There's been a push from government agencies, navigation equipment companies, and smartphone manufacturers to wring additional accuracy and performance out of the Global Positioning System's network of satellites. Part of that effort has included using a network of GPS-fixed reference stations to build an ever-improving model of satellite orbit data, which can be used to help GPS systems pick the right satellites to connect to get a quicker fix.

But there's a new emerging class of "near-real-time" GPS applications that promise to both squeeze additional accuracy out of GPS signals and in some cases dramatically reduce the power consumption of GPS sensors. The National Geospatial-Intelligence Agency (NGA), which supplies data on GPS satellite orbits to the Department of Defense and the scientific community, is looking to create these even more accurate calculations of GPS satellite orbit data by collecting data from a wider range of GPS reference stations. But rather than adding more of its own ground stations, NGA is looking to acquire more orbit data, called Precise Ephemeris (PE), through a sort of crowdsourcing—collecting the data from a constellation of GPS ground stations operated by private companies and other institutions.

The agency hopes the resulting network of GPS reference points will speed the gathering of precise location data, create extremely accurate GPS tracking for some GPS receivers, and allow for lightweight tracking devices that can last for months or even years on a single battery charge. The same approach could be used for commercial applications such as smart phone location services, improving their precision while dramatically reducing the power drain from GPS by pushing the computation of location up into the cloud.

Location services that don't suck (your battery)

Currently, most standalone GPS receivers get Ephemeris information (their current orbital position) and almanac data (the current course information for all the GPS satellites) from the satellites themselves as part of their broadcast. Because of the slow data rate of the transmission—in some cases, as low as 50 bits per second—it can take 30 seconds for a standalone GPS receiver to get the complete information required from each satellite used in establishing a fix, which requires at least three satellites (and optimally, five). In the process, GPS receivers expend a lot of power doing the signal processing needed to lock onto satellites' weak signals and adjust for the Doppler shift of those signals as satellites zoom past in their orbits.

Most smartphone GPS systems and Internet-connected GPS receivers use Assisted GPS (A-GPS) to speed up the process. A-GPS uses the smartphone's network connection to obtain Ephemeris and almanac data over the air and guide which satellites to watch for. Smartphone navigation apps also draw on other location data, including cell tower triangulation and a database of Wi-Fi access point MAC addresses (such as those maintained by Google Maps and Skyhook), to speed up getting a precise fix. Processing all of that data to get a fix can be a big drain on battery life.

But a new approach to handling GPS data takes a big bite out of that power consumption—by offloading all the computation to a server in the cloud, where the computational cost is lower and power is plentiful. In a paper published by Microsoft Research in November, researchers reported that a prototype "cloud offloaded GPS" sensor they developed, called CLEO, pushed raw GPS signal information to a remote server for processing. The result: CLEO was able to generate high-precision location information while reducing the amount of power consumed by the sensor by three orders of magnitude. "In other words," the researchers wrote, "with a pair of AA batteries, CLEO can theoretically sustain continuous GPS sensing (at 1s/sample granularity) for 1.5 years."

A little help from friends

The Ephemeris data used in most smartphone and commercial GPS applications is usually based on predictive data, and is good enough for most navigation. But it’s not nearly as accurate as the historical "post fit" PE data published by NGS, the National Geodetic Survey, and by NASA on behalf of the International Global Navigation Satellite Systems Service (IGS). Scientists and engineers use PE data for a number of applications requiring high levels of location precision, such as geodesy, wildlife tracking, monitoring sea levels and glacial movements, and creating geospatial records of the location of sample collections. After the 1994 Northridge, California earthquake, scientists were able to use PE data in combination with information from GPS ground stations to measure deformations to the Earth's crust in the region with a precision of millimeters.

Those applications aren't exactly near real-time—scientists typically access the data over FTP download to perform their analysis of GPS records. To support near real-time applications such as cloud offloaded GPS at the sort of precision NGA is hoping to provide, the agency would need a much more accurate predictive model that could be served up to applications—not just the Precise Ephemeris collected from its 17 stations around the world, which is published daily.

That's where looking for outside help comes in. The NGA's request for information, issued December 4, announced that the agency is looking to "augment its real time GPS data feeds," and that "consideration is being given to acquisition of GPS data from non-DOD entities." The data would also be used to generate the daily Precise Ephemeris, but it wouldn't be used by the Air Force for GPS's Control Segment—the network that monitors the health of the GPS system and sends commands to satellites for course corrections. Theoretically, the program could tap into some of the more than 350 ground stations operated by members of IGS internationally, as well as GPS ground stations operated by other government agencies and research entities.

At the moment, there's no word on whether a version of the services created by NGA would be offered through the National Geodetic Survey, which is responsible for distributing GPS data to citizens and private companies; NGA's spokesperson did not respond to an Ars request for comment.