Learning from the time and length redefinitions, and the metre demotion

After discussing several issues in a future redefinition of the kilogram, this paper considers the lessons that one might have learned from the analogous redefinitions of the metre and the second. The progress of length metrology was slow and steady, from seven digits reproducibility with the 1889 X-shaped metre prototype, to nine digits with Kr lamps, to 11 digits with the 1983 redefinition of the metre using the speed of light. With laser cooling, the Cs clock improved to 15, now 16, digits (and so also astronomical distance measurements could improve). Laser-cooled ions, and now atoms captured and cooled in an optical lattice, enable accuracy capability of three different optical frequency references to exceed 17 digits, i.e. better than time itself. The optical comb and related techniques vastly simplify frequency comparisons. Such progress stimulates a new satellite experiment, the STAR Mission (Space–Time Asymmetry Research). The goal is to test at the 1E–18 level frequency shifts owing to spatial anisotropy, position, gravitational potential and boost. The onboard optical clock will use stabilization to a molecular transition in I 2 or HCCH or CO 2 . The length etalons will be multiply redundant, with stability at the thermo-mechanical mirror motion limit. For a ULE glass etalon spacer (1987), I measure length creep approximately −1.5E–12/d, i.e. below 1E–14 over the 500 s satellite spin period.

OSO-8 X-Ray Polarimeter and Bragg Crystal Spectrometer Observations

The OSO-8 satellite contains a focusing mosaic graphite crystal X-ray polarimeter that is oriented along the spin axis of the wheel section of the satellite. The polarimeter operates at 2.6 and 5.2 keV. Polarization in a source appears as a modulation of the counting rate at twice the satellite spin frequency. The amplitude and phase of the modulation are simply related to the polarization and position angle, respectively. Two independent polarimeters are employed, and their axes are offset by 128°. Focusing is achieved by mounting the crystals on a parabolic sector, which reduces the background without reducing the sensitivity. The low background that results from the focusing not only improves the statistical quality of the data but also substantially reduces the danger that an asymmetry in the charged particle background may produce a spurious polarization result. This is particularly important in the case of weak sources. The instrument has been described in detail elsewhere (Novick 1975); here we will briefly discuss the results obtained on the Crab Nebula, Cyg X-l, and Cyg X-2.

Topside ionosphere electron density measurements on Ariel 4

The accuracy and validity of the electron density measurements are discussed in terms of inherent system errors and those arising from satellite spin modulation effects. Comparative measurements with other independent techniques support the ± 5 % error figure quoted for the accuracy of relative electron density measurements and suggest that overall agreement to within individual experimental errors would be achieved for measurements of absolute electron density if the present values were about 10 % greater.