The effects of orbital precession on remote climate monitoring

The effect of the diurnal cycle when monitoring the climate from low earth orbit is examined briefly. Equations are derived that relate the harmonics of the diurnal cycle to temporal sampling error and drift rates in that err or. Special attention is given to nodal precession of satellite orbits. Usi ng an insolated blackbody as a simple model for the diurnal cycle, roughly simulating subtropical desert surface temperature, the effects of orbital p recession are examined numerically. From an initial configuration, wherein satellites are evenly spaced in nodal crossing time, minor differences in p recession rates lead to biases proportional to the amplitude of the semidiu rnal cycle and inversely to the square root of the number of satellites. Ov erall biases for a single mission can be dramatically reduced by flying in a formation wherein the satellites' orbits are evenly distributed in their equator-crossing times. To monitor surface temperature, it is suggested tha t at least six satellites be flown in formation and that their precession r ates be controlled to well within 25 min. The tolerance for monitoring any other variable can be scaled according to the size of its semidiurnal cycle .