Spin Axis Direction Research Articles

The Cretaceous-Tertiary Boundary: Chronostratigraphic Position and Sequence of Events

The Cretaceous-Tertiary boundary has always been considered to be exceptional and linked to special events and mechanisms where catastrophes have often been advocated. The boundary stratotype, Stevns Klint in Denmark, has been paleomagnetically analysed. The C/T boundary is now given an absolute age of 66.7 m.y. The boundary is shown to have occurred under stable geomagnetic conditions, about 165,000 years before the polarity reversal of the base of Anomaly 29, and about 0.7-1.0 m.y. before the extinction of dinosaurs in New Mexico. Significant disturbances in the Earth's rate of rotation and direction of spin axis at the passing from the Cretaceous into the Tertiary are strongly contradicted. The CAT boundary events cannot be explained by a single large catastrophe. A sequence of different events and some gradualism need also to be included in present and future scenarios of the boundary events.

The long-term motion of comet Halley

The orbital motion of comet Halley has been numerically integrated back to 1404 BC. Beginning with an orbit based upon the 1759, 1682 and 1607 observations of comet Halley, our numerical integration was run back in time with full planetary perturbations and non-gravitational forces taken into account at each 0.5 day time-step. The non-gravitational forces were assumed due to the rocket effect of an outgasing water ice nucleus. Small empirical corrections were made to the computed perihelion passage time in 837 and to the osculating orbital eccentricity in AD 800. In nine cases, the perihelion passage times calculated by Kiang from Chinese observations have been redetermined; we have used the unusually accurate observed perihelion passage times in 837, 374 and 141 to constrain the computed motion of the comet. Osculating orbital elements are given at each apparition from 1910 back to 1404 BC. The dynamic model used to compute the long-term motion of comet Halley successfully represented the ancient Chinese observations over nearly two millennia. This model assumed the comet's non-gravitational forces remained constant from one apparition to the next. Hence it seems likely that comet Halley's spin axis direction and ability to outgas has also remained relatively constant with time over its observed interval. ‘You see therefore an agreement of all the Elements in these three, which would be next to a miracle if they were three different Comets… . Wherefore, if according to what we have already said it should return again about the year 1758, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.’ E. HALLEY (1752)

スピン衛星の新しい姿勢決定アルゴリズム

In a conventional method for attitude determination of a spinning satellite by processing so called cone angle data, iterative calculation is used to minimize the weighted squares of the difference between the observed and calculated cone angle. This method requires a considerable amount of calculation. As another conventional method, KALMAN filtering technique is used. This method requires the knowledge of the satellite attitude dynamics.In this paper, a new algorithm for determining the attitude of a spinning satellite is proposed. The proposed algorithm offers a simple method for processing cone angle data of earth sensor and makes calculation program extremely simple and reduces the required CPU time remarkably.A method for estimating wobble angle and phase using sun and earth sensors is also proposed.An example of the results of its application to CS satellite is presented. This example shows that the earth sensor data of 2/3 day period is sufficient to estimate the spin axis direction and sensor bias.

Ultra-low magnetic field apparatus for a cryogenic gyroscope

We have completed the construction of an apparatus for continued Earth-based testing of a gyroscope system to be used in a satellite test of general relativity. The immediate goal is a readout capable of measuring the direction of the gyroscope spin axis to an angular resolution of one arcsecond over a limited range. A combination of SQUID magnetometers and persistent current loops are used to measure the London moment of the spinning superconducting rotor, which is levitated electrostatically. The apparatus makes use of a new magnetic field shielding technique for obtaining large superconductor shielded regions below 10 −7 gauss to obtain a trapped flux signal in the gyroscope sufficiently smaller than the London moment signal. A recently prepared shield provides a gyroscope environment of 2.3 × 10 −7 gauss; a factor of nearly 100 improvement over our previous ability using conventional μ-metal shielding.

A high accuracy gyroscope readout test facility for the relativity gyroscope experiment

We are building an apparatus for Earth-based testing of a gyroscope system to be used in a satellite test of general relativity. The immediate goal is a readout capable of measuring the direction of the gyroscope spin axis to an angular resolution of one arcsecond over a limited range. A combination of SQUID magnetometers and persistent current loops are used to measure the London moment of the spinning superconducting rotor levitated electrostatically. To obtain a trapped flux signal in the gyroscope sufficiently smaller than the London moment signal, the apparatus makes use of a new magnetic field shielding technique for obtaining large superconductor shielded regions below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> gauss.

