National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: GPS Transmit Antenna Calibration

« Previous: Spot Beams
Suggested Citation:"GPS Transmit Antenna Calibration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 133

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.

TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 133 conflict, the desired signal power at the receiver could be increased. For example, if a 3-meter, steerable-reflector L-band antenna (or phased-array antenna) could be added to the spacecraft, then a gain of approximately 20 dB would be obtained, which would increase range-to-jammer penetration by a factor of 10.12 While adding a 3- meter steerable antenna to the GPS satellites is a very significant change with attendant complexity, weight, and cost penalties, this is clearly a preferable approach to simply boosting the overall L2 transmitter power. In summary, in addition to increasing the L2 transmitted power, military anti-jam capabilities can be further improved by using a new, very wide-band signal (approximately 200 MHz), a spot beam, or some combination of both. ENHANCEMENTS FOR HIGH-PRECISION USERS GPS Transmit Antenna Calibration High-accuracy users of GPS rely on differential carrier phase measurements to obtain millimeter- to centimeter-level results. High accuracies are obtained because for the differential measurements, most satellite- based errors are common mode errors and cancel in the differencing process. One error, however, that does not cancel is the error due to variations of the effective location (phase center) of the transmitting GPS antenna. These variations are a function of the angle to the user, primarily the angle off the GPS antenna array boresight. Satellites that require precise orbit determination, such as Topex/Poseidon, are vulnerable to this error because the satellites view the GPS antenna from large angles off boresight. The maximum boresight angle to receivers on the ground is about 13 degrees, while the angle to a satellite in an orbit at 1,300 kilometers altitude is about 17 degrees. Variations of a few centimeters in the GPS transmit antenna phase center would induce variations of about 10 centimeters in the altitude of the Topex orbit. Even for ground-based measurements, these effects may contribute a small (~10-9 x baseline length) error. Phase variations are expected to be much greater at larger boresight angles. For such applications, knowledge of the transmit antenna phase variations is needed to reliably obtain centimeter, or subcentimeter, accuracy. By measuring the transmit antenna phase center, the error currently limiting accuracy of very-high precision users can easily be eliminated. Since elaborate antenna measurements are already being made prior to launch, it should be relatively simple to make the measurements required to determine the actual phase center.13 12 Since the nominal GPS antenna has a gain of + 11 dBiC L and + 13 dBiC L , at 14.3 degrees off axis, the benefit of the 2 1 postulated spot beam is about 20 dB. 13 B. R. Schulper, B. L. Allshouse, and T. A. Clark, ''Signal Characteristics of GPS User Antennas," Navigation: The Journal of the Institute of Navigation 41, no. 3 (1994).

Next: Improved L1 Signal Reception at Angles Below the Earth's Horizon »
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!