[Community] Wifi, Modem and GPS

Dr. H. Nikolaus Schaller hns at goldelico.com
Tue Jul 15 10:44:42 CEST 2014

Am 15.07.2014 um 10:30 schrieb Christoph Mair:

> Hi Sven,
> On Tue, Jul 15, 2014 at 10:15 AM, Sven Dyroff <S.Dyroff at phytec.de> wrote:
>> But please don't forget the possibility that a non-free firmware would allow
>> direct communication between UMTS and GPS (if supplied with power) without
>> sending any noticeable signals to the outside of the modem.
> Yes, the modem firmware could do this, but the modem-integrated GPS
> receiver has no connection to any GPS antenna. To route GPS
> information from the external GPS chip to the modem you need to
> exploit the host CPU as there is no direct connection between these
> two modules.

And it is questionable if this "feature" would reveal additional information to someone
trying to observe you.

The reason is that an observer already knows where you are located by
triangulation from the base stations. GPS may give a better precision (1-2 meters
instead of 10-20) but is not that reliable. Especially in buildings you may be able
to use 2G/3G but have no GPS reception. And tracking people works diffferently
(IMSI catcher).

So protecting against this type of risk is IMHO only a very gradual improvement - if at all.

Well, to be honest, the poor MH370 system should have had a telemetry channel
to permanently report the real GPS location (avoiding the triangulation by Inmarsat
data that was never designed to be used this way - and just fortunate that it was
logged at all).

>>> The Neo900 includes additional circuitry to detect such bad
>>> behavior by measuring the current consumption of the module.
>> Is this power observation also included within the power line of the modem's
>> GPS receiver?
> AFAIK the modem has only one shared power line for all internal
> modules. I can't remember if the Neo900 will also include an extra GPS
> chip to ensure no GPS information is passed to the non-free modem
> firmware.

Someone responsible should give a definitive answer - AFAIK there is currently
no separate GPS receiver module on the block diagrams.


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