FIGURE 5.1 Horizontal and vertical dilution of precision above 45° latitude. Dilution-of-precision values of 1 to 2 are ideal. Values of 2 to 5 represent the minimum required for reliable navigation. Values above 5 represent low fix quality and may not be acceptable for certain military operations. SOURCE: Courtesy of The Boeing Company, Seal Beach, Calif.

FIGURE 5.1 Horizontal and vertical dilution of precision above 45° latitude. Dilution-of-precision values of 1 to 2 are ideal. Values of 2 to 5 represent the minimum required for reliable navigation. Values above 5 represent low fix quality and may not be acceptable for certain military operations. SOURCE: Courtesy of The Boeing Company, Seal Beach, Calif.

Similarly, depending upon the placement of the antenna on a projectile versus the position of the GPS satellites on the horizon, guided munitions performance could also be adversely affected. Increased VDOP can also affect targeting when inaccurate height of target can transpose into horizontal error, depending upon the trajectory of the weapon. Figure 5.1 shows both horizontal dilution of precision (HDOP) and VDOP above 45° north latitude.1

Ionospheric Effects

Errors introduced by ionospheric delays are more pronounced in higher latitudes because of the reliance on low-elevation satellites. The ionosphere can affect GPS receivers by degrading the signal strength, in some cases causing code delay, phase advance, and loss of carrier lock. Additionally, irregularities in electron density, known as scintillation effects, can lead to significant phase and amplitude fluctuations in GPS signals as they pass through the ionosphere.

1

For a more detailed discussion of GPS performance issues, see Dennis Milbert, 2009, “Improving Dilution of Precision,” GPS World, November 1. Available at http://www.gpsworld.com/gnss-system/algorithms-methods/innovation-improving-dilution-precision-9100?page_id=3. Accessed August 2, 2010.



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