GPS System Segments - Understanding GPS: Principles and Applications - page 109

CDU. The CDU permits operator data entry, displays status and navigation solu-
tion parameters, and usually accesses numerous navigation functions, such as
waypoint entry and time to go. Most handheld units have an integral CDU. Other
installations, such as those onboard an aircraft or ship, may have the I/O device
integrated with existing instruments or control panels. In addition to the user and
operator interface, applications such as integration with other sensors (e.g., INS)
require a digital data interface to input and output data. Common interfaces are
ARINC 429, MIL-STD-1553B, RS-232, and RS-422.
3.4.1.5 Power Supply
The power supply can be either integral, external, or a combination of the two. Typ-
ically, alkaline or lithium batteries are used for integral or self-contained implemen-
tations, such as handheld portable units; whereas an existing power supply is
normally used in integrated applications, such as a board-mounted receiver
installed within a server to provide accurate time. Airborne, automotive, and ship-
board GPS set installations normally use platform power but typically have built-in
power converters (ac to dc or dc to dc) and regulators. There usually is an internal
battery to maintain data stored in volatile random access memory (RAM) inte-
grated circuits (ICs) and to operate a built-in timepiece (date/time clock) in the event
platform power is disconnected.
3.4.2 GPS Receiver Selection
At the time of this writing, there were well over 100 GPS set manufacturers in the
United States and abroad. While some, like SiRF, offer a few different chip set
receivers for integration with other electronic functions, other companies like
GARMIN and Trimble Navigation have many different end products ranging from
handhelds to automobile and aircraft navigators to complex survey receivers. GPS
receiver selection is dependent on user application. The intended application
strongly influences receiver design, construction, and capability. For each applica-
tion, numerous environmental, operational, and performance parameters must be
examined. A sampling of these parameters follows:
• What are the shock and vibration requirements, temperature and humidity
extremes, as well as atmospheric salt content?
• If the receiver is to be used by government or military personnel, PPS opera-
tion may be required. PPS operation usually dictates that a dual-frequency set
with a cryptographic capability is needed.
• The necessary independent PVT update rate must be determined. As an exam-
ple, this rate is different for aircraft precision approach than it is for marine oil
tanker guidance.
• Will the receiver have to operate in a high-multipath environment (i.e., near
buildings or on an aircraft where satellite signals are reflected by various fuse-
lage surfaces)? If so, multipath mitigation signal-processing techniques may
be required. (Detailed descriptions of multipath and multipath-mitigation