Earth Planets Space, Vol. 50 (No. 3), pp. 229-234, 1998
Hiroshi Oya and Takayuki Ono
Department of Astronomy andGeophysics, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan
(ReceivedSeptember 24, 1997; RevisedDecember 15, 1997; AcceptedJanuary 12, 1998)
Abstract: A new altimeter has been developed for the observation of the land shape of a planet from an orbiting spacecraft by transmitting RF pulses. The present system utilizes an RF sounder for observing the electron density profile of the topside ionosphere by selecting a transmission frequency fhigher than the local cutoff frequency of the ionosphere by adding upper and lower side band modulations with a frequency p; i.e. at frequencies f+ pand f- p. The phase difference of the propagating signals at the upper and lower side band frequencies gives a fine delay time with a range resolution of 100m for p= 50 kHz and 20m for p= 250 kHz with f= 9 MHz corresponding to a resolution of 6° of the phase detector of the system onboard the spacecraft. Using 600 Watts transmitting power applied to a 52m tip-to-tip dipole antenna, the altitude of the spacecraft and the land shape can be measured while the spacecraft is in the range below 300km altitude at the planet.