A high accuracy all-angle gyroscope readout using quantized flux

Means are described to use SQUID magnetometer flux counting and the London moment of a spherical, superconducting gyroscope to read out the gyroscope spin axis direction to an accuracy of at least 23 bits per quadrant. The system is discussed in analogy to optical fringe counting as applied to distance measurement. Several methods of applying both analog and digital SQUID magnetometers to the readout problem are given, as well as limitations on each. Described are two methods of increasing the flux available for measurement: magnetizing the gyroscope with a trapped field, and optimizing readout circuit inductances. Finally, the same principle on which the gyroscope readout is based is applied to a description of a high accuracy, flux counting, digital angle encoder.

Hard X-ray observations near Ariel 1118–61

OBSERVATIONS made by Ariel V in the region of Cen X-3 have revealed a new transient X-ray source with a periodicity of 6.75 min (refs 1 and 2). Tentative identifications are: with a Mira-type variable star by Fabian et al.3, who suggest the 6.75 min periodicity is associated with the rotation period of a compact companion; and with a heavily reddened, variable ultraviolet star4. As well as the proportional counter observations during the period 1974 December 17 to 1975 January 31, the Imperial College crystal scintillator, sensitive to the photon energy range 26 keV to 1.2 MeV, viewed the same region of the sky. This detector has a sensitive area of 8 cm2, an opening angle of 8° FWHM defined by an active collimator and is offset from the spacecraft spin axis by an angle of 3°. A source a few degrees from the direction of the spin axis can be detected by determining the modulation in the counting rate seen in four equal angle spin sectors with a time resolution of 512 s. A single channel (50–190 keV) mode with a 48 s time resolution was also used. Since a spurious modulation is induced in the apparatus by cosmic rays, two pointing direction offsets on opposing sides of the source have to be used, and the X-ray contribution is obtained by a subtraction technique.

Mars physical parameters as determined from Mariner 9 observations of the natural satellites and Doppler tracking

Mariner 9 Doppler tracking data and television photographs of Deimos and Phobos were analyzed to determine the gravity field, mass, and spin-axis direction of Mars and the natural satellite orbits. The solutions agree with previously published results. Radio data consisted of an apoapsis state vector for each of revolutions 5-195 obtained from one-revolution fits of Doppler data. Optical data consisted of TV photographs of Phobos and Deimos taken between revolutions 25 and 221. A first-order analytical theory, extended to include dominant second-order resonance effects on the Mariner 9 orbit, was used to calculate the motion of the spacecraft, Deimos, and Phobos. The feasibility of combining radio and optical data in long-arc solutions for accurate determination of orbits and physical parameters is demonstrated. The analytical theory developed for the evolution of a highly eccentric orbit in shallow resonance is accurate to plus or minus 1 km in the apoapsis state vector of Mariner 9 over a period of 200 revolutions.

Active damping of free-rotor gyroscopes during initial spin-up

The free-rotor gyro is so named because the nearly spherical gyrorotor is not mechanically constrained to rotate about a particular axis. Therefore, it may be initially spun up about any rotor fixed axis. As seen in the rotor, the subsequent motion of the angular velocity vector after spin-up is the well-known classical polhode motion about either the maximum or the minimum principal axes of inertia. The maximum axis of inertia is the desired spin axis direction because it is the only stable free-rotor spin direction in the presence of minute energy losses during flexures of the rotor. Consequently, the rotor-fixed markings used for spin axis readout of many of these gyros are referenced to the maximum axis of inertia. The principle delays in activating typical gyros are 1) waiting for a passive d.c. field to damp the gyro spin axis into the maximum axis of inertia of the rotor after it has been spun up (this requires typically five times the spin-up time) and 2) waiting for the thermal transient (of spin-up and damping) to subside. This paper describes a method of actively controlling the spin axis in the rotor, while leaving it unchanged with respect to a case-fixed reference. A control algorithm, called Hemispheric Torquing is used to actively damp the spin axis to the proper rotor fixed direction. It offers the advantage of being able to reduce damping times to less than is required for initial spin-up.

