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How the S3 BTGPS Works
The Global Positioning System (GPS) is a space-based radio-navigation system.
This system consists of 24 satellites, which orbit the Earth at an altitude of
approximately 17,500 kilometers. Each of the 24 satellites is deployed in 6 orbital
planes and circles the earth twice a day. These systems of satellites continuously
transmit signals containing precise information, 24 hours a day in any weather
condition, everywhere around the world. By processing the signals received, the S3
BTGPS receiver provides users with helpful information of position, velocity, and
time for navigation and location based services anytime, anywhere on the Earth.
The time it takes the GPS to find the satellites is called the Time to First Fix
(TTFF). The GPS receiver needs to lock on to at least 3 satellites for a position fix.
The S3 BTGPS Receiver acquires a position fix typically within 10 seconds from a
hot-start state and roughly 80 seconds from a cold-start state. The initial state of
the receiver, (i.e. the latest status in the memory of the receiver) primarily
determines the time of TTFF. If you have not used your GPS unit for several
months, the almanac data for the satellites may be out of date. The unit is capable
of recollecting this information on its own, but the process can take as long as 3 to
5 minutes. The TTFF will also be longer the very first time the user first turns on
the GPS receiver, when a GPS receiver has lost memory, or has been moved over
300 miles (500 km) from its last location will also be longer.
Both TTFF and positional accuracy can be affected by obstructions such as tall
buildings, narrow streets & passageways, protective film on glass, heavy foliage,
large cliffs, and other situations where GPS satellite signals are blocked. An
external antenna provides better reception and faster time to fix when operating in
these conditions.
Once there is a position fix, the S3 uses the satellite signals to calculate an
exact geodetic location through triangulation method, contained in 10 meters CEP
accuracy devoid of Selective Availability (SA). The position data is then converted
within the receiver to latitude and longitude coordinates, which is usually provided
in the geodetic datum on which GPS is based (WGS 84). The map datum selected
on a GPS receiver needs to match the datum listed on the corresponding paper map
in order for the position readings to match. Using the wrong datum can result in
position offsets of hundreds of meters or much more. In addition to using the
wrong datum, accuracy can be limited by satellite orbiting errors, multi-path signals,
atmospheric delay and receiver clock timing.