Info

User

• AH Intermediate steps occur In real time

• Accuracy requirement can be reduced to that needed lor real time support

• "Direct to User" data flow Is both feasible &

economical

Downlink data (or vertHcafion aa needed

• Needs high accuracy to support long term orbit propagation

1 Many opportunities lor Communications or Operations Errors

• AH Intermediate steps occur In real time

• Accuracy requirement can be reduced to that needed lor real time support

• "Direct to User" data flow Is both feasible &

economical

Downlink data (or vertHcafion aa needed

Fig. 2-3. Comparison of Traditional vs. Autonomous Approach to Satellite Navigation.

Use of autonomous operations may significantly reduce mission complexity and thereby Increase reliability.

the payload, but we would use some automatic operations to save money or to make the operator's job easier.

Controlling the attitude of the spacecraft and its appendages is done autonomously on board for nearly all satellites. Controlling the attitude from the ground is too expensive and too risky. The attitude control system on board most spacecraft provides various attitude control modes and can work over extended periods with little or no intervention from the ground.

Ground control has remained strongest in orbit maintenance and control, in which virtually all thruster firings intended to change the orbit are set up and enabled by ground command. This ground control will probably continue whenever large rocket engines produce orbit maneuvers such as when a kick stage moves the satellite from a parking orbit into a geosynchronous transfer orbit Once in their operational orbit, however, many satellites either leave the orbit entirely uncontrolled or simply maintain the orbit at a given altitude or within a given window. In this case, low-thrust propulsion is both feasible and desirable because it is much less disturbing to the normal spacecraft environment Low-thrust orbit maneuvers have been used on geosynchronous spacecraft for a long time so normal satellite operations can continue during the course of these stationkeeping maneuvers.

With low-thrust propulsion and current technology for autonomous navigation, autonomous orbit control is cheap, easy, and inherently less risky than autonomous attitude control. If the attitude control system stops working for even a short time, the spacecraft can have various potential problems, including loss of power, loss of command, and pointing of sensitive payloads toward the Sun. In contrast, if we lose low-thrust orbit control for a while, nothing disastrous happens to the spacecraft The spacecraft proceeds in its orbit drifting slowly out of its predefined position. This is easily detected and corrected by the ground, assuming that the orbit control system didn't fail completely.

The major problem facing autonomous orbit control and, therefore, with autonomous satellites as a whole, is tradition. The ground does it mostly because it has always been done that way. However, there are some signs of change. Both UoSAT-12 and EO-1 are planning experiments in autonomous orbit control and several of the low-Earth orbit communications constellations have baselined autonomous orbit control to minimize both cost and risk.

Current satellite technology allows us to have fully autonomous, low-cost satellites. Autonomy can reduce cost and risk while enabling mission operations people to do what they do best—solve problems, handle anomalies, and make long-term decisions. We believe fully autonomous satellites, including autonomous orbit maintenance, will come about over the next decade as lower costs and risks, validated by on-orbit experiments, begin to outweigh the value of tradition.

2.13 Mission Timeline

The mission timeline is the overall schedule from concept definition through production, operations, and ultimately replenishment and end of life. It covers individual satellites and the whole system. Table 2-3 lists the mission timeline's main parts and where they are discussed. Notice that two distinct, potentially conflicting, demands can drive planning and production. One is the demand for a particular schedule or time by which the system must be operational. Thus, a Halley's Comet mission depends on launching a satellite in time to rendezvous with the comet On the other hand, funding constraints frequently slow the mission and cause schedule gaps which add both further delays and cost Of course, funding constraints can affect much more than timelines. They can determine whether we will do a mission, as well as its scope.

TABLE 2-3. Principal Elements of the Mission Timeline. Key milestones in the mission or project timeline can have a significant effect on how the space system is designed and operated.

Element

Typically Driven By

Where Discussed

Planning and Development

Funding constraints System need date

Sec. 1.2, Chap. 1

Production

Funding constraints Technology development System need date

Chap. 12

Initial Launch

Launch availability System need date

Chap. 18

Constellation Build-up

Production schedule Launch availability Satellite lifetime

Sec. 7.6.1

Normal Mission Operations

Planned operational life

Satellite lifetime (planned or failure constrained)

Chap. 14

Replenishment

Production schedule Launch availability

Satellite lifetime (planned or failure constrained)

Sec. 19.1

End-of-Life Disposal

Legal and political constraints Danger to other spacecraft

Sec. 21.1

If the mission involves a constellation of satellites, a key timeline driver is the need to have the full constellation up for most of the satellite's lifetime. If a single satellite will last 5 years and we need a constellation of 50, we'll never get a full constellation with a launch rate of 5 per year. If having the full constellation is important, we must deploy the initial constellation within 20-25% of an individual satellite's lifetime. This schedule allows some margin for almost inevitable stretch-out as difficulties arise during the mission. If the constellation must remain complete, we need to plan for regular replenishment of satellites. We can replenish on a predefined timeline as satellites wear out or become technically obsolete, or we can respond to on-orbit failures or other catastrophic events which "kill" a particular satellite.

Two areas of the mission timeline typically do not receive adequate attention during concept exploration: performance with less than a full set of satellites while building up the constellation, and end-of-life disposal. In a constellation of satellites we would like to increase performance levels as we add satellites. If FireSat is a constellation, we want to achieve some protection from fires with the first satellite launch and more with each added launch until all satellites are in place. As described further in Sec. 7.6, designers of constellations often concentrate only on the full constellation's performance. However, the period of time before the constellation is brought fully into place can frequently be long and may well be a critical phase since a large fraction of the funding has been spent and yet full capability has not been achieved. Thus it is particularly important for constellation design to take into account the problem of performance with less than a full set of satellites. In addition, we want graceful degradation, so a satellite failure will still allow strong performance until we replace the failed satellite. These issues are important during concept exploration because they may significantly affect the design of the constellation and the entire system.

There is now growing concern with disposal of satellites after their useful life in orbit. We have already recognized this need for geosynchronous satellites because the geosynchronous ring is rapidly filling. But it is also very important for low-Earth orbit constellations in which debris and spent satellites left in the pattern can threaten the remaining constellation. Again, we must address this issue eariy in concept definition.

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