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Spacecraft Emergencies and Pointing Modes

An overview

You may at times notice that some of the SOHO real-time images do not update at all, and that some of them look very, very strange, like these:

There may also be a notice above the images that "SOHO is in ESR (or CRP, RMW, or 'safe') Mode".

As with most spacecraft, SOHO occasionally experiences temporary interruptions to normal operations, with the spacecraft going into a "safe mode" that is designed for one thing only: To make sure that the spacecraft is not harmed or lost.

In order to operate safely, the most important aspect for most spacecraft is to make sure that its solar panels are oriented towards the Sun, ensuring sufficient electrical power to its critical subsystems (e.g. thrusters, thermal control, communication, etc).

The following describes some of the features of SOHO's pointing modes:

NM (Normal Mode)

In NM, the attitude and roll is controlled by spinning up or braking down three "reaction wheels", oriented in a triangular fashion to each other. If one wheel is spun up, the spacecraft will rotate in the opposite direction around the axis of that wheel, due to the conservation of angular momentum. The inputs to the control mechanism - the Attitude and Orbit Control Subsystem (AOCS) - come from the Fine Pointing Sun Sensor (FPSS) and a star tracker, a small telescope oriented 90 degrees away from the Sun. The "Roll Steering Law" (RSL) dictates the movement of the stars in the star tracker to keep SOHO oriented exactly in line with the Sun's North-South axis.

With full knowledge of SOHO's orientation, the High Gain Antenna (HGA) can be steered to point at the Earth, providing a High Rate dowlink of telemetry using 26-meter ground stations. Note: Due to a failure in the HGA Z-axis, SOHO experiences telemetry 'keyholes' every 3 months.

ESR (Emergency Sun Reacquisition) Mode

This is the "ultimate safety net" for SOHO. In ESR, the spacecraft attitude is controlled entirely by hardware that senses the approximate position of the Sun and fires hrusters autonomously to ensure that the spacecraft is pointed towards the Sun (plus/minus 2 degrees on each axis). The spacecraft roll is not controlled by the hardware, but it can be controlled by ground intervention.

In ESR mode, the reaction wheels are not used autonomously by the spacecraft - they spin down after a while (in the process they impart their angular momentum on the rest of the spacecraft, which is counteracted by the thruster firings when the pointing reaches the 2-degree limites of the "error box". After the wheels are spun down, the spacecraft pointing "bounces around" within the error box.

Through ground intervention, however, the reaction wheels can be spun up (this is part of a normal recovery). They then provide gyroscopic stability to the spacecraft. When tuned correctly, the spacecraft can be left in ESR mode with wheels spun up with preactically no thruster firings. In this mode, the pointing is not strictly controlled, it will drift slowly (and occasionally bounce) within the "error box", or it can be adjusted by ground control (spinning up/down reaction wheels).

In this mode, the High Gain Antenna (HGA) cannot be used, because the spacecraft does not know how to point it at the Earth. Instead, an omnidirectional Low Gain Antenna (LGA) is used to transmit vital information to ground controllers.

With 26-meter ground stations, only Low Rate (LR) telemetry can be transmitted through the LGA, giving only the bare minimum of information about the health, safety and condition of the spacecraft and instruments. No science data is available in LR.

On 34-meter ground stations, however, Medium Rate (MR) telemetry can be downlinked through the LGA, giving full science data while the spacecraft is in contact. With 70-meter stations, even High Rate telemetry can be achieved, making it possible to dump recordings of what goes on between station passes.

ESR mode can be triggered by a number of "safety features". The last two years (up to April 2004), the trigger has typically been the so-called Fine Sun Pointing Attitude Anomaly Detector (FSPAAD). This is entirely independent of the AOCS, and is supposed to trigger when the Sun is outside a 5-degree radius error circle. All of the latest triggers have been false events, however, and it has now been disabled from triggering ESRs. There is a similar sensor called the CSPAAD (Coarse SPAAD) that triggers at a 5-degree radius error circle, which is still enabled.

When an ESR is triggered, a message about the fact is sent to most of the instruments. The reason is that some instuments do not tolerate even minimal deviations from the correct attitude without encurring damage (e.g. LASCO, which blocks out direct sunlight with an occulter, would quickly overheat if the Sun was suddenly shining directly into the sensor part of the instrument). Other instruments are very sensitive to contamination from the thruster firing (e.g. CDS, SUMER, CELIAS/STOF).

EIT is not directly affected by a loss of nominal attitude, but it shares its electronics box with LASCO, so its instruction queue is supposed to get flushed as a result of an ESR flag. During recovery, however, if Medium Rate telemetry is available, EIT can resume observations by sending up new commands. In case of only Low Rate telemetry their observation queue gets "backed up", potentially endangering the instrument when it tries to catch up later (too frequent exposures might heat up the shutter mechanism).

So, sometimes, when SOHO is in ESR, you'll see some very funny images from EIT: The Sun is "bouncing around" relative to its field of view, causing blurred images with the Sun appearing in the strangest locations. In some cases, the Sun is not even visible at all. Since the ground software processing the images has no clue about the ESR condition, it faithfully processes the images the normal way: correcting the images for burn-in and scaling it to make sure intensity details are visible. This can cause very strange visual effects, as shown in the images above.

CRP (Coarse Roll Pointing) Mode

In this mode, the first step "up" from ESR, the reaction wheels are used for attitude and roll control (i.e. no thruster firings). Pitch and yaw is determined through the FPSS, and the roll rate is measured with the reaction wheels. It may seem a contradiction that the roll is both controlled and measured through the reaction wheels, but this is an ingenious software substitute for the gyroscopes (which are no longer functional): Due to the absolute conservation of angular momentum (external torques are almost zero), you can tell if SOHO starts spinning by the fact that the reaction wheel speeds must be changing! The roll control in this mode is in fact two orders of magnitude better than the old modes where SOHO used its gyroscopes!

In CRP, SOHO can be commanded to point correctly at the Sun, and it is stable enough to do a star mapping through the star tracker, so the absolute roll can be determined. It can then be commanded to roll back to near nominal roll orientation, and the HGA can be oriented towards Earth. When the HGA Z-axis drive was still functional, rolls could be performed while keeping the HGA pointed at Earth. This is no longer possible.

RMW (Roll Maneuver Wheels) Mode

In RMW, the intermediate mode between CRP and Normal Mode, SOHO is controlled by the FPSS and the star tracker. In other words, SOHO has full knowledge and control of its orientation. However, the steering laws are a bit more "relaxed", and a strict alignment with Solar North-South is not maintained. This mode is used for thruster maneuvers: Station Keeping (SK) maneuvers are used to trim SOHO's orbit, and Momentum Management (MM) maneuvers are used to spin up/down the reaction wheels when necessary (as time goes by and SOHO rotates around the Sun, the speeds evolve slowly by exchanging angular momentum between each other and the spacecraft). The wheel speeds must be kept within certain ranges (not too slow, not too fast) to maintain stable control.