Accelerometers, Gravimeters, Gradiometers and Drag-Free Systems



COURSE NO.: 1010



This course takes you on a walk through a class of inertial instruments that tread the fine line between acceleration and gravity. The basic theory of each instrument type is described and explained, together with relevant history and several actual, often truly ingenious designs. There will be a detailed discussion of error sources, including instrument noise, scale factor errors, g-squared errors, self gravity, thermal distortion, vibration rectification and charge effects. Spacecraft accommodation issues receive a great deal of attention, including the many ways to achieve calibration. Applications to the fields of navigation, hazard avoidance, gravity measurement, gravity wave detection, aeronomy and fundamental physics are described. Where possible, actual hardware will be available for examination.



Lecture notes containing all theory covered, short resumes of the error source discussions and diagrams of actual instruments will be provided. Photos and models of actual hardware will be shown where possible.



While the designers of earthbound instruments would find the course interesting and useful, it is primarily aimed at those concerned with the design of spacecraft that include such instruments, and how the cost of accommodating them can get out of hand if not addressed early in the program.



History and design principles of this extraordinary class of instruments, errors they are subject to and how to control them, plus methods for calibrating them in flight. Applications to the fields of navigation, gravity measurement, gravity wave detection, aeronomy and fundamental physics.




The fundamental physics. Linearization and rebalancing. Scale factor error sources. The g-squared errors. Controlling thermal distortion. Instrument noise. Self gravity. Proof mass charge estimation and control. Spacecraft accommodation issues. Ground and in-flight testing and calibration. One, three and six-axis designs. Applications to navigation and attitude and orbit determination. Gravimeters. Comet probe hazard detection. Satellite constellation gravity wave detection.


Gravity Gradiometers.

The gradient tensor. Different accelerometer types – the Metzger, Paik, Bernard and Fuligni designs. Direct measurement types – the Forward, Tregesar and Van Kann designs. DOD, NASA and ESA programs. The intrinsic tensor. Scale factor errors. Vibration rectification. Self gravity. Ground and in-flight testing and calibration. Extracting gravity fields. Correcting gravity errors in navigation. Satellite attitude determination.


Drag-free Systems.

Chasing truth. The Stanford and APL programs. Solar Probe, Gravsat and Einstein programs. Internal position detection. Control philosophy. Self gravity revisited. Charge estimation and control. Thermal distortions. Miscellaneous error sources. Calibration by tracking and self gravity variations. Applications to navigation. Applications to gravity field measurement. Semi-drag-free accelerometers and aeronomy. The care and feeding of exquisitely sensitive instruments. Interplanetary gravity wave detectors. STEP.



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