System Feature Overview 47
68007024085 April 2011
2.4.3.1 Performance Specifications
The definitions for some of the terms stated in the table above are as below:
• Cold start – A cold start scenario occurs when the radio is first powered up, and the
GPS receiver is attempting to acquire its first position lock. In this scenario, the GPS
receiver only has a valid almanac stored; it does not have any valid satellite ephemeris
data nor valid real-time clock synchronization. Almanac data is stored in a non-volatile
(persistent) memory, and is valid for approximately one year. The GPS receiver regularly
updates the almanac data; therefore it will always be valid unless the radio is powered
off for more than one year. The almanac data provides a mapping of the GPS satellites’
position in the sky in relation to a real-time clock.
• Hot start – A hot start scenario occurs when the GPS receiver attempts to acquire a
new location fix after a previous fix had occurred recently. In this scenario, the GPS
receiver has valid satellite ephemeris data, a valid almanac, and valid real-time clock
synchronization.
• TTFF – Time to First Fix indicates the time the GPS receiver takes to determine its first
or subsequent position lock. This is determined largely by the time taken to download a
full satellite ephemeris or satellite orientation packet with a data rate of 50 bits per
second (bps), as well as, how long it takes for the GPS receiver to reach the relevant
satellite in its Scan List. In a cold start, the Scan List includes all of the 24 orbiting
satellites. The GPS receiver samples each satellite for a certain amount of time to
determine if it is visible or not before moving to the next satellite. The receiver continues
to do this until it detects a certain number of visible satellites and can determine an
approximate location, thus helping the receiver to truncate the Scan List. In a hot start,
the receiver already has most, if not all, the data needed to calculate its position.
Therefore, no scanning is needed and minimal downloading is necessary to calculate
position, resulting in a lower time to acquire a positional fix.
• Horizontal Accuracy – Horizontal Accuracy indicates a radius length from the reported
point location. The latitude and longitude reported is equivalent to a point in the center of
a circle, with the horizontal accuracy value as the radius of the circle. The true position
should be within this location range.
GPS Transmitter Portable Mobile
TTFF (Time to First Fix) Cold Start < 2 minutes < 1 minute
TTFF (Time to First Fix) Hot Start < 10 seconds
Horizontal Accuracy < 10 meters
Note: Accuracy specifications are for long-term tracking (95th percentile values > 5 satellites visible at
a nominal -130 dBm signal strength).
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