======================================================================== Visual Satellite Observing F A Q Chapter-04 What Are The Different Kinds Of Satellite Orbits? ======================================================================== ---- 4.0 What are the different kinds of satellite orbits? Satellite orbits are "grouped" into general categories because a major characteristic of a particular orbit in the "group" produces a highly desired ground track or an aspect of the orbit which is needed to accomplish the main purpose of the satellite. In general, a satellite orbit gives rise to particular desirable ground track. For example, a communications satellite needs to stay where it can always be seen from the ground, a weather satellite needs to view the earth with the sun in the same relative position every time the satellite passes over a country. Thus, a satellite is placed in an orbit which capitalizes on an aspect of the orbit which helps the satellite meet its mission, be that scientific, military, or commercial. ---- 4.1 Low Earth Orbit (LEO) Any orbit, in which the satellite completes one full orbit around the earth (the "period") in less than 225 minutes, is called a "low earth orbit." In some documents these orbits are called "near earth" orbits. The reason for the 225 minute definition is the factors which affect the satellite in orbit. Satellites so low that they orbit in fewer than 225 minutes are far more susceptible to the earth's atmosphere and earth gravitational anomalies than any other source of disturbance. Satellites with a period greater than 225 minutes are more likely to be affected by the gravitation of the sun, moon and planets, and the earth's natural radiation belts. ---- 4.1.1 Low-Inclination Orbits This has to be one of the poorest choices of terms in the satellite industry. A "low-inclination" orbit is whatever the term means to the user. The inclination of a satellite, defined as the angle between the orbital plane of the satellite and the equatorial plane of the earth, manifests itself in the highest north or south geographic latitude the satellite reaches in its orbit as viewed from the ground. The inclination for a particular satellite is a particular number having a clear meaning and mathematical significance. When referring to a "low-inclination orbit", there simply is no established definition or mathematical significance. There is no military or civilian definition of a "low-inclination orbit". One finds a low-inclination orbit can be somewhat arbitrarily defined as an inclination less than 45 degrees but no accepted source of authority, including the USA Space Command (USSPACECOM), has defined the term. ---- 4.1.2 Polar orbits Strictly defined, a "polar orbit" is when the inclination is exactly 90 degrees. Some latitude (no pun intended) is allowed so that any orbit within a few degrees of 90 is considered a polar orbit. ---- 4.1.3 Sun-synchronous orbits The sun-synchronous orbit is one of the special categories alluded to in the opening paragraph of this section. All satellites, at any inclination other than exactly 90 degrees, are affected gravitationally by the fact that the earth is not a perfect sphere. This "mass asymmetry" of the earth causes the orbit of the satellite to change. The greatest effect is on the argument of the perigee, and the right ascension of the ascending node. In simple terms, not only does the satellite go around the earth on its orbit, but the orbit itself rotates, or "regresses", around the earth. This is "nodal regression" and is greatly dependent on the satellite's orbital altitude and inclination. At 185 km (100 nm) altitude, 40 degrees inclination, the nodal regression is about 6.8 degrees per day westward. For 555 km (300 nm) altitude, 130 degrees inclination, the nodal regression is 4.7 degrees per day eastward. One can take advantage of nodal regression and launch a satellite into an orbit where the nodal regression nearly exactly cancels out the daily change in the position of the sun over any point on earth, caused by the earth's orbit around the sun. This means that every day, when the satellite passes over a point on earth, the position of the sun in relation to the satellite and the earth would be the same. This is a very useful thing to do for a weather or surveillance satellite. The satellite always "sees" the point on the earth, when the sun is shining on the earth from the same angle - the same "sun time". The orbit which has this unique characteristic is called "sun-synchronous" and is an orbit where the combination of orbit altitude and inclination causes a nodal regression of 0.98 degrees per day eastward. This turns out to be, depending on the altitude of the satellite, about 95 