects.
GPS now has several competing and complimentary “constellations,” such as GLONASS and GALILEO. Multi-constellation receivers (with increased accuracy) are becoming more common. They are not marketed heavily in the US as the Federal Govt frowns upon ceding its control over the positioning market under national security grounds.
A further deep dive can be found at Navipedia and Wikipedia
Track Circuits
Track circuits (or similarly “wayside detection”) are a location source from a rail signaling system. They record the presence or passing of a train or piece of equipment at one fixed point. For many rail AVL systems, information from the circuit is aggregated by a central signalling system, which matches the circuit location with a geographic location. Data quality is impacted by several factors, including the distance of circuits from one another, the accuracy of the map between circuit and geographic location, and the update interval from the central system.
Augmentation Technologies
When primary location is not available or of low quality, augmentation can boost reliability.
Dead Reckoning
Dead reckoning (DR) is the most common solution for augmenting GPS accuracy on vehicles. Augments GPS continuously calculating the vehicles current location based on the last known location and direction along with its current speed. The vehicle’s location is estimated by DR when the GPS location is unavailable or below a quality threshold.
Signals-Based
Signals-Based location, or hybrid positioning systems, relies on interrogating additional signals visible by the device, such as cellular towers, WiFi devices, and even radio/tv signals. This is the primary location method for smartphones. Google, Apple, Skyhook, and Mozilla provide services for geolocation.
Satellite-Based
Satellite-Based Augmentation Systems (SBAS) provide additional meta-data on GPS quality. Designed for aviation, SBAS systems are also used in land-vehicle GPS units.
RFID and Induction Loops
RFID systems use readers and “tags” to determine the location of an object. Some systems and configuratons can provide incredible (to the centimeter) accuracy.
Induction Loops use wires embedded in pavement to detect vehicles passing over them. Some systems can exchange additional information (e.g. Vehicle number and status). RFID in AVL can provide certain advantages, such as.
- Lower cost, less complexity and maintenance per vehicle. An RFID tag is both far less expensive and complex to install and maintain than other location + communication systems. Smaller numbers of more-complex readers are also easier to maintain.
- Precise location or departure detection. Systems that require instantaneous and accurate location data, such as Traffic Signal Priority benefit greatly.
DGPS and RTK
Differential GPS and Real-Time Kinematic GPS are higher-cost, survey grade technologies. These have been used in a specialized public transit context, just not often enough to warrant further discussion.