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2.4.4. GPS Usage
When you turn on the GPS, the receiver starts searching for radio signals from the
satellites and as soon as one is found, data starts flowing from the satellite to the
receiver. The data includes a list of the satellite positions and the GPS time. The GPS
time is highly accurate because each satellite has an atomic clock on board.
The data flows quite slowly and usually a little less than a minute is needed for the
calculation of the first position estimate. To minimize the duration, you should maintain
an unobstructed view of the sky right after turning on the GPS. You need the satellite
positions and the GPS time before you can perform the actual position calculation and
before the GPS becomes functional. After the first position fix has been established, it
still takes additional 10 minutes before all relevant data has been received from the
satellites. The position estimate accuracy improves during this time as more satellites
can be used for position calculation.
The downloaded data is valid roughly for the next four hours during which the re-
ceiver starts more quickly, typically in less than 10 seconds if it was turned off in
between.
You should turn off the GPS when, for example, you have a longer break. This is
because satellite tracking consumes large amounts of power. Suunto G9 uses the
lowest power consuming GPS technology available but you should still turn off the
GPS when you do not need it.
2.4.5. Versatility
The GPS system is run by the United States Department of Defense and has been fully
operational since 1995. At least 24 operational satellites circle the globe every 12
hours in 6 orbital planes with 4 satellites per plane to provide a true global coverage.
A worldwide ground control network monitors the health and status of the satellites.
The system works 24 hours a day regardless of the weather. It can provide service to
an unlimited number of users and it is free of charge.
