E L S E V I E R Measurement 15 (1995) 245 249

Measurement

An interferometer for measuring surveyor's pillars in the open air

N. Brown *, J.G. Winter, I. Fujima National Measurement Laboratory, CSIRO Division of Applied Physics, P.O. Box 218, Lindfield, NSW2070, Australia

Abstract

Electronic distance meters are performance tested on a 650 m long baseline at the National Measurement Laboratory, Sydney, Australia. The distances between the pillars which make up the baseline must be measured in a way which is traceable to the international definition of the metre. A laser interferometer has been developed to measure one of the intervals in a very accurate manner. It has some novel features to allow the use of a simple carriage-way to transport the reflector and has demonstrated a measurement accuracy of 0.2 ppm over a distance of 80 m in the open air.

Keywords: Length measurement; Interferometer; Electronic distance meter

1. Introduction

Modern techniques for measuring distances in the open are constantly improving and current electronic distance measuring (EDM) instruments achieve remarkable precision over long distances (0.3 mm in 650 m or 0.5 ppm) [1,2]. The task of testing their performance against the national standard of length is becoming increasingly difficult. One facility for doing this is a line of seven pillars which extend over a distance of 650 m at the CSIRO National Measurement Labora tory in Sydney, Australia. This paper des- cribes and reports on the performance of a laser measurement system designed to calibrate the 80 m pillar separation. The 80 m interval can then be used to provide a scale for the baseline which is traceable to the international definition of the metre, as the laser frequency can be directly compared with an iodine-stabilised laser (the most popular international laser for realising the defini- tion of the metre [3,4]).

* Corresponding author.

0263-2241/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 2 6 3 - 2 2 4 1 ( 9 5 ) 0 0 0 0 8 - 9

2. Description of the interferometer

The laser interferometer is a frequency offset homodyne interferometer which has some novel features to improve its performance in the open air. It is designed to tolerate quite large lateral shifts (_+ 10 mm) in the laser reflector as it is moved between the pillars and can track the reflector at speeds up to 3 m/s. The tolerance to lateral move- ments means that an inexpensive wire track can be used to guide the reflector between measurement positions.

The laser system is shown in Fig. 1. A frequency- stabilised HeNe laser provides a 1 mW single- frequency light beam. This beam is divided into a reference beam and a measurement beam. The measurement beam is frequency-shifted 80 MHz by an acousto-optic modulator (AOM), then reflected by a polarising beam splitter through a quarter wave-plate and a beam expander. A retro- reflector returns the beam, which, after passing through the quarter wave-plate, has the correct polarisation to pass through the beam splitter and combine with the reference beam. A polariser is

246 N. Brown et al./Measurement 15 (1995) 245 249

PHOTO- DIODE

A REFERENCE!

BEAM :

................... ............... I

POLARISING RETRO- BEAM

BEAM REFLECTOR SPLITTER EXPANDER

WAVE PLATE

/ ~ MEASUREMENT • BEAM

EQUENCV [ STAB~ISED

LASER

Fig. 1. The laser interferometer design.

required to mix the two beams as they are orthogo- nally polarised. A photo-detector monitors the beat signal which is 80 MHz plus any Doppler shifts introduced by movements of the retro- reflector.

The interferometer records the change in path length which occurs when the reflector is moved from its mounted position on pillar one to its mounted position on pillar two. The laser measure- ment beam must be carefully aligned to pass through the centres of these two positions. While the retro-reflector is being moved the interfer- ometer only records the component of the move- ment which results in a Doppler shift. Lateral movements will have no influence on the measure- ment provided there is enough signal returned. The measurement beam is expanded to a 50 mm diameter so the interferometer can tolerate quite large lateral shifts of the retro-reflector. For exam- ple a movement of 10 mm from the measurement axis displaces the return beam 20 mm and the beat signal falls by 12 dBm. This does not affect the counting electronics which can cope with a 30 dBm signal reduction (equivalent to a beam displace- ment of about 30 mm).

