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MEASUREMENTS OF STRUCTURAL DEFORMATIONS OF LARGE REFLECTOR ANTENNAS

 Michael J. Brenner
Engineering Metrology Services
Tucson, Arizona  USA
 
 

ABSTRACT

Optical surveying techniques with theodolites have been utilized for many years for static measurements of reflector antennas. This paper reports on updated optical surveying systems used to measure the accuracies and structural deformations of reflector antennas. Deformations of large Cassegrain tracking antennas during elevation rotation and a fixed, billboard-style compact range reflector over time are discussed.

A simple surveying method is shown for the integrated measurement of Cassegrain antennas (both primary and secondary reflectors) from near the primary vertex. Other topics covered include accurate prediction of interpolated gravity deformations of rotating reflectors based on a small measurement sample, and a method for taking differences between measurements.

The use of EDM (Electronic Distance Measurement) theodolites as well as angle-only devices is described, along with software which manages both the measurement and data-reduction systems.

Keywords: Reflector Structures, Reflector Metrology, Errors, Compact Ranges
 
 

1.0 INTRODUCTION

Most large, multi-panelled reflectors are aligned, at least initially, using some variant on the theodolite-tape method [1][2][3], in which a vertex-mounted theodolite is used to measure angles, and a radial arm of some sort measures target distance. This paper describes extensions of this general technique to the measurement of structural deformations of reflectors, including the position of the Cassegrain subreflector. The use of integrated EDM (Electronic Distance Measuring) instruments is discussed.

These techniques were applied during three recent field installations, including a 45-foot diameter Cassegrain tracking antenna used for millimeter-wavelength radio astronomy, a 31 x 22 foot billboard-style compact range reflector, and an 86-foot diameter centimeter-wavelength tracking radar antenna.
 
 

2.0 CHANGES OVER TIME
A large, serrated-edge compact range reflector surface was measured using multiple triangulating theodolites on two occasions separated by several weeks. During that time, the temperature in the anechoic chamber changed substantially, and there was possible subsidence of the reflector's foundation. Figure 1 shows the difference in the surface between the two measurements.

Surface deformations normal to the reflector surface based on separate measurements are determined by taking the vector dot product of the displacement vector with the surface-normal unit vector. This in effects projects the displacement into the surface-normal direction. Symbolically this can be written as 

Figure 1 Compact Range Reflector Deformations
Figure 1 Compact Range Reflector Deformations 
dn = D dot product with n 
where
dn = surface-normal displacement

D = displacement vector

n = parabola normal unit vector 

3.0 CASSEGRAIN OPTICS MEASUREMENT

Cassegrain secondary reflectors (subreflectors) are often constructed with a central targeting mirror normal to the axis of rotation with etched cross-hairs, and 2 or more perimeter targets at known locations. A vertex-mounted theodolite is commonly used to align the subreflector to its theoretical location with respect to the primary. Such a targeting system with a theodolite can also be used to determine the position of the subreflector. Figure 2 shows a schematic layout of the theodolite and the subreflector with a central mirror target and one perimeter target. One additional perimeter target can provide information in the orthogonal axis. More targets provide redundant information, which can be used for error checking and statistical evaluations.
Geometric evaluation of Figure 2 shows that given R_targ, Z_targ and Z_offset (target coordinates) from the subreflector manufacturer, R_th, Z_th, tilt_th (theodolite position) calculated from field measurements of the primary (a standard 3 degree-of-freedom parabola bestfit as described below will provide these data), th1 and th2 theodolite measurements of subreflector targets, the subreflector position R_subr and Z_subr can be calculated. The subreflector tilt angle tilt_subr is based on the theodolite angle measurement from autocollimation (or autoreflection) on the subreflector mirror.

Unlike the use of a theodolite to measure the primary reflector targets from the vertex region, subreflector measurements require the use of near-zenith look angles, which can compromise accuracy. This accuracy can be critical due to the sensitivity of the subreflector focal distance calculation to small changes in angle. Therefore, it is important that the inherent instrument errors be removed during the measurements. The best way to do so is to take both face-1 and face-2 theodolite measurements. By proper combination of measured angles, the theodolite vertical index error and axis inorthogonality can be removed. 

