Orbital parameters for Ariel 4 and Prospero have been determined at the Royal Aircraft Establishment and made available for use with the respective telemetry analysis programs. Ariel 4 orbit determinations were based on N.A.S.A. Minitrack observations, and Prospero orbit determinations on U.S. Navy observations and a small number of visual observations. Both orbits are near polar (inclination 83 degrees for Ariel 4 and 82 degrees for Prospero) but not otherwise similar. The initial perigee and apogee heights were, respectively, 500 and 600 km for Ariel 4, as against 550 and 1600 km for Prospero. Hence Ariel 4 has experienced much more drag than Prospero and orbital parameters had to be determined at much closer intervals for the former than for the latter, 3 days as against 7 days. The Ariel 4 orbit is being analysed to study the effects of 15th-order tesseral harmonics in the Earth's gravitational field, and the Prospero orbit has been used in a recent determination of odd zonal harmonics.