# Multiwavelength Interferometry for Geodetic Lengths

Working Group 5.42

### The 50m Comparator

The geodetic base is a comparator for the calibration of material measures and length measuring systems up to a length of 50 meters. The comparator is composed of a comparator bed, to which measuring tapes or line scales can be applied, and a measuring carriage running on the comparator bed.

Fig. 1: View of the geodetic base (50m comparator)

The length measurement is performed with a HeNe laser interferometer. Instead of a triple mirror (three mirrors in the form of a cube corner) which is usually used in an interferometer, the reflector consists of three single mirrors which are arranged on the surface of an imaginary cube. A material measure to be tested (e.g. a measuring tape) lies in the center of the reflector on the measuring axis of the interferometer by which Abbe errors are minimized. For the calibration of measuring tapes and line scales, each one optical and one photoelectrical microscope is arranged on the measuring carriage for determination of the line positions. The achievable measurement uncertainty is considerably influenced by the refractive index of the air and, possibly, by the body temperature of the product to be measured. The refractive index is calculated with the aid of the Edlen formula from air temperature, air pressure and air humidity.

• 21 air temperature sensors at a distance of 2.5 m on the comparator bed (PT 100 - Thermometer)
• 21 material temperature sensors at a distance of 2.5 m on the on the measuring tape (PT 100 - Thermometer)
• one barometer for measurement of the air pressure
• one hygrometer for measurement of the relative air humidity

Fig. 2: Selection of typical test specimens on the 50m comparator

### Calibration possibility for:

• Measuring tapes (supported): Measuring tapes are carried in a low-friction bearing on rolls on the comparator bed at a distance of approx. 50 cm. The required elasticity of the measuring tape is given by a mass which is applied via a steel band and two rolls. Tapes of up to 50 m in length can be measured as a whole. It is also possible to measure longer tapes in several fragments.
• Line scales: Line gauges are usually applied to their Bessel points after the supporting rolls for the measuring tapes have been removed. As the comparator is aligned for large lengths, no competitive measurement uncertainty can be provided for line gauges.
• Distance measuring equipment: In the case of distance measuring equipment, a distinction must be made between two types. Many systems work with plane reflectors (special reflector foils or simply a white surface) which must - due to their size - be fixed on top of the measuring carriage. The measuring axis thus lies above the measuring axis of the interferometer which leads to an increase in the Abbe errors. Some distance measuring instruments work with triple reflectors which in some case may be applied to the measuring axis of the interferometer.
• Laser interferometer Use of another measuring carriage with a larger (complete) triple reflector allows laser interferometers to be calibrated. By using a heat bath and a pressure chamber, the complete system (interferometer and weather station) can be calibrated about a useful range of different external conditions.

For further details see following articel PTB-Mitteilungen 2/2010

Further details are shown in the table: Calibration of length measuring devices.

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# 600m Baseline

### The 600 m baseline

For the calibration and verification of – as well as for the research on and for the advancement of – length measuring devices with ranges above 50 m, PTB has a 600 m baseline at its disposal which – in the years 2010 and 2011 – was equipped with an elaborate weather monitoring system.

The baseline is installed at the northern border of the PTB premises, along a cycle path (Fig. 3). To the south, it is limited by a wooded area, and to the north by the outer fence which is shielded with laser-tight foil. It consists of eight posts which allow all distances between 50 and 600 metres to be realized in steps of 50 metres. The reference post distances can be traced back to the SI definition of the metre.

Fig. 3:  First segment of the extended 600 m baseline of PTB

To calibrate an optical distance meter under open-air conditions, it is of utmost importance to know the ambient conditions, as these define the refractive index and, thus, the scale of the optical devices. For the determination of these ambient conditions, the baseline is equipped with a dense network of calibrated environmental sensors. The temperature is determined every ten metres at beam height and the air humidity every hundred metres. The air pressure is determined by at least two barometers arranged along the baseline. For the measurement, the weather data are read out centrally and simultaneously per optical fiber, and the relevant effective values are determined. From the standard deviation determined, conclusions on the measurement uncertainty – which can be achieved concretely on this day – can be drawn. Monitoring is achieved by means of an easily controllable computer programme (cf. Fig. 4) which can be controlled on the whole baseline by means of a laptop.

Fig. 4:  Screenshot of the weather station of the 600 m baseline.

For research and development purposes, an air-conditioned external laboratory is available at the first post (Fig 5). On the complete site, laser protection class 4 can be maintained.

Fig. 5:  External and internal view of the air-conditioned external laboratory at the zero post.

When carrying out tests in accordance with the Festpunktfelderlass ("Fixed-Point-Field Decree") and the Administrative Provision regarding Property Measurements (LiegVermErlaß), PTB is collaborating with the Institute for Geodesy and Photogrammetry of Braunschweig Technical University.

# Calibrations for probe

### Calibrations for probe

On request are calibrations available in special geometry for probe developed and offered.