Orbits Used by High
Altitude Satellites
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Satellites use a wide variety of orbits to fullfil their missions. The orbit chosen for a satellite is a comprimise between the mission requirements, the capabilities of the rocket used to launch the satellite & orbital mechanics. A detailed discussion of orbital mechanics is beyond the scope of this web page, but the key parameters to remember are;
The graphics on this page were generated with Satspy from orbital elements unless otherwise indicated. In each case the following annotations are used on the images
Two typical low earth orbits, that of the Mir space station and of the Landsat 7 satellite are show for reference to high altitude satellite orbits.
Mir orbit
Mir is in a 350km altitude, near circular, 91 minutes period orbit of 51.6° inclination. This orbit was used for most Russian Space Stations, since the Baikonor launch site is situated at 46°N and 51.6° is the lowest inclination possible from this site (A 46° inclination is theoretically possible, but the practical concerns of overflying China prevent a launch due east). The International Space Station uses a similar orbit to permit launches to the Station from the Baikonor cosomodrome.
These images show
that a 350km altitude orbit is infact only a little larger than the diameter
of the earth. As a result the region of earth visible from the Mir space
station is relatively small (as shown in the ground trace below). Conversly,
the Mir space station is only visible for a few minutes at a time from
any location and only on perhaps two or three orbits per day. Mir also
spends a significant part of its orbit in darkness during sometimes of
the year , although for a few days each year the orbit is positioned such
that it is permanently illuminated. Therefore, the viewing possibilities
for satellites such as Mir are often quite limited. In general from mid
northern latitudes Mir is visible in the evenings during a period of several
every 6 weeks or so.
Landsat 7 orbit
Landsat 7 is an earth resources spacecraft which images the earth's surface in visible and infrared light. Therefore this satellite orbit is optimised for earth observation. For this reason a near polar orbit of 700km, 98.8° inclination, 98 minute period is used which ensures that the satellite can (at least in theory) observe the entire globe. Several other features of this orbit make it especially useful for remote sensing satellites
View of orbit from ascending node
In theory an
orbit should remain fixed in space whilst the earth rotates beneath the
satellite. In reality the earth is slightly bulged and the effect of this
bulge is to shift the point of perigee and the ascending node for any orbit
which has an inclination other than 90°. This effect is known as nodal
regression, the result of which is that the plane of the orbit rotates
or precesses. However, this effect is used to advantage here to shift the
orbit at exactly the same rate as the daily change in position of the sun
over any point of the earth. So the satellite always passes over the earth
on the sunlit part of its orbit at the same local time of day (for example
at 9 am local time). This ensures that lighting conditions are similar
(ignoring seasonal differences) for images taken of the same spot on the
earth at different times. Additionally the orbit is resonant with the rotation
period of the earth, meaning that the satellite passes over the same point
on the earth at the same time of day at regular intervals (which may be
daily or every 2 or more days depending on the resonance). In the case
of Landsat there are 14.5 orbits per day or 29 orbits every 2 days. Again
this is a very useful feature for remote sensing applications.
A geosynchronous orbit is an orbit which has an orbital period close to that of the earths rotation. A geostationary orbit is a special case of the geosynchronous orbit where inclination = 0° and the period is equal to the rotation period of the earth (approx 1436 minutes), corresponding to a cricular orbit of approx. 35,700km altitude. A satellite in this orbit appears essentially stationary in the sky, which is why this orbit is used extensively for telecommunications & weather satellites. In reality lunar & solar gravitational influences perturb the satellites orbit, so that through the day the satellites position shifts slightly. Below is shown the orbit of the TDRS-7 satellite, one of a series of NASA satellites which used to provide a near continous communications link with the Space Shuttle, International Space Station & other spacecraft such as the Hubble Space Telescope.
Compared with the
LEO orbit of Mir a much larger portion of the earth's surface is visible
from the TDRS-7 spacecraft. The zone of visibility of the spacecraft has
been highlighted by a cone. Approximately 40% of the earths surface can
be viewed at any one time from geostationary altitude. Additionally, the
spacecraft orbit is sunlight apart from a small zone which passes into
the earths shadow. Actually, geostationary satellites only experience eclipses
at two periods of the year - for a few weeks at a time at the spring and
autumn equinoxes. The reason for this is simple. The earths rotation axis
is inclined with respect to the ecliptic, hence the earth's shadow cone
misses the plane of a zero inclination geostationary orbit apart from the
times when the suns declination is close to zero. This occurs twice a year,
once at the spring equinox and once at the autumn equinox.