The retro-reflector and pillar mounting are shown in Fig. 2. A light-weight carriage holds the reflector and is designed to both run along two steel wires on three wheels, and be supported in a kinematic mount at each pillar position. The

mount is supported on a Kern levelling module which attaches to the pillar fitting. The effective reflecting centre of the retro-reflector is located directly above the centre of the Kern mount. The mounting assembly must be levelled and rotated to align the reflector with the pillars being mea- sured. Only one mounting has been made; it is moved between the pillars while the reflector is being moved.

The retro-reflector and its carriage weigh 265 g. Three wheels allow it to run along two wires which are tensioned to 1 kN. Supports are placed every 20 m to reduce the sag due to the weight of the carriage to 12 mm. Simple camera tripods are used for this task as they can be readily adjusted.

3. Signal processing

Fig. 3 shows how the reference signal and the interferometer output are mixed with a local oscil- lator to produce two low-frequency signals, F and F + D, where D is the Doppler shift introduced by the movement of the retro-reflector along the meas- urement beam. F can be chosen to optimise the interferometer for a particular task. It must exceed expected negative Doppler shifts, but should be as low as possible if fringe fractions are to be mea- sured. In this interferometer F is 625 kHz and is obtained by dividing the 80 MHz AOM drive

N. Brown et al./Measurement 15 (1995) 245-249

SIDE VIEW FRONT VIEW

247

RETRO-REFLECTOR

MOU~_~ ]

+> +> KERN

LEVELLING MOUNT

Fig. 2. The retro-reflector and its carriage (shaded) and pillar mounting.

80 MHz REFERENCE [ INTERFEROMETER SIGNAL

WA F ~ D (80+F)

DISTANCE OWN COUNTER UP

80+D (80-D)

) F+D (F-D)

Fig. 3. lnterferometer signa]t processing for the case where D is positive (negative case is shown in brackets).

signal by 128. The maximum negative Doppler shift is therefore 625 kHz (or a retro-reflector veloc- ity of -0 .18 m/s) as this would reduce the inter- ferometer signal ( F + D) to 0 Hz. The maximum positive Doppler signal is much higher and funda- mentally limited to being less than 79.4 MHz but a practical bandwidth is about 10 MHz, which is equivalent to a mirror velocity of 3 m/s.

The local oscillator is generated by mixing the 80 MHz AOM drive signal with F in a side-band generator using the phasing method. It is quite simple to produce either the upper ( 8 0 + F ) or lower ( 8 0 - F ) side-band by changing the phase of F with respect to the 80 MHz signal. If the lower side-band is chosen, the mirror velocity must fall within the range 3 m/s to - 0.18 m/s, while selecting the upper side-band switches this to 0.18 m/s to - 3 m/s. Knowing which way the mirror is to travel, therefore, allows the interferometer to track velocities between +3 m/s. The signals F and F + D are integrated in an up/down counter which effectively subtracts F from F +D, resulting in a counter output which is the integral of D and is therefore the displacement of the mirror.

Fringe fractions are measured by a second counter which measures the phase shift between F and F+D. It does this by counting a 2 0 M H z signal (generated by dividing the 80 MHz signal by 4) and is switched on when F crosses zero and off when F + D crosses zero. The maximum count is 32 (20 MHz/625 kHz) representing a resolution of 1/32 of a fringe or 10 nm. While this is unneces- sarily accurate for this task, it represents a useful resolution for other applications.

248 N. Brown et al./Measurement 15 (1995) 245 249

The distance counter, phase measuring counter and interface circuits were programmed into two programmable logic devices (Altera EPM 5064 and EPM 5128).

4. Static tests of the system

Initial tests of the interferometer were carried out by mounting the retro-reflector on the 80 m pillar and recording short-term changes in the apparent position of the reflector caused by changes in the refractive index of the atmosphere. These are mainly due to temperature variations in the air path. Air temperatures were monitored at four positions along the path and the air pressure and humidity were recorded at one station close to the path. The local refractive index was deter- mined [5,6] and a corrected path-length calcu- lated. The most stable result for the 80 m path occurred just before sunrise with a total cloud cover and showed a maximum change of 16 ~tm over a 20 minute period. This represents a measure- ment uncertainty of 0.2 ppm and is probably the practical limit for this technique as it represents an uncertainty in the air temperature of +0.2°C. This could only be improved by a technique which integrated the air temperature along the path, rather than monitoring it at a finite number of stations.

