useful info for those interested:
Almanac and Ephemeris Data as used by GPS receivers
(4 July 199
The satellites broadcast two types of data, Almanac and
Ephemeris. Almanac data is course orbital parameters for all
SVs. Each SV broadcasts Almanac data for ALL SVs. This Almanac
data is not very precise and is considered valid for up to
several months. Ephemeris data by comparison is very precise
orbital and clock correction for each SV and is necessary for
precise positioning. EACH SV broadcasts ONLY its own Ephemeris
data. This data is only considered valid for about 30 minutes.
The Ephemeris data is broadcast by each SV every 30 seconds.
When the GPS is initially turned on after being off for more than
30 minutes, it "looks" for SVs based on where it is based on the
almanac and current time. With this information, appropriate
SVs can be selected for initial search.
When the GPS receiver initially locks onto a SV, the Garmin
display then shows "hollow" signal strength bars. At this time,
the Ephemeris data has yet to be completely collected. Once the
ephemeris data is collected from EACH SV in turn, the associated
signal strength bar will turn "solid" black and then the data
from that SV is considered valid for navigation.
If power is cycled on a GPS unit, and when turned back on, the
Ephemeris data is less than 30 minutes old, lock-on will be very
quick since the GPS does not have to collect new Ephemeris data.
This is called a "warm" start.
If it is later than 30 minutes, this is considered a "cold"
start and all Ephemeris data will have to be recollected.
If the GPS has moved more than a few hundred miles or accurate
time is lost, the Almanac data will be invalid and if you are
far enough off, none of the SVs that the Almanac thinks should
be overhead will be there. In such case, the GPS will have to
"sky search" or be reinitialized so it can download a new Almanac
and start over.
(Note: Yes! We know this is somewhat simplified information.
Yes, we know that the Ephemeris data may not have to be updated
as often as the G-12XL does it to get data to the G-12XL
accuracy.)
Joe Mehaffey
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http://gpsinformation.net/main/almanac.txt