thoughts on A-GPS offline

[mqy]

It is not a problem that user account is required when download u-blox online
aiding data.

The download protocol is fairly simple, can be implemented within several
hours in C.
(1) appaly for a free account by sending a email to u-blox 
(2) format request package by filling in user/password/lat/lon and other
optional data such as pacc,
(3) submit request and get response data, 
(4) parse aiding data from the reply (it's a block of binary data after
header lines),
(5) write the binary data to GPS device. The binary data block contains
about 50+ individual
    aiding messages

The account is free. AFAIK, u-blox AsssistNow service should be one of the
most open/free 
A-GPs services, and the up-to-14 days offline data is very attractive
comparing to others.
What I'm concern is: don't know how to eat the "big cake" without knowing
the format -- 
unlike online data, there is no ubx binary message headers (0xB5, 0x62)
inside offline file.

I guess, each alp file (N days) should be contains the following data:
time range(from, to): each file contains data of [1,2,3,5,7,10,14] days
region IDs: the earth is divided into R regions each cover a (lat/lon?)
area.
satellite IDs: 24 satellites each has it's unique orbit.
region visible satellites: at any time only several satellites visible to a
user.
differential almanac correction data: for each satellite (at the unit of
hours).

Nothing helps regardless where the data come from,
if either don't know how to extract aiding data from offline data, or data
format in-compatible.


Rikard Nilsson wrote:
> 
> would it be possible to get the satelite information that we get from
> u-blox from somewhere else?
> somewhere that dont require user account?
> 
> /rikard
> 
> On Tue, Feb 17, 2009 at 12:41 PM, Timo Juhani Lindfors
> <timo.lindfors at iki.fi> wrote:
>> mqy <meng.qingyou at gmail.com> writes:
>>> I think get locations by cell id is especially useful in case of weak or
>>> no
>>> GPS signals.
>>> I'll have a look at opencellid, thanks.
>>
>> Note also that ogpsd of frameworkd supports sending this off-line
>> aiding data.
>>
>>
>>
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