Individual geopotential harmonic coefficients of order 15 from 30 resonant satellite orbit analyses

The results from 30 15th-order resonant satellite orbit analyses are used to determine individual tesseral harmonics of order 15 in the Earth 's geopotential. The equations are solved by a modified least-squares method to determine 11 C and nine S tesseral harmonic coefficients of odd degree and 11 C, S pairs of even degree. For degree 15 to 23 the average standard deviation is 9 x 10 -10 , corresponding to an error of 6 mm in geoid height. The resonance values thus provide a strict independent standard of comparison for comprehensive geopotential models. The models emerge well from such tests, and the most recent models, GRIM4-C1, GEM-T3 and JGM -1, are closest to the resonance values.

Critical Remarks on the Possibility of Determining Variations of the Geocenter Position Using Geostationary Satellite Observations

The possibility of determining the variations of the Earth’s cejiter (expressed in terms of the geopotential harmonic coefficients , , or of some functions of them), using geostationary satellites, was discussed by Burŝa and Ŝíma (1978) after discerning the difference between observed and theoretical positions of libration points. Although their effect on geostationary orbits cannot be expected to be of high magnitude, the question of determination of these terms arose. It was suggested that the “resonant phenomenon” observed here might be analogous to the resonant effects in close satellite orbits, because the satellite is at the 1/1-resonance (1 nodal revolution per 1 sidereal day).

Cosmos 462 (1971-106A): Orbit determination and analysis

Cosmos 462 (1971-106A) was launched on 3 December 1971 into an orbit inclined at 65.75° to the equator, with a perigee height of 230 km and apogee height of 1800 km. The satellite remained in orbit for 40 months and decayed on 4 April 1975. Orbital parameters have been determined at 85 epochs by using the R.A.E. orbit refinement program PROP, with 6635 radar and optical observations, including 197 from the Hewitt cameras. The average standard deviation in eccentricity and inclination corresponded to a positional accuracy of about 100 m. In addition, orbits of similar accuracy were determined daily for the last 15 days of the life, from 2000 NORAD observations. During its slow decay, the orbit passed through 14:1,29:2 and 15:1 resonances with the Earth’s gravitational field. The variations in inclination and eccentricity at these resonances have been analysed in detail to evaluate lumped geopotential harmonic coefficients of order 14, 29 and 15. The variation of inclination between resonances has been analysed to obtain four values of the average atmospheric rotation rate at heights of 200-250 km in 1972-5. The values of A show a seasonal dependence, being greater in winter than in summer, and the average rotation rate is lower than in the 1960s, being near 1.0 rev/day. Analysis of the inclination in the last 15 days of the satellite’s life indicates a weak west-to-east wind at high latitude (54-62° N). The variation of perigee height has been analysed to obtain 24 values of density scale height H , including eight in the last 15 days. Comparison with values from CIRA 1972 shows a bias difference of only 1 % and r.m.s. difference of 10%, so CIRA 1972 provides a good approximation to the values of H in 1972-5.