105 lines
4.4 KiB
Markdown
105 lines
4.4 KiB
Markdown
---
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title: Telemetry, tracking and command
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tags: tt-c, telemetry, telecommand, ranging, pass, pass-prediction
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summary: The operational chain that turns a spacecraft into something that can be flown, and where each part of it fails.
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---
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# Telemetry, tracking and command
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Telemetry, tracking and command is the chain by which a spacecraft is
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observed and controlled. It is a measurement chain, an estimation problem,
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and a scheduling problem, and the failure modes of each are different.
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## Definition
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- **Telemetry** is the stream of spacecraft state sent down: attitude, orbit
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parameters, power, temperatures, and a health status.
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- **Tracking** is the measurement of where the spacecraft is and how it is
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moving, from range and range rate.
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- **Command** is the instruction stream sent up, including the telecommands
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themselves and the upload of a schedule.
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The chain is: a station knows where the spacecraft ought to be, measures
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where it is, compares, and uploads the difference. Every stage has a latency,
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and the control design is largely about what to do about the total.
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## Mathematics
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### Orbit determination from tracking
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A range measurement constrains the position along one line of sight; a range
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rate constrains it along the derivative. A single station with both obtains
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a two-dimensional state for a three-dimensional orbit, which is why
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determination needs geometry and time rather than one station.
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The estimate is a weighted least squares over the observations in an arc, and
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the covariance of the result says how well the arc determined the orbit. This
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is the number to look at: an orbit with a large covariance will drift
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predictably and lose the arc that would have fixed it.
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### The measurement error budget
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Each measurement has a variance from noise, from the atmosphere, from the
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clock, and from the station geometry. The geometry term is usually largest
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and is a property of the orbit: a low-inclination orbit seen from a station
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near its plane is well determined along the orbit and poorly determined
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across it.
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### Pass prediction and scheduling
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A pass is a visibility interval for a station. Prediction is a forward
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propagation of the orbit, and it inherits every error of that propagation,
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which is why a predicted pass that is 50 km long is worth less than its
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nominal length suggests.
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Scheduling is the assignment of passes to stations to meet the requirements
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per spacecraft, which is a scheduling problem with hard deadlines.
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### Command latency and safety
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The round trip from a command decision to it taking effect is dominated by
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propagation delay and by a ground processing time of tens of seconds to
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minutes. A safety rule that must act inside that latency must therefore be
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implemented on the spacecraft, where it is bounded by the reaction of the
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processor rather than by the link. This is the origin of the spacecraft's
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autonomous safe mode and of the monitor-based command verification that
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precedes execution.
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## Unit conventions
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- Range in kilometres, range rate in metres per second or kilometres per
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second.
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- Pointing angles in degrees when passed to the spacecraft, because that is
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the convention of the telecommand dictionaries.
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- Time is a spacecraft clock, usually TAI or GPS time; the offset from a
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ground reference is part of the tracking problem, not a detail of it.
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## Applicability
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The chain degrades in different ways at different bands. A Ku band station
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can close a link in weather that Ku cannot; an S band ranging system trades
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accuracy for robustness and works in rain. A mission that needs both carries
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both, because the requirement that fails in the worst weather is usually
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tracking, not commanding.
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The clock is a limiting element throughout, and a mission that ignores it
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discovers this when its tracking residuals stop improving.
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Commanding is also where the interaction with interference appears: an uplink
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that is jammed is not merely degraded, and the ground segment needs a way to
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know whether a command arrived.
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## Standards
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- Telemetry and telecommand coding: CCSDS Space Packet Protocol, and the
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telemetry and telecommand dictionaries in the CCSDS Blue Book
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- Time and frequency: CCSDS Time and Frequency Formats, which define the
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onboard time format
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- Station keeping and pass prediction: not standardised; mission practice
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## See also
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- [[doppler-and-rate]] for the measurement the chain depends on
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- [[link-budget]] for whether a pass exists
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- [[operations-planning]] for what the chain is planned against
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- [[interference]] for the uplink |