to 100 degrees inclination. ---- 4.1.4 What is an n-th resonant orbit? A resonant orbit is one that completes an integral number of complete revolutions in one day. For example, a 15th-order resonant orbit has exactly 15 revolutions in 24 hours. Resonant orbits can be used in geophysical studies because the satellite passes over the same locations every day, and minor gravitational irregularities accumulate and become measurable. The repeated passing over the same location characteristic is also taken advantage of by classified "spy" satellites (which also can use "two day" resonances, such as 31 revolutions in 2 days). Lastly, the Space Shuttle sometimes uses a 16th order resonant orbit because it permits same-day synchronization of work and sleep schedules. ---- 4.2 Geostationary Orbit A geostationary orbit is a special case of a geosynchronous orbit. Put the satellite in a very nearly circular orbit (no eccentricity) and give it zero inclination and the satellite will stay over the same point of the earth's equator - in other words, appear to be stationary in the sky. This is the ideal condition for a communications satellite. One would simply point their ground antenna to the spot in the sky where the satellite appears. Unfortunately, orbits are easily perturbed through natural causes, and a geostationary satellite soon drifts from the position and must be forced back to position by firing thrusters. This uses up the satellite's fuel, and soon, with all fuel exhausted, the satellite drifts permanently away from its geostationary point. Communication satellite owners get around this problem by allowing the satellite to have a small inclination and a very small eccentricity. Ground antennae "see" a large enough area of the sky that if the satellite stays within that area, communications is retained. Thus, all "geostationary" satellites are really allowed to be geosynchronous. They make tiny figure eights in the sky instead of staying in exactly one place. It takes less fuel to let the satellite wander around a little, and the lifetime supply of fuel on the satellite is extended. The communications industry somewhat whimsically refers to geostationary communications satellites as "wobblesats." All geostationary satellites must be located along the celestial equator as viewed from the earth. An international commission "assigns" who gets to put a satellite on a particular longitudinal subpoint. Interestingly, a perturbation, caused by the Earth's oblateness, causes a longitudinal acceleration of a satellite in geostationary orbit. The acceleration is zero at 75 degrees east longitude (over the Indian Ocean) and 255 degrees east longitude (over the eastern Pacific Ocean). The lucky owners of these slots get to put their satellites where they are least likely to need fuel to maintain position! All other satellites must use fuel to retain their positions, or they will drift toward these two stable longitudinal points! ---- 4.3 Geosynchronous Orbit A geosynchronous orbit is achieved when the satellite completes one orbit around the earth in one sidereal day. (A sidereal day is the time it takes the earth to rotate once with respect to the stars (not the sun). A sidereal day is 23 hours 59 minutes, 4.091 seconds, compared to a mean solar day which is 24 hours.) This gives the satellite an altitude of about 35,786 km (19,300 nm or 22,236 statute miles). A satellite in a geosynchronous orbit, pretty closely matches the earth's rotation and "appears" from the ground to stay overhead at all times. It is only "pretty close" because, though the satellite has a sidereal period, nothing has been said about the inclination or eccentricity. Give the satellite a nearly circular orbit, but some inclination, say 10 degrees, and the satellite will, over the course of an entire day, appear to inscribe a line in the sky - 10 degrees above the celestial equator to 10 degrees below it. Change the eccentricity a little and the apparent path of the satellite can be changed to some rather odd shapes, from lopsided figure eights to a circle. Communications and surveillance satellites use geosynchronous orbits. ---- 4.4 Molniya Orbit In addition to nodal regression discussed above under sun-synchronous orbits, the earth causes the perigee of the satellite orbit to change its position with respect to the stars. The perigee of the orbit (satellite's lowest altitude) literally moves along the plane of the orbit at a rate dependent on the inclination. The condition is known as "rotation of the apsides." A satellite launched such that the perigee is in the southern hemisphere will soon find its perigee in the northern hemisphere. It turns out that at two special inclinations, the apsidal rotation rate