High-Resolution Image

An unexpected characteristic of the observational pattern of the Tropical Rainfall Measuring Mission (TRMM) satellite was discovered after it was launched: the satellite's radar, even after a year of orbiting, missed certain points on the earth at specific hours of the day, but could observe them at times just a few hours before or after the prohibited time. The top panel of the image, taken from Negri et al. (2002), shows a map of the number of times the TRMM radar saw each point of an area within Brazil, during a year of orbiting, as a function of the local time (LT) at that point. For instance, the top-leftmost map labeled "00 LT" shows the visit count between midnight and 1 AM local time. No observations at all were made in the white patches at the center of the blue areas. Two hours later, however, at "02 LT", the unobserved patches are well observed. This might matter in areas where rainfall statistics change a lot with the time of day, such as can happen in coastal areas or over continental areas during the summer, because the satellite could miss the peak rainfall period by a few hours.

This peculiarity of TRMM's observational pattern is extraordinarily sensitive to the satellite's altitude. The two panels at the bottom of the image show the visit counts for a simulated satellite like TRMM, with the satellite at TRMM's actual altitude (350 km) on the left, and at an altitude just 0.3 km lower on the right. (Note that the map shows an area much larger than the maps in the upper image.) This tiny change in the altitude erases the gaps!

Finding the right orbit for an Earth-observing satellite is often a tough balancing act. It has to be close enough to the Earth to see "clearly", and far enough away to get as much coverage as possible. The changes in the illumination of the Earth by the sun at the times the satellite will observe—day vs. night, for example—are an important consideration. For satellites like TRMM, the subtle interaction of the Earth's period of rotation with the satellite's orbit has to be taken into account as well.

By the way, TRMM's orbit altitude was increased to 402.5 km in August, 2001, to save fuel and extend TRMM's life, and the gaps are no longer present at its new altitude.

A thorough description of the phenomenon can be found in the paper by Negri et al., 2002: Sampling of the diurnal cycle of precipitation using TRMM. J. Atmos. Oceanic Tech., 19, 1333–1344.


(Submitted by T. L. Bell.)