Re: Assisted GPS UBlox7

[Greg Troxel]

"Gary E. Miller" <gem@rellim.com> writes:

>> This slows down time to first fix and that if a data connection
>> exists, one can download this data from other sources and pass it to
>> the GPS receiever.
>
> For older receivers, yes, newerecievers, no.

I don't follow this.

A 12-channel receiver (counts as "older" today) needs to know where to
listen in doppler and code space, so having alamanac lets it figure that
out.  A 160-channel receiver can perhaps just listen to the whole space
fast enough that almanac doesn't matter.  However, almanac lasts a
while, so unless you are leaving it off for months and then powering it
up and caring about TTF, this is very unlikely a real issue.

Receivers also need ephemeris, to calculate satellite positions.   That
can be had by decoding 30s of data, or AGPS.   This means that a modern
(160 or so channels; exact number of course not critical) can often get
a fix in just over 30s.  But with AGPS, I think the first fix can be in
a few seconds.