Inclined orbit operation of geostationary satellites

Explanation of how geostationary satellites eventually run short of fuel and start to move north-south and how tracking antenna may be used to extend operational life.

Once a satellite has been placed accurately into its geostationary orbit position it gradually starts to drift north-south on a daily basis due to the influence of the sun and moon.  There is a gradual increase in the inclination of the orbit.  If left alone, a satellite that has initial zero inclination will have its inclination increase at the rate of 0.8 deg per year.

It is usual not to let the maximum north south movements exceed +/- 0.15 degree to allow the use of large numbers of fixed pointing moderate sized and VSAT antennas.  It is accepted that  the largest teleport stations need to have tracking capability.  If nothing were done, the 0.15 deg limit would be exceeded after about a month and moderately sized, but fixed, dishes would start to see signal increases and decreases during each day. If left much longer, even VSATs would need tracking motors.

To solve this problem, satellites start their life in the geostationary orbit with a substantial amount of rocket fuel which is used periodically every few weeks to correct the trend toward orbit inclination increase.  Typically a brief burst firing of the north-south thrusters will be made as the satellite crosses up across the equator so that instead of continuing gradually upwards it instead goes gradually downwards, effectively into negative inclination.  The inclination then reduces over the next few weeks to zero and then increases again, by which time another thruster firing is needed.

The net result is that over a period of 10 to 15 years the station-keeping fuel is gradually used up, but for the whole of this period the satellite is maintained within +/- 0.15 deg north-south of the equator.

A small proportion of the total fuel is used for east-west orbit adjustments, since there is a tendency for satellite to very slowly drift sideways due to the triaxial nature of the earth. Gravity is slightly stronger at three points around the equator.

Once a satellite is getting near the end if its normal north-south station-keeping the operators decide to stop and concentrate the remaining fuel on the much more economic east-west station-keeping so as to extend the life by several more years.  During this periods the satellite is kept in is east-west position so that interference to adjacent satellites is avoided, but its inclination is allowed to increase to say +/- 5 deg over 6 years.  The communications payload continues to operate, with some loss of performance at the edges of the coverage beams since they no longer always point accurately at the countries on the ground all of the time.

During the inclined orbit years earth stations must must have tracking systems so that their pointing is adjusted to aim at the satellite all during the day.  The beam pointing movement is classically a lissajous "figure of 8", but might at times be a tall slim elliptical shape or tilted elliptical shape.

The figure above shows the nominal satellite orbit position along the geostationary orbit.  This view refers to a satellite on the same longitude as you, so the equator appears horizontal. Normally the whole image appears tilted relative to your local horizon. Either side and to the north and south are the 0.15 deg station keeping box limits.  The example satellite path shows a figure of eight movement over one day.  The diameter of the beam from a 1.2m 14 GHz VSAT earth station dish is shown surrounding the area.  Calculator for VSAT satellite dish gain and beam width. The satellite remains within the beam.  A large teleport earth station may have a beam 10 times smaller diameter and would need tracking, even to follow the approx +/- 0.1 deg diurnal movement shown.

The figure above shows three satellites in inclined orbit.  One has only a small inclination, the other two much larger.  The scale here is exaggerated for clarity.

Result of having a badly pointed VSAT antenna, which sees a big signal drop for a few hours each day when the satellite has moved to the north easterly position. This satellite is very slowly drifting to the west.

Implications for earth station inclination tracking

To avoid loss of service, the earth stations need to track the satellite following the daily sinusoidal movements.  If you are located on the same longitude as the satellite the north-south daily movement will be up and down.   If you are on the equator then all the satellites are in a straight line across the sky from east to west, via directly overhead.  North-south movement of all these satellites will be a sideways movement.   Anywhere else and you have daily diagonal movements to contend with, which means using two motors for an azimuth-elevation mount or a declination only motor on a polar mount dish.

Due to the problems with tracking and the uncertainty of operation of old satellites that have exceeded their regular life, the prices charged for satellite transponder capacity are lower.