Popular Operational Orbits

Now that we have a grasp of the three principal distinguishing characteristics of orbits—shape, size and orbital inclination—we can begin to look at the Earth orbit types that are most commonly used by spacecraft operators. Obviously, if we allow all possible variations in these three characteristics, then there is an infinite number of resulting Earth orbits to choose from! The popular orbits that we are about to introduce, therefore, are a small subset of this vast number of possibilities, and these are widely used simply because they have useful properties that enhance the performance of scientific and applications spacecraft. In writing this chapter, I found it difficult to decide what to include and what to leave out. No doubt other experts would say, "Well, what about such and such an orbit, which is often used for this or that?" I guess the reader has to accept that sometimes things are simplified and generalized a little to aid clarity.

Bearing in mind these comments, five types of popular operational orbit are identified, and these are summarized in the following box for quick reference.

Some popular operational Earth orbits

1.

Low inclination LEO

A circular Low Earth Orbit with an orbit plane varying from equatorial

up to about 50° inclination.

2.

Near-polar LEO

A circular Low Earth Orbit with an orbit plane inclined near 90°.

3.

HEO

A Highly Eccentric Orbit.

4.

GEO

A circular, equatorial orbit at a height where the orbit period is 1 Earth

day. This is referred to as a Geostationary Earth Orbit.

5.

Satellite constellation orbits.

A network of usually identical circular, inclined orbits, often

accommodating a large number of satellites.

Low-Inclination LEO

This is a circular low Earth orbit, with an orbit plane that is near-equatorial (Fig. 2.7). However, the simplicity of this statement is deceptive, and what we mean by "low'' and "near-equatorial" requires qualification.

Figure 2.7: Low-inclination low Earth orbit (LEO). Large vehicles, such as space shuttles and space stations, are often accommodated in this type of orbit. (Image courtesy of the National Aeronautics and Space Administration [NASA].)

Surprisingly there is much debate among experts about the meaning of the word low, but my working definition is altitudes below about 2000 km (1240 miles). The phrase near-equatorial similarly gets a wide interpretation, meaning orbit planes ranging up to around 50 degrees in orbital inclination. The kinds of spacecraft found in this type of orbit are often large, that is, massive, manned vehicles such as shuttles and space stations, or large unmanned spacecraft. An example of this class of unmanned vehicle is the well known Hubble Space Telescope, which is about the same size and mass as a double-decker bus—around 11,000 kg (24,000 lb). The mass of space vehicles and the type of orbit in which they are accommodated are related. As we will explain in more detail in Chapter 5, it is much easier to launch large spacecraft into low, near-equatorial orbits.

Also, given that the plane in which the planets orbit the Sun is close to Earth's equatorial plane, spacecraft destined to probe distant planets are often launched into a near-equatorial LEO. This is then used as a kind of parking orbit, to check out the spacecraft's onboard systems, before a rocket is fired to take the probe to its ultimate destination.

Near-Polar LEO

This orbit is used mostly by operators of Earth observation and surveillance spacecraft (Fig. 2.8). It is a popular operational orbit, particularly at altitudes in the region of 700 to 1000 km (435 to 620 miles), and this is mainly driven by a need to get a global perspective on environmental issues such as climate change. Consequently, many national and international space agencies have launched (and are planning to launch) an armada of spacecraft equipped with powerful instrumentation directed downward to the Earth's surface. Earth observation also has a military dimension, and many military agencies are launching surveillance satellites to gain the new military "high ground.'' It is perhaps not well known that the biggest spender on space in the world is the U.S. Air Force, and details of most of their spacecraft and activities are classified. However, to get a feel for the capabilities of their optical surveillance satellites, you have to imagine a spacecraft with similar imaging power to the Hubble Space Telescope, but directed down instead of up!

It is easy to see why near-polar LEOs are good for Earth observation. Figure 2.8 demonstrates that there is potential for our spacecraft to see most of the planet's surface if we wait long enough; this is called global coverage. As our spacecraft orbits once every 100 minutes (typically), and Earth rotates once every 24 hours beneath the orbit plane, the spacecraft operators can image most targets of interest worldwide within a day or two. The targets of interest can vary substantially in character, from the health of a crop of maize to tank movements on a battlefield.

Figure 2.8: Near-polar LEO. This is commonly used by Earth-observation satellites, like the Landsat spacecraft shown. (Image courtesy of NASA.)

If we compare the near-polar LEO with the low inclination LEO, we can get a good idea of why the near-polar orbit is so well suited to Earth observation missions. It is obvious from Figure 2.7 that if we launched an Earth observation satellite into a low-inclination orbit, we would get a good look at the near-equatorial regions of Earth, but not much else.

