The Effects of Orbital Precession on Remote Climate Monitoring

Abstract

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 error. Special attention is given to nodal precession of satellite orbits. Using an insolated blackbody as a simple model for the diurnal cycle, roughly simulating subtropical desert surface temperature, the effects of orbital precession are examined numerically. From an initial configuration, wherein satellites are evenly spaced in nodal crossing time, minor differences in precession rates lead to biases proportional to the amplitude of the semidiurnal cycle and inversely to the square root of the number of satellites. Overall 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 that at least six satellites be flown in formation and that their precession rates 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.