Question About Garmin "Hot Fix Data"

Hoping someone can elucidate. Is the following a correct understanding
of the Garmin Hot Fix data download?

"It is my understanding that for a "cold" startup, the GPS does not know
where it is. It therefore has to "scan" all the possible codes and
frequencies in order to acquire the required number of satellites.

Subsequent "turn-ons" should be able to make use of the last known
position, timezone, etc. in order to narrow the search in terms of
satellite ID, codes and carrier frequencies. If my understanding is
correct, then what does the hot fix data entail? For example would it be
a dataset consisting of any updated codes/carrier frequencies, a table
of most "visible" satellite ID's (ie., codes, carrier frequency) based
upon timezones and possible last known GPS position? I'm just curious as
to what it would be?"

Thanks,
Harvey

Harvey Gratt wrote:


Gdenerally speaking, in a "cold start" situation, the handheld unit will
often ask for approximte location.(For instance "southern Québec" or
"Eastern Australia".

You need to remember that GPS units use Doppler a LOT.

Consider a satellite "27" moving from south west pacific to north east
pacific.

If you are in australia, the satellite is moving away from you, if you
are in california, it i moving towards you. So you would need to tune in
to a different frequency to hear satellite 27's signal depending on
where you were.

Without a general idea of where you are, the unit has to scan
frequencies for any/all satellite signals. And if its almanach is not up
to date, it takes longer before the GPS can confirm a location based on
satellites it has acquired so far.

If you are moving, *in my experience*, it takes longer for the GPS to
get a fix because until it has a fix, it doesn't know you are moving,
and doppler effect of your move results in a satellite not being heard
at the expected frequency so the unit has to scan more before it hears
satellites.

JF Mezei wrote:
O.K., based on what you stated, I think my original understanding is
correct. The "hot-fix" dataset is probably a set satellites indexed by
the location (initial or otherwise) present in the GPS unit which the
GPS unit would have a line-of-sight to. This would then narrow down the
code/frequency search to a much smaller number than the two dozen + that
a complete search would need, thus speeding up the whole process.

Thanks,
Harvey

On Tue, 03 Aug 2010 23:16:19 -0400, JF Mezei


But not as part of the calculations that provide your position, right?

Everything I've read says that GPS receivers use the time signals from
the various satellites in current view, or more specifically, they use
the *elapsed* time of those various time signals, to calculate the
position of the receiver.

Sunshine wrote on Aug 4, 2010:


My understanding is that Doppler shift is used in the calculation of the
current velocity rather than your current position

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com