Taking GPS down and deep

Frank van Diggelen
Wireless Asia

As we all know, GPS is practically perfect in every way . . . so long as it's outside and unobstructed. Even cell phones can now produce meter-level accuracy under open sky. And, with Assisted GPS (A-GPS), those cell phones have mitigated the two great deficiencies of the original GPS: slow time to first fix (TTFF), and outdoor-only operation. A-GPS receivers can produce TTFF as fast as one second after a cold start, and (sometimes) work indoors.

However, there are still many deficiencies in the state of the art of location, particularly in deep urban canyons and inside large buildings. In the latter you will soon notice that even if your A-GPS operates in your house, it does not operate everywhere. The term "indoor GPS" is rather like "off-road vehicle": your four-wheel drive may let you cruise down the beach, but you certainly cannot use it to climb every mountain nor ford every stream. Similarly "indoor GPS" denotes the presence of a capability ?not the absence of all limitations.

And so what is the future of urban and indoor navigation, and which technologies will prevail? The short answer is: more satellites and more sensors. In this article we'll look at the technologies that will move us from the era of GPS-only into the future of GPS-plus.
 
Other GNSS

The most likely addition to GPS will be the other global navigation satellite systems, and all GPS receivers will be replaced by true, multi-system, GNSS over the next two to three years. Not because this will ever fully solve indoor location, but because of the outdoor problem in deep urban canyons.

Of the various GNSS systems, those with the most influence in the next few years will be Russia's GLONASS, because it is there, and Japan's Quasi-Zenith Satellite System (QZSS) because it is high. The first QZSS satellite recently began functional transmission. So let's use QZSS as an example of why extra satellites are so important in the deep urban canyon.

Tokyo's Shinjuku district is a typical deep urban canyon and a terrible place for GPS because there are not enough satellites in direct view. This puts receiver designers in an insoluble dilemma: Track only strong satellites, and you will not have enough; or track weak satellites, and you will measure reflections with large measurement errors because of the extra path length of the reflection. Moreover, the reflected signals can be indistinguishable from direct signals in their characteristics, especially in mobile phones where the antennas are poor, and directional ?so that signal strength is not a reliable indicator of whether a signal is direct or not.

Extra-high satellites can improve horizontal dilution of precision (HDOP). In this case the HDOP improves by about 20 times, from 58 to 3. It is easy to find many similar examples using GPS + GLONASS or any other GNSS combination. More often than not, extra satellites improve the situation significantly.

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