The Inner Edge: Who Holds the Key to Indoor Nav?

April 16, 2013  - By

The FCC released in March results of intensive indoor location trials of various technology solutions to this most difficult of PNT problems — yet the one that will unlock the greatest remaining untapped potential. The results will shape FCC-mandated position-reporting requirements for cell phones, and will drive future development of all indoor positioning applications. This story gives a top-level view of the results. For in-depth exploration, tune in to the free webinar this Thursday to hear critical information, insight, and perspective on this groundbreaking study from four key participants.

The April 18 webinar is free, but you must register beforehand. A downloadable file of the webinar will be available roughly one week afterwards, in case you miss the live presentation. Speakers include Khaled Dessouky from TechnoCom Corporation, a company that supervised the trials; Ganesh Pattabiraman from NextNav and Norm Shaw from Polaris Wireless, two companies whose technologies underwent rigorous testing in the trials; and Greg Turetzky from CSR, a company closely involved in the process.

Testing Overview

Conducted by the Communications Security, Reliability, and Interoperability Council (CSRIC) of the Federal Communications Commission (FCC), Working Group 3 (WG3), the tests trialled thousands of attempted location fixes in four representative morphologies (dense urban, urban, suburban, rural) and various building types.

The massive R&D movement focus on consumer-level applications, that is, cell phones, but this work will also ultimately affect professional and high-precision uses of GNSS. Those involved in machine control for warehousing, industrial assembly, indoor and even underground mapping, construction both above- and underground, underground mining, utility work, and even forestry will find this of particular interest — any activity in areas where sky-view is limited or negligible.

Today, well more than half of mobile phone calls are made inside buildings. The number of emergency calls roughly parallels that, and both figures are only projected to rise. The FCC has a clear mandate to bring E-911 capability to indoor calls.

The 2001 regulations governing such emergency calls required that both landlines and cellphones should provide the location of callers to within specific accuracy levels. Location information was to be sent transparently to public safety answering points (PSAPs), to dispatch fire/rescue/police personnel to the source the 911 call, and not just to the right street address, but to the right floor of a multi-storied building. That’s the driver for all this.

Widespread application of successful technology/ies meeting the indoor requirement, once determined, is the key to significant revenue for many parties, not least of them GNSS manufacturers and location-based services (LBS) providers.

GPS and augmented GPS technologies were only part of the cellphone solution, and other implementations included use of the cell signal itself along with an extensive database which can contain amongst other things signal attributes and network asset locations.

The WG-3 Locations Based Services (LBS) sub-group set about finding what technologies exist, how well they work and how they could be applied to E-911. Click here for the full report.

In the tests, Polaris Wireless used an RF pattern-matching/fingerprinting technique, Qualcomm used a hybrid assisted-GPS (A-GPS)/advanced forward link trilateration (AFLT) system, and NextNav used wireless beacon technology.

WG3 selected the San Francisco Bay Area for the Stage-1 Indoor Test Bed. The methodology centered on indoor testing in sample buildings within the most common wireless use environments, called morphologies: dense urban, urban, suburban, and rural.

Bldg. 2: One Front Street, San Francisco, California.

Dense urban: Bldg. 2: One Front Street, San Francisco, California.

Bldg. 18: Super 8 Motel on O’Farrell St., San Francisco, California.

Urban: Bldg. 18: Super 8 Motel on O’Farrell St., San Francisco, California.

Bldg. 13: Gilroy Gaits, Beige Stable Building, Hollister, California.

Rural: Bldg. 13: Gilroy Gaits, Beige Stable Building, Hollister, California.

Polygons surrounding areas containing 19 buildings were selected; the distribution of buildings tested was 6 dense urban, 5 urban, 6 suburban and 2 rural. 75 test points were selected by TechnoCom within these 19 buildings. Statistically significant samples of stationary test calls were placed from each test point using multiple test devices for each of the 3 location technologies under test by NextNav, Polaris Wireless, and Qualcomm.

More than 13,000 valid test calls were collected across the test points for each of the three technologies. Broad, representative wireless industry participation in the test bed meant that Polaris’ results were aggregated over AT&T’s and T-Mobile’s networks; Qualcomm’s results were aggregated over Sprint’s and Verizon’s networks; and NextNav operated essentially as a standalone overlay location network.

A certified land surveyor provided indoor ground-truth accuracy to compare test-call locations. The certified accuracy was +/-1 cm horizontal and +/-2 cm vertical.

The test results show the location-performance attributes under test: horizontal location accuracy, vertical accuracy, yield, time to first fix (TTFF), and reported uncertainty.

NextNav Summary Indoor Accuracy Statistics.

NextNav Summary Indoor Accuracy Statistics.

Polaris Summary Indoor Accuracy Statistics.

Polaris Summary Indoor Accuracy Statistics.

Qualcomm Summary Indoor Accuracy Statistics.

Qualcomm Summary Indoor Accuracy Statistics.

