Table of contents

A high-precision absolute radiometer with a thermally stabilized cavity as receiving element has been designed for use in solar irradiance measurements. The State Special Standard of the Solar Irradiance Unit has been built on the basis of the developed absolute radiometer. The Standard also includes the sun tracking system and the system for automatic thermal stabilization and information processing, comprising a built-in microcalculator which calculates the irradiance according to the input program. During metrological certification of the Standard, main error sources have been analysed and the non-excluded systematic and accidental errors of the irradiance-unit realization have been determined. The total error of the Standard does not exceed 0.3%. Beginning in 1984 the Standard has been taking part in a comparison with the Å 212 pyrheliometer and other Soviet and foreign standards. In 1986 it took part in the international comparison of absolute radiometers and standard pyrheliometers of socialist countries. The results of the comparisons proved the high metrological quality of this Standard based on an absolute radiometer.

In optical interferometers illuminated by laser sources the shift of the phase and changes in the curvature of the wave fronts lead to systematic errors, to which there must be added the error caused by misalignments. In the study described in the present article, the interference pattern was obtained from the actual amplitude and phase of the interfering wave fronts at the exit aperture of the interferometer. With reference to plane-wave illumination, the excess of the period of the output signal was obtained in a closed form in the case of small misalignment and negligible difference between the optical paths.

Determination of the differences between the Kelvin Thermodynamic Temperature Scale (KTTS) and the International Practical Temperature Scale of 1968 (IPTS-68) in the range from 230°C to 660°C has been accomplished by gas thermometry. The estimated random uncertainty of the results ranges from ±0.005°C to ±0.008°C; the major uncertainties appear to lie in the determination of the volume of the gas bulb and the determination of its IPTS-68 temperature. The present results differ noticeably from earlier gas-thermometric determinations in the range of temperature overlap. The value found for the thermodynamic temperature of the freezing point of Al is (660.342 ± 0.015)°C.

The usual error model for calibration experiments is extended to situations where there are both short-term and long-term random errors of measurement. Such error models are useful where short-term errors are related to instrumentation, and long-term errors are related to operating procedures, environmental factors or changes in the artifacts themselves. The concept of a check standard is advanced for estimating variability and maintaining statistical control of the measurement process.

A general technique for estimating the effect of modelling errors on linear least-squares problems is discussed and applied to the specific example of determining the systematic effect of the vertical gravity gradient on free-fall absolute gravity measurements. The effective measurement height of an absolute gravimeter occurs nominally at the height corresponding to one third of the drop. However, this rule of thumb can be in error by several centimeters due to the initial velocity. The true measurement height is also sensitive to the number of points used and the distribution of the points selected for the least-squares solution.

Two active hydrogen masers constructed at the CSIRO Division of Applied Physics have been contributing to the laboratory's frequency and time scales since 1982. Design and performance details are discussed. Vacuum-pump lifetimes necessitate servicing of the masers every few years and these operations can cause frequency drifts of 2 × 10^-15 per day due to contamination of the maser bulb surface. However, recovery, to 2 × 10^-14 per year occurs over a year or two. By staggering service operations, continuous frequency and time scales are achieved which compare favourably with such scales in other countries.

A method is described for measuring oscillatory displacements in the range 2 nm to 255 nm, using a laser interferometer. The reference mirror is moved at a constant velocity, modulating the photoelectric signal at 130 Hz. The modulated signal is digitally sampled over a prescribed interval, and a Fast Fourier Transform is taken. After correcting the harmonic amplitudes for the so-called `picket-fence error', their ratios are measured. The displacement is then calculated by reference to the ratios between Bessel functions J₂/J₀, J₃/J₁ and J₄/J₂. An application is described for automated calibration of accelerometers at high frequencies. The latter is shown to have accuracy comparable to that of methods which establish discrete amplitudes by reference to zeros of Bessel functions.

A pyrometer is described which is simple and easy to construct, yet fulfils the requirements of a standards laboratory which wishes to perform radiation thermometry to very high accuracy. A test of its performance is described in which it was used to measure the ratio of the spectral radiances at 650 nm of black bodies at the melting points of silver and gold. Taking the gold-point temperature to be 1064.43 °C, this measurement gave a result of (961.961 ± 0.017) °C (99% confidence level) as the value of the silver-point temperature.

International atomic time references, TAI and UTC, are computed by the BIPM from the data of atomic clocks running in various laboratories; they are made available to the time/frequency community and to the potential users. Furthermore, by mid-1988, 15 independent atomic time scales had been established by the laboratories with their own data.

From the user's point of view, it is worth wondering whether these independent time scales possess specific qualities with respect to the international references.

The long-term performance of two active hydrogen masers developed at the National Measurement Laboratory of the CSIRO is presented. Six years of comparisons between the masers, the United States Naval Observatory (USNO) and the Bureau International des Poids et Mesures (BIPM) are summarized. Operating as independent clocks the masers showed outstanding long-term performance and consistently high weightings in the coordination of major time-keeping centres carried out by the BIPM.