As can be seen from this graphic a perfectly geostationary satellite stays over the same spot on the equator all day. However, if we were to look closely we would see that the satellite does appear to change position, generally describing a small figure of 8 or an arc due to the effect of lunar / solar pertubations dragging the satellite into a slightly elliptical, slightly inclined orbit. There are many non operational satellites in "graveyard" orbits slightly above or below a true geostationary orbit. Since the orbital period is slightly more or less than the earths rotation period these satellites appear to drift slowly around the earth. This drift combined with a small inclination gives thes satellites a zig-zag ground trace as shown below for Gorizont 23
There are many rocket boosters which are observable in "transfer" orbits. These are the orbits used to transfer the satellite from an initial low earth orbit to the final orbit. The orbit used for transfer to geostationary orbit is named appropriately enough a "geostationary transfer orbit" (GTO). A standard GTO is an orbit which requires the minimum energy to reach geostationary altitude (A Hofmann transfer ellipse). The perigee corresponds to the altitude of the initial low earth orbit parking orbit, the apogee the geostationary orbit altitude and the inclination is usually the inclination of the initial parking orbit. At apogee the payload usually fires an on-board motor to circularise the orbit and adjust the inclination to zero. The GTO orbit of the Intelsat 4-2 rocket (An Atlas-Centaur) is shown below. This is a 600x35,700kmx28° inclination orbit Note that the orbit is very elliptical. The perigee is in the southern hemisphere, so it is possible for observers over a narrow latitude range (centered on -28°) to see this object at a range of only a few hundred kilometers. However, the satellite spends most of its time, due to the "equal areas" rule of orbital dynamics at high altitudes. The apparent trajectory of Intelsat 4-2 rk with respect to Alt / Azimuth is shown below. The rocket appears to trace out a hairpin loop - the shape is a combination of the orbit & the roation of the earth as can be seen from the ground trace.
In recent years modified versions of the GTO orbit have been used. A supersynchronous orbit is one where the apogee is significantly greater than geosynchronous altitude. Why send the payload higher than the target orbit altitude? The reason is probably because the payload still has to adjust its inclination from the launch inclination (anything from 5° to 51°) to 0°. This manouvre is very expensive in terms of energy, much more so than an in plane change of orbital altitude. The energy required to do this manoevre decreases with orbital altitude, so it probably requires less fuel to perform this plane change at high altitude (eg. 60,000 or 70,000km) and then descend to a geostationary orbit rather than do the plane change at geostationary height. Many supersynchronous transfer orbits also have very low perigee altitudes, probably to accelerate their decay & reduce the amount of debris in orbit. Subsynchronous trasnfer orbits also exist where the rocket only sends the payload part of the way to geostationary height, the payload then uses its own propulsion system to reach the final orbit.
Many Russian cities are at high northern latitudes where it is impractical to use geostationary satellites for telecommunications since the satellite would appear either low on the horizon on not visible at all. To overcome this problem Molniya satellites are used for communications in these regions. The orbit used by these satellites is a 12h, high inclination elliptical orbits. The orbit of Molniya 3-47 is shown below. This is a 1470 x 38900km, 63.4° inclination orbit. Again this orbit has special features which make it well suited for telecommunications. First the period is 12h, so there are 2 orbits per day. As a result the ground track of the orbit repeats at the same time of day each day. Since the orbit is elliptical, the satellite spends most of it's time near apogee (where its velocity is slowest), so for 11h of each orbit the satellite is above the horizon for high northern latitudes. Additionally, for several hours per day the satellite moves only very slowly across the sky (as can be seen from the ground track), making it easy to follow with a communications antenna. There is a special reason for the 63.4° inclination. Normally, the as described above for the sun-synchronous orbit of Landsat 7, the oblate shape of the earth causes a gradual shift of the orbits perigee along the orbit. As a result the perigee of this orbit would shift from the southern hemisphere into the northern hermisphere. However, the rate of shift depends on the inclination of the orbit and at certain inclinations the perigee does not move . For 63.4° inclination orbit with a perigee in the southern hemisphere the position of perigee remains fixed, which is why this inclination is used for Molniya orbits.
Left: General view of the orbit of Molniya 3-47 with cone of visibility. Centre: View normal to the plane of the orbit. Right: View of orbit from ascending node
Mid earth orbit (MEO) is a term used to describe 12h period, medium inclination orbits generally used for Global Positioning Satellites. With a constellation of 24(?) appropriately spaced satellites in approx 20000km near circular orbits it is possible to ensure that at least 4 satellites are visible from any one location at any time to ensure reliable navaigation using signals from these GPS satellites. The orbit is resonant with the earths rotation period (2 orbits per day) so the orbit track repeats itself each day.
A number of scientific satellites, particularly orbiting observatories, use highly eccentric orbits with apogee's of over 100,000km. The reason for using these orbits is generally to permit continous observations of celestial objects without the earth blocking the view every 30 to 40 minutes. Additionally, some instruments need to operate outside of the earth's radiation belts. The orbit of the Chandra X-ray observatory is shown below. This is a 9600 x 139000km, 28.4° inclination orbit of 3809 minute period (63.5h orbit). The ground trace of Chandra is quite interesting. For a large part of the orbit the satellite appears to move only very slowly with respect to the stars, so its ground track is largely the result of the earths rotation apart from the brief passage through perigee which appears as a "loop" in the southern hemipshere
Under construction!
Links to further information on satellite orbits
SeeSat-L Frequently Asked Questions, Chapter 4
NASA JPL Spaceflight Basics