Figure 2 Secondary Reflector Position Measurement
Figure 2 Secondary Reflector Position Measurement 

Having measured the primary reflector surface targets in the usual manner [1][2][3], the subreflector position information can be combined to provide information on the full Cassegrain optics [4][5]. Such an analysis provides information on where the subreflector may be moved to peak the gain (i.e. minimize the RMS of the path length errors [6]), the rf beam pointing position before and after subreflector adjustment, and the adjustments at the primary reflector screw points which will produce the most accurate paraboloid of the desired focal length.
 
 

4.0 MEASUREMENT OF GRAVITY SAG

With minor adjustments to the theodolite and a comfortable working platform for the operator, primary surface and subreflector position measurements as described above can be performed at any elevation angle for a tracking antenna. For antenna structures which behave in a linear elastic fashion, measurements at 3 separate elevation angles provides sufficient information to determine the relative positions of all targets at any other angle. (A common example of non-linear structural deformation is the bending of bolted flanges.) The process requires the intermediate calculation of the face-up (zenith pointing) and face-side (horizon pointing) gravity deformations of the reflector, and their subsequent vector superposition. The best results will be achieved if the elevation angles at which measuree ments are mad are well separated, such as 0, 45 and 90 degrees.

The gravity deformation of a linear elastic structure at elevation angle can be derived by vector superposition of the face-up and face-side gravity vectors as
d = ds cosq + du sinq

where
ds = face-side gravity deformation

du = face-up gravity deformation 

The face-up and face-side deformations described here are of the type derived from a computer structural analysis in which gravity is "turned on" from a particular direction. As such, they cannot be directly measured in the real world.

For a linear elastic structure which rotates from angle q1 to q2 the relative deformation, or motion, will be d2 - d1 as shown above. For 3 measurements, two independent linear equations can be written for d2 - d1 and d3 - d1. Simple matrix algebra shows that
du =

ds

sinq3 -sinq2cosq3 -cosq2-1

sinq3 -sinq1cosq3 -cosq1

d3 - d2

d3 - d1

Now that du and ds are known, for any angle q, the predicted alignment error at a target is the gravity deformation travelling from a measured reference angle ( q2) to angle superposed onto the actual measured alignment error at the reference angle. This can be expressed as
d(q) = ds (cosq - cosq2) + du (sinq - sinq2)

For targets on the primary reflector, the surface errors ë can be determined using the vector dot product method described above.
These concepts were recently applied to a 45-foot diameter millimeter-wavelength radio telescope. For that project, upon completion of the primary surface alignment effort, measurements were taken at 0, 45 and 90 degrees elevation. The measurements included the subreflector position as was the primary reflector surface at 480 target locations. Figure 3 shows the calculated RMS surface accuracy for 5 cases.

The primary reflector surface measurement data was analyzed using 3 and 6 Degree-Of-Freedom (DOF) least squares parabola bestfit routines. The 3-DOF solution calculates the required translation along the boresight axis (Dz), as well as rotations about the X and Y axes (ax , ay), while the 6-DOF solution additionally solves for the bestfit translations of the X and Y axes ( D x, D y) and the bestfit focal length (F). Additionally, a full Cassegrain optics analysis was performed [4][5], both for the case of the subreflector in the as-measured position and for the calculated bestfit subreflector position. As expected, the 6-DOF primary analysis gives identical results to the full optics analysis with subreflector motions allowed. The only difference between them is that the full optics analysis accounts for path length effects of the feed misposition, which in this case was zero. 

Figure 3 Surface Accuracy of 45-foot Radio Telescope
Figure 3 Surface Accuracy of 45-foot Radio Telescope

Figure 3 shows that the reflector was well-aligned at the selected optimal elevation angle of 45 degrees, and that the residual (after bestfitting) RMS gravity deformations of the primary are greater than 7 mils. In this case (and many others in the author's experience) the measured gravity deformations are greater than those predicted by analysis by substantial factor. The figure also shows that the subreflector is misaligned to optimize performance at 75 degrees elevation instead of 45 degrees.
 
