Why space matters: Imaging satellite operations, part 8–synchronizing flying pumpkins and suns

Sorry, this isn’t about a tribute band for 90’s alternative/grunge music, at least not “Today.”  This is all about polar orbits and polar sun-synchronous orbits.

But before I get into those topics, let me warn you that if you delve too far into this, understand the math involved, the terms used, and worst of all worlds—enjoy it–you risk becoming pariah.  It’s one of the reasons I chose not to be as smart as some of you readers—I couldn’t risk becoming a giant among men—it’d be socially irresponsible.  So once you’re done reading and you understand all the references, maths, pictures, my writing, etc., go have a beer and kill some brain cells.  For the “normals,” what I’m about to write might be a little complicated.  I sometimes have trouble with it myself and the funky symbols certainly don’t help.

Polar orbits are cooler than cucumber daiquiris and perhaps more useful (depending on your need).  With one imaging satellite, you can take a boatloads (payloads?) of pictures and eventually get a portrait of the whole planet’s surface.  But how do you guarantee that the location on the surface will be in sunlight at a particular local time of day?  You put the satellite into a polar sun-synchronous orbit.

Both Digitalglobe and RapidEye use this orbit for their satellites (there’re, as of this writing, five satellites per company, remember).  Even though the orbit is close to polar, it’s not quite there—more like a little past polar.  For RapidEye and Digitalglobe both, the inclination is listed as around 97 (97.8 for RapidEye and 97.2 for Digitalglobe) degrees.  Sun-synchronous orbits can be as low as 96.5 degrees from the Earth’s equator, to as high as 102.5 (JPL paper page 1 Intro, para 2).  Why would they choose such weird inclinations for their satellites?

Well, one reason both companies are so inclined (see what I did there?) is because what I told you two paragraphs ago:  to ensure the satellites will always cross the same latitude (the line that parallels the Earth’s equator) at the same local time for that part of the planet.  If you’re trying to take atmospheric measurements, document changes on the Earth’s surface, etc., this sort of orbital consistency helps take a variable or two out of the imagery equation.

Another reason to use these inclinations is because of the Earth’s shape.  Most studied space scientists now agree the Earth is shaped more like a squashed pumpkin (oblate spheroid) rather than an orange (sphere).  So there is a little extra mass along the Earth’s equatorial area (the Earth has a beer belly!).

This extra mass causes some interesting influences to a satellite’s orbit, especially one inclined between 96.5 and 102.5 degrees.  Remember, most polar low earth orbiting satellites are moving in the direction of the Earth’s rotation, and they are zipping over the Earth’s surface, completing one orbit, or period, between 90 to 120 minutes.  But, their orbit stays in place, relative to the Earth’s rotation.  Sun-synchronous low earth orbiting polar satellite orbital planes are actually moving slightly BACKWARDS relative to the Earth’s rotation.

So, why is this helpful to satellite imagery operators?  Okay smart guy (gal?), before you answer, go have a beer and get back to me later.