Global Positioning System (Gps) Section; Gsm Technology - Motorola iDEN i930 Field Service Manual

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Global Positioning System (GPS) Section

The U.S. Department of Defense (DoD) has built a 24-satellite constellation for the purpose of
pinpointing locations anywhere on the planet. The satellites circle the earth every 12 hours,
transmitting radio signals at a fixed frequency of 1575.42 MHz. The satellites share the 1575.42
MHz spectrum with each other through the use of CDMA modulation.
To determine a location on earth, a receiver must be built that can receive the simultaneous signals
from the satellites that are in view of the receiver and use that information to calculate the location
of the receiver. The receiver must pick up the signals from 4 satellites. Once the 4 satellites are
located, the receiver measures the time it took for the satellite signals to arrive. From this timing
information, the distance between the receiver and each satellite can be calculated. The four
satellites' ephemeris data provide the satellite's X, Y, and Z positions. The range, R, is the receiver
measurement made by calculating the time it took for the signal to reach the receiver. The user's
position, (Ux, Uy, Uz), and the clock bias, Cb, is then calculated.
To use the GPS, a receiver that can receive the spread-spectrum signals must be built. The detected
signals are then converted from RF signals into appropriate digital input formats. These digital
inputs are processed and converted into position information. Once the position coordinates are
determined, they are communicated to the handset microprocessor and displayed on the screen or
transmitted to the base station as required.
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GSM Technology

Unlike previous cellular systems, GSM uses digital radio techniques and operates in the frequency
range 890-960 MHz. The GSM system has the following advantages over previous analog systems:
International Roaming - Due to international harmonization and standardization, it will be possible
to make and receive calls in any country that supports a GSM system.
Digital Air Interface - The GSM phone provides a digitally encoded RF link between the telephone
and the base station, which is, in turn, digitally links into the switching subsystems and on into the
PSTN.
ISDN Compatibility - ISDN (Integrated services digital network) is a digital communications
standard that many countries are committed to implementing. It is designed to carry digital voice
and data across existing copper telephone cables. The GSM phone offers similar features to the
ISDN telephone.
Security and Confidentiality - Telephone calls on analog systems can easily be overheard with a
suitable radio receiver. GSM offers vastly improved confidentiality because of the way in which
data is digitally encrypted and transmitted.
Better Call Quality - Co-channel interference, handover breaks, and fading will be dealt with more
effectively in the digital system. The call quality is also enhanced by error correction, which
reconstructs lost information.
Efficiency - The GSM system will be able to use spectral resources in a much more efficient way
than previous analog systems.
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OVERVIEW: Global Positioning System (GPS) Section
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