The geometry of a typical highly eccentric orbit is shown in Figure 2.9, which is inherently useful for a variety of missions.

Figure 2.9: The highly eccentric orbit (HEO). One type of mission that it accommodates is astronomical observatories, such as the XMM Newton X-Ray Telescope. (Image courtesy of the European Space Agency [ESA].)

The HEO has accommodated many scientific spacecraft, for example, the European Space Agency Cluster mission, dedicated to exploring Earth's magnetic field, and the energetic atomic particles that are trapped within it.

The source of these particles is the solar wind, a stream of high-speed ions— atoms stripped of their electrons—emanating from the Sun. When these encounter the magnetic field of Earth, some of them are trapped in near-Earth space to form doughnut-shaped regions filled with energetic particles (see Chapter 6). These regions were named the Van Allen radiation belts in 1958 after their discoverer, and the particle radiation they contain is hazardous to both man and spacecraft systems. Understanding this hazard has been an important endeavor, and the HEO provides the best means of doing this, as a spacecraft in this orbit is able to sample the magnetic field and particles over a wide range of altitudes on each orbit.

The HEO is also a popular orbit for space observatories. An observatory at the apogee of a HEO has good sky viewing efficiency, as the distant Earth obscures only a small part of the sky. Also the relatively slow speed at apogee means the spacecraft spends most of its time there, providing good opportunities for extended periods of communication with the ground. This allows ground operators to command the telescope and receive its data as if it were effectively on the ground in a dome next to the control room. This way of operating is referred to as observatory mode operation and is an important attribute for space telescopes. Also the high apogee of the HEO means that the observatory spends most of its time above the Van Allen radiation belts, which is beneficial for some instruments that cannot operate in a high radiation environment.

The HEO has also been used extensively as a communications orbit, mainly by the former Soviet Union and by Russia today. A HEO inclined at 63 degrees to the equator, with an orbit period of 12 hours, is called a Molniya (Russian for "lightning") orbit after a series of communications satellites accommodated in this orbit. The Soviet Union began to use this orbit in the 1960s for communications between ground sites at high northern latitudes, by positioning the apogee of the orbit above the Northern Hemisphere. The low speed of the spacecraft in the apogee region means that it spends the majority of its orbit period high in the sky above these northern regions, giving an opportunity for extended, uninterrupted periods of communication with terrestrial users. Between 1964 and 1998, around 170 spacecraft were launched into Molniya orbits to provide telephone communication and satellite TV to high-latitude regions bordering the Arctic Ocean.

Figure 2.10: The geostationary Earth orbit, shown to scale. There are many communication satellites in this orbit, such as the Intelsat spacecraft illustrated. (Image courtesy of EADS Astrium.)

The geostationary Earth orbit is a widely used operational orbit, mostly for communications, but also for scientific and Earth observation satellites (Fig. 2.10). An example of a GEO orbit Earth observation satellite is Meteosat, which provides those impressive weather pictures we see each evening on the television weather forecast. The invention, if such it can be called, of the GEO is attributed to the science-fiction author Arthur C. Clarke in 1945. Unfortunately, he failed to patent the idea; if he had done so, he would probably be very rich today! The reason for the popularity of the GEO as an operational orbit is its unique characteristic that satellites in this orbit appear to be stationary when seen from Earth's surface—hence the name. To achieve this, the orbit needs to be circular and equatorial, but in addition the orbit height has to be such that the spacecraft orbits Earth in the same time as it takes for Earth to rotate once on its axis.

There is often confusion about the meaning of the term geosynchronous orbit (GSO) and how it relates to the GEO. GSO is the name used for any orbit that has an orbital period equal to one Earth rotation, so the GEO is a special case of the GSO. There are obviously a whole bunch of GSOs with a 1-day period, but having an elliptical shape, or a plane inclined to the equator, or both. The important distinction is that a spacecraft in one of these GSOs does not appear stationary relative to a ground-based observer.

Usually the orbit period of a GEO is said to be 24 hours, but it is actually a little shorter than that—23 hours and 56 minutes. The day on which we base our calendar is the familiar 24 hours, which is called the solar day, and this is the time it takes for Earth to rotate once with respect to the Sun. If the Sun is precisely due south (or due north if we live in the Southern Hemisphere) and we measure the time it takes for it to return to the same position in the sky the next day, we will find it to be the familiar 24 hours. The period of the GEO satellite, 23 hours 56 minutes, is called the sidereal day, which is the time it takes for Earth to rotate once with respect to the distant stars. The reason for the difference is Earth's orbital motion around the Sun; because of this, the Sun appears to move relative to the stars. As Earth rotates, it takes 23 hours and 56 minutes to do one revolution with respect to the stars, and then it has to rotate for an extra 4 minutes to catch up with the Sun, as the Sun's position has changed from the day before.

