Measurement system for the Green Bank Telescope
IEEE Antennas and Propagation Society International Symposium. 2001 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.01CH37229)
The National Radio Astronomy Observatory (NRAO) is constructing a 100-meter aperture, 60-meter focal length offset paraboloid, fully steerable, radio telescope in Green Bank, West Virginia. The Green Bank Telescope (GBT) 1 will be the world's largest fully steerable telescope when completed in late 1999. The GBT stands 150meters at its peak and has a moving weight of 8000 tons suspended on 16 conical steel wheels on a 32-meter radius track. The goal is to obtain a reflector surface accuracy of
... urface accuracy of 0.25 mm RMS and a pointing accuracy of 1 arcsecond, under benign weather conditions, in order to work up to a frequency of 100 GHz (3 mm wavelength). To accomplish this goal, a number of unique features are included in the design of the GBT. The surface is composed of 2004 reflector panels mounted on motor operated actuators to correct the surface for thermal and gravity deflections of the support structure. Custom designed and automated Electronic Distance Measurement (EDM) instrumentation 2 3 is included to provide closed-loop feedback of the surface shape at the 2209 actuator locations, and measurements from fixed ground-based instruments provide closed-loop measurements for pointing error corrections. Six instruments are mounted on the structure above the main reflector looking into 2209 retroreflectors mounted on the surface, to produce a complete survey of the reflector every 8 minutes. Twelve ground-based instruments, equally spaced on a 120-meter radius around the telescope, track retroreflectors mounted on cardinal points on the telescope structure 4 . The two systems are tied into a survey network referenced to the 12 ground-based instrument monuments. The instruments use a laser diode modulated at 1500.000 MHz and a digital phase measurement technique to produce a measurement modulo 100 mm. A servo-controlled mirror is used to switch between targets. The instrument accuracy is around 0.100 mm at 100 meters (1 ppm) with a measurement speed of 5 points per second for near neighbor retroreflector targets. All coordinates are calculated by trilateration, i.e., only distances are used in the reduction. The group refractive index, the major limit to accuracy, is corrected by a combination of refractometer paths between ground monuments; temperature, pressure, and humidity profile measurements; and possibly acoustic thermometry. For example, a 1°C change in temperature produces a 1 ppm change in the group refractive indexthe entire error budget if uncorrected. The 18 instruments are interconnected via Ethernet to a central computer that orchestrates the required measurements and computes the coordinates. The central computer tracks the telescope motion from 24-bit encoders and a finite element analysis model of the structural deformations as a function of telescope elevation angle. These measurements will be used in the early stages to study deflection of the structure 5 and refine the finite element analysis model. 6 In operation, the measurements will be used to correct the surface shape and absolute pointing.