Matteo Luccio
“What’s new with GPS?” people often ask me when I tell them my job. Recently, I have been responding by telling them about the other three GNSS constellations now fully available. However, as reflected every month in these pages, that is but one of many developments that combine to make satellite navigation ever more accurate, reliable and ubiquitous.
While the GPS program is old by the standards of the digital age, it has never been static. In the 1970s, when GPS was developed, the expected accuracy for civilians was tens of meters, though pioneering commercial users began right away to chip away at the system’s limitations by developing differential GPS (DGPS), carrier-phase positioning, and other techniques. By the end of the next decade, better signal processing and the implementation of DGPS had brought civilian accuracy to about one meter. In the 1990s, phase-ambiguity resolution made real-time centimeter accuracy standard for surveyors.
As the adoption of cell phones exploded, it became imperative to locate them to preserve the 911 system. Initially, this was done using the time-of-arrival of signals to handsets from towers, because it was assumed that GPS receivers could not be made sufficiently small, cheap, fast, power-efficient and accurate to work in cell phones. The implementation of assisted GPS, now standard in all smartphones, largely solved those problems.
Precision for civil GPS users increased by an order of magnitude in May 2000, when President Clinton ordered the removal of Selective Availability, and substantially once enough satellites began to broadcast the L2 civil (L2C) code, enabling ionospheric corrections. Later, the modernized signals in the L5 band enabled sub-meter accuracy without augmentations and very long-range operations with augmentations. There are now more than 80 signals in that band, on GPS, Galileo and BeiDou satellites. On the military side, the effort to deploy M-code signals, cards and receivers continues.
Over the years, in addition to modernized satellites and signals, improvements have included the development of PPP, RTK and hybrid techniques; the proliferation of local, regional and global correction services; improved jamming and spoofing detection; and the increasing integration of GNSS receivers with other RF receivers as well as with inertial, optical, radar, lidar and other sensors.
Future improvements may include:
■ signal authentication
■ commercial systems in low Earth orbit that would have a signal strength on the surface three orders of magnitude greater than current GNSS, greatly boosting indoor reception and protection from jamming
■ inertially aided extended coherent integration, a.k.a. “supercorrelation,” which makes moving GNSS receivers more sensitive to signals they receive directly than to reflected ones
■ 3D-mapping-aided GNSS, which enhances the positioning algorithms by identifying non-line-of-sight signals; this is being pioneered by Google in nearly 4,000 cities, relying on its 3D city models and machine learning.
The moment I send this month’s issue to the printer, I will think of more past and future improvements. As soon as you receive it, many of you will think of yet more. What’s new with GPS? A lot.
Matteo Luccio | Editor-in-Chief
mluccio@northcoastmedia.net