Getting back to our GEO spacecraft, we can calculate the orbit height corresponding to this orbit period using Kepler's third law of planetary motion (see Chapter 1). If we do this, we get a precise altitude for our GEO of 35,786 km (22,237 miles). If we have a circular, equatorial orbit at this height, a satellite initially positioned above a particular geographical feature on the equator will remain above that feature as it orbits; it appears to stand still in the sky from the point of view of someone on the ground.

This property is the key to its popularity. It makes communication with the spacecraft easier, as you don't need to track the satellite with your dish antenna—you just point it in a fixed direction. And of course it also means that the communications link with the spacecraft is uninterrupted. This is a familiar idea, as evidenced by the large number of small satellite TV receiving dishes we see bolted to the exterior walls of houses, staring fixedly at a particular point in the sky where the service provider's invisible GEO satellite resides.

The GEO orbit is most commonly used by communication spacecraft, and there are literally hundreds of active communication satellites (comsats) on the GEO arc. People routinely use this technology day to day, without really noticing, which is of course the way it should be. If I pick up the phone to say, "Hi, this is Graham'' to a friend on another continent, then my electronic voice will transit over land lines or microwave links to the nearest satellite ground station, where it will be transmitted into the sky to a GEO

comsat by a large fixed dish antenna. This signal will be received and amplified by the spacecraft, and then transmitted down to another ground station in the region of my friend's home, ultimately arriving at his telephone handset. When he responds by saying, "Oh hello! How are you?,'' the whole process begins again in the reverse direction—amazing technology that is transparent to the user!

The nature of the GEO arc is that it is literally a one-dimensional line in space, and as such it is a limited natural resource that needs to be protected and managed for the future, like any other. Unfortunately, as well as all the active comsats on GEO, there are many defunct spacecraft that are essentially debris polluting the orbit. Because of the pressure of use on the GEO arc, spacecraft operators are now expected to boost their comsats to a higher graveyard orbit—200 or 300 km above GEO—when they reach the end of their operational life.

Satellite Constellation Orbits

The orbits associated with a satellite constellation are usually a network of identical inclined circular orbits, often accommodating a large number of satellites. A typical constellation geometry is illustrated in Figure 2.11, where the black dots represent the orbiting satellites comprising the constellation. Constellations have been most commonly used over several decades for satellite navigation (satnav), which uses satellites to determine your position on the ground (or on the ocean, in the air, or wherever you happen to be). More recently, constellations have been used for satellite communications, and there is currently an interest in using them for Earth observation as well.

Perhaps the best known example of a constellation is the global positioning system (GPS) navigation system. Navstar GPS satellites (see Chapter 1) are operated by the U.S. Department of Defense, mainly for use by the U.S. military. However, satnav in automobiles is becoming commonplace, as well as in leisure activities such as hiking and sailing, giving the user's position with an accuracy on the order of 10 m (32 feet). To triangulate a user's position on the ground, the receiver needs to access signals from at least four GPS satellites simultaneously. To make this work, the constellation must be designed so that the user can see at least four GPS satellites from any location on Earth's surface at any time. This ground coverage requirement leads to the design of the geometry of the satellite constellation. In this case, the required ground coverage is achieved by the operation of 24 satellites in the constellation. The resulting geometry of the constellation consists of six circular orbit planes at 20,200 km (12,500 miles) altitude, spread out around the equator. Each orbit plane is inclined at 55 degrees to the equator and accommodates four satellites. Figure 2.12a shows

Figure 2.11: An illustration of a typical LEO constellation geometry for a communications system. The black circles represent the orbiting satellites that comprise the system.

the GPS constellation geometry, illustrating well the old saying that a picture is worth a thousand words! A typical GPS satellite configuration is shown in Figure 2.12b.

However, the Navstar GPS system is unlikely to be the future of space-based navigation systems because of its military ownership. Quite reasonably, the Department of Defense reserves the right to limit the signal strength, to erode the positional accuracy of the GPS system, and to shut down public access to GPS completely in times of military conflict. This political aspect of the GPS system has overridden the amazing technological benefits, and means that civilian agencies have been reluctant to embrace space-based navigation systems wholeheartedly. Just think how useful satnav would be if fully utilized for things like air traffic control, particularly now when the density of air traffic is growing at such a phenomenal rate. To overcome these political difficulties, the European Union has proposed launching a new satellite navigation constellation called Galileo, which will have civilian ownership. This system, comprising 30 satellites, is due to be operational in around 2012, and should allow space-

Navstar Global Positioning System
Figure 2.12: (a) Schematic of the global positioning system (GPS) Navstar constellation geometry, (b) A typical GPS satellite configuration. (Image courtesy of Lockheed Martin.)

based navigation techniques to become more fully integrated into all aspects of human activity where high precision positioning is required.

