Centimeter-accurate GPS for self-driving vehicles

  • 02-Nov-2016 02:44 EDT
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In many prototype self-driving vehicles, the cameras, lidar and radar array work together to provide safety-redundant 360° sensing.

The list of technologies that could deliver the expected level of real-world performance and safety for robotic cars and trucks is long. Beyond today's advanced driver assistance systems (ADAS), artificial intelligence-based machine learning, V2V/V2I communications, high-definition 3D street maps and digital-twin controls, what other technologies will find their way aboard the autonomous vehicle?

Next-generation GPS (Global Positioning System) satellite navigation technology that can reliably and precisely locate a car or truck within around 5 cm (1.9 in) in real-time, could be a game-changer, providing back up, experts believe. That's because over time sensors will at some point deliver a degraded return or fail entirely. Vehicle designers deploy multiple technologies for this reason.

Should the cost of centimeter-accurate GPS location drop below $30 per unit, it could provide effective, reliable redundancy on a high-volume basis.

Pinpoint GPS

Such low-cost, precision GPS/GNSS (global navigation satellite system) receivers are nearly at hand, according to tech market analysts ABI Research, which recently issued a forecast on super-accurate sat-nav technology.

By 2021, precision GPS will begin to find its way into driverless cars, drones and even smartphones, said Patrick Connolly, principal analyst at ABI.

“Various competing technologies are now under investigation by the auto industry, but we believe that it will move to a hybridized approach, combining multiple sensors, sensor fusion technology and precision GNSS,” Connolly noted. “As the receivers’ unit price drops below $50, we expect to see a market develop for location technology services, such as artificial reality and head-up displays in higher-end vehicles.” V2V communications is another probable application.

GPS is not the primary sensor, but the back-up, he said. Precision sat-nav capabilities are  expected to provide the essential redundancy to complete the safety systems for autonomous vehicles. Of course, GPS has its own inherent intermittency problems: signal blockages, multipath interference and so forth, Connolly pointed out. But if the price were right it could be an invaluable addition to the technology arsenal required for SAE Levels 4 and 5 driving.

How GPS works

GPS/GNSS satellites circle the earth in well-defined orbits and transmit satellite and signal information to the ground. GPS receivers take this data and use a "trilateration" process to calculate the user’s location. Essentially, the GPS receiver compares the time a signal was transmitted with the time it was received.

The time difference, or "time-of-flight," tells the GPS receiver how far away the satellite is. With distance measurements from at least four satellites—three for position and one to estimate any offset in the receiver’s clock—the receiver can triangulate the user’s position.

Most handheld GPS receivers use single-frequency GPS signals to achieve accuracies of about 3 m (10 ft), whereas commercial grade mapping units get within about a meter (39 in), according to Todd Humphreys, associate professor at the University of Texas at Austin and director of its Radionavigation Laboratory.

The most accurate type of GPS system, a differential survey-grade location system, requires a base station and a rover, each of which must receive signals from at least four satellites, he explained. Survey-grade GPS units, which are typically dual-frequency, have accuracies within 1 cm (.39 in) horizontally and 2 cm vertically, but cost $1,000 to $2,000 and more.

If no base station is nearby, Humphreys said, dual-frequency receivers can still achieve centimeter-level positioning via a technique called Precise Point Positioning, which integrates external information such as precise satellite clock timing, ancillary "ephemeris" predictions and a model of the current state of the ionosphere.

“The catch is that PPP can take tens of minutes to converge to a sub-10-cm solution,” Humphreys explained. Another such approach is real time kinematics (RTK).

Sensor assistance

ABI's Connolly noted that precision GNSS achieves sub-meter accuracy through a variety of methods, including a network of reference stations. He said the biggest question mark today is not cost-related. Rather, it is "how to achieve reliable, worldwide satellite navigation coverage using supplementary location information from reference stations or other sources to support correction techniques, such as RTK or PPP methods. This is a costly undertaking, currently with no guarantee of a return on investment, he said.

Engineers and technology researchers are interested in how far they  can improve sensing to reduce the burden on the machine-learning aspect of the autonomous control systems in vehicles. “If you can cheat with better sensing, you can more readily solve many problems with ADAS technology,” noted Humphreys. Sensors and image recognition can perform poorly, for example, if lane markers or road margins are obscured by rain, snow or fog, even nighttime.

Next-gen GPS would be something like instrument-flying on the road, Humphreys indicated. Beyond much lower device cost, “the other ‘miracle’ would be to greatly decrease the time it takes for these systems’ calculations to converge to a corrected solution.”

Last year UT Austin’s own start-up Radiosense demonstrated precision sat-nav positioning on smartphones in a project supported by Samsung. The solution was "fragile," however, Humphreys reported.

The team uses sophisticated signal processing techniques to process the incoming data and reduce the errors that derive from multipath signals and RF signal blockage. Current testing involves a low-cost vehicle lane departure warning system that receives real-time corrections from a network of a dozen base stations set around town.

Next-gen predictive GPS

Competition in the location technologies segment ranges from crowd-funded startups to Internet giants Google and Alibaba. The cast includes precision receiver vendors NovAtel and Trimble Navigation, consumer receiver makers u-Blox and Skytraq Broadcom, Qualcomm and ST Microelectronics, and start-up firms such as North Surveying, NVS Technologies, REACH and Swift Navigation, ABI reported.

California-based Swift Navigation, for example, just introduced its Piksi Multi GNSS receiver, said Rob Hranac, vice president for business development. The software-defined receiver’s dual-frequency operation offers rapid RTK convergence times and reliable, centimeter-accurate positioning results at an increasingly affordable price, he stated.

An Israel-based start-up called EXO Technologies has developed software-based technology that generates corrections in a predictive manner, thereby reducing the dependency on connectivity and the need for additional computations, said Isaac Zafarani, co-founder and chief marketing officer. EXO’s device delivers better satellite position data, he claimed, which means faster acquisition time.

“Corrections are applied in real-time to most GPS receivers, resulting in 5- to 10-cm [up to 4 in] accuracy," Zafarani said. "And our data is provided through the internet, enabling global coverage.”

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