98 lines
3.7 KiB
Markdown
98 lines
3.7 KiB
Markdown
---
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title: Operations planning
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tags: operations, planning, scheduling, fuel-budget, collision-avoidance
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summary: The day-to-day problem of keeping a spacecraft where it should be, and the three budgets that decide it.
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---
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# Operations planning
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Once a spacecraft is flying, someone has to decide what it should do. That
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is a scheduling problem with three hard budgets, and the whole of operations
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is the negotiation between them.
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## Definition
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- The **fuel budget** bounds the total manoeuvre and station-keeping propellant
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available over the mission life.
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- The **contact budget** bounds the number of passes available from the
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ground segment, which bounds how much commanding and how much orbit
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determination are possible.
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- The **safety budget** bounds the number of collision avoidance manoeuvres
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that can be executed, because each one consumes fuel and contact time.
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Planning is the allocation of these. A spacecraft that is short on contact
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cannot be flown to its accuracy targets; one that is short on fuel will end
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early.
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## Mathematics
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### Station keeping cost
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For a low Earth orbit in a plane that is not exactly sun-synchronous, the
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plane drifts because the oblateness and the solar and lunar gravity produce
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a nodal rate the design did not ask for. Holding the plane costs a
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station-keeping manoeuvre at some interval,
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$$
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\Delta v_{\text{keeping}} \sim \left\lvert \dot\Omega \right\rvert \Delta t \, a
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$$
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so the cost is linear in the drift rate and in the interval. Halving the
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allowed drift doubles the rate, and halving the allowed drift interval
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doubles the number of manoeuvres: the two multiply.
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### Collision avoidance
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With many satellites in similar shells, conjunction assessment produces
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candidate approaches, and each confirmed one needs a manoeuvre. The cost per
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manoeuvre is small and the number is not controlled, which is why the
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modern practice is a few centimetres of uncertainty per satellite rather
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than tens of metres: an approach that cannot be confirmed does not need a
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manoeuvre.
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### Orbit determination as a budget
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The fuel budget and the contact budget meet here. Accuracy requires arc
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length, arc length requires contact, and contact is the scarce resource. A
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spacecraft that cannot be observed often enough will drift, will need
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manoeuvres it cannot afford, and the failure appears as a budget rather than
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as an orbit error.
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## Unit conventions
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- Propellant in kilograms, or in delta-v which is what it is spent for.
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- Contact time in minutes per day per spacecraft.
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- Drift rates in degrees per day.
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- Conjunction screening volumes in kilometres; the uncertainty that
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determines them in the same.
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## Applicability
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The budgets are not independent. Spending fuel to buy accuracy because the
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contact budget allowed more tracking is a legitimate trade, and it is the
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trade a plan makes explicitly or implicitly. A plan that does not state the
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allocation has made one by accident.
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The hardest case is the crowded shell, where the collision avoidance budget is
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consumed by events nobody planned for, and the response is a rule rather than
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a decision: the manoeuvre is executed automatically when the probability
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exceeds a threshold, because the decision cannot be made in the time
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available.
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Operating a spacecraft in a crowded shell is therefore partly a matter of
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the uncertainty with which it is catalogued, which is a design decision made
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years before launch.
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## Standards
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- Conjunction assessment and avoidance practice: the standards of the
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international coordination bodies for the relevant regime, plus mission
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practice
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- Ground segment scheduling interfaces: the CCSDS mission operations
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standards
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## See also
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- [[maneuvers]] for what a manoeuvre costs
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- [[tt-and-c]] for the contact budget
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- [[scenario-simulation]] for rehearsing a plan before flying it |