1. The antenna that must find Earth

Arthur C. Clarke’s 2001: A Space Odyssey has a quiet but persistent preoccupation with pointing. The Discovery One maintains its alignment with Earth so the long-range antenna can keep the link. The shuttle must align with the station’s docking port. Even HAL’s calm is, in part, the calm of a machine that knows which way it is facing. Pointing is not dramatic until it fails, and then it is the only drama.

Entry 259 said ADCS is the negotiator between subsystems. This entry asks what each subsystem demands from the negotiator.

2. Where the requirements come from

Every attachment that cares about direction makes a request of ADCS. The requests are not all compatible, so the first step is to write them down honestly.

  • Imager: nadir pointing to within roughly 0.1 degree during exposure; stability low enough that motion blur is smaller than a pixel.
  • Solar arrays: sun pointing to within roughly 5–10 degrees to capture most of the available power; finer tracking is better but not critical.
  • Radiators: cold-sky pointing to within roughly 10–20 degrees; Earth and Sun must stay out of the radiator field of view.
  • Star trackers: a clear view of the star field, which means the tracker boresight must stay away from Earth, Sun, and Moon by typically 20–30 degrees.
  • GPS antennas: Earth-facing, with the antenna pattern covering most of the Earth disc below.
  • Communication antennas: Earth-facing or ground-station tracking, depending on link budget.
  • Thrusters: body orientation must keep plume cones clear of deployables during burns.

These requirements are loose because the desktop’s payload is not yet final. The important thing is to keep them visible so they do not get quietly ignored.

3. The operating modes

From these requirements, the desktop needs at least the following modes. Each mode is a contract between ADCS and the rest of the spacecraft.

Nadir pointing

The body z-axis points at Earth’s centre, usually with the velocity vector along the body x-axis. This is the default observation mode and the mode for much of routine operations. The imager looks straight down. GPS and communication antennas see Earth. The solar arrays may not be optimally sunlit, so this mode is often combined with a slow rotation around the nadir vector to keep arrays roughly sun-facing.

Sun pointing

The solar array normal tracks the Sun. This is the default power-maximising mode, especially during eclipse exit or battery recovery. In this mode the imager is usually not Earth-pointing, so observation campaigns must pause.

Inertial pointing

The body stays fixed relative to the stars. This is useful for astronomy, for some calibration campaigns, and for momentum dumping when the reaction wheels need to be unloaded. It is also the simplest mode to implement, because the reference frame does not move with the spacecraft orbit.

Target tracking

A body-mounted instrument or antenna tracks a specific target: a ground point, another spacecraft, or a celestial object. This is more demanding than nadir pointing because the target may move across the sky faster than the orbit rate. The ADCS must generate a target quaternion in real time and follow it with higher-rate commands.

Sun-safe / survival

A fallback mode that keeps the spacecraft power-positive and thermally safe while a fault is diagnosed. Usually this means solar arrays roughly sunward, radiators shaded, and non-essential loads off. It is the mode you enter when something has gone wrong and you need time to think.

4. Accuracy and stability budget

Accuracy is the static error between commanded and actual attitude. Stability is the dynamic error during an observation or a burn. The desktop does not need extraordinary performance, but it needs to know its limits.

A rough budget for the desktop:

Mode Accuracy Stability Notes
Sun pointing ±5° ±1° Enough for power capture; arrays can tolerate coarse tracking.
Nadir pointing ±0.5° ±0.1° Good enough for medium-resolution imager and routine comms.
Target tracking ±0.1° ±0.05° Needed for narrow-field instruments or directed antennas.
Inertial pointing ±0.1° ±0.05° Useful for calibration and star tracker operation.
Safe mode ±20° ±5° Only needs to keep arrays sunlit and radiators shaded.

These numbers are not final; they are a starting point for choosing sensors and actuators. If a customer later demands arc-second pointing, the budget will move and the hardware will change.

5. Mode transitions and conflicts

The desktop cannot be nadir-pointing and sun-pointing at the same time unless the Sun happens to be in the right direction. Mode transitions therefore need to be scheduled. A typical sequence might be:

  1. Sun-pointing to recharge after eclipse.
  2. Slew to nadir pointing for an observation window.
  3. Slew to target tracking for a specific ground site or spacecraft.
  4. Return to sun-pointing to recharge.
  5. Enter safe mode if an anomaly occurs.

Slews consume power and time. They also disturb the thermal state because different faces see the Sun or Earth. ADCS must coordinate with the power and thermal subsystems so that a slew does not coincide with a peak power demand or a critical cooling period.

6. What jitter costs

Jitter is the small, rapid attitude motion that remains after the coarse pointing error is removed. For an imager, jitter smears pixels. For a laser communicator, jitter spreads the beam. The desktop’s imager is not a telescope, but even a consumer-grade camera on a moving platform needs the line of sight to be steady during the exposure.

A simple rule: the angular jitter during an exposure should be less than one-tenth of the imager’s pixel field of view. If the imager has a 1-degree field of view and 1000 pixels across, one pixel is 0.001 degree, so jitter should be below roughly 0.0001 degree, or 0.36 arcseconds. That is finer than the coarse pointing budget, which means the imager may need its own stabilisation or very short exposures.

This is the kind of detail that makes ADCS interesting: the coarse and fine requirements live in the same system, and the actuator choice must satisfy both.

What this changes

  • The desktop needs five core pointing modes: nadir, sun, inertial, target tracking, and safe mode.
  • Pointing requirements come from the imager, solar arrays, radiators, star trackers, GPS, communication antennas, and thrusters.
  • A rough accuracy and stability budget is set, with safe mode deliberately coarse and target tracking the most demanding.
  • Mode transitions must be scheduled with power and thermal constraints in mind.
  • Jitter may force the imager to use short exposures or its own stabilisation.
  • The next entry will choose the sensors needed to measure attitude.