Re: GPS unable to lock while moving

I would disagree - I have had otherwise excellent retail GPS receivers be
unable to get enough information to get a fix on startup in a moving
vehicle. When the vehicle is stopped (without changing the GPs position
inside the car) it will rapidly get a fix. I have seen the same happen on a
bicycle! I believe it is a function of individual satellites being lost to
view while the almanac is downloaded (maybe?). Once the almanac is
downloaded and a fix is obtained, then movement has little impact. I'd
suggest getting a current poition fix before getting under way. In my very
subjective experience the same seems to also apply to WAAS "lock" as well -
best to establish a WAAS connection at standstill.


"C.P Kurz" <cpkXSPAM@xxxxxxxx> wrote in message
news:dc5b24$ll7$01$1@xxxxxxxxxxxxxxxxxxxx
> Peter schrieb:
>> I use a variety of GPS systems, on the ground and in aircraft.
>>
>> The ground ones, usually Emtac (SIRF) based ones, also Sysonchip
>> (possibly SIRF) and others, tend to have big problems locking on while
>> moving e.g. on a train. Once the train stops, the thing locks
>> immediately.
>>
>> Is this normal, or is it poor software? The aircraft systems
>> (Honeywell KLN94 GPS) don't seem to have any of these problems.
>
>
> Velocity is not an issue. Remember, receiver and satellites are already
> moving at high speeds with respect to each other (earth rotation and
> satellite orbit). The train or aircraft velocity is comparably small
> against that.
>
> I don't know if the Honeywell uses an external antenna mounted outside the
> aircraft, but I guess it does.
>
> On the train, you will only be able to use the GPSR internal antenna, with
> three major issues:
>
> - the train and train furniture will block line of sight to most of the
> available satellites.
> - most train windows are metallized to prevent heating up through
> sunlight. These windows will block or dampen GPS signals
> - as a result, the receiver will only see few satellites, and those with
> lower elevations. Receiption of these signals is intermittent because of
> the moving train passing bridges, poles, tunnels, etc.
>
> The receiver needs a certain amount of uninterrupted receiption for every
> satellite in order to compute an initial position, typically at least
> 20-40s.
>
> That's easy to accomplish on the aircraft with an external antenna, but
> very difficult on a train.
>
>
> You should try to obtain an initial position fix while you are outside the
> train and let the receiver run while you enter it.
>
> - Carsten
>


.

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