Gear & Gadgets / Product News & Reviews

Inside assisted GPS: helping GPS help you


Assisting GPS beyond satellites

There's a whole other method of GPS assistance that doesn't fall precisely under the AGPS rubric: these are methods in which another form of time and space signal is used instead of, or mixed in with, GPS data. WiFi positioning is the most fully developed and deployed of these, but television broadcasts, inertial sensors, gyroscopes, odometers, speedometers, compasses, and other inputs can also be used.

Skyhook Wireless was the first firm to push public fixed WiFi base stations as a tool for positioning, and they currently have partnerships with Broadcom, Qualcomm, and SiRF, among other firms, to integrate their data into GPS platforms.

Skyhook's XPS system

Skyhook's approach relies on measuring billions of points of data by driving specially equipped vans along thousands of city streets. The vans constantly measure all WiFi network signals that are detected, pairing those snapshots with an onboard GPS receiver. WiFi networks that aren't set to be "closed" (suppressing their network name) continually broadcast a unique network address as part of their find-me beacon. (That's the BSSID or basic service set identifier, which is typically the MAC or Media Access Control address built into a base station. This address is uniquely set at the factory. Hackers and hobbyists can change the MAC, but few regular users would ever touch the setting.)

"We've gone out and built a massive database of WiFi access points and we've positioned those around the world," said Mike Shean, Skyook's co-founder and vice president of business development. "We use that as the foundation of a positioning system." Skyhook began capturing cell base station information on these drives last year as well.

Skyhook also corrects its data by using WiFi snapshots—the current signal environment around a user—provided by the devices and hardware the company supports or licenses to. With the technology built into the iPhone starting in version 2.0, Skyhook receives undisclosed ("You wouldn't believe the level of usage we're getting from the iPhone," Shean said) but huge numbers of snapshots. These snapshots let them supplement the driving.

But, Shean notes, the driving is critical because "you have to drive to detect how access points propagate their signal in the real world." Signals from access points high in buildings, reflections across bodies of water, and other real-world conditions can only be ferreted out through their driving data.

Skyhook offers pure WiFi positioning to partners that want it, but also offers what they call a Hybrid Positioning System (XPS), which culls data from a GPS receiver, a cellular radio, and WiFi radios. As opposed to assisted GPS, which typically allows a system to better decode GPS data faster, "We're taking actual raw GPS measurement data and we are incorporating that with cellular data and WiFi data to produce one location," Shean said.

This lets Skyhook use error-ridden data from as few as two satellites, which isn't uncommon in the concrete canyons of a major city, and then pop in both cellular base station locations and WiFi access points. "The idea is that XPS can produce a location that's better than any of the individual technologies could have," Shean said.

For different applications, Skyhook's system can deliver a rough result, which the application displays while waiting for the more precise result that may require a GPS to listen for quite a while.

On cellular handsets, Skyhook has the advantage of being able to use either the cellular data side to pass cell and WiFi scans, or the WiFi side to pass the same data. For standalone devices, Skyhook can create and transfer a relatively compact database which can be sectioned into regions based on a user's current location. "We bring down local caches of data, and use that local data for very rapid lookups on devices," said Shean.

Beyond WiFi, many firms are considering integrating data from whatever is best available, especially in rural areas, deep indoors, or other challenging locations. SiRF's strategy for its market, where the manufacturers that use their chips typically develop standalone devices, is to use as many sources as they can.

Sirf's founder, Chadha, said their system can "take assistance or sensing information from MEMS [micro-electro-mechanical system] sensors like an accelerometer, odometer sensor, gyro, ABS [anti-lock brake system] sensor," as well as pull data from WiFi or a TV signal. (One firm, Rosum, is characterizing television broadcasts to act as a primary location positioning tool, as well as a way to assist GPS.)

In automobiles especially, Chadha noted, there are few methods for bringing in outside assistance, and thus reliance on sensors is far higher.

Qualcomm's Presutti said, "We're looking at how sensors will augment the overall GPS performance," by providing augmentation for tasks like pedestrian navigation. "We haven't even begun to touch on the additional things on a phone that you'll be able to do with [additional sensors]," she said.

Expand full story

You must to comment.