Centimeter-Accurate Low-Power Mobile Positioning
GPS and other GNSS (Global Navigation Satellite System) chipsets are getting smaller, cheaper, and more energy efficient. They are now ubiquitous in smartphones and tablets, enabling a host of location-based services. But the underlying positioning accuracy of consumer-grade GNSS receivers has stagnated at approximately 2-3 meters. The next revolution in consumer-grade mobile positioning will take us to centimeter accuracy, but will require some clever tricks.
The latest clock, orbit, and atmospheric models have improved location precision by a meter or so, leaving receiver-dependent multipath- and front-end-noise-induced variations as the dominant sources of error in current consumer devices. Under good multipath conditions, we can expect 2-to-3-meter-accurate positioning at present; under adverse multipath, accuracy degrades to 10 meters or worse. Unfortunately, dramatic improvement on these numbers is simply not possible using standard code-ranging techniques, as manifest by the flattening of precision improvements over the past few years.
Yet outside the mainstream one can find GNSS receivers capable of centimeter---even millimeter---accuracy. These non-consumer, industrial-grade receivers are used routinely in geodesy, agriculture, and surveying. The key to their exquisite accuracy is a radically different approach to positioning in which the standard code-phase (or pseudorange) technique is replaced by differential carrier-phase positioning. So why not adopt this high-precision carrier-phase-based technique for consumer-grade mobile devices? Why not mainstream centimeter-accurate positioning? For the past decade or so, there have been three primary answers:
- The antennas on mobile handsets and tablets are little better than smashed paper clips. Their poor quality (15-20 dB below that of even a cheap patch antenna and dismal multipath mitigation) makes it extremely challenging to extract carrier phase measurements accurate enough for fast fixing of the integer ambiguities that arise in the carrier-phase differential technique. Keep in mind that mobile users are impatient: they may be persuaded to wait 30 seconds for a cm-accurate position fix, but only a resolute few would hold out for 5 minutes. So why not replace mobile "paper clip" antennas with better ones? The historical answer has been that real estate on mobile devices is at such a premium, and cost-cutting is so severe, that even paper clip antennas risk being too big and too expensive. To their credit, GNSS chipset manufacturers like Broadcom, Qualcomm, and CSR have eeked surprisingly good code-phase-based location performance from these wretched antennas by clever signal processing. But none has yet dared to take on carrier-phase-based positioning.
- Differential carrier-phase-based positioning is power hungry compared with standard code-phase positioning. On a mobile device, milliwatts matter.
- Lack of a killer app. The reasoning goes that users are content with accurate directions to the nearest Starbucks. What would they do with centimeter-accurate positioning?
Note that (3) influences (1) and (2): finding a killer app for cm-accurate mobile positioning would motivate antenna improvements and justify additional power allocation.
In WNCG's Radionavigation Laboratory, we're working on ways to deal with both (1) and (2). As for (3), we believe the killer app is this: convincing, globally-referenced, spontaneous augmented reality.
Figure: Demonstration of cm-accurate positioning using the antenna on a mobile phone. To our knowledge, this achievement is the first of its kind. Many challenges remain, such as reducing the time to ambiguity resolution, but this first demonstration has profound significance.
Over the next 3 years, we intend to address each of the above three objections and thereby beat a path to cm-accurate mobile location in outdoor environments. Once mobile device manufactureres and users catch the vison of what can be done with cm-accurate positioning, a new ecosystem of applications and capabilities will emerge to harness its potential.
Todd Humphreys's TED talk articulates our vision for mainstreaming cm-accurate location.
We demonstrate here an early prototype of our augmented reality system.
K.M. Pesyna Jr., Z.M. Kassas, R.W. Heath Jr., T.E. Humphreys, "A Phase-Reconstruction Technique for Low-Power Centimeter-Accurate Mobile Positioning," IEEE Transactions on Signal Processing, 2014.
D.P. Shepard and T.E. Humphreys, "High-Precision Globally-Referenced Position and Attitude via a Fusion of Visual SLAM, Carrier-Phase-Based GPS, and Inertial Measurements," IEEE/ION PLANS, Monterey, CA, May 2014.
K.M. Pesyna, R.W. Heath, and T.E. Humphreys, "Precision Limits of Low-Energy GNSS Receivers," Proc. ION GNSS+, 2013.
D.P. Shepard, K.M. Pesyna, and T.E. Humphreys, "Precise Augmented Reality Enabled by Carrier-Phase Differential GPS," Proc. ION GNSS, Nashville, TN, 2012.