NAVIOR-24S
™
CHAPTER IV
Operation and Performance
54
NAVIOR-24S ™
Revision
V1.00 20.05.2008
Satellite Acquisition and Time to First Fix
Cold-Start
The term “cold-start” describes the performance of a GPS/GLONASS receiver at power-on when
no navigation data is available. “Cold” signifies that the receiver does not have a current almanac,
satellite ephemeris, initial position, or time. The cold-start search algorithm applies to a NAVIOR-
24S GPS/GLONASS receiver which has no memory of its previous session (i.e., is powered on
without the memory backup circuit connected to a source of DC power). This is the “out of the box”
condition of the NAVIOR-24S GPS/GLONASS module as received from the factory.
In a cold-start condition the receiver automatically selects a set of 24 satellites and dedicates an
individual tracking channel to each satellite to search the Doppler range frequency for each satellite
in the set. If none of the selected satellites is acquired after a predetermined period of time (time-
out), the receiver will select a new search set of 24 satellites and will repeat the process, until the
first satellite is acquired. As satellites are acquired, the receiver automatically collects ephemeris
and almanac data. The NAVIOR-24S GPS/GLONASS receiver uses the knowledge gained from
acquiring a specific satellite to eliminate other satellites, those below the horizon, from the search
set. This strategy speeds the acquisition of additional satellites required to achieve the first position
fix.
The typical time to first fix is less than 90 seconds.
A complete system almanac is not required to achieve a first position fix. However, the availability
and accuracy of the satellite ephemeris data and the availability of a GPS/GLONASS almanac can
substantially shorten the time to first fix.
