Advantech PCM-3291, User Manual

                                                     PCM-3291      

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USER MENU 

1.GPS INTRODUCTION 

GPS is fully operational and meets the criteria established in the 1960s for an   

    optimum positioning system. The system provides accurate, continuous , worldwide 

    , three-dimensional position and velocity information to users with the appropriate   

    receiving equipment. GPS also disseminates a form of Universal Time Coordinated 

    (UTC). The satellite constellation consists of 24 satellites arranged in 6 orbital planes 

    with 4 satellites per plane. A worldwide ground control/monitoring network monitors   

    the health and status of the satellites. This network also uploads navigation and other   

    data to the satellites. GPS can provide service to an unlimited number of users since   

    the user receivers operate passively(i.e., receive only). The system utilizes the concept   

    of one-way Time Of Arrival (TOA) ranging. Satellite transmissions are referenced to   

    highly accurate atomic frequency standards onboard the satellites, which are in   

    synchronism with an internal GPS system time base. The satellites broadcast ranging   

    codes and navigation data on two frequencies using a technique called Code Division   

    Multiple Access (CDMA); that is, there are only two frequencies in use by the system,   

    called L1 (1575.42MHz) and L2 (1227.6MHz). Each satellite transmits on these   

    frequencies, but with different ranging codes than those employed by other satellites.   

    These codes were selected because they have low cross-correlation properties with   

    respect to one another. The navigation data provides the means for the receiver to   

    determine the location of the satellite at the time of signal transmission, whereas the   

    ranging code enables the user’s receiver to determine the transit (i.e., propagation)   

    time of the signal and thereby determine the satellite-to-user range. This technique   

    requires that the user receiver also contain a clock. Utilizing this technique to measure   

    the receiver’s three-dimensional location requires that TOA ranging measurements be   

    made to four satellites. If the receiver clock was synchronized with the satellite clocks 

    , only three range measurements would be required. However, a crystal clock is     

    usually employed in navigation receivers to minimize the cost, complexity, and size of   

    the receiver. Thus, four measurements are required to determine user latitude,   

    longitude, height, and receiver clock offset from internal system time. If either system   

    time or altitude is accurately known, less than four satellites are required. GPS   

    provides two services : the Standard Positioning Service (SPS) and the Precise   

    Positioning Service (PPS). The SPS is designated for the civil community, whereas   

    the PPS is slated for U.S. authorized military and select government agency users.   

    Access to the GPS PPS is controlled through cryptography.