1. INTRODUCTION

Since polar orbiting Sun-Synchronous satellites like TERRA pass the equator at approximately the same local time each orbit, the local Mean Solar Times computed for TRMM can be used to identify overflights at any location that will provide roughly coincident observations with these other satellites.

For any selected site for TRMM, the local times of overflights observed over the site will roughly cycle over a 24 hour period every 46 days. Thus there are a few days each month and a half with opportunities for close coincidences. These opportunities occur on both sides of the sun synchornous satellite orbit path, e.g. on both the day side and night side.

2. TERRA OVERFLIGHTS -- COINCIDENCE CRITERIA FOR PASSING WITHIN AN ORBIT IN TIME

For TERRA the descending node is at about 10:30 a.m. (for the southbound equator crossing) and the asscending node is at about 10:30 p.m. (22:30) (for the northbound equator crossing).

Since the Sun Synchronous orbit inclination is about 98 degrees,there is some local time adjustment for locations north and south of the equator, but that adjustment is relatively small for the TRMM range of latitudes, +/- 35 degrees. This correction is less than about 20 minutes.

Thus to find TERRA coincidences, one can search for the days when the local solar time at your selected latitude is between 10:00 and 11:00 local mean solar time for daytime overflights, and search for the days when the local solar time is between 22:00 and 23:00 for for nightime overflights at your selected site.

This will give coincidence events where TERRA and TRMM pass the same location within one half orbit in time (within 50 minutes). Only for some of these passes will TERRA and TRMM fly over the same location within a few minutes. The pattern in the relative timing of passing a given location is discussed in the next section.

3. TERRA OVERFLIGHTS -- CLOSE TIME COINCIDENCES

Because of the different orbit periods --about 91 minutes for TRMM and 99 minutes for TERRA-- the time delay between the ground track (subsatellite path) intersection crossings varies between 0 and 45 minutes. This changes about 8 minutes each orbit (based on the time difference in the orbits), so that the closest time approaches reoccur about every 12 TRMM orbits, or about every 18 hours.

4. MORE INFORMATION

More notes about the coincidence patterns between TRMM and TERRA overflights will be added at this site at a later date.

Questions about TRMM data collection coincidence with TERRA and other satellites can be directed to Steve Bilanow at