Antenna Phase Centre Offset and Variation (Physical Influences of GPS Surveying)

Satellite Antenna Phase Centre Correction

The geometric distance between the satellite (at signal emission time) and the receiver (at signal reception time) is in fact the distance of the phase centres of the two antennas. However, the orbit data, which describes the position of the satellite, is usually referred to the mass centre of the satellite. Therefore, a phase centre correction (also called mass centre correction) has to be applied to the satellite coordinates in precise applications.

A satellite fixed coordinate system shall be set up for describing the antenna phase centre offset to the mass centre of the satellite. As shown in Fig. 5.15, the origin of the frame coincides with the mass centre of the satellite, the z-axis is parallel to the antenna pointing direction, the y-axis is parallel to the solar-panel axis, and the x-axis is selected to complete the right-handed frame. A solar vector is a vector from the satellite mass centre pointed to the Sun. During the motion of the satellite, the z-axis is always pointing to the Earth, and the y-axis (solar-panel axis) shall be kept perpendicular to the solar vector. In other words, the y-axis is always perpendicular to the plane, which is formed by the Sun, the Earth and satellite. The solar-panel can be rotated around its axis to keep the solar-panel perpendicular to the ray of the Sun for optimally collecting the solar energy. The solar angleis defined as the angle between the z-axis and the solar identity vector(see Fig. 5.16). Denoting the identity vector of the satellite fixed frame asthen the solar identity vector can be represented as

is needed for computation of the solar radiation pressure in orbit determination.

Fig. 5.15.

Satellite fixed coordinate system

Fig. 5.16.

The Sun vector in satellite fixed frame

Denoting r as the geocentric satellite vector andas the geocentric solar vector (Fig. 5.17),

then in a geocentric coordinate system one has

or

Fig. 5.17.

The Earth-Sun-satellite vectors

Table 5.3.

GPS satellite antenna phase centre offset

Satellites of	*	y	z
Block 1	0.2100	0.0	0.8540
Block ll/IIA	0.2794	0.0	1.0259
Block MR	0.0000	0.0	1.2053

Suppose the satellite antenna phase centre in the satellite fixed frame is (x,y, z), then the offset vector in the geocentric frame can be obtained by substituting Eqs. 5.175, 5.177 and 5.178 into the following formula:

which may be added to the vector r.

GPS satellite antenna phase centre offsets in the satellite fixed frame are given in Table 5.3.

The dependence of the phase centre on the signal direction and frequencies is not considered for the satellite here. A mis-orientation of the ey (ex too) of the satellite with respect to the Sun may cause errors in the geometrical phase centre correction. In the Earth’s shadow (for up to 55 minutes), the mis-orientation becomes worse. The geometrical mis-orientation may be modelled and estimated.

Receiver Antenna Phase Centre Correction

In the case of receiver antenna phase centre correction, the dependence of the phase centre on the signal direction and frequencies has to be taken into account. Both the phase centre offset and variation should be modelled. Generally, the phase centre corrections can be obtained through careful calibration. Receiver antenna phase centre offset is also antenna type dependent. For a GPS network, antenna phase centre corrections are usually predetermined and listed in a table for use.