Satellite constellations have also been proposed for communications, and in this application low-altitude orbits are required. An example of this type of LEO constellation is shown in Figure 2.11. We have already discussed the usefulness of the GEO orbit for global communications, so why is there a need to propose a whole new way of doing the same thing? Well, the spur for this development is the craving we have for communications using small hand-held terminals, or, put another way, mobile phones. This love affair with mobile phone technology has created a huge expansion in terrestrial cell phone networks. As we roam around the planet, the communications with our friends or business associates is achieved by the links between our phones and a network of fixed, ground-based, mobile phone antenna masts. This works well most of the time, as the cell phone network operators have located the fixed phone masts in places where population is concentrated. This is, of course, driven by market forces. However, we all know that if we go for a hike in a remote mountain region and we wish to contact someone, the mobile phone will probably not work as we are out of range of the terrestrial network of mobile phone masts. What do we do if we find ourselves on top of Mount Everest and wish to contact a friend who is traversing the Gobi Desert? Yes, I know—a rather unlikely scenario—but it makes the point. In this situation we can adopt a satellite solution to the problem of mobile communications. Instead of having a network of fixed masts on the ground, we can establish a network of satellites in the sky that perform the same function. If we design the orbital geometry of the sky network or satellite constellation appropriately so that we always have line of sight to at least one satellite member of the constellation, then truly global mobile phone communications become possible. The sort of constellation geometry we see in Figure 2.11 is again driven by the coverage requirement that we need to see at least one satellite from all terrestrial locations, and at all times.

Why do we need a LEO constellation of satellites for mobile phone communications? Why can't we just talk through the existing network of GEO satellites? The simple answer is that the GEO satellites are just too far away. If our mobile phones had enough microwave transmission power to reach the 36,000 km (22,400 miles) or so to GEO, then they would literally fry our brains in the process—quite a major physiological constraint on the technology!

If we take account of all these various considerations, then mobile phone constellations usually end up comprising a network of near-polar LEOs. Perhaps the best-known example of this type of communications constellation is the Iridium system. The original proposal for this constellation was to have 77 active satellite members, equal to the number of electrons of an iridium atom, thus giving rise to its name. Subsequently this was reduced to 66 active satellites in a network of near-polar, circular LEOs at a height of approximately 780 km (485 miles). Because of competition from terrestrial mobile phone networks, Iridium has had a checkered commercial history, which has inhibited the growth of space-based mobile communications. However, if the economics can be gotten right, then, in the words of the ad, the future's bright for this type of space application!

The third main application of constellations is Earth observation, which is currently the least well-developed, although there may be military developments of which I am ignorant! When we briefly discussed Earth-observation satellites in the near-polar LEO section, we commented that the spatial resolution of current imaging payloads were amazing. Objects the size of a fraction of a meter can be easily seen from orbit with the right payload equipment, provided that the ground is not obscured by cloud. However, one problem with conventional Earth-observation spacecraft is their temporal coverage. As the satellite orbits every 100 minutes or so, and Earth rotates once every 24 hours beneath the orbit plane, it may take a while for the spacecraft to have an opportunity to over-fly and image a particular ground target of interest. The temporal coverage is not good using a single satellite. Overcoming this limitation is particularly important in military operations, where uninterrupted strategic battlefield information may be a requirement. Various civilian applications, such as disaster monitoring, would also benefit from improved temporal coverage, and using constellations of Earth-observation satellites is a way of achieving this. In principle, a continuous line of sight to a ground target of interest is achievable using a constellation provided that sufficient imaging satellites are launched.

From the above discussion, it is obvious that there a lot of advantages to using satellite constellations. Another one that we have not mentioned is graceful degradation. If you launch one satellite to provide a service, such as communications or Earth-observation, and it suffers a serious system or payload failure, then the service it provides is abruptly interrupted. However, if the function of providing the service is distributed among a large number of constellation satellites, then clearly the failure of one satellite means that the service may be compromised a little, but nevertheless it can be maintained. This characteristic of a more robust operation, associated with constellations, is particularly important in military space activities, where an adversary may be actively seeking to interrupt normal service!

Finally, constellations have the disadvantage of the cost of manufacturing, launching, and operating the many satellites in a constellation system, which is much higher than the cost of a single satellite system. However, as we have seen with navigation and mobile communications services, this burden of the cost has been taken on by the operators, as the many-satellite attribute of a constellation is essential in achieving the objective.

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