Further thoughts on GPS receivers as I remember being lost on Market Street.

So I landed in San Francisco, and took the BART to Market Street to go to my hotel on 3rd street. And I’m fully prepared, too: iPhone 3G, Garmin Legend GPS, laptop computer, Kindle, and a large bag full of other, more mundane things. (Pants, shirts, shampoo, toothpaste.) Bag is large and unwieldy, laptop bag is digging a trench into my shoulder, but I’m there at Market Street.

I pick what I think is the nearest BART exit to my hotel (a selection which was made by an uneducated guess), I go up the stairs, and I’m now on Market Street.

With the location of my hotel programmed into my cell phone and into my Garmin, I turn both on in order to get my current location and figure out which way I need to go.

The Garmin does it’s little satellite animation status thingy. I watch as it captures and syncs with one satellite, two, three,… finally, it knows where I am: near Pier 29.


I’m standing here with a huge pile of stuff in a large bag, a clean version of the random homeless who have taken an interest in extracting my spare change from me, I’m tired, and there is only 15 minutes left before early registration closes (because my flight was an hour late), and my Garmin thinks I’m two miles from where I know I am–somewhere on Market street?!?

So on comes the iPhone. Same result.


Here’s the problem: the way GPS works is by triangulating your position between multiple satellites in orbit around the earth. Each GPS is an ultra-precise atomic clock and a transmitter: in the transmission signal are the Orbital Elements of each satellite and the precise time to the nanosecond. (The time transmitted, by the way, is adjusted to the Earth’s surface relativistic frame. It turns out that the effects of gravity and the motion of the satellites means that the GPS satellites are moving 45 microseconds faster in the orbital relativistic frame than time moves on the surface of the earth. Which means the time on the GPS satellites has to be tuned slower so that the transmitted time matches the time on the surface of the Earth. Yes, that blows my mind too.)

So your receiver gets the ultra-precise time to the nanosecond relative to the satellite, the ultra-precise position of the satellite to within a foot, repeats with five more satellites, and using the fact that the speed of light travels one foot per nano-second, does a little bit of geometry and figures out where you are.

But if you are in the canyon of buildings that is Market Street, you run into signal reflections. Signal reflections create noise on the signal, so you don’t necessarily get the ultra-precise time, and they add distance to the signal’s travel path, which screws up your location. Thus in the canyon of Market Street GPS is useless: the GPS receiver does the math and thinks you’re around Pier 29, give or take fifty feet, when in reality you’re two miles away.

The error circle for the GPS doesn’t help, either: that error is calculated based on the calculated precision assuming you have a clear sky. The GPS error calculation cannot take into account building reflections, because the GPS receiver has no way of knowing if the signal bounced around before being received.

The upshot of this is that I had two GPS receivers in complete agreement, which were utterly useless, and worse: left me lost.

And resorting to the crudest of navigational tools: reading street signs and walking two blocks in order to get my bearings.

A generation from now our children will forget about street signs and we’ll become like the streets of London: their street signs were put on the sides of buildings rather than made into stand-alone sign posts. Over the years, renovations caused the building owners to tear down the signs but never replace them–which means in many places in London there are no street signs at all.

If everyone in the United States has GPS receivers–they’re very cheap and are starting to be included in every phone–then when will city governments decide to save a little money and stop replacing street signs? Will we get to a day and age where the only way to figure out where you are is to pull out the electronic gadget from your pocket and ask it?

And what happens if you’re like me, lost on Market Street, and there are no street signs? Do you just find a warm grate along the sidewalk while your hotel room goes unfilled?

More importantly, what does that mean for the location based marketing software and location-based navigation tools of tomorrow, not to mention the new iPhone “Find my phone” service, when the phone lies about where it’s located?

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