Know your orbit

(Satellite Broadband) Ask the average layman to describe a satellite orbit and he’ll probably include the phrase whizzing around up there while gesticulating in a circular fashion above his head.

This characterization might be seen, by the PC-police, as insulting to the technologically challenged, but, let’s face it, some satellite company CEOs could be classed as laymen when it comes to describing the technology on which they build their empires.

Indeed, satellite orbits can be very complex things. They are the result of a delicate balance between the speed of the satellite (which tends to shoot the satellite off into the universe) and the gravitational pull of the Earth (which tends to hold it back). The schoolboy analogy is the mass on a string, whirled around the head at high speed: the string (i.e. gravity) provides the force to make the object move in a circle.

Moreover, with satellites, the string is elasticated: Variations in the Earth’s gravitational pull, coupled with the pull of the moon and the sun, and the pressure of the solar wind, mean that satellites rarely travel in a perfect circle. So an orbit is always an approximation.

Having said that, for most practical purposes, there are only a few types of orbit that can be used for satellites, and all of them can be described by only a few parameters.

Orbit types

The most common orbits are:

1. geostationary orbit (GEO), used for communications and weather satellites

2. low Earth orbit (LEO), used for communications constellations, scientific satellites and manned vehicles

3. medium Earth orbit (MEO), used for comms and navigation constellations

4. sun-synchronous orbit (SSO), used for Earth observation and weather satellites.

All orbits are defined by a small number of orbital parameters which describe their size, shape and orientation in space, the main ones being the height of the orbit above the Earth’s surface, its eccentricity and its inclination.

The height of an orbit is more likely to be quoted than its diameter (which is measured from the center of the Earth), because it is directly related to practical matters, such as how far radio signals have to travel and the breadth of the coverage area. Diameter is, however, more often used in mathematical equations.

Eccentricity is a measurement of the ellipticity or non-circularity of an orbit and is given a value between zero and one, so a circular orbit has an eccentricity of zero. If an orbit is elliptical, it has a high point and a low point above the Earth which are known, respectively, as the apogee and the perigee.

Inclination is a measurement of the angle between the plane of the orbit and the equatorial plane of the orbited body, so an equatorial orbit has an inclination of zero.

There are, of course, other parameters of interest, such as orbital period (the time it takes a satellite to make one revolution), but these can largely be derived from equations using the above parameters. In a nutshell, the higher the orbit, the longer the orbital period.

So, let’s have a look at each of those orbit types.

Geostationary Orbit (GEO)

Most of the world’s communications satellites are placed in geostationary orbit, a circular orbit in the same plane as the Earth’s equator. It is 35,786 km (22,237 miles) above the Earth’s surface and has a circumference of 264,924 km (164,624 miles), so the couple of hundred satellites up there have a bit more room than most diagrams suggest. Since it’s a circular orbit, its eccentricity is nominally zero and, since it lies in the plane of the equator, its inclination is zero degrees.

Geostationary orbit is commonly abbreviated to GEO or sometimes GSO (for Geostationary Satellite Orbit or simply GeoStationary Orbit). The term geosynchronous is often heard in place of geostationary. In fact, GEO is a special type of geosynchronous orbit (i.e. equatorial with a 24-hour period).

Satellites in GEO circle the Earth in exactly the same time it takes the Earth to turn on its axis, so they appear to be stationary compared with the Earth (hence the term geostationary. This means that ground stations do not have to scan across the sky to track the satellites, and that the satellites can be given orbital positions related to the line of longitude above which they are stationed (e.g. 19°W, 26°E). Each one-degree slot of orbital space is about 736 km (457 miles) wide, so the chances of collision are minimal.

When Arthur C. Clarke wrote about GEO in 1945, the technology required to use it was some 20 years away and communications satellites were thought of as manned space stations. Since reliable electronic components were not even science fiction, a crew was required to change the valves in the amplifiers. Even Clarke would have found it difficult to predict how important geostationary orbit would become.

