GPS System Segments - Understanding GPS: Principles and Applications - page 110

techniques are contained in Chapter 6. The contribution to the GPS error bud-
get is described in Chapter 7.)
• Under what type of dynamic conditions (e.g., acceleration and velocity) will
the set have to operate? GPS sets for fighter aircraft applications are designed
to maintain full performance even while experiencing multiple “Gs” of accel-
eration, whereas sets designated for surveying are not normally designed for
severe dynamic environments.
• Is a DGPS capability required? (DGPS is an accuracy-enhancement technique
covered in Chapter 8.) DGPS provides greater accuracy than stand-alone PPS
and SPS. Most receivers are manufactured with a DGPS capability.
• Does the application require reception of the geostationary satellite-based
overlay service referred to as SBAS broadcasting satellite integrity, ranging,
and DGPS information? (SBAS is discussed in Chapter 8.)
• Waypoint storage capability as well as the number of routes and legs need to
be assessed.
• Does the GPS set have to operate in an environment that requires enhanced
interference rejection capabilities? Chapter 6 describes several techniques to
achieve this.
• If the receiver has to be interfaced with an external system, does the proper I/O
hardware and software exist? An example would be a user who requires a
blended solution consisting of GPS and other sensors, such as an IMU and
vision system.
• In terms of data input and display features, does the receiver require an exter-
nal or integral CDU capability? Some aircraft and ships use repeater units such
that data can be entered or extracted from various physical locations. Display
requirements such as sunlight-readable or night-vision-goggle-compatible
must be considered.
• Are local datum conversions required, or is WGS-84 sufficient? If so, does the
receiver contain the proper transformations?
• Is portability for field use required?
• Economics, physical size, and power consumption must also be considered.

As stated earlier, these are only a sampling of GPS set selection parameters. One
must carefully review the requirements of the user application prior to selecting a
receiver. In most cases, the selection will be a tradeoff that requires awareness of the
impact of any GPS set deficiencies for the intended application.

References

[1] Bate, R., et al., Fundamentals of Astrodynamics, New York: Dover Publications, 1971.

[2] U.S. Department of Defense, Global Positioning System Standard Positioning Service Per-

formance Standard, Washington, D.C., October 2001.

[3] U.S. Coast Guard Navigation Center, http://www.navcen.uscg.gov.

[4] Riley, W. J., Rubidium Atomic Frequency Standards for GPS Block IIR , ION-GPS-92,

Albuquerque, NM, September 1992.