The Pointing System

The pointing control system is the same as the one successfully used by FGE. The entire gondola frame rotates in azimuth (yaw) to point at the Sun, while the telescope is tilted in elevation to point at the target. Azimuthal pointing of the entire gondola offers the advantages of simplicity.

The pointing control system has four tracking states, each using a different and gradually more sensitive error sensing mechanism:
Track-state 0: No tracking. This state does nothing to orient the telescope and is used during a major portion of the balloon ascent to avoid expending power fighting the windmill effect when rising through the dense portion of the atmosphere.

Track-state 1: Coarse tracking. Four photodiode sensors mounted at 90° intervals around the gondola provide the Sun's position in azimuth, while an encoder on the elevation shaft provides elevation information. A calculation of the ephemeris based on GPS time and gondola position is used to target the solar elevation.

Track-state 2: Intermediate tracking. Two detectors mounted on the front of the guider telescope each consisting of a cylindrical lens mounted in front of a position-sensing photodiode are used to measure azimuth and elevation errors. The field of view of these detectors is approximately ±20° and they provide an accuracy of about 0.25°.

Track-state 3: Fine tracking. A small guiding telescope rigidly mounted to the main telescope cage produces the fine pointing error signal. The guiding telescope projects an image of the full solar disk that is 1 cm in diameter onto a lateral-effect-diode (LED) used as a position sensor. The LED has a metal disk that occults the inner 90% of the Sun's image to amplify the error signal, thus increasing the fine pointing sensitivity, when the Sun is at or in the vicinity of the LED center. When the occulting disk is fully illuminated, the pointing error is measured to 0.05 arc-seconds RMS. The servo-loop (described in the next section) always maintains the solar image on the center of the LED. An X-Y motion stage moves the LED in the image plane of the guiding telescope to affect offset pointing of the Main Telescope from Sun center.

The digital control system is handled by the MAX3 computer, it uses a two-pole, two-zero equalizer (equivalent to a PID controller with an extra pole available) to determine motor drive entirely from position errors and runs at a sample rate of 40 Hz. Each track state has four control coefficients per axis and can be adjusted via radio commands. Hand-off between the various track states was achieved by gradually blending the control output from one state to the next. This provides an extremely robust and fast acquisition of fine pointing.

The remainder of the pointing control system consists of a momentum-transfer-unit (MTU), two torque motors for steering, a digital control system, and the supporting electronics. The MTU serves both to minimize disturbances introduced through the balloon suspension cable and to shift accumulated momentum from the azimuth reaction-wheel to the balloon. It also acts as the support and attachment point between the gondola and the parachute-balloon system. Systematic azimuthal torque disturbances are common in balloon flights. They are caused by balloon rotation and wind shear forces. These disturbances cause the reaction-wheel to accumulate significant angular momentum that must be "dumped" from the wheel, otherwise the system loses its ability to produce torque (in one direction at least) against the fast-moving wheel. This fact necessitates a transfer of momentum from the spinning wheel to the balloon. A motor is connected between the wheel and the suspension cable to perform this action. It acts as a generator rather than a motor, and a short-circuit load is switched on and off by the control system computer.