Observing Geostationary Satellites


Surprisingly, given dark enough skies, it is possible, armed with binoculars or a telescope, to spot some of the satellites nestling in the geostationary ring (known as a Clarke orbit, after Arthur C. Clarke who first suggested the usefulness of such an orbit).

Strictly a geostationary satellite would be in an orbit of 0 degrees inclination, zero eccentricity and a mean motion of 1.002701 revolutions per day (the Earth rotates once in about 23 hours and 56 minutes; the remaining 4 minutes allow the Earth to rotate further, compensating for the apparent change in position of the Sun. This arises from the movement of the Earth in it's orbit about the Sun). In fact most geostationary satellites are really geosynchronous. Having mean motions between 0.9 to 1.1 revolutions per day they are allowed to drift across a box before corrections are made by on board thrusters. The size of this box is dictated by mission requirements. For example the box for a TV broadcast satellite is determined by the beamwidth of the reception dishes used.

The drift from the ideal position arises due to anomalies in the Earth's gravitational field, at this altitude atmospheric drag is not a consideration. The gravitational influence of the Moon provides an out-of-plane force too, which gradually increases the orbital inclination towards that of the Moon about the Earth (which itself varies between 18 and 29 degrees). The satellite now tends to describe a figure-of-eight ground track; ground controllers aim to restrict this to the box mentioned earlier given that enough orbit-keeping fuel remains. This wandering has been allowed to grow unchecked in the case of a few communications satellites in order to provide better coverage of the polar regions which is otherwise poor (from the poles a geostationary satellite would almost graze the horizon). Net connectivity to US research stations in the Antarctic was recently achieved in this manner.

Due to the popularity of this orbit (geostationary slots over many regions are highly crowded and greatly valued) some agencies deorbit their satellites into a graveyard orbit some 500 to 1000 km above their operational altitude. Failure to do so could put some of the valuable active satellites at risk once station-keeping fuel was exhausted and the dead satellites are at the mercy of the gravitational forces described above.

Observational Considerations

Most of these birds are communications satellites of one description or another. A common design is that of a spin stabilised cylinder. The Hughes HS376 series are typical: the main body being some 3 metres long and 2 metres in diameter. This grows to around 6 metres in length once on orbit with the extension of the communications antennae and an extra skirt of solar panels. These supplement the cells which already cover the main body, making a very nice specular reflector. This skirt and the main body rotate about the long axis, typically at around 55 r.p.m., whilst the antenna and equipment shelf are despun so as to maintain contact with their ground targets.

The Intelsat 603 satellite (left) which was rescued during the STS-49, shuttle mission (May 1992) is a Hughes HS type satellite. Here the skirt and communications antennae are still in their unfurled launch positions. An attitude gas thruster is visible on the left lower side of the satellite. After installing a new perigee kick motor (the original failed to fire, stranding the craft) this satellite was placed into its operating orbit using a new supersynchronous insertion method. Whereas most geosynchronous satellites are delivered to orbit via a geo-stationary transfer orbit (that is an initial orbit of around 300 by 36000 km where the perigee is then raised to 36000 km), here the initial orbit was around 300 by 82000 km. A series of burns both lowered the apogee and raised the perigee until the 36000km high orbit was attained.

Unlike objects in low Earth orbit, geostationary satellites are visible throughout every night of the year, only entering the Earth's shadow for up to 70 minutes per day, around a couple of weeks either side of each equinox. During the same period the satellite tends to brighten over several days, twice a year, when the satellites orientation favours the 'beaming' of the Sun in the direction of the observer.

Typically the satellite will be in the mag. +11 to +14 range (or dimmer), but brightening by several magnitudes when the geometry is favourable (around mag. +5 to +6 is not untypical). One satellite is reported to have briefly been visible to the naked eye at mag. +3 !

Two line elements can be obtained for nearly all these satellites, bar the classified military ones such as the MAGNUM/VORTEX signals intelligence and the DSP early warning satellites. They can be used to generate a series of positions for the satellite in right ascension and declination (RA and dec) for the time of observation. This can then be plotted on star map to form a finder chart; the guide stars will help identify the satellites location. Turning off the motor of a driven telescope will maintain the satellite in the field of view whilst the stars drift out courtesy of the Earth's rotation. By either doing this or tracking the stars instead during a wide angle photographic exposure one can provide a nice illustration of the geostationary ring as either the satellites are fixed and the stars trail, or vice versa. As pointed out above it is more rewarding to carry this out around the equinoxes when the satellite will be more apparent. Observations over successive nights before and after this time will allow you to view the brightening of the object, plus its entry and exit from the Earth's shadow. Of course this will also avoid the disappointment of searching for it whilst it is in eclipse!

Recent Observational Results

At the moment the BWGS have been involved in observations of Insat 1B (83-069B). In April 1994 this satellite exhibited an interesting light curve which possessed a maximum of mag. +5.6 dropping to invisibility, with a secondary maximum of mag. +9. The flash period was some 38 seconds.

Photometry by A. B. Giles and K. M. Hill of the University of Tasmania (reported in SpaceFlight, Vol. 31, September 1989) indicated the rotation rates of the Aussat A1 and A2 HS376-type satellites (85076B/15993 and 85109C/16275). When the light curve of each satellite (during the brightening period around the equinoxes) was analysed, frequencies corresponding to various surface features (such as solar cells and thermal radiator mirrors) were evident.

Though three-axis stabilised satellites may become more abundant there are sufficient spin stabilised satellites available for various studies; the spin rates can yield insight into perturbing forces, whilst the reflected solar spectrum could infer the degree of degradation of the solar cells.

Produced by Neil Clifford & Bart De Pontieu n.clifford@physics.oxford.ac.uk & bdp@mpe.mpe-garching.mpg.de

The original source of this document is: http://www.ipp-garching.mpg.de/~bdp/vsohp/geosats.html

Click here to return to the Lesson Map