Evaluation of a New Prototype Geodetic Astrolabe for Measuring Deflections of the Vertical

Details Evaluation of a New Prototype Geodetic Astrolabe for Measuring Deflections of the Vertical Evaluation of a New Prototype Geodetic Astrolabe for Measuring Deflections of the Vertical

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.g23c0701s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #G23C-0701

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[1229] Geodesy And Gravity / Reference Systems, [1244] Geodesy And Gravity / Standards And Absolute Measurements, [1294] Geodesy And Gravity / Instruments And Techniques

Scientific paper

During the last three years, the National Geospatial-Intelligence Agency (NGA), with assistance from the U.S. Naval Observatory (USNO), sponsored the development of a new electronic geodetic astrolabe for measuring deflections of the vertical (DoV). NGA’s current operational astrolabes, built in 1995, have a number of undesirable features including the need for a pool of liquid mercury as a reflecting surface. The new state-of-the-art prototype instrument, completed by Trex Enterprises in early 2009, was designed to meet a 0.2 arcsec accuracy requirement. It reduces the weight, eliminates the mercury, and dramatically reduces observation times. The new astrolabe consists of a 101 mm aperture telescope with a 1.5° field of view and an inclinometer mounted inside a 92-cm high, 30-cm diameter tube, an external GPS receiver for timing, and a laptop computer that controls and monitors the instrument and performs the computations. Star images are recorded by an astronomical-grade camera with a 2,048 x 2,048 pixel CCD sensor that is externally triggered by time pulses from the GPS receiver. The prototype was designed for nighttime observation of visible stars equal to or brighter than magnitude 10.0. The inclinometer is a system of two orthogonal pendula that define the local gravitational vertical, each consisting of a brass plumb bob suspended from an aluminized polymer ribbon set between two electrodes. An internal reference collimator is rigidly tied to the inclinometer and projects an array of reference points of light onto the CCD sensor. After the astrolabe is coarsely leveled to within 20 arcsec, voice coil actuators automatically adjust and maintain the inclinometer vertical to within 0.02 arcsec. Independent images are collected at 6 second intervals using a 200 msec exposure time. The CCD coordinates are determined for each star and a collimator reference point on each image. Stars are identified by referencing a customized star catalog produced by USNO. A plate model is fitted to the topocentric coordinates of the stars, and then used to solve for the astronomical latitude and longitude of the vertical reference point on the CCD. The average of 100-150 individual image solutions (10-15 minutes) defines the astronomical position for the observation session. In order to remove an azimuthal orientation bias, the astrolabe is rotated 180°, a new observation session solution is produced for that orientation and then averaged with the first solution to get the final astronomical position of the site. By combining these coordinates with GPS-derived geodetic latitude and longitude, one obtains the DoV. Initial testing of the prototype at a known astronomic position has been completed. The tests evaluated the session-to-session and day-to-day repeatability of the solutions, the number of observations required for a solution, the accuracy with respect to the known position, and the operational robustness of the hardware and software. Based on the field tests, Trex will make improvements to the prototype hardware and software and then produce operational units for use by NGA.

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