is zero. The inclinations are 63.4 and 116.6 degrees. If a satellite is at 63.4 degrees inclination, and the perigee is in the southern hemisphere, the perigee stays in the southern hemisphere. One can take advantage of this. Say you live at a northern latitude where a geostationary satellite is too low in your sky to be any use, yet you want a communications satellite to be high in your sky as long as possible. If you launch a satellite in a highly eccentric orbit, at 63.4 (or 116.6) degrees inclination, putting the perigee in the southern hemisphere, then the satellite path during the time it is at or near apogee will spend most of its time in the northern hemisphere. If you further give this satellite a useful period, say, 12 hours (2 revolutions per day), then the satellite spends a majority of its time in the northern hemisphere over your country. Russia uses this technique for many of its communications satellites, and these orbits have become known as molniya orbits. Molniya orbits typically have large eccentricities (0.73 for example) and perigee altitudes of 200 to 1000 kilometers, keeping the satellite in view of the high northern latitudes for around 11 hours of every 12. ---- 4.5 Mid-Earth Orbit (MEO) Mid-earth orbit is also known as Semi-synchronous. Satellites said to be semi-synchronous have a period of 1/2 a sidereal day. Thus, they orbit the earth two times per day. Geopositioning and navigation satellites, such as GPS and GLONASS, use this type orbit. ---- 4.6 What is a "transfer orbit"? What does GTO mean? When it is desired to change the altitude of an orbit, to raise the perigee of a nearly circular orbit to a higher perigee for example, it is accomplished through a "transfer orbit." Orbital maneuvers can take place anywhere in an orbit, but you can accomplish the change using the least amount of propellant, if the maneuver is performed at certain points on the orbit. To raise the satellite to a higher orbit, you fire a thruster to increase its speed, wait for it to arrive at its new apogee, then fire a thruster to adjust the satellite's speed to the new orbit. The satellite literally was put in a new orbit until the thruster was fired again to achieve the desired end orbit. That temporary new orbit was the transfer orbit and was an elliptical orbit which intersected the old orbit and the new orbit. A special case of a transfer orbit is the Geosynchronous Transfer Orbit or GTO. The GTO is the elliptical orbit needed from earth launch to geosynchronous altitude. At geosynchronous altitude, either the payload or the final stage of the launch vehicle conducts a velocity change burn to correct the speed of the payload to that needed for the geosynchronous orbit. ---- 4.7 High-Earth Orbit (HEO) A high-earth orbit is any orbit greater than geosynchronous. Thus, if the period is greater than a sidereal day, it is a high-earth orbit, also known as supersynchronous. These orbits are often highly inclined and highly elliptical to get the satellite out of the earth's natural magnetosphere. Many satellites designed for astronomical work are placed in HEO. ---- 4.8 Solar Orbit The earth is in solar orbit. Any satellite given enough energy to leave earth orbit, but not enough energy to leave the solar system, will enter solar orbit. Many science satellites are placed in a solar orbit - Ulysses and Galileo for example. The two USA Pioneer spacecraft of 1972 and 1973 are in a highly elliptical solar orbit. One special case is a "Halo" orbit. A Halo orbit relies on a gravitationally stable point between the earth and the sun, one of the "Lagrange Points." A satellite placed at the Lagrange point between the earth and sun, approximately 1.6 million kilometers from earth, will execute a three dimensional elliptical orbit about the Lagrange point as the Earth, moon, and satellite system orbit the sun. The satellite ISSE 3 was put in this orbit to detect solar wind products and thus provide an early warning to observers on the ground when solar flare protons were heading toward the earth. ---- 4.9 Beyond Solar Orbit Given enough energy, a satellite can be in orbit about nothing, literally free flying through space until it encounters an object with enough gravity to change the energy of the satellite. The new path may be a highly elliptical orbit about the gravitational source, or the satellite may have been given more energy and a new direction of travel. The two USA Voyager spacecraft are currently leaving our solar system. They have enough energy to be free of the sun's gravity and used the gravity of Jupiter to gain energy. ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in February 1998. ========================================================================