Letters to the Editor: Smartphone Revolution

Nice article on “The Smartphone Revolution” in the December issue. I am not so tech-y about the working of GPS/indoor GPS, but I am interested more in this technology specific to indoor GPS (repeaters).

Can the smartphones get indoor GPS signals correctly and quickly? If the smart phones are really smart that they can connect to GPS satellite from indoor locations, then do the GPS repeater products become obsolete?

— Saad B.

Author Frank van Diggelen replies:
The simple answer to your first question is yes, many of these smartphones can get GPS signals indoors. But indoors is a big and varied place, and the more complete answer is that the term “indoor GPS,” like “offroad vehicle,” describes the presence of a capability, not the absence of all limitations. So even if your GPS receiver works indoors in some locations, there will always be other places it doesn’t. And it will generally work better where there are stronger signals, like outdoors.

Similarly, for your second question, high-sensitivity GPS will work some places indoors, but not everywhere, so there is a role for repeaters. However, GPS repeaters are like a long cable from the repeater’s receiving GPS antenna; so any GPS receiver that gets signals from a repeater will compute the position of the repeater’s receiving GPS antenna.

Nice article. One comment and one question.

Comment: The IGS ultra-rapids that started in 1999–2000 were from the beginning available for the future. They always contained 24 hours of estimated orbits and 24 hours of predicted orbits usable in real time. As I was responsible for generating these products within the IGS at that time, I am pretty sure that was the case.

Question: I do not understand why you write that turning off SA (Seletive Availability) was an enabler for A-GPS!? I know that one possible feature of SA was an artificial degradation of the satellite ephemerides but this option was never exercised to my knowledge. So using a global network to obtain broadcast ephemerides and predict them into the future was always possible. Nothing fundamentally changed when SA was turned off!?

— Tim Springer

Frank van Diggelen replies:
Just to be clear, the article enumerated seven enabling technologies for the revolution of GPS in cell phones (they are: A-GPS; massive parallel correlation; high sensitivity; coarse-time navigation; low-power TOW decoding; host-based GPS; and RF-CMOS), and a dominant spin-off technology: long-term orbits good for many days into the future.
The demise of SA made it easier to predict long-term orbits for two reasons: technical and commercial.

Technically, if you used the code- or carrier-phase measurements in your orbit modeling, then it was easier if these measurements were not degraded. Of course, you could have corrected them differentially, but the article makes the point that things were harder (not impossible) when SA was on.

On the other hand, if all you did was use the broadcast ephemeris in your predictions, then as you suggest nothing changed, technically, if the ephemeris degradation option was not exercised. But the fact that this degradation option existed made it a more difficult commercial proposition to develop a system for predicting many days of ephemeris. Thus the end of SA certainly helped facilitate the commercial availability of predicted ephemeris that is valid for many days into the future.