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, 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 system 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 systems 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 (now a unit of John Deere Company), 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.