 

5.0 USE OF ELECTRONIC DISTANCE MEASUREMENT
In some cases, it is necessary to measure targets on a primary reflector surface where the radial target locations are not known, such as the upgrade an older antenna. For these applications, the use of a precision tape is not practical. Fortunately, in recent years commercially available coaxial (with the sighting telescope) Electronic Distance Measuring (EDM) theodolites have become sufficiently accurate to be of use in the measurement of some of the most accurate large reflector antennas. The RMS distance accuracy of well-calibrated instruments is about 10 mils (0.010 inch) over the ranges of interest here. A typical vertex-mounted theodolite views perimeter targets on a large parabolic primary reflector offset by no more than 17 degrees from the surface tangent. The RMS measurement error normal to the surface due to the EDM is therefore approximately 10 mils*sin(17 deg) = 3 mils, which is good enough for most applications.

Recently, an EDM theodolite was used to measure 1200 targets on the primary reflector of an 86-foot diameter tracking radar. Measurements were made at 4 elevation angles. The surface deviations derived from the measurements were combined in 4 ways, thus providing 4 separate calculations of the face-up and face-side gravity deformations. The differences among these results gives an indication of the degree to which the assumption of linear elastic behavior is valid, as well as a check on the measurement accuracy.

The three most reliable data sets were combined as described above to determine the face-up (90 deg) and face-side (0 deg) gravity deformations, as shown in Figure 4. Two features are apparent in the figure. First, as expected, in the face-up orientation the reflector open up, increasing the focal length. while in the face-side measurement the opposite is true. Second, a ring structure is apparent, which indicates the action of the reflector backstructure. 

Figure 4 Face-Up and Face-Side Gravity Deformations

Gravity Deformations at 0 Degrees Elevation

 

Figure 4 Face-Up and Face-Side Gravity Deformations 

6.0 COMPUTERS AND SOFTWARE

Clearly, the techniques described above require the collection and analysis of large amounts of data. Custom software has been developed for field use which downloads theodolite angles and distance measurements to a laptop computer via a serial connection. In practice, the operator aims at a target and clicks a trigger switch to measure a target location. The software provides information needed to take all of the necessary measurements, and performs all of the necessary field calculations. It provides surface maps as shown above, RMS accuracies based on various degrees of bestfitting, as well as the present location and adjustment requirements to the subreflector position which will optimize antenna performance.
 
 

7.0 CONCLUSIONS

These techniques have proved useful both in the installation of new equipment and the upgrading of existing facilities. They can be used with finite element structural analysis tools to qualify new designs or to upgrade existing facilities.

While the full optics analysis of Cassegrain antenna systems was described, these general techniques would apply as well to prime focus or Gregorian systems.
 
 

REFERENCES

[1] M. J. Brenner, A. Joel Ellder, M. S. Zarghamee, "Upgrade of a Large Millimeter-Wavelength Radio Telescope for Improved Performance at 115 GHz", Proc of the IEEE, vol. 82, no 5, pp. 734-741, May 1994.

[2] M. J. Kesteven, B. F Parsons, D. E. Yabsley, "Antenna Reflector Metrology: The Australia Telescope Experience", IEEE Transactions on Antennas and Propagation, vol. 36, no.10, pp.1481- 1484, October 1988.

[3] A. Greve, "Reflector Surface Measurements of the IRAM 30-m Radio Telescope", International Journal of Infrared and Millimeter Waves, vol. 7, no. 1, 1966.

[4] M. S. Zarghamee, J. A. Antebi, "Surface Accuracy of Cassegrain Antennas,", IEEE Transactions on Antennas and Propagation, vol. AP-33, no. 8, pp. 828-837, August 1985.

[5] M. S. Zarghamee, "Peak Gain of a Cassegrain Antenna with Secondary Position Adjustment," IEEE Transactions on Antennas and Propagation, vol. AP-30, no. 6, pp. 1228-1233, November 1982.

[6] J. Ruze, "Antenna Tolerance Theory - A Review", Proc of the IEEE, vol. 54, pp. 633-640, 1966.
 
 

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