SymTop is a numerical model of the FAME spacecraft. The dynamical model underlying the numerics is a modified symmetric top, an approximation for a spin-stabilized spacecraft for purposes of analysis and data simulation. The model includes a solar shield in the form of a conical "skirt" attached to the top of the spacecraft. The spacecraft is embedded in a pressure field, which can be either the solar wind or the solar radiation pressure or both. The pressure field acts on the top of the spacecraft and on the solar shield, producing torques that drive the precession of the spin axis about the instantaneous symmetry axis of the pressure field. (The solar wind direction fluctuates.) The sketch at right (click on the thumbnail image) shows the solar shield at bottom, the spacecraft bus, and the FAME instrument on top, with its two viewports visible. (See the FAME web site for information about the FAME spacecraft.)

SymTop is a flexible and easily extensible program designed to

• address spin dynamics issues and
• generate simulated stellar observations

SymTop will probably form the core of the "calculate spacecraft orientation" processes of the FAME data reduction pipeline. (You'll need to use a browser that can handle JavaScript in order to view this link. MSIE 4 & 5 and Netscape 4.7x seem to work; Netscape 6 does not. Also, to minimize pixellation problems, the GIF images are optimized for a screen resolution of 1024x768.) A PDF version (72kB) of the current set of software reduction pipeline diagrams is here.

• A study of the distributions of observation density and scan angle. These distributions play a large part in determining the mission accuracies of the FAME astrometry measurements (position, proper motion, parallax).
• Some further results on the magnitude of the attitude perturbations and along-scan timing errors due to stochastic fluctuations of the solar radiation pressure (8 May, 2000). These calculations are an extension of those shown in the documents below.

• Ecliptic Coordinates of the FAME Viewports and Symmetry Axis
  PDF of Maple Worksheet (184 kB)
  Maple worksheet  (846 kB)

• Rigid Body Dynamics of a Truncated Cone
  Maple worksheet, where all the algebra is worked out in detail -- for the stout of heart  (135 kB)
  PDF of Maple worksheet  (418 kB)

• Solar Wind and Radiation Effects on the Spin Dynamics of FAME [draft]
  PDF (259 kB)

• Harnessing Radiation Torques to Drive the Precession of a Spin-Stabilized Spacecraft
  Oral paper given April 3, 1998, at the 29^th annual meeting of the AAS Division on Dynamical Astronomy, Charlottesvile, Virginia
  HTML/Java   PDF (1151 kB)  

• Some (and Only Some!) Aspects of FAME Data Analysis
  Talk given November 11, 1999, at the U.S. Naval Observatory
  HTML/Java   PDF (1826 kB)