GPS World - April 2010 - (Page 6)
OUT IN FRONT
What’s in a Number?
omputers killed a trusty companion of my teenage years. That is, after those proto-computers known as pocket calculators knocked him out and left him unconscious on the cooling floor. But I come to praise my slide rule, not to bury him. I marveled at the way he worked. You had a tactile relationship with numbers on a slide rule. You could see — and feel — how a small adjustment here effected a big change over there. With computers, it’s just numbers in, numbers out. Maybe that high-tech approach led both the GPS Wing and the Government Accountability Office into trouble with constellation gaps. GPS satellites have proven themselves very hardy in space, outlasting their
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Trusty companion of my teendom.
predicted lifetimes. The GPS Wing has grown to lean on those longer lives a bit, and what with Congress and the Administration booting budgets a year or two to the right with addictive regularity, the Air Force has saved money by replenishing upon need. And need has been not all that great, so replenishment, and the contract awards and manufacturing that feed the replenishing line, have been allowed to relax. But not the mathematical models
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that someone has held to more conservative standards. Those models use the shorter predicted satellite lifetimes. When those models were projected against the real-world timelines for IIF and Block III — whoa GAO! Some black gaps suddenly yawned. Now we learn that GAO and the Wing will re-undertake this exercise, factoring instead the longer lifetimes that the satellites have proved capable of. Tinker a small adjustment here, see a big change out there. Speaking of numbers, I’ve grown fond of 20, and lately enamored of 200. The former being the number of years we have published this magazine, the latter the new world record for GNSS technical articles, attained by one Richard B. Langley. With characteristic Canadian unbravura, Langley fidgets and frets that we have made too much of him on this magazine’s cover and page 42. It looks too braggy for him and he feels uncomfortable with it. But I have prevailed upon him to swallow his humility, to take one for the team. We bask in his reflected glory. Quick, what’s the difference between 160 and 144.5? Not in absolute terms, but in tactical advantage. If I add a metric, east longitude, geosynchronous orbit, does that help? I’m puzzling out why Compass would move its G1 satellite from one location to another after only ten days in space. Better ground control might be the answer. But more mystifying, why China’s spokespersons at the Munich Summit would proffer the first location, when they must know very well — in fact, they so admitted when I confronted them with it — that the second is actually the case. Numbers don’t obfuscate. People do.
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GPS World | April 2010
Table of Contents for the Digital Edition of GPS World - April 2010
GPS - April 2010
Out in Front
Jamming: A Clear and Present Danger
Galileo Test User Receiver
GPS by the Numbers
Lost Graves, Trail of Tears
GPS World - April 2010