Debunking the Wi-Fi only iPad Tethered iPhone 4 GPS Myth
So like everyone else I heard that you could tether your Wi-Fi only iPad to the iPhone 4 and it would get the GPS signal, albeit at a slow rate.
Many other sites, blogs and tweets picked this up and ran with it, including Daringfireball. I got excited since I was currently debating picking up a 3G iPad 2 and thought this would help me decide to save the 130 bucks and just tether a Wi-Fi model.
So I decided to give it a try. I had a 30 minute trip to take to pickup my daughter from a location I had never been to. Normally I would just use Google Maps on my iPhone. Instead I also grabbed my iPad 1st gen, tethered it up, and headed to the car. I started up Google Maps on the iPad and it found my location as it always does when I’m at home on my Wifi. I keyed in the destination and took off. Sure enough a little way down the road it updated my location. Cool. I kept driving and it seemed like it was getting updates about once a minute - just like I had read in various blog posts. But as my trip continued it started updating less frequently. Pretty soon it had gone for over 6 minutes with no update, I started to get skeptical. Now I live in a small town, and this trip was taking me through country roads with very few people around. I started to notice that each time it DID update it seemed that some houses were around. Eventually it stopped updating for a long enough stretch that I missed my turn and gave up. I pulled over and switched back to the iPhone.
Later I wondered if maybe the device just got grumpy and stopped updating for maybe it only worked properly with the iPad 2.
After finally scoring a 32M Wi-Fi iPad 2 and continuing to read more about this alleged ‘feature’ I decided to get to the bottom of it:
From my my first experiment my theory was that this was just Wi-Fi location tracking, nothing more. Since I had seen multiple videos of this ‘working’ by now I new that ‘something’ was happening, but what? I decided the best way to test this was to start by NOT tethering at all. What would the iPad do?
Test 1: Wi-Fi iPad 2 Not Tethered
Loaded up google maps zoomed in on my (small) town and headed for the car. I tapped the location dot and it found where I was via my Wifi signal. I then proceeded to drive around. I figured as long as I stayed within the map that was currently on the screen I had a chance of picking up locations and sure enough I did. As I drove down the road my location periodically updated. That’s right it was updating with no GPS and no tethering. (Actually how it was able to determine the GPS locations from the Wi-FI SSIDs without a data connection is a bit of a mystery, but one I’ll save for another day. Maybe it is cached in the Maps app.) As I drove around it seemed to update at the same rate and accuracy as when I was tethered days before. After a couple of miles I pulled over to try something different.
Test 2: Wi-Fi iPad 2 Not Tethered, ‘Ask to Join Networks’ disabled.
Two things I had heard to help ‘prove’ that the GPS information was NOT coming from the Wi-Fi were:
1. That it worked when ‘Ask to Join Networks’ was disabled. and
2. That when driving down the road you were going to fast to pickup Wi-Fi networks anyway.
Anyone who has done any war-driving will tell you that #2 is just plain false. Back in my war-driving days (PocketPc) I was able to map about 40 open Wi-Fi spots while driving 60MPH down the freeway on the way to work. If anyone out there still has a copy of WiFiFoFum go try this out.
As for #1, this was the perfect time to test that theory. I disabled ‘Ask to Join Networks’ and turned around and backtracked home. Sure enough the location updated in exactly the same spots as it had in the first trip. So ‘Ask to Join’ doesn’t disable the Wi-Fi location services at all.
Test 3: Wi-Fi iPad 2 Tethered to iPhone 4 through a Wi-Fi dead zone
In this test I wanted to prove that I wouldn’t get a location if there were no Wi-Fi access points around. Luckily I live out in the country so that wasn’t hard. First I headed out again without tethering, and with ‘Ask to Join Networks’ disabled. Again the location updated at the same parts of the road as before. I kept driving down a back road until the location stopped updating, then I drove a few miles more for good measure. I pulled over and tethered to the iPhone. Nothing happened, no location. I had heard that maybe it got the location once on start of the maps app, and then periodically afterwards. I exited the maps app, then killed it with the multi-tasking bar and restarted it. No location. I waited a few minutes, no location. I then started the maps app on the iPhone to make sure it knew where I was, yep, no problem. I then left it tethered, turned around and drove home. After a few miles it picked up the location in the exact spot where the last update was on the way out, and continued to pickup location at each spot on the way home that I was now very familiar with.
So there you have it. As far as my experience goes this simply doesn’t work. Myth busted.
If you want to try this yourself, give it a go without tethering first to see how that behaves first (as long as you stay within the range of your map). If you can get to a spot where it hasn’t updated in a while (might be hard if you live in the city), pull over and tether up. See if it gives you a location. A variation is to drive while tethered until it hasn’t updated for awhile, then pull over and wait for a location ‘update’. My money says you’ll be waiting a long while.
Test 4: The final conclusive proof - disable Wi-Fi and tether with Bluetooth
As I was writing up this post I thought of another test. What if you could tether the iPad to the iPhone over Bluetooth and disable the Wi-Fi altogether? If the GPS info was sent to the iPad over the tethering IP connection, then the method of connection should have nothing to do with it. I didn’t know if is was possible to tether to the iPhone over bluetooth. As I recall the original iPad’s didn’t support tethering to the iPhone at all. But as I had recently discovered testing our iPad app, Remote Conductor, iOS 4.3 had made a PAN profile available to the iPad. I gave it a try. Sure enough I was able to disable Wi-Fi and tether to my iPhone using bluetooth. Once I paired I headed to the car to take another test run. I didn’t get far. As soon as I brought up the Maps app I was hit with the dialog:
"Cannot Determine Location"
That’s it folks, there’s nothing to see here, move along.
[Update 3/24/2011]: I have created a YouTube video on this topic entitled: “Wi-Fi iPad Tethered GPS Myth Debunked”.