I'm still puzzled as to why a transponder can be unavailable at certain hours and then suddenly become available at other hours. Is the elevation of the satellite from here in Mexico so low on the horizon that part of the satellite is physically blocked until certain orbital irregularities come into play? My antenna is pointed almost horizontally. Could that be a clue?
This has nothing to do with blockage, also has nothing to do with how horizontal your antenna is.
The answer has to do with the natural movements that all Geo satellites do and that the downlink antennas are aimed to serve the intended area so that you will notice it more in a period of time, satellites move like a figure 8 in a daily basis. People within the intended area will not suffer this effect, the effect is noticed more by users located in fringe.
For more deep explanation on natural movements of satellites in Geo-Orbit:
Extracted from wikipedia:
A perfect stable geostationary orbit is an ideal that can only be approximated. In practice the satellite drifts out of this orbit (because of perturbations such as the
solar wind,
radiation pressure, variations in the Earth's gravitational field, and the
gravitational effect of the
Moon and
Sun), and thrusters are used to maintain the orbit in a process known as
station-keeping.
Drift pressures are significant if uncorrected. East-West drift occurs because the equator is not perfectly circular, so satellites drift slowly towards one of two longitudinal stable points.
Solar radiation pressure, caused by the transfer of momentum from the Sun's light and infrared radiation, periodically flattens and disturbs the orientation of the orbit. Other factors, such as local irregularities in the gravitational field, also contribute less systematically to drift pressures.
Due to luni-solar perturbations and the ellipticity of the
Earth equator, an object placed in a GEO without any station-keeping would not stay there. It would start building up
inclination at an initial rate of about 0.85 degrees per year. After 26.5 years the object would have an
inclination of 15 degrees, decreasing back to zero after another 26.5 years.Therefore, a lot of energy has to be devoted to maneuvers that compensate this tendency. This part of the GEO station-keeping is called North-South control.
Part of GEO station-keeping, called East-West control, requires significantly less fuel than North-South control, so some aging satellites are only East-West controlled. This still guarantees that the satellite is always visible to a permanent parabolic antenna. (no need for Steerable antenna while communicating with a Geo-Stationary Satellite, because the satellite exists in one permanent spot above the earth).