An Active Magnetic Attitude Control System for a Spinning Sun Oriented Satellite

This paper states the basic relations for the active magnetic orientation of the spin axis of spinning satellites. A magnetic coil around the spin axis is used for the generation of the control torques. The coil current is controlled by a simple logic control circuit, the input of which are the measuring signals of a magnetometer and a sun sensor. The spin axis direction can be controlled only at certain switching points. The present paper shows the example of a special satellite mission providing the use of air coils as the most unfavorable case. In this case the electrical energy consumption is less than 0.08 Wh per orbit.

Determination of the attitude of the Ariel III satellite

The three radio experiments on the Ariel III satellite require a knowledge of spin axis direction in order to deduce the fields measured by their loop aerials. Solar aspect sensors provide the angle between the direction of the spin axis and the Sun, which defines a conical surface containing the spin axis. Some other frame of reference is required in order to find the exact position of the spin axis on this cone and a method was therefore proposed based on that used on Telstar. Six echelon mirrors were placed on the satellite from which glints of solar reflexions could be monitored by visual, photographic or photoelectric techniques on the ground. At the instant when glints are received the position of the satellite is known in relation to the observer and to the direction of the sun, as also is the angle of the mirror to the spin axis of the satellite; a second cone is therefore determined on which the spin axis lies. From the intersections of these two cones the final attitude of the spacecraft is calculated. Any one observation yields two possible directions, but successive observations enable the true direction to be determined. Excellent results have been obtained and the spin axis direction has been monitored to an accuracy of better than 2°. Results from a photoelectric camera have enabled the coning angle, or angle between dynamic and geometric axes, to be determined to better than 0.1°.

Theoretical considerations in the determination of the attitude of Ariel III by optical means

Observations of the glints of sunlight reflected from surfaces on the Ariel III spacecraft are used to determine the satellite’s spin-axis direction. This paper describes the methods of analysing the observations.

Attitude determination of the Ariel III satellite

ARIEL III, the third UK/US satellite, was successfully placed into orbit on May 5, 1967, with apogee and perigee approximately 600 km and 500 km, respectively, and inclination 80°. It had been suggested some time before launch that the spin axis direction required for the interpretation of data from some of the experiments could be monitored by observing fluctuations in the telemetry AGC recordings as the satellite rotated; this method had been adopted for the Ariel II satellite1, but the analysis proved difficult and because the telemetry polar diagrams for Ariel III were found to be nearly circular this technique was regarded as unpromising. An alternative method was therefore proposed similar to that used on Telstar2—namely, observing the glints of solar reflexions from the solar cell panels and from special mirrors placed on the spacecraft. Solar aspect sensors already included in the payload would determine the angle between the spin axis of the satellite and the direction of the Sun, thus defining a conical surface containing the spin axis. To position the spin axis on this cone another frame of reference would be required and the mirror glints would define this. The method was adopted successfully for Ariel III, and spin axis direction to better than ±2° has been calculated together with coning angle and spin rate.

Attitude Determination of the Ariel III Satellite

ARIEL III, the third UK/US satellite, was successfully placed into orbit on May 5, 1967, with apogee and perigee approximately 600 km and 500 km, respectively, and inclination 80°. It had been suggested some time before launch that the spin axis direction required for the interpretation of data from some of the experiments could be monitored by observing fluctuations in the telemetry AGC recordings as the satellite rotated; this method had been adopted for the Ariel II satellite1, but the analysis proved difficult and because the telemetry polar diagrams for Ariel III were found to be nearly circular this technique was regarded as unpromising. An alternative method was therefore proposed similar to that used on Telstar2—namely, observing the glints of solar reflexions from the solar cell panels and from special mirrors placed on the spacecraft. Solar aspect sensors already included in the payload would determine the angle between the spin axis of the satellite and the direction of the Sun, thus defining a conical surface containing the spin axis. To position the spin axis on this cone another frame of reference would be required and the mirror glints would define this. The method was adopted successfully for Ariel III, and spin axis direction to better than ±2° has been calculated together with coning angle and spin rate.