Thu October 11, 2012
TheEC: Solar Outages
By Aaron Read
Providence, RI – This week on the Engineer's Corner we talk SOLAR OUTAGES. I know, it sounds like the next (terrible) Michael Bay movie but it's really far more benign.
It has to do with our satellite downlink from NPR. We have a hefty 13-foot-diameter satellite dish, located in North Providence; there's no room for it at One Union Station! It points to "Galaxy 16," a telecommunications satellite in "geostationary" orbit that all NPR stations use.
"Geostationary," also popularly referred to as "geosynchronous," means that the satellite orbits the Earth in sync with the Earth's rotation...about 6800 MPH. That's pretty fast, but the key is that it's the SAME speed for both. So from our perspective here on Earth, the satellite just floats there, not moving, over 22,000 miles up in the sky! In Galaxy 16's case, the satellite is at 99.0 degrees west longitude, meaning it's fixed above a point on the equator over the Pacific Ocean, about 500 miles west of the Galapagos Islands (near Ecuador).
In general, geostationary orbits work great for communications satellites like for NPR, but there's a catch: twice a year there are a few days when, the Sun appears to pass right "behind" the satellite for a few minutes each day. The Sun puts out so much energy on ALL frequencies that it completely swamps the satellite's own signal, so we lose all NPR, BBC and other satellite programming.
To put it another way, imagine it's nighttime and someone 100ft away shines a handheld flashlight at you. You can see the flashlight just fine, right? Now, imagine that a bit behind the guy with the flashlight, there's another guy who fires up one of those giant 20ft-wide super-bright searchlights you see in movies. That searchlight is so blindingly bright you can't possibly see the flashlight anymore, even though it's still there.
The actual dates/times of the outages vary depending on your latitude and longitude. For RIPR they happen around 2:00pm each day from March 1st through the 4th, and around 2:36pm each day from October 6th through the 10th, each time for 5 or 10 minutes. Why only those dates? For the same reason we have seasons! The tilt of the Earth changes the relative angle of the Sun as the planet revolves around it.
Fortunately there's an easy solution to solar outages: teamwork! NPR stations in different parts of the country have different longitudes, and thus outages at different times. So, for example, West coast stations can partner with East coast stations, each providing the other with a program feed during their respective outages. In RIPR's case, thanks to our history with WBUR, we get our feed of "Talk of the Nation" from them and they get their feed on a special ISDN line (high-fidelity digital telephone lines) from NPR in Washington.
So if things go right, you'll never know the difference. But now you know a little more about solar and orbital dynamics...thanks to solar outages!