Buying a new kind of GPS requires a new kind of specification.
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Financial Grade GPS, Regular GPS or Other Choices?
The choice of technology depends on the service you wish to measure. Historically the choice was been between GPS and other wireless technologies, which are highly flexible and can work everywhere the car goes but with accuracy troubles, and fixed roadside infrastructure, which measured financial transactions accurately. Most operators settled for fixed gates and meters. Unfortunately, they are limited to observing a single spot, making them inflexible and hugely if not prohibitively expensive to use on a large scale. The development of Financial Grade GPS, provides operators with a third option.
GPS Charging Reliability: Always vs. Usually
Most GPS chip manufacturers claim accuracy to 1 yard (or meter) or better. Why is this not good enough for charging? The problem is that these figures describe the average, not the exceptions. Errors occur in GPS because of atmospheric effects, satellite geometry and the landscape you are in (as well as controllable factors like where the receiver is placed and how the device is engineered). A receiver that is reliably accurate to 1 yard (or meter) in an Australian desert can be wrong by 200 yards (or meters) in downtown New York.
Even asking “how many meters is your system accurate to” is the wrong question. Bills are measured in dollars, not in latitude and longitude. There may be many steps between the GPS chip and the final customer bill. What matters is the aggregate overcharge (e.g., 0.1%) and undercharge (e.g., 1.0%), and the magnitude and frequency of the largest billing errors. These numbers will further depend on the exact geometry of the testing location and route, making specification challenging.
The industry furthest along in specifying charging reliability is the road user charging industry. Good work is being done by, among others, the GNSS Metering Association for Road User Charging.
GPS Operational Reliability is Commonly Available
The concept of charging reliability is new to telematics. Historically, everyone used the same GPS chips with roughly similar accuracy. As a result, historically one differentiated telematics suppliers based on the operational reliability of their devices: how many systems did they have running, could they minimize failure rates and so on. This resulted in differentiating firms based on their ability to test, identify and reduce design weaknesses and manufacturing errors. While important, the required skills are commonly available in any competent device design team.
Specific GPS-Related Terms
The GPS road-user charging industry particularly has a number of specialized terms and strategies to be aware of. All firms struggle to deal with the problem of GPS errors. The most common way to deal with this is map-matching, a process that pulls the GPS points to the closest road. A process often used in parallel is inertial navigation, typically drawing information from the car OBD port. Both processes, while dramatically improving the quality of car navigation, have weaknesses in the essential area of evidence.
Related to map-matching is a second debate: the debate of a “thick client”, where a map is kept on board - leading to difficulties in proving that the map is always up to date – or a “thin client”, where raw GPS positions are streamed off the car, violating privacy, using large amounts of bandwidth and creating a demand for massive data center capacity. Skymeter’s Financial Grade GPS does not use map-matching, and is liberated from these requirements. The overwhelming majority of the calculations are performed on board, without maps. The single reference pricing map is kept in a datacenter and accessed through cooperative processing techniques.
