A known location receiver measures its position and calculates a correction for each satellite that is passed to other GPS receivers in the local area. Correcting bias errors using a known location accomplishes this. Position accuracy may be improved through the use of differential GPS processing. ECF is a right-hand orthogonal Cartesian coordinate system with the origin at the center of Earth, the z-axis increasing through the rotational North Pole of Earth, the x-axis increasing through the prime meridian (Greenwich, England) at latitude zero and longitude zero, and the y-axis increasing through 90 degrees longitude and 0 degrees latitude. The x, y, and z estimates are computed in Earth-centered fixed (ECF) coordinates. A precise estimate of the position of each space vehicle in view is determined from the broadcast ephemeris data. Four satellites must be in view to estimate x, y, and z coordinates along with a time estimate. An intersection of multiple range spheres determines where the GPS receiver is located. Given the ToA information measured from the correlation peak and the GPS time embedded in the signal, the GPS receiver can measure range to each satellite in view. Time of arrival (ToA) information is extracted when a correlation peak is measured. This signal is tracked using a phase locked loop (PLL), and the 50 Hz navigation message is demodulated from each satellite. This despreading produces a full-power signal. Because each satellite uses a different Gold-word spreading code, when the receiver has a peak correlation it knows which satellite sent the signal. The known spreading codes are very short and may be generated or stored in memory. The GPS receiver correlates known coarse acquisition spreading codes (with a 1-millisecond period of 1023 chips) from each of the GPS satellites with the processed signal from the GPS satellites. Fette, in Cognitive Radio Technology (Second Edition), 2009 Signal Processing of GPS Signals The height is the distance from the nearest point normal on the assumed altitude. The longitude is then found by rotating around the y-axis until the x-axis coincides with the vector from the center of the earth to the position. The latitude is found by rotating around the z-axis until the x-axis crosses the projection from the position on to the x–y-plane. The system takes its basis in the ECEF rectangular frame. This system expresses position in latitude, longitude and height, and is given in the spherical coordinates. The other representation is called ECEF geodetic frame. Its x-axis points through the intersection of the prime median (0° longitude) and equator (0° latitude), its z-axis towards the true North Pole (parallel to the earth’s rotational axes), and the y-axis to complete the right hand rule through the intersection of 90° longitude and equator ( Fig. 11.3). This is named the ECEF rectangular system but is usually just referred to as the ECEF system. The first representation frame gives its position in Cartesian coordinates, based on its distance from the center according to each axis. Of all the possible combinations of ECEF coordinate systems, two are of particular importance. This frame has its center in the center of the earth, and the frame is stationary relative to the surface.
The earth centered-earth fixed (ECEF) frame is mostly used in the case of global-positioning based navigation. Baudoin, in Using Robots in Hazardous Environments, 2011 Earth centered-earth fixed frame