Earth-Centered, Earth-Fixed Coordinate System - Fundamentals of Global Positioning System Receivers: A Software Approach - page 67

t oe : reference time ephemeris.
C us , C uc : amplitude of the sine and cosine harmonic correction term to the
argument of latitude, respectively.
C rs , C rc : amplitude of the sine and cosine harmonic correction term to the
orbit radius, respectively.
C is , C ic : amplitude of the sine and cosine harmonic correction term to the
angle of inclination, respectively.
e : longitude of ascending node of orbit plane at weekly epoch.
˙ : rate of the right ascension.
i : inclination angle at reference time.
ω : argument of perigee.
idot: rate of inclination angle.

4.12 SUMMARY

This chapter takes the satellite position calculated in Chapter 3 and transforms
it into an earth-centered, earth-fixed coordinate system because this coordinate
references a fixed position on or above the earth. Since the satellite orbit cannot
be described perfectly by an elliptic, corrections must be made to the position of
the satellite. The information for correction is contained in the ephemeris data
transmitted by the satellite. This information can be obtained if the GPS signal
is decoded. The GPS time t at time of transmission needs to be corrected for the
transit time as well as from the ephemeris data. Obtaining the coarse GPS time t c
at time of transmission corrected for transit time will be discussed in Section 9.7.
Finally, the coordinate system of the satellite must be adjusted to accommodate
the transit time.

REFERENCES

1. Riggins, R. “Navigation using the global positioning system,” Chapter 6, class notes,
Air Force Institute of Technology, 1996.
2. Bate, R. R.,

Mueller, D. D., White, J. E., Fundamentals of Astrodynamics , pp.

182 - 188, Dover Publications, New York, 1971.
3. Britting, K. R., Inertial Navigation Systems Analysis , Chapter 4, Wiley, New York,
1971.
4. “Department of Defense world geodetic system, 1984 (WGS-84), its definition and rela-
tionships with local geodetic systems,” DMA-TR-8350.2, Defense Mapping Agency,
September 1987.
5. Global Positioning System Standard Positioning Service Signal Specification , 2nd ed.,
GPS Joint Program Office, June 1995.
6. Spilker, J. J. Jr., “GPS signal structure and theoretical performance,” Chapter 3 in
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tions , vols. 1 and 2, American Institute of Aeronautics and Astronautics , 370 L'Enfant
Promenade, SW, Washington, DC, 1996.