pointing. The gondola itself is suspended beneath a standard NASA rotator to
enable large angle azimuth targeting and coarse azimuth stabilization.
Sub-arc-second pointing derives from the mechanical design of the WASP gimbal hubs. A pair of hubs on opposing sides of the gimbal are utilized to establish
each articulated axis of rotation. Each hub uses high-precision angular contact bearings to float the rotor side and stator side of the hub on a central shaft, and the central shaft in each hub is itself rotated by a small diameter torque motor through a
gear box to eliminate static friction. The shafts in each hub pair are counter- rotated
to minimize the residual kinetic friction that the control system must correct.
A large-diameter brushless direct current torque motor is utilized to provide the
torque for each control axis. The current in the three electrical phases in each of
the control motors is commutated in software and set by using power in a motor
interface circuit. The rotor-to-stator angle for each torque motor is determined by
the use of resolvers and positional encoders, with one pair for each control axis.
After the initial test flight, a number of system modifications and improvements
were made to the WASP and a new gondola was designed. The system changes
included incorporating the star tracker into the control system loop, changing the
mock telescope from a 24 foot steel tube to a 16 foot tube with significantly less
inertia, moving the avionics deck from the outer frame onto the mock telescope,
and adding angular positional encoders to the resolver hubs.
For a briefing on the WASP, go to:
https://core.ac.uk/download/pdf/83529822.pdf
Star Tracker
High altitude balloons are capable of supporting astronomical observations with
virtually no image degradation from atmospheric turbulence. To take advantage of
this space-like “seeing”, a telescope must be pointed and stabilized with sub-arcsecond precision. This involves providing an error signal, then using that to correct the pointing.
The University of Colorado Aerospace Capstone Program created a star tracker
called DayStar with support from Southwest Research Institute, an independent
non-profit applied R&D organization headquartered in Antonio, TX. It is meant to
improve upon the pointing accuracy and daytime performance of the ST5000, the
star tracker commonly used by NASA’s sounding rockets. The ST5000 was shown
to work on a balloon at night but it failed to acquire stars during the day. DayStar
overcomes this by filtering light below a wavelength of 620 nm and by using a
sensor with high red-performance and resolution that attenuates most of the sky
background. In combination with custom star identification algorithms, this
enables stars be seen during the day.
To validate modeling and demonstrate daytime acquisition, a DayStar prototype
flew on a high altitude balloon in September 2012. The filtered camera typically
saw three stars during daytime, proving its ability to operate diurnally. Additional
3.4 Support Equipment 57
enable large angle azimuth targeting and coarse azimuth stabilization.
Sub-arc-second pointing derives from the mechanical design of the WASP gimbal hubs. A pair of hubs on opposing sides of the gimbal are utilized to establish
each articulated axis of rotation. Each hub uses high-precision angular contact bearings to float the rotor side and stator side of the hub on a central shaft, and the central shaft in each hub is itself rotated by a small diameter torque motor through a
gear box to eliminate static friction. The shafts in each hub pair are counter- rotated
to minimize the residual kinetic friction that the control system must correct.
A large-diameter brushless direct current torque motor is utilized to provide the
torque for each control axis. The current in the three electrical phases in each of
the control motors is commutated in software and set by using power in a motor
interface circuit. The rotor-to-stator angle for each torque motor is determined by
the use of resolvers and positional encoders, with one pair for each control axis.
After the initial test flight, a number of system modifications and improvements
were made to the WASP and a new gondola was designed. The system changes
included incorporating the star tracker into the control system loop, changing the
mock telescope from a 24 foot steel tube to a 16 foot tube with significantly less
inertia, moving the avionics deck from the outer frame onto the mock telescope,
and adding angular positional encoders to the resolver hubs.
For a briefing on the WASP, go to:
https://core.ac.uk/download/pdf/83529822.pdf
Star Tracker
High altitude balloons are capable of supporting astronomical observations with
virtually no image degradation from atmospheric turbulence. To take advantage of
this space-like “seeing”, a telescope must be pointed and stabilized with sub-arcsecond precision. This involves providing an error signal, then using that to correct the pointing.
The University of Colorado Aerospace Capstone Program created a star tracker
called DayStar with support from Southwest Research Institute, an independent
non-profit applied R&D organization headquartered in Antonio, TX. It is meant to
improve upon the pointing accuracy and daytime performance of the ST5000, the
star tracker commonly used by NASA’s sounding rockets. The ST5000 was shown
to work on a balloon at night but it failed to acquire stars during the day. DayStar
overcomes this by filtering light below a wavelength of 620 nm and by using a
sensor with high red-performance and resolution that attenuates most of the sky
background. In combination with custom star identification algorithms, this
enables stars be seen during the day.
To validate modeling and demonstrate daytime acquisition, a DayStar prototype
flew on a high altitude balloon in September 2012. The filtered camera typically
saw three stars during daytime, proving its ability to operate diurnally. Additional
3.4 Support Equipment 57