At 11:43 AM 12/03/2013, you wrote:
On 12/03/2013, at 11:57 AM, Dave Long & Cath Lincoln
<[email protected]> wrote:
> Hi Emilis,
>
> I suspect that the GPS unit in the iPad is not as sophisticated as those we
> are used to using in aviation.
GPS is GPS. There hasn't been a huge amount of "sophistication" for most of
the last 20 years.
Except for SA getting turned off in 2000 and the boys and girls
running their system have learned how to tweak their satellite clocks
for accuracy and later Block satellites being launched with higher
power transmitters.
The algorithms in GPS receivers have also been greatly improved. With
some early ones you needed a time or an approximate position to
initialise the receiver. No longer required as most modern GPS
modules will find themselves in 45 seconds from a cold start. Also
more sensitive front ends and parallel processing internally, not
sequential and far greater number of correlators. If the thing has a
window in a room it is likely to find itself.
There is also the issue of receiver optimisation for the dynamics of
the vehicle it is in. With advanced modules it is possible to select
maximum accelerations etc for various classes of vehicles and also
any averaging or dead reckoning times. Put something in an aircraft
that was optimised for pedestrians, boats, cars etc and you may see
some anomalies under some conditions. I suspect the GPS in the iPad
isn't aviation optimised. Try turning off the wifi and 3G or LTE
connection on the iPad and see what happens when flying.
Glonass is being rejuvenated, there's a Chinese system and the
European Galileo. In the next few years I think most "GPS" receivers
will use all available satellites/systems. You can buy high end
receivers that do this already.
BTW GPS can give you 3 D velocities straight from the doppler shifts
accurate to mm per second. It isn't done by differencing the positions.
Mike
You're probably actually seeing the opposite. The GPS in most mobile devices,
regardless of vendor, is deployed on the understanding that it'll spend quite
a lot of its life indoors, where it'll be virtually impossible to
get a satellite
lock.
So it's an "assisted" GPS. It uses last known GPS position,
triangulation with
visible 3G towers, and observations of which geolocated WiFi base
stations are
in range to provide additional fidelity, which is why it does a pretty good
job of showing where you are even when you're inside a building, and
can provide
a location almost immediately instead of waiting for satellites like your
panel mounted GPS does when you turn it on.
Then the compromises set in: the device designers subsequently figure that
since they can supplement poor GPS reception with other data, they didn't need
to pay as much attention to the GPS antenna. So the device has one which is
smaller, lighter, more physically constrained than a "real" GPS device.
In built up areas you don't notice the compromise, because even though the GPS
isn't necessarily receiving a great signal, there are plenty of other location
cues and you still get accuracy to within a handful of metres.
But travel out into the middle of nowhere, shield the already compromised GPS
receiver under a few layers of FRP, and take yourself 50 km from their nearest
3G base station and 200km from the nearest WiFi and the antenna
limitations become
obvious.
Different vendors will suffer to different extents, but none of them get off
scott free in this respect.
- mark
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