High-Resolution Image

There are many satellites orbiting the earth, in very different orbits. For those satellites making similar or complementary observations (of rainfall, for instance), it is valuable to be able to compare their measurements when the satellites are viewing the same spot on the earth at the same time. The figure gives an example of the frequency and locations of co-observations by two satellites similar to ones currently in orbit. It shows where on the Earth (latitude and longitude) the two satellites visit within 15 minutes of each other during one year. In this example, intercomparisons over much of the globe turn out to be impossible. (Animation with varying altitudes described below.)

Simultaneous observations can also make calibrating one satellite against another much easier. This is an important feature of a planned constellation of satellites designed to improve precipitation measurement, the Global Precipitation Measurement (GPM) Mission. The ÒcoreÓ GPM satellite will carry an exceptional panoply of instruments that can be used to validate and improve rain estimates made by the other less well endowed (but cheaper) satellites in the constellation.

The Tropical Rainfall Measuring Mission (TRMM) satellite orbits the earth at 35¡ inclination relative to the Equator. The Aqua satellite is in a polar orbit with inclination 98.2¡. Both can measure rainfall, but TRMM carries a radar and its measurements are believed to be more accurate. Comparisons between TRMM and Aqua estimates of rainfall are therefore valuable. The plot in the figure is from a simulation for TRMM at a hypothetical altitude of 410 km and Aqua at its current altitude. (Since the TRMM satellite orbit does not take it to latitudes north of 35¡N or south of 35¡S, no co-observations poleward of 35¡ are possible.) The figure indicates that if the TRMM satellite orbited at this altitude, large areas of the earth would never be overflown by both satellites simultaneously. TRMM at present orbits the earth at an altitude of 402.5 km. It was raised to this orbit from an altitude of 350 km in August 2001 in order to extend its life.

The co-observation pattern depends on the altitudes of the two satellites. An animation shows the remarkable changes in the pattern with the altitude (340Ð420 km) of the TRMM-like satellite. Making sure that the GPM core satellite cooperates well with the other satellites in the constellation is one of many factors that must be considered in choosing its orbit.

The Aqua satellite is part of a constellation of satellites, the ÒA-TrainÓ, that solves the co-observation problem by placing all the satellites in exactly the same orbit, lined up like the cars of a train.

(Submitted by T. L. Bell; simulations generated using Mathematica^¨.)