National Academies Press: OpenBook
« Previous: Receiver Errors
Suggested Citation:"Availability." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 162

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

APPENDIX C 162 Dilution of Precision Dilution of precision, or DOP, is a term that describes the effect of satellite geometry on positioning, timing, and velocity accuracy. Any positioning system that relies on pseudoranging will be affected by the angular spacing between the known points that are used to measure from. The GPS constellation has been designed to give users at least four satellites in view with good geometric spacing, but terrain and man-made structures can occasionally block a receiver's view of some satellites, especially those near the horizon, making the dilution of precision less than ideal. IMPROVING THE CAPABILITIES OF GPS Even before the implementation of SA in 1990, many potential GPS users envisioned a need to improve the accuracy of the system, as well as some of its other specified characteristics. Although GPS accuracy has just been discussed, other characteristics such as integrity, availability, continuity of service, and resistance to radio frequency (RF) interference require further elaboration. Integrity Integrity, as defined by the Federal Radionavigation Plan, is the ability of a navigation system to provide timely warnings to users if and when the system should not be used. The integrity function of a navigation system involves monitoring the system's errors and, if specified protection levels are estimated to be exceeded, giving a warning to the user that the system cannot be used for navigation. In the case of GPS, integrity is maintained by monitoring the signal emanating from each satellite and determining if the pseudorange accuracy meets specified performance criteria for a given application. Two statistical measures of integrity are often used. One measure relates the probability that a hazardously misleading error will occur and the probability that this error will go undetected (1 minus PHE times PD, where PHE is the probability of hazardous error and PD is the probability of missed detection). The second measure of integrity is simply the time a navigation system takes to warn the user that a hazardous error exists (time-to- alarm) There is currently no specified integrity value for either the GPS SPS or the PPS. Availability The availability of a navigation system, which is also defined in the Federal Radionavigation Plan, is the percentage of time that the services of the system are useable. Availability is an indication of the ability of a system to provide useable service within the specified coverage area. For GPS, "useable service within the specified coverage area" means

Next: Resistance to RF Interference »
The Global Positioning System: A Shared National Asset Get This Book
×
Buy Paperback | $61.00 Buy Ebook | $48.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!