In fact, until the dawn of the 1990s, GEO was more or less synonymous with satellite communications, that is until the reinvention of the satellite constellation operating from low or medium altitude orbits.

Low Earth Orbit (LEO)

There are many different types of LEO and no firm definitions as to what constitutes a low-altitude orbit. However, low Earth orbits tend to be circular and up to about 1,000 km in altitude. Unlike geostationary orbit, they are not confined to the equatorial plane, they can have any inclination from zero to 90 degrees and they can also be elliptical. Satellites in LEO tend to circle the Earth in about 100 minutes, which means that ground antennas must be equipped with active tracking systems.

Individual communications satellites are not stationed in LEO, but in recent years several low-orbiting constellations of satellites have been proposed, mainly for mobile telecommunications services. The best-known constellations now deployed in LEO are Iridium, Globalstar, and Orbcomm (the latter designed for data communications and messaging services as opposed to voice services).

Some scientific satellites and all current manned space vehicles are launched to LEO. Most Earth observation spacecraft, including some weather satellites, use the high-inclination, polar orbits. The chief advantage of LEO for communications satellites is that they are relatively close to Earth, which means that signal delay is kept to a minimum.

Of course, geostationary satellite operators insist that this advantage has been over-hyped by LEO operators desperate to cite any advantage they have over the geostationary establishment. The problems faced by Iridium and Globalstar tend to imply that LEO is not the satcoms panacea they once promoted.

Medium Earth Orbit (MEO)

Like LEOs, medium-altitude Earth orbits are not closely defined, but lie between LEO and GEO, typically at altitudes between about 5,000 km and 25,000 km. One of the best-known users of MEO is the Global Positioning System (GPS) constellation whose satellites are based in six 19,150 km-high circular orbits spaced 60 degrees apart around the globe at an inclination of 55 degrees.

The type of MEO used by GPS is also called an intermediate circular orbit (ICO), and it is this term which ICO Global (now joined with Teledesic) chose for its mobile satphone constellation. It will be based in two orbits of altitude 10,390 km and inclination 45 degrees. The terms ICO and MEO are often used synonymously, but the MEO classification is not restricted to circular orbits.

The orbital period of a satellite in MEO is between those of satellites in LEO and GEO, and is measured in hours rather than minutes, but is less than 24 hours. MEOs are typically inclined somewhere between the polar and equatorial planes, but can have any inclination and be either circular or elliptical.

Most types of unmanned spacecraft could operate from MEO, but other orbits tend to offer greater specific advantages. For example, the ability to remain above a given line of longitude in GEO, or to provide regular coverage of the whole Earth from polar orbit.

Sun-Synchronous Orbit (SSO)

An orbit used particularly by Earth observation or remote sensing satellites is the sun-synchronous or heliosynchronous orbit. The SSO is a type of high-inclination, polar orbit, which is synchronized with the sun so that lighting conditions remain the same for each successive pass over a given area of observation. In technical terms, the sub-satellite point (the point on the Earth directly below the satellite) remains approximately fixed at the same local time on Earth.

This type of orbit has a typical altitude of between about 600 and 800 km and an inclination of about 98.5 degrees (i.e. close to a 90 degree polar orbit). It is used by both Earth observation/remote sensing satellites and some weather satellites (such as the American NOAA series). Now that satellites offering ground resolutions down to 1 meter are available, Earth observation is becoming a true commercial space application -- perhaps one day something that will rival the success of satellite communications, who knows?

Orbital Legacy

The satellite which marked the dawn of the Space Age, in October 1957, was Sputnik 1. It was placed in an elliptical low Earth orbit with an apogee of 939 km and a perigee of 215 km. It bleeped for 21 days, until its battery discharged, and re-entered the Earth's atmosphere, as planned, some 10 weeks later. Since then, we have learned to utilize the laws of physics -- at least as far as orbits are concerned -- to our social and economic advantage. If physics had not given us the legacy of the orbit, there would be no satellites. So, if you see that layman whizzing and gesticulating, take him quietly to one side and tell him a tale of GEOs, LEOs and MEOs.