Principles Of Operation - BMW E90 Manual

Principles of Operation

Each satellite transmits messages with the following (simplified) information:
• Number of the satellite
• Current position of the satellite
• Message time
The satellite additionally broadcasts so-called orbit (path) and almanac data. These orbit
and almanac data are stored in the navigation system and used for subsequent calcula-
tions to enable fast acquisition of the currently available satellites. Almanac data contain
information on the orbits of all satellites, time correction factors and atmospheric delay
parameters.
To determine the position, the GPS data are received by the GPS antenna in the antenna
base and routed via a coaxial cable to the navigation computer (CCC). The GPS data are
decoded.
To determine the location, the navigation system compares the time at which the signal
was sent with the time at which the signal was received. The distance of the satellite can
be calculated from this time difference. The time calculations are based on a highly accu-
rate atomic clock (UTC: Universal Time Coordinated).
The current position can now be determined by adding measurements from other
satellites. The GPS receiver can determine its position on the earth's surface with the
aid of at least three satellites. This process is known as "2D position fix". Two dimensional
because the receiver must assume that it is located directly on the earth's surface. The
absolute spatial position can be determined with the aid of four or more satellites ("3D
position fix"). The altitude is necessary, for example, on multi-tier road junctions and
intersections.
Note: A clear view of the sky is necessary for the reception of GPS data. Rain or
snow normally do not interfere with reception. However, a very dense
cover of wet leaves or shadow areas such as high-sided buildings, rows
of trees may impair reception.
In addition, depending on the time of day, there may be fluctuations in
satellite reception at one location during a 24 hour period (example:
Munich 5-10 satellites). This is due to the orbits of the satellites.
From these satellite signals, the navigation system calculates
• longitude
• latitude
• altitude from the satellite data.
Based on repeated measurements, the direction of movement and speed can also be
calculated by way of GPS.
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E90 Communication