July 18, 2011 by onstarconnects
The GPS Advantage
If you’re an OnStar subscriber and your vehicle is stolen, OnStar can locate it using GPS even if it is out of plain sight. Or, if you’re in a crash and can’t ask for help, OnStar can still dispatch emergency services to your location using GPS. And if you’re lost, OnStar can help you find your way, again thanks to GPS. But how does GPS work and why does it play such a crucial role in the OnStar suite of services?
Curtis Hay, core technology engineering manager at OnStar in Detroit, explains that GPS — or the Global Positioning System — is technically superior to an alternative cellular-network-based technology called Advanced Forward Link Trilateration — or AFLT — that is used for handset 911 calls and by some other automakers’ telematics systems.
GPS is more accurate, faster
Hay says that accuracy tops the list of GPS benefits for OnStar. “Typically, we’ll know the location of the vehicle within a few meters with GPS,” he says. “Because cell towers weren’t designed to be precision navigation instruments, AFLT accuracy is an order of magnitude worse. We really can’t be confident with AFLT that that location would be within 50 or 100 meters.” According to Hay, at any one time, only three to four cell towers might be in range of the vehicle, compared to eight or nine of about 30 satellites in orbit.
Related to this is latency — the time the OnStar system would have to wait to get a latitude/longitude fix on the vehicle’s location on Earth. In tracking mode, a GPS receiver computes a new fix at least once per second, whereas “with AFLT that latency could be between 10 and maybe 30 seconds,” Hay says. Moreover, he explains, AFLT might fail in rural areas where cell network towers might be spaced far apart. “If I’m lucky I can establish a connection with one tower for a cell phone voice call, but the ‘T’ in AFLT stands for trilateration because at least three are needed to triangulate the vehicle’s position, and the likelihood of having three cell towers when I’m out in a remote part of America is not very high,” he adds.
Heads and shoulder above AFLT
“In fact,” Hay says, “there is only one instance when OnStar will turn to AFLT for location guidance, and that is to comply with a federal government mandate: when the OnStar button is pushed to call 911. Even then, he explains, unless the vehicle is inside a building without a clear view of the sky, GPS will also be used to compute the location.”
Still, OnStar does not rely solely on GPS information to provide its subscriber services. Another aspect of OnStar’s system, called “dead reckoning,” ties GPS to the vehicle’s on-board computers and sensors to gather data about the wheels’ rotation rate and turning angle, for example, to supplement the GPS satellite signals and more accurately determine the vehicle’s whereabouts. This is especially helpful for determining a location when GPS data is temporarily unavailable, such as when passing through a tunnel that blocks the vehicle’s visibility to the satellites.
And, Hay adds, other than being fast, accurate and reliable, GPS is, of course, global — allowing OnStar to be available around the world as well. Currently, OnStar operates in the U.S., Canada and China, but, Hay notes, the service is slated to expand to other countries, too.
No Related Posts
Tags: Emergency Advisor, Stolen Vehicle Assistance, Turn-by-Turn Navigation