Dense Urban Environment

Satellite signals (in this instance, GPS) have, of course, significant challenges in penetrating large buildings. Consequently, AGPS fall-back modes, such as AFLT, were experienced frequently. Accuracy degraded as expected when GPS fixes were not attained. While a surprising proportion of hybrid fixes were experienced, even at test points where one would not expect a satellite signal to penetrate, the quality of the hybrid fixes was in general significantly degraded compared to GPS fixes.

RF finger-printing experienced its best performance in the dense urban setting. This is probably a combination of a confined environment that could be extensively calibrated and many RF cell sites and handoff boundaries that could be leveraged in creating a good RF fingerprint map of the dense urban center.

The best observed performance in the dense urban setting was that of the dedicated terrestrial (beacon) location system — a new infrastructure. However, due to multipath, location fixes that may be relatively close in absolute distance (for example, 40 meters away) are often located in a building across the street, in a neighboring building, or even across a few blocks from the test point.

Urban Environment

Each individual test building in the urban morphology produced different challenges, and the three technologies under test met them in varying degrees.

A major-league baseball stadium created a situation where AGPS fallback fixes could be very far away due to the exposed RF propagation outside the structure in which the test points were located. Stadium structure created challenges to RF fingerprinting at some test points.

A convention center created in some cases an environment that was deep indoors but with very strong cellular signal from cell sites inside the building. This made the beacon-based location system perform poorer than in most other test points, since attenuation to different directions in the outside world was particularly strong in those scenarios. AGPS and RF fingerprinting relied on the cell sites inside the structure to create adequate location fixes.

An older building of comparatively heavy construction, with a large atrium in its middle, produced widely varying results based on distance from windows or the atrium. Again, the phenomenon of apparent location in a building across the street was seen for both NextNav and Qualcomm. RF fingerprinting fixes appeared to cluster about the larger reflectors in this urban corner of San Francisco, which happened to be mostly across the streets from the target building.

A motel building demonstrated the unique challenge with indoor location: absolute distances (like 50 or 150 meters) which may have meant much in assessing outdoor performance mean less for the indoors, since emergency dispatch to the wrong building or even the wrong block could be easily encountered at those distances. A location across the street is certainly better than one a few or many blocks away but it may still leave some human expectations unmet.

A tall condominium building in a (non-dense) urban downtown San Jose created relatively poor AGPS performance, uneven beacon system performance, and RF fingerprinting performance that degraded with the height of the test point. All of the above factors related to each of the urban buildings, combined with a generally lower cell site density for fall back (than in dense urban), resulted ultimately in an aggregate urban performance that is slightly worse than the dense urban performance.

Suburban Environment

The effect of smaller buildings with lighter construction and more spacing between buildings quickly became evident. Outstanding GPS performance, almost as good as outdoors, can be achieved inside single-story homes. Similarly outstanding performance is achieved on average by the beacon-based location technology under similar circumstances. RF fingerprinting appears to suffer from performance degradation compared to more dense morphologies in the city.

The AGPS performance predictably changes as the suburban buildings become bigger and higher. The terrestrial beacon-based network continues to perform well in the larger suburban. RF finger-printing shows some enhancement relative to the smaller suburban buildings, but still shows most of the location fixes along the roads, highways or reflecting buildings.

Rural Environment

Large one-story structures with metal roofs limited the available number of satellite signals available for trilateration. In these cases more hybrid fixes were experienced with a concomitant increase in the spread of the location fixes about the true location. The performance of the beacon-based network was less impacted by the metallic roof (since that roof had more impact on sky visibility rather than on side visibility towards terrestrial beacons). Consequently the performance was somewhat better than for AGPS. The performance of the beacon-based network would of course depend on the density of its deployed beacons covering the rural area, which was sufficient in the case of the rural test polygon.

RF finger-printing showed reduced performance relative to the suburban environment due to the large spacing between surveyed roads (where calibration is done) and the rural structures as well as the lower density of cell sites.


Finally, the report concludes: “Stage-1 of the test bed contained in the end only three technologies to test. With the complexity of the task at hand, this created a good learning opportunity for both CSRIC WG3 members and the test house. However, there are a number of technologies that are either in use for location based services (LBS) or that are emerging which should be evaluated for their potential to contribute to the improvement of indoor wireless E911.

“Indoor wireless E911 is a critical public safety issue that will only increase.”

One key factor that the report does not at all address is relative cost of implementing these respective solutions. The same can be said for timeline. While some observers have concluded that “NextNav came out on top,” this solution in particular can be presumed to face much greater challenges for full or nationwide implementation than the other two, which rely largely on already existing infrastructures.

Another round of E-911 test-bed activities will ensue once funding and management issues are resolved. See CSRIC WG 3 LBS Subgroup member Greg Turetzky’s “Expert Advice” column from GPS World for perspective and a forward look.

Once again, for an up-close and personal look at the CSRIC Bay Area indoor tests, register beforehand here for Thursday’s webinar, April 18. A downloadable file of the webinar will be available roughly two weeks afterwards, in case you miss the live presentation.


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About the Author: Alan Cameron

Alan Cameron is the former editor-at-large of GPS World magazine.