Choosing the Best Orbit

The five types of orbit we have discussed are popular with spacecraft operators, but how do we select the best of the huge number of possible orbits to choose from for a particular spacecraft mission? This question is central to the activities of the team of project engineers tasked with the job of the spacecraft mission analysis.

The quick answer is that the spacecraft needs to be in the right place, that is, the right orbit, so that the spacecraft payload can most effectively achieve its mission objectives. We need to pause a moment to reflect on this concise but not so simple statement. First, what is the spacecraft payload? Essentially it is the part of the spacecraft that fulfills the mission objectives—the business end of the spacecraft. For example, the payload of an Earth-observation satellite will be the camera instruments used to acquire the image data, or the payload of a communications satellite will be all the telecommunications equipment and antennas needed to maintain the desired communications service. Second, what is a mission objective? This is the purpose behind the whole project, its raison d'être. Some typical spacecraft mission objectives are the following:

1. The provision of high-resolution imagery of Earth with global coverage

2. The provision of a telecommunications service for the Australasian region using large, fixed ground antennas

3. The acquisition of high-resolution astronomical imagery

The process of linking the mission orbit selection with the mission objective usually involves the following steps:

• Definition of the spacecraft mission objective: the formulation of a precise statement defining the prime purpose of the spacecraft.

• Choice of payload instruments or equipment, usually done by a group of experts who can produce a detailed specification of the payload hardware required to achieve the objective.

• Development of payload operational requirements: How does the payload hardware need to operate to best achieve the objective? This includes where the payload needs to be physically located to maximize its effectiveness.

• Finally, the consideration of the payload's location leads naturally to the selection of an appropriate, or even optimal mission orbit.

All this may sound rather formal and complicated, but we can return to our example mission objectives above to show that sometimes the process can be rather straightforward.

If we look again at mission objective 1, above, related to Earth observation, the requirement for high-resolution imagery of Earth means that the imaging payload instruments need to be close to their terrestrial targets of interest, which in terms of an orbit translates to a LEO. The need for global coverage means that our LEO must be near-polar in inclination to provide the instruments the opportunity to "see," after some period of time, the Earth's entire surface. Without too much difficulty, the use of a near-polar LEO (see Fig. 2.8) seems the obvious choice in this case. Similarly, it is easy to see that the choice of "best" mission orbit for mission objective 2, above, is a GEO.

These two examples illustrate well the process of how the mission objective drives the choice of mission orbit for a particular spacecraft project. However, just to muddy the waters a little, we can look at an example that shows that sometimes the choice of the mission orbit is not quite so obvious. The third example, mission objective 3, above, relates to the operation of an orbiting astronomical observatory, and if we look at the orbits of such spacecraft in current operation we find them in a variety of orbits. For example, the Hubble Space Telescope can be found in a LEO, the XMM Newton X-Ray telescope orbits in a HEO, and the Hipparcos observatory was designed to fly in GEO (although, due to a rocket engine failure, it did not make GEO and went into a HEO instead!). This variety suggests that the choice in this case is perhaps not quite so simple.

In cases such as this, a more detailed analysis is required to select the orbit, involving consideration of both spacecraft payload and system requirements which influence the decision. This kind of process is illustrated in Table 2.1, which is a simplified version of a trade-off table that engineers might use to help make an orbit selection for a space observatory. In a typical trade-off process, the objective is to make a choice among a number of different options; in this case the orbit options are LEO, HEO, and GEO (the right-hand columns of the table). To make this choice, a number of criteria or trade-off parameters, are specified (in the left-hand column of the table) against which the options are judged.

In our choice of parameters, three are related to the payload (telescope) operation, and three are related to spacecraft system operation. You may

Table 2.1: A simple orbit trade-off table for an orbiting astronomical observatory

Parameter

Type of parameter

Favoured orbit

Payload System LEO HEO GEO

Observatory mode operation (duration / of ground communications link) Uninterrupted source observation /

Sky viewing efficiency /

Radiation exposure Ease of orbit acquisition In-orbit repair and maintenance

recall, from the discussion about observatories in HEOs earlier, the explanation of observatory mode operation and sky viewing efficiency. A third payload parameter, uninterrupted source observation, is introduced, which relates to how long the telescope can point at a particular nebula or galaxy without interruption. To maximize sensitivity, telescopes often operate in a kind of time-exposure mode, where they point at the object of interest for long periods of time to collect as much light as possible. When you are looking at extremely distant objects at the edge of the universe, collecting every photon of light counts. If you think about an observatory in a LEO with a period of around 100 minutes, this kind of operation is difficult to achieve because Earth can get in the way of the telescope for about 30 minutes on each orbit revolution. Table 2.1 shows that high orbits—HEO apogee and GEO—are favored when we consider the payload parameters.

On the other hand, when we look at our system-related parameters, the LEO is the preferred option. The radiation exposure in LEO is less severe than in the higher orbits, so the reduced degradation of the spacecraft systems caused by particle radiation favors the LEO. The ease of orbit acquisition parameter relates to the amount of propulsive effort required to reach the mission orbit. Again the LEO orbit is favored as it requires the least amount of rocket fuel to get there, compared to the higher orbits. And savings in fuel mass in getting to your mission orbit can be usefully invested in increasing the payload mass, resulting in an improvement in the overall effectiveness of the spacecraft in achieving its objective. The third parameter, in-orbit repair and maintenance, is a means of effectively increasing the mission lifetime of the observatory. This maintenance is usually done by space-walking astronauts, and since manned vehicles can only visit LEOs routinely, this form of maintenance can only be performed on LEO spacecraft.

After all that, when we look at the checked boxes on the right-hand side of Table 2.1 we may be disappointed to find that the choice of mission orbit for our observatory is still unresolved! In truth, the process illustrated above is too simple to make any real progress, but it is useful in illustrating the tradeoff process. In a real project situation, the trade-off exercise would be much more finely tuned to the specific spacecraft characteristics. Also, some of the parameters that are considered to be particularly important would be weighted in the trade-off process. For example, in choosing the best orbit for the Hubble Space Telescope, the issues of ease of orbit acquisition and inorbit maintenance were paramount. The sheer size and mass of the telescope meant that LEO was the only real option for the orbit. Also, the desire to extend its useful lifetime by the process of in-orbit maintenance requires the use of astronaut repairmen taken aloft by the U.S. Space Shuttle, so the telescope's orbit is again constrained as the shuttle is unable to operate above LEO. As a consequence, the Hubble Space Telescope ended up in a LEO, even though we can see from Table 2.1 that it is not the best orbit for a space observatory!

In this chapter we have come a long way from Newton's cannon to popular operational orbits for modern scientific and applications spacecraft, and in the process have acquired an understanding of the nature of orbital motion. In Chapter 3, which discusses real orbits, we delve into this topic a little more deeply, to discuss the mysteries of orbital perturbations and how they influence the process of mission analysis. Give it a try, but if you find it hard going, skip it and move on. The rest of the book is not crucially dependent on the content of Chapter 3.

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