5. Pillar measurements

The pillar mount was locked in place on the first pillar, with care taken to remove any slack in the mounting by applying a sideways pressure to the mount as it was clamped. The control computer selected the upper side-band to accommodate the negative Doppler frequency shift for the first move and set the interferometer's distance counter to 1 m, the distance it was mounted behind the first pillar. One hundred readings of the mirror position, taken at 1 s intervals, were then recorded while measuring the air temperature, pressure and humidity. The retro-reflector carriage was then lifted onto the wire track and moved at a steady pace to the second pillar. While it was being moved

the pillar mount was removed from the first pillar and installed on the second. It was levelled and aligned with the wires (which run between the pillars and therefore define the measurement axis). The retro-reflector was then installed on the second pillar. One hundred readings were again taken of this position while monitoring the atmospheric parameters. These two readings were subtracted to determine the pillar separation. The pillar mount was then repositioned on the second pillar by unclamping and reclamping, levelling and aligning. The control computer then switched the local oscillator frequency from the upper side-band to the lower side-band to accommodate the positive Doppler frequency shift expected on the return journey. A measurement of the pillar position was then taken before moving the reflector back to the first pillar where a final pillar measurement was made. Initial and final positions of the first pillar should agree to within the setting accuracy of the pillar mount.

The Kern mount has a three-fold symmetry allowing 120 ° rotations of the instruments mounted on it. On our baseline the Kern fittings have been installed so that different rotations are required on each pillar to align the retro-reflector with the measurement axis. It was therefore necessary to determine any offsets caused by rotating the retro- reflector stand and this was done by measuring the apparent pillar position with the pillar mount installed in the three possible positions.

Initial measurement were carried out on two pillars 20 m and 60 m apart in a long underground laboratory. The standard deviation for twelve read- ings was _+ 6 ~tm for the 20 m interval and _+ 9 ~tm for the 60 m interval. Measurements on the outside baseline were much harder to carry out as the atmospheric conditions were only suitable on rare occasions. Unfortunately Australia has been suffering from a drought and the required overcast conditions only occurred on about three occasions during the month of October when measurements were carried out. Eleven measurements of the 20 m interval resulted in a standard deviation of + 11 ~tm. The 80 m interval required near perfect conditions and eight measurements were selected using the criteria described above (initial and final

N. Brown et al./Measurernent 15 (1995) 245-249 249

pos i t ions agreeing) f rom a set of 28, resul t ing in a s t anda rd dev ia t ion of + 18 ~tm.

6. Conclusion

research project between C S I R O and the Na t iona l Research L a b o r a t o r y of Met ro logy , Japan. The au thors would like to thank P.E. C i d d o r for advice and M. Blanco for designing the mechanica l com- ponents and assist ing with the measurements .

The basel ine p i l lar dis tances of the C S I R O E D M basel ine are at 20 m, 80 m, 195 m, 472 m, 548 m and 649 m. This technique has been used to measure the 80 m interval and may be ex tended to the 195 m interval . Wi th the 80 m interval ca l ib ra ted and t raceable to the in te rna t iona l defi- ni t ion of the meter , a scale factor is p rov ided for conven t iona l measurements of the basel ine made by E D M ins t ruments such as the K e r n M e k o m e t e r ME5000.

Acknowledgments

Ichiro F u j i m a worked on this pro jec t dur ing 1992-93 as par t of an ongo ing co-opera t ive

References

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[-3] BIPM Proc.-Verb. Corn. Int. Poids et Mesures 51 [1983) 25-28, Recommandation 1 (CI-1983).

[-4] Documents concerning the new definition of the metre, Metrologia 19(1984) 163.

[-5] B. Edlen, The refractive index of air, Metrologia 2 ~ 1966) 71. [-6] K.P. Birch and M.J. Downs, The results of a comparison

between calculated and measured values of the refractive index of air, J. Phys. E: Sci. Instrurn. 21 (1988) 694.