• Existence of Altitude Plateaus We should evaluate each constellation to see if plateaus exist in which the number of orbit planes or other key characteristics make a discrete step. Plateaus may be different for different instruments and operating modes, but usually are fonctions of the swath width for each instrument or operating mode.

There are no absolute rules for choosing the proper constellation. Selection is based on the relative importance of the various factors to the owners and users of the constellation. A summary of the most common rules and the reason for them is given in Table 7-13. As with all aspects of mission design, we must document our selection, our reasons, and the coverage characteristics. It is critical to keep in mind possible alternatives and to reevaluate orbits with advances in mission definition and requirements.

Finally, one of the most important characteristics of any constellation is collision avoidance. The reason for this is not merely the loss of the satellites which collide because we anticipate losing satellites for many reasons in any large constellation. The fundamental problem is the debris cloud that results from any satellite collision. The velocity imparted to the particles resulting from the collision is small relative to the orbital velocity. Consequently, the net effect of a collision is to take two trackable, possibly controllable satellites and transform them into thousands of untrackable

TABLE 7-13. Rules for Constellation Design. While there are no absolute rules, these broad guidelines are applicable to most constellations.


Where Discussed

1. To avoid differential node rotation, all satellites should be at the same inclination, except that an equatorial orbit can be added.

Sec. 6.2.2

2. To avoid perigee rotation, all eccentric orbits should be at the critical inclination of 63.4 deg.

Sec. 6.2.2

3. Collision avoidance is critical, even for dead satellites, and may be a driving characteristic for constellation design.

Table 7-14

4. Symmetry Is an important, but not critical element of constellation design.

Sec. 7.6.1

5. Altitude Is typically the most important of the orbit elements, followed by inclination. Zero eccentricity Is the most common, although eccentric orbits can improve some coverage and sampling characteristics.

Sees. 7.4,7.6.1

6. Minimum working elevation angle (which determines swath width) is as important as the altitude in determing coverage.

Sec. 5.2, Fig. 5-21

7. Two satellites can see each other if and only if they are able to see the same point on the ground.

Sec. 7.6.1

8. Principal coverage Figures of Merit for constellations:

• Percentage of time coverage goal Is met

• Number of satellites required to achieve the needed coverage

• Mean and maximum response times (for non-continuous coverage)

• Excess coverage percent

• Excess coverage vs. latitude

Sec. 13.

9. Size of statlonkeeping box is determined by the mission objectives, the perturbations selected to be overcome, and the method of control.

[Wertz, 2001]

10. For long-term constellations, absolute statlonkeeping provides significant advantages and no disadvantages compared to relative statlonkeeping.

[Wertz, 2001]

11. Orbit perturbations can to treated In 3 ways:

• Negate the perturbing force (use only when necessary)

• Control the perturbing force (best approach if control required)

• Leave perturbation uncompensated (best for cyclic perturbations)

[Wertz, 2001]

12. Performance plateaus for the number of orbit planes required are a function of the altitude.

Sec. 7.6.1

13. Changing position within the orbit plane is easy; changing orbit planes is hard; implies that a smaller number of orbit planes Is better.


14. Constellation build-up, graceful degradation, filling In for dead satellites, and end-of-life disposal are critical and should to addressed as part of constellation design.

Sec. 7.6.2

15. Taking satellites out of the constellation at end-of-life is critical for long-term success and risk avoidance. This Is done by.

• Deorbiting satellites in LEO

• Raising them above Vie constellation above LEO (Including GEO)

Sees. 6.5,7.6.2

particles that spread out with time in the same orbits as the original satellites. Because the energy is proportional to mv2, even a small piece of a satellite carries an enormous amount of kinetic energy at orbital velocities.* Because the debris cloud remains in the constellation orbit, it dramatically increases the potential for secondary collisions which, in turn, continues to increase the amount of debris and the possibility of making the orbit "uninhabitable." The implication for constellation design is that we should go to great lengths to design the constellation and the spacecraft to avoid collisions, explosions, or generation of extraneous debris. Methods for doing this are summarized in Table 7-14.

TABLE 7-14. Key Issues In Designing a Constellation for Collision Avoidance.

Approach or Issue


1. Maximize the spacing between satellites when crossing other orbit planes.

May impact phasing between planes and, therefore, coverage.

2. Remove satellites at end-of-life.

Either deorbit or raise them above the constellation, if still functioning.

3. Determine the motion through the constellation of a satellite that "dies In place."

Constellations at low altitude have an advantage.

4. Remove upper stages from the orbital ring or leave them attached to the satellite.

Do not leave uncontrolled objects in the constellation pattern.

5. Design the approach for rephaslng or replacement of satellites with collision avoidance in mind.

All intersatellite motion should assess collision potential.

6. Capture any components which are ejected.

Look for explosive bolts, lens caps, Marmon clamps, and similar discards.

7. Avoid the potential for self-generated explosions.

Vent propellant tanks for spent spacecraft


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