Arc-Second Pointer for Balloon-Borne Astronomical Instrument
Something in common to many balloon missions is the requirement to accurately
point instruments at desired targets. The Arc-Second Pointer for Balloon-Borne
Astronomical Instrument is a control system to keep a balloon-borne instrument
pointed toward a celestial object within an angular error of an arc second or less.
The design was intended to be adaptable to a large range of instrument payloads.
The initial payload prospect was a telescope. This was modeled as a simple thinwalled cylinder 7.3 m (24 ft) long, 0.91 m (3 ft) in diameter and weighing 680 kg
(1,500 lb). The instrument would be mounted on a set of motor-driven gimbals in
a pitch-yaw configuration. The motors on the gimbals would apply the torques for
fine adjustments of the instrument in pitch and yaw. The pitch-yaw mount would,
in turn, be suspended from a motor mount at the lower end of a pair of cables that
hung from the balloon. The motor mount would be used to effect coarse azimuth
control of the pitch-yaw mount.
A notable innovation incorporated in the design was a provision for keeping the
gimbal bearings in constant motion. This would eliminate the deleterious effects of
static friction. This is important when attempting to achieve the necessary arc- second
precision. Another innovation was using linear accelerometers to provide feedback to
achieve the early detection and counteraction of disturbance torques before they could
integrate into significant angular velocity and angular position errors. The control software that processed the sensor data had to be capable of distinguishing between translational and rotational accelerations. The output of the accelerometers was combined
with that from angular position and angular velocity sensors for the pitch and yaw
torque motors. The preliminary analysis determined that with appropriate gains, the
power demand of the control system would be sufficiently low for storage batteries to
be charged by solar cells while in the daylight portion of a mission.
See Section 3.4.2 for details of the Wallops Arc Sec Pointer (WASP) developed
by the Wallops Flight Facility.
From Balloon to Spacecraft
The fundamental discoveries made by NASA scientific balloons gave rise to the
maturation of technologies for spacecraft. Instruments for the Compton GammaRay Observatory (CGRO) satellite were developed on the basis of precursors on
balloon flights. So too were instruments for the Cosmic Microwave Background
Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) to
study Big Bang phenomena. Detectors on the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) mission were first developed and demonstrated
with scientific balloons. The scintillating fiber trajectory detector on the Cosmic Ray
Isotope Spectrometer for the Advanced Composition Explorer (ACE) was proved
on a balloon flight. These are just a few examples. The same can be said for various
components of instruments. Often the components or instruments are simply reconfigured for the required dimensions of the spacecraft and the launch vehicle.
7.4 Instrument and Technology Development 159
Something in common to many balloon missions is the requirement to accurately
point instruments at desired targets. The Arc-Second Pointer for Balloon-Borne
Astronomical Instrument is a control system to keep a balloon-borne instrument
pointed toward a celestial object within an angular error of an arc second or less.
The design was intended to be adaptable to a large range of instrument payloads.
The initial payload prospect was a telescope. This was modeled as a simple thinwalled cylinder 7.3 m (24 ft) long, 0.91 m (3 ft) in diameter and weighing 680 kg
(1,500 lb). The instrument would be mounted on a set of motor-driven gimbals in
a pitch-yaw configuration. The motors on the gimbals would apply the torques for
fine adjustments of the instrument in pitch and yaw. The pitch-yaw mount would,
in turn, be suspended from a motor mount at the lower end of a pair of cables that
hung from the balloon. The motor mount would be used to effect coarse azimuth
control of the pitch-yaw mount.
A notable innovation incorporated in the design was a provision for keeping the
gimbal bearings in constant motion. This would eliminate the deleterious effects of
static friction. This is important when attempting to achieve the necessary arc- second
precision. Another innovation was using linear accelerometers to provide feedback to
achieve the early detection and counteraction of disturbance torques before they could
integrate into significant angular velocity and angular position errors. The control software that processed the sensor data had to be capable of distinguishing between translational and rotational accelerations. The output of the accelerometers was combined
with that from angular position and angular velocity sensors for the pitch and yaw
torque motors. The preliminary analysis determined that with appropriate gains, the
power demand of the control system would be sufficiently low for storage batteries to
be charged by solar cells while in the daylight portion of a mission.
See Section 3.4.2 for details of the Wallops Arc Sec Pointer (WASP) developed
by the Wallops Flight Facility.
From Balloon to Spacecraft
The fundamental discoveries made by NASA scientific balloons gave rise to the
maturation of technologies for spacecraft. Instruments for the Compton GammaRay Observatory (CGRO) satellite were developed on the basis of precursors on
balloon flights. So too were instruments for the Cosmic Microwave Background
Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) to
study Big Bang phenomena. Detectors on the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) mission were first developed and demonstrated
with scientific balloons. The scintillating fiber trajectory detector on the Cosmic Ray
Isotope Spectrometer for the Advanced Composition Explorer (ACE) was proved
on a balloon flight. These are just a few examples. The same can be said for various
components of instruments. Often the components or instruments are simply reconfigured for the required dimensions of the spacecraft and the launch vehicle.
7.4 Instrument and Technology Development 159