
iPhone vs. Droid GPS signal quality - dlokshin
http://rnr.davidlokshin.com/post/825290568
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antrover
I have a Droid incredible. Here is my experience:

If you have "use wireless networks" and Gps turned on, things don't always
work right. If you have just Gps turned on the experience seems to work
better.

I live in Madison, wi and having both turned on makes my phone think I'm in
appleton (100 miles away). Using Gps, my location is reported correctly.

When out of Madison, and turning on "use wireless networks", things work
correctly.

I have no idea why!

~~~
Niten
I'm not experiencing this issue with my own HTC Incredible, for what it's
worth. Maybe there's some glitch with Google's wireless network database...

As for the Incredible's GPS performance, I couldn't be happier. It's accurate
enough that when I go cycling and let My Tracks record my ride, I can zoom in
and see where I headed out on one side of the road and came back on the other.
The biggest downside is the elevation inaccuracy, but from what I can tell
that's an issue with all devices that gather altitude data from GPS (rather
than an internal barometer).

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wglb
My droid is rather awsome in this regard. But haven't tried any iphone for
this.

I am wondering why you say _decided that the antennae was of too low quality_?
I am curious why you think this is an antenna issue?

Also as another poster said, it can be an application issue.

~~~
buss
Somewhat unrelated to the original article, but: I got great results with GPS
on my droid in north Florida, but I recently moved to Seattle and the
performance decreased. It takes me 2-3 times longer to get a GPS fix, and it's
often off by as much as a block. Could it be the latitude?

~~~
celoyd
In theory, GPS fix quality is roughly the same in all mid-latitudes[0]; any
difference between Florida and Washington should be lost in other factors.

Probably, either your sky is more covered in Seattle (by trees and hills, for
example), or there’s more reflection and interference in the radio signal.

0\. <http://blogs.agi.com/navigationAccuracy/?cat=4> has some nice
illustrations. “DOP” is a sort of ideal fix quality, with smaller numbers
meaning more accurate fixes (under various assumptions); Wikipedia will
explain the math.

Edit: <https://gps.afspc.af.mil/gpsoc/PerformanceReports.aspx> is a live-ish
map. When I check it, FL and WA are both in the lowest (best) category.
However, it’s worst-case DOP for only the 4 best satellites; under a clear
sky, a good receiver will usually see several more. I often get DOPs of 1.5 in
residential neighborhoods.

~~~
buss
Thanks for that interesting info. I guess it's all of the buildings that are
interfering.

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sambeau
As an iPhone 4 user I find this depressingly disappointing but not surprising.
Anecdotally I have found the GPS performance of the 4 to be far superior yo
the 3G but, sadly nowhere near sub-street-lvel accuracy.

~~~
cheald
The problem with the iPhone 4 seems to be low sample size. Honestly, I would
not be at _all_ surprised if Apple lowered the sample rate of the GPS to
extend battery life. GPS is a terrible power-sucker, and you probably won't
know the phone is skimping on GPS readings unless you do comparisons like in
the article, since most consumers are pretty happy with ~20-foot accuracy.
People recording precision data, like surveyors or geocachers, though, would
do well to get a device with a high sample rate and good accuracy.

My N1's GPS accuracy is easily good enough for my purposes; if I pull up
Google Maps, it shows my location as about where my desk is on my lot in my
neighborhood. I can't really complain.

~~~
Niten
I haven't done any iPhone development, but can't the GPS sample rate be
overridden in software? I know it can on Android -- My Tracks will let you
tweak the sample rate if you want, for example.

~~~
cheald
I'm not an iPhone developer either, but I can't find anything in the docs that
would seem to allow you to set a sampling rate. What you do is tell the OS
that you want location data, and give it a desired accuracy
(AccuracyBestForNavigation, AccuracyBest, AccuracyNearestTenMeters,
AccuracyHundredMeters, AccuracyKilometer, AccuracyThreeKilometers) and the OS
decides when to give you updates. It could be that Motion-X isn't using an
appropriate resolution, but I'm somewhat dubious that that's the case, given
its intended purpose would need sampling at more than a 30-foot resolution.

Relevant docs:
[http://developer.apple.com/iphone/library/documentation/User...](http://developer.apple.com/iphone/library/documentation/UserExperience/Conceptual/LocationAwarenessPG/CoreLocation/CoreLocation.html#//apple_ref/doc/uid/TP40009497-CH2-SW1)

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albertzeyer
I have similar experience on the iPhone 3GS right after I start some GPS-using
application (like TomTom). It is sometimes really terrible in the first 10-15
minutes. Though after that, it becomes quite accurate. Often also earlier.
Also it is often better when I start it already when being outside of the car
and not when being in the car.

I guess that it searches each time for new satellite signals if the last usage
of GPS was too long ago.

~~~
bigiain
That sounds a lot like the time required to acquire the complete almanac from
the GPS signals. See the almanac section of:

<http://en.wikipedia.org/wiki/GPS_signals>

If a GPS receiver has no current satellite orbit data, it'll need to listen
for 12.5 minutes to determine the current set of satellite orbit information.

There's a useful explanation of the various possiblilities of GPS startup
(specifically talking about Garmin models and terminology, but more broadly
applicable too) here:

<http://gpsinformation.net/main/warmcold.htm>

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kgroll
How did you process/present that data after collection? Those look like Matlab
plots - did you use it analyze the raw GPS data, or just use some GPS
visualizer altogether?

If it would be helpful to you, I'd gladly repeat this experiment using my HTC
Hero (CDMA) + Handy Runner, and a Garmin for reference.

~~~
dlokshin
Hi kgroll. Yes I used matlab to process the raw GPS data (lat and long). It'd
be really helpful if you could try and replicate this test with a Hero. Do you
mind emailing me please? Email: me [at] davidlokshin [dot] com?

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hackermom
I think the test should've been performed with more care, by using multiple
GPS loggers instead of just one, to see if the result holds true. It might be
an application issue at show here.

~~~
lokshin
I used Run Keeper for the iPhone and it gave the same results as MotionX,
which is why I continued using just one of them to collect data (and then
write up the blog post). There's nothing to say, though, that both programs
may just be crap...

