GPS performance may affected in several ways:
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SIGNAL IN SPACE is a category of problems in satellite transmission, code generation, satellite upload, satellite position, space environmental degradation, etc.
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BLOCKAGE / MASKING occurs when radiated SV signals are unable to reach the receiver antenna systems, such as when traveling through a tunnel, under a dense forest canopy, when indoors, etc.
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INTERFERENCE can occur when high electromagnetic conditions exist.
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EQUIPMENT FAILURES (either hardware or software) can include loss of power supply, software bugs, physical damage to the system, etc.
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INTEGRATIONS are sources of discrepancies due to interfaces with host vehicles, embedded systems, power, antenna orientation, output formats, etc.
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OPERATOR ERROR occurs by inadequate training, miss-keying, improper installation or use and set-up, etc.