Schmidt still scanning the skies 50 years after defining the quasar Fifty years ago this Friday Dutch astronomer Maarten Schmidt revolutionized his field with a paper on quasars, the energy sources that now act as terrestrial guide points in our exploration of the cosmos. His initial 1963 paper was disputed by some of the biggest names in astronomy, got him on the cover of Time magazine ( …
We read:
Because quasars are so far away, they are incredibly stable in their positioning, and as some of the brightest objects in the sky they are easily recognizable.
Hold on, what!
The brightest quasars have magnitude above 14 (i.e. are weaker than 14). THAT'S NOT BRIGHT, LIKE AT ALL. Pluto is 14.
I have heard the bizarre "GPS sats track by quasar" story earlier, but I frankly could not believe it. It's very much far easier to track by bright star, and update the map every 20 years than to carry supertelescopes on those satellites to detect random points at magnitude 14 lost in the haze of galaxies.
Do they track by RADIO BRIGHTNESS? I haven't seen a radio telescope on GPS sats either.
Sounds like an urban legend to me.
But your orbit will be subject to random errors as time progresses. So you have to correct it via on-board methods. Maybe ground control uses the plotted position of the satellite against the quasar map to tell the satellite how to fire thrusters. Maybe. But I would like to see a complete explanation of how that is supposed to work.
"quasars have magnitude above 14". You are thinking about visible light, in radio astronomy about 10% of quasars sit well above the noise floor of a modest radio telescope.
The GPS systems don't use quasars directly. Rather quasars are the reference points for surveying the earth's position in space (you use two interconnected radio telescopes half the world apart to form a baseline and then measure the different times of arrival of the quasar's signal, triangulating the position of earth in space, the jargon word is "eVLBI geodesy").
This field of research was very interesting to the US Air Force during the Cold War as it was directly relevant to the accuracy of their ICBMs. That work continues to be used to accurately place GPS and surveillance satellites, which are flip sides of the same triangulation problem.
>US Air Force during the Cold War as it was directly relevant to the accuracy of their ICBMs.
Of course one of the rare examples they tout of why its a good idea for us (the US) to spend more than but a handful of countries entire GDP on weapons and corporate death merchants for "defense". Even the sun shines on a dog's balls occasionally.
Well, it seems Quasar-Based Earth Attitude Tracking was set up in the early 90's, so clearly it wasn't used for the Cold War.
What was used for the cold war was pretty good integrators on ICBMs (integrate over the acceleration to find out your current speed, integrate over speed to find out where you are). An upgraded V2, basically. Even GPS tracking came later, with Cruise Missiles in the Gulf War 1 (earliest). Prior to that, Cruise Missiles had integrated terrain recognition systems.
Even the Minuteman III was "inertially guided":
http://www.fas.org/nuke/guide/usa/icbm/lgm-30_3.htm
Even GPS tracking came later, with Cruise Missiles in the Gulf War 1 (earliest).
They were still using terrain guidance on Cruise then, at the very least for final guidance onto target.
I remember a hack who was in Baghdad saying he was standing at his hotel window and a Cruise Missile went past the window, following the street. He said it turned left at the junction down the way and then obliterated the TV centre round the corner.
He also said that he almost expected it to stop, as the lights at the junction were red.
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