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