Sunday, June 19, 2011

Space Weather

It was March 13, 1989, in Utica, NY. My apartment was rather spare but it had a nice view North across the Mohawk Valley. There was plenty of privacy - the nearest prying eyes would have been 15 miles away - so I often left the shades up to sleep.

That night I had to close the shades. My bedroom was fully lit, but not with a steady glow. The greenish light danced and leapt around the room; it might have kept me up all night. Looking out the window revealed the most intense aurora borealis I have ever seen.

I had it lucky. The province of Québec lost all electrical power that night. The solar storm that caused the aurora also disabled the power grid.

That was my introduction to space weather, the study of the bands of charged particles that surround the Earth (the ionosphere), which are just as intricate and fascinating as the atmosphere. Its nature wasn't really understood until 1958 when the Explorer satellites made in situ observations. Prior to that, we knew that something caused the auroras at the poles. Admiral Byrd in his book Alone talked about measuring the aurora australis by triangulating. He spent the austral winter alone in a cabin (where he nearly died from CO poisoning), making observations, which were later compared with observations made at Little America, his Antarctic base.

A recent article in the New York Times was surprisingly shallow, except in making pilots look bad. To quote from the article:

"...a space weather forecaster we know at the National Oceanic and Atmospheric Administration often tells a story of a conversation he had with a pilot:

"Pilot: 'What do you do for a living?'

"Forecaster: 'I forecast space weather.'

"Pilot: 'Really? What’s that?'”



So, why should pilots care about space weather? Start with the South Atlantic Anomaly (SAA, a permanent feature). NASA pays close attention to the SAA, to the point that it is prominently displayed on the screens at Mission Control. For reasons that I do not quite understand, the Van Allen radiation belts are lower in this region, exposing orbiting spacecraft (and their occupants) to more radiation. This disrupts communications; actually, it disrupts anything electrical. (What I don't understand is, given the Earth's rotation, why this does not occur at every longitude.)

Wish as we might, most of us aren't astronauts. But every time you press DIRECT on a GPS you are a rocket scientist. Well, maybe not you, but there are bunch of rocket scientists in the picture.

The problem is that the GPS signals have to go through the ionosphere, which slows them down. Remember that GPS receivers calculate a pseudorange to each satellite, putting you on a gigantic sphere, and the intersection of four of these spheres defines your position in space. Then, using the WGS-84 model of the Earth (or one of its refinements), the receiver calculates your latitude and longitude.

The GPS position calculation assumes that the signals travel at the speed of light, and, knowing the position of the satellites from the ephemeris, determines which sphere your on. Originally, the system used two ionospheric corrections each day, but that was too crude. Here's the space weather: ionospheric storms that can really slow the GPS signal can last as little as fifteen minutes! So a twice-daily correction didn't cut it.

If your receiver can use two frequencies it can estimate the ionospheric delay, but older civil receivers only had access to one frequency. (Many of these are still in use.) So, again we were out of luck.

A lot of smart people put a lot of work into figuring out how to monitor this and other effects on the GPS "Signal In Space", and the final result is WAAS, the Wide Area Augmentation System. WAAS monitors the GPS signals at many locations, calculates corrections, and broadcasts them from a geosynchronous satellite. Most new GPS receivers are WAAS capable, and this leads to unbelievable accuracy and reliability.

We are approaching the peak of the sunspot cycle, although the Sun has been uncharacteristically quiet the past couple of years. More sunspots means more interactions with the ionosphere and more chances for a massive GPS outage.

Check out Spaceweather.com if you'd like to learn more.

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