iPhone 4s and iPad 4th Generation GPS Cold Start Test With No

iPhone 4s and iPad 4th Generation
GPS Cold Start Test With No Cell Service
Initial report - Update will follow in late February
By: Joseph Elfelt
http://www.MappingSupport.com
January 28, 2016
Summary
Test 1
An iPhone 4s and 4th generation iPad with (1) no almanac data and (2) iOS 9.2.1, can
download the almanac data from the GPS satellites and then determine its position using
satellite data.
•
Downloading the almanac from the satellites takes about 20 minutes.
•
The first coordinates returned have poor accuracy.
•
After an additional 2-3 minutes the coordinates are as good as they will likely get.
•
The compass app that is standard on iPhones can display the user’s coordinates.
Test 2
Coming in late February 2016.
Background
In order for a cell phone or other GPS-enabled device to use the satellite data to report a position,
the device must have current almanac data. A handheld GPS (Garmin, Magellan, etc)
downloads the almanac data from the satellites. Due to the slow transmission rate, the download
takes at least 12.5 minutes and 20 minutes is a more practical estimate. Fresh almanac data is
required whenever (1) the almanac data in a device is about two weeks old or (2) the device is
moved more than a few hundred miles.
If a cell phone has location services turned ‘on’ then the phone downloads the almanac data very
quickly from cell towers. This is referred to as A-GPS or assisted GPS.
Test purpose
The purpose of this testing is to discover if iDevices can download the almanac data from the
satellites when the device is at a location where there is no cell coverage from any provider.
Device configurations to be tested
This 2-part test will look at two iDevice configurations using an iPhone 4s and 4th generation
iPad. Both devices will be using iOS 9.2.1.
Test 1:
An iDevice with no almanac data. A new iDevice that has never had
location services turned on has no almanac data. Test completed.
Test 2:
An iDevice with out-of-data almanac data. This circumstance can come
about if (A) the iDevice is left off for about two weeks or (B) the iDevice
is turn on but location services are left off for about two weeks. Test
planned for 2/20/2016 or shortly thereafter.
In 2015 while in Canada I conducted a test with my iPhone 4s with
iOS 8-something and an out-of-date almanac. My phone never reported a
position until much later in the day when we got within range of a cell
tower. I concluded that my phone was not able to download the almanac
data from the satellites. Next month I will retest using the latest iOS.
Test 1 details - no almanac data
In order to put my devices into an “as new” condition, I backed them up and then did a factory
reset. That reset deleted the existing almanac data and put the devices into an “as new”
condition.
A brand new iDevice displays a “Hello” message and must be initialized. During the
initialization process I declined the offer to turn on location services in order to avoid having
almanac data downloaded into the iDevices from a cell tower.
After initialization I started Safari and loaded two browser apps:
Gmap4
http://www.mappingsupport.com/p/gmap4.php?usng=10U_EU_9073_271
8&tilt=off&z=14&t=t4
I used this one since I developed this enhanced Google map viewer and so
I know exactly how it does geolocation. The above link opens at the test
site. I dragged the map so the screen showed a location about 10 miles to
the east. Dragging the map caused these map tiles to be saved in the
browser’s cache.
USNGapp
http://usngapp.org
I used this one since it is minimalist code that eliminates the possibility for
user error.
Then I drove to the test site which is a known cell phone dead zone. For the first five minutes I
simply monitored my phone to ensure that it said “no service” and never showed any bars. Then
I turn location services on in both devices and the test began.
After about 15 minutes I used Gmap4 and did Menu ==> My Location. Gmap4 would think for
a bit and then report that geolocation was not available. I would wait about a minute and then try
again.
2
Around minute 21 of my test when I did Menu ==> My Location, the map suddenly re-centered
itself and now showed the area where I was parked. Taking about 20-21 minutes to download
the almanac is the same time period that I experienced with a Magellan Meridian GPS when I
was using it a bunch.
This initial coordinate (center of the map) was wrong by about 1/3 of a mile or so. (The phone
likely had about a minute of earlier coordinates that were wrong by larger amounts.) Over the
next ~2 minutes the geolocation steadily improved until it was basically right where I was
parked.
After Gmap4 was correctly showing my location, I looked at the Safari tab for USNGapp. That
app still showed question marks instead of coordinates. Fortunately that browser app is designed
so you can reload the app even when you are offline. I reloaded USNGapp and it immediately
displayed coordinates.
I also started the compass app that is standard on iPhones (but not included on the iPad). The
compass app is with some other apps labeled “Utilities”. You have to do a minor calibration
step. Since (1) location services were ‘on’ and (2) my phone now had a fresh almanac, the
compass app reported my position in degrees, minutes and seconds.
The coordinates displayed by the iDevices were basically the same as the coordinates displayed
by a Garmin Oregon 600 GPS.
Test 1 conclusions
1.
The factory reset I did successfully erased the existing almanac data in the iDevices.
2.
The iPhone 4s and 4th generation iPad with (1) iOS 9.2.1 and (2) no almanac data, can
download the almanac data from the satellites and then report a position when the devices
cannot see any cell tower.
Key things to remember:
1. It takes ~20 minutes to download the almanac data from the satellites.
2. It will take another 2-3 minutes for the location data to become as accurate as possible.
3. Every iPhone since at least the 4s has a compass app that can display the user’s coordinates.
-end-
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