Making GPS work: Service providers augment navigation data

Systems can slash time for the initial location fix and increase precision to inches instead of yards

Ever wonder how that GPS chip in your cell phone or personal navigation device or some other gizmo used by your field workers actually knows how to find the orbiting satellites and grab their positioning data?

It has to hunt for them, and that's where a number of frustrating GPS problems lie. To solve them, a group of service providers, including U.S. silicon vendor Broadcom Corp., are using different techniques to augment the basic GPS satellite system.

To hunt for the several navigation satellites it needs, a GPS chip first needs a bunch of information about the satellites themselves, especially about their orbits. Then, it uses that information to find their signal and download their ranges and positions at a given point in time. Only then can the chip crunch the data and plot its own longitude and latitude. All of that, but especially the first step, takes time, which can stretch to minutes.

And if you've got a weak signal or are out of range or indoors, you're completely out of luck. If even one bit is lost in the satellite transmission, the process has to start all over. If the signal is too weak for that initial data, the chip won't get the initial data it needs ... unless you can make use of a kind of GPS device for the satellites themselves: some data that tells the GPS chip in your handset in effect "look here for this satellite, and here for this other one and here. ..." Such a system could slash the time needed for that first fix to seconds, and provide the necessary starting data where a signal might be too weak to pull it down.

Giving a boost to GPS

It turns out there are systems that in various ways augment data from navigation satellites. The augmentations are used to slash the time for the initial location fix; to correct for the impact of variables such as transmission delay created by distortions in the ionosphere and troposphere, "clock drift" onboard the satellites, even tidal fluctuations; and to increase precision to inches instead of yards; or a combination of these.

Differential GPS devices focus on high precision. One of the best known is NASA's Global Differential GPS System (GDGPS), developed originally by the Jet Propulsion Laboratory (JPL), run by California Institute of Technology under a NASA contract. NASA claims it's the biggest such network, originally built to support its own terrestrial, airborne and space operations, but now available to government and commercial customers.

Some are mainly research oriented, like that of NASA's JPL.

Another example, using a different approach, is the StarFire Network from Navcom Technology Inc. (now a unit of John Deere Co.), which specializes in very high-precision (10 centimeters) GPS applications such as land and aerial surveying, precision agriculture and machine control. Navcom says it's the first satellite-based augmentation system, providing higher accuracy and the ability for users to roam anywhere without being "tethered" to a nearby ground station for augmentation data.

What's called "assisted GPS," however, doesn't aspire to this level of accuracy. Instead, these services typically use the standard satellite signal to help client GPS chips get that initial orbital data either faster or more reliably or both. Usually these services are aimed at cellular carriers or device makers.

Broadcom's location gambit

One provider of assisted GPS is Broadcom, the big U.S. silicon manufacturer whose inventory includes GPS chips. It's sort of like Starbucks buying a burlap-sack company to make sure there are enough bags to transport all those coffee beans. Other providers include Andrew Wireless Solutions, a division of CommScope Inc., with its Geometrix MLC service, and TeleCommunication Systems Inc., with its Xypoint service.

Broadcom's WorldWide Reference Network was originally launched in 2000, created by Global Locate Inc., which was itself an early GPS chip start-up. Global Locate realized it needed a reference network to create a reliable, consistent and fast source of orbit tracking data if the chips were actually to deliver the goods, says David Murray, director of marketing with Broadcom's GPS products group. Murray was deeply involved in Broadcom's $143 million acquisition, finalized in June 2007.

Broadcom's network consists of a global chain of GPS receivers at ground stations, clustered around the Earth's equator. The company won't say how many receivers or stations it has, only that there are at least three receivers tracking at all times each of the 28 satellites now in orbit, according to Murray.

The receivers constantly monitor the GPS satellites and track their orbits, feeding data in real time to two redundant servers -- one in New Jersey and one in New York -- run by a third-party data center operator. Today, about 20 million devices of various types access this data via an IP connection, eliminating the slow process of satellite hunting and creating almost immediate fixes.

Broadcom charges for the service, which is usually bought by carriers or handset makers, and which works with other brands of GPS chips. The service pays for itself, but Broadcom won't disclose revenues or profits. "Our plan is to make money off of this business," says Murray.

How it works

Here's how it works. A fleet driver needs to find the closest service station that has a contract with his company for diesel fuel. He pulls out his company smart phone, and presses an icon to load Google Maps. The phone has a small application that uses an industry protocol -- Secure User Plane Location -- to access the Broadcom reference network via an IP connection and request the orbital data. That typically takes eight to 10 seconds, according to Murray. Once it knows where to find the satellites, the GPS chip can get the range and timing data it needs in a couple of seconds.

That speed and reliability is critical. Murray points out that 2G cellular networks support e911 emergency calls by using a triangulation technique based on cell phone tower signals to calculate a rough location. But 3G networks use the much more precise GPS, which can narrow the position to within a few meters. Broadcom supports both techniques, part of a plan to create a service that eventually will automatically use several techniques to locate a mobile device, depending on the precision needed and on the availability of the various techniques.

As part of this plan, Broadcom recently announced a deal with Skyhook Wireless Inc., which has a database of tens of millions of Wi-Fi access points, and software that quickly calculates a Wi-Fi user's position in relationship to them, within a 30-to-60-foot range. Wi-Fi positioning will shortly be offered as an additional location option in the Broadcom service. Eventually, says Murray, Broadcom will integrate Skyhook with its own GPS chips and reference network infrastructure, enabling devices to use one integrated, hybrid positioning system.

The reference network also uses the data it stores to run complex mathematical models that can accurately predict where each satellite will be up to seven days in advance (Broadcom is working to extend that to 30 days). The models factor in such variables as tides, which can shift satellite orbits by many meters, says Murray.

This predictive data creates an intriguingly paradoxical element of the reference network. Customers can download this predictive data from a Web site and use it whenever they lack a data connection to the reference network. This data works as a kind of virtual satellite system conjured by mathematics.

This story, "Making GPS work: Service providers augment navigation data" was originally published by Network World.

Copyright © 2008 IDG Communications, Inc.

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