Analysis of 27 satellite orbits to determine odd zonal harmonics in the geopotential

Abstract

The odd zonal harmonics in the geopotential are the terms independent of longitude and antisymmetric about the Equator: they define the ‘pear-shape’ effect. The coeffecients J 3, J 5, J 7,…of these harmonics have been evaluated by analysing the variations in eccentricity of 27 orbits covering wide range of inclinations. We use again most of the orbits from our previous (1969) evaluations, but we now have the advantage of 3 accurate orbits at inclinations between 60° and 66°, where the variations in eccentricity become very large, and 3 near-equatorial orbits, at inclinations between 3° and 15°, whereas previously there were none at inclinations lower than 28°. The new data lead to much more accurate and reliable values for the coeffecients. Our recommended set, which terminates at J 17, is 10 9J 3 = −2531 ± 7 10 9J 11 = 159 ± 16 J 5 = −246 ± 9 J 13 = −131 ± 22 J 7 = −326 ± 11 J 15 = −26 ±24 J 9 = −94 ± 12 J 17 = −258 ± 19 . With this new set of values the pear-shape tendency of the Earth amounts to 44.7 m at the poles, instead of the previous 40 m, though the new geoid is within 1 m of the old at latitudes away from the poles.