Crossing Times

Polar Orbits

A polar-orbiting satellite is placed in a circular sun-synchronous orbit, typically at a low altitude of 700 to 800 km. It usually takes about 100 minutes to make one trip around the earth, allowing for just over 14 orbits daily. These satellites cross the equator at the same local solar time each day, once ascending (traveling from south to north) and once descending.  Sun-synchronous orbits are often described by their equatorial crossing times. The equatorial crossing times remain nearly constant throughout the year. However, orbit degradation can cause a slow change in the value over time.

 In the sections below, we provide crossing time information for all the satellites we work with. We provide data from the DMSP polar orbiters hosting the SSM/I instruments and the QuikScat platform hosting the SeaWinds scatterometer,as well as data from WindSat and mirowave sounders hosted on NOAA, EUMETSAT and NASA platforms.  To keep the plots from being overly complex, we divide the satellites into two classes, those that provide wind and SST data (imaging radiometers and scatterometers) and those that provide atmospheric temperature data (microwave sounders).  The TRMM platform, which hosts the TMI instrument and the GPM platform which hosts the GMI instrument are two special cases.  Both of these satellites, TRMM and GPM, are in near-equatorial orbits, and thus does not have fixed equatorial crossing times.

Imaging Radiometers and Scatterometers (SSM/I, SSMIS, AMSR-E, AMSR2, QuikScat, WindSat)

Listed in the table below are the ascending and descending equatorial crossing times for each currently functioning instrument on our web page. The plot shows the change over time of the ascending equatorial crossing times for all instruments. Some satellite orbits are more stable than others and little change occurs over the years of operation.

Ascending Local Equator Crossing Times: F10, F11, F13, F14, F15, F16, F17, AMSR-E, AMSR2, WindSat
Descending Local Equator Crossing Times: F08, QuikSCAT (dashed lines)


Microwave Sounders (MSU/AMSU on NOAA, EUMETSAT, and NASA platforms)

In the figure below, we plot the local ascending node crossing time for the microwave sounding instruments that we use in our upper air temperature products.  The local crossing time is important for sounding measurements because they generally need to be adjusted to account for changes in local observing time.  Because the AQUA and METOP-A satellites are in controlled orbits, their crossing times do not drift substantially.

Near Equatorial Orbits (TMI and GMI)

The TMI and GMI instruments have near-equatorial orbits. This type of orbit crosses the equator at varying times throughout the day.