VLSI%DEC-MARLBORO@sri-unix.UUCP (01/28/84)
From: John Redford <VLSI at DEC-MARLBORO>
So Gerard O'Neill is going to put up his own communications/navigation
satellite. I find this sort of idea really exciting. Information is the
one commodity that we know can be produced in space. Advanced materials
processing may or may not work, and solar power satellites may or may
not be a economical, but space info is already a billion dollar industry.
But if O'Neill's system can actually let you find your position to within
meters, then the government may not let him build it. The DoD is already
building an elegant navigation system, the Global Positioning System or Navstar,
and wants to keep it to itself. They figure that if they can find their way
around with it then so can the enemy. There's a good description of the system
in the October '83 Proceedings of the IEEE. The original proposal for GPS had
several classes of equipment. There were to be expensive receivers with accuracies
of under ten meters and cheap backpack units with accuracies of a hundred
meters. Then when they actually tested the equipment they found that the
backpack guys were good to forty meters. There was rejoicing among the
engineers, but the military became worried. The portable units can be built
and used by anyone. There's less advantage to knowing where you are if your
enemy also knows where he is. They decided to degrade the accuracy of the
signals that the portable units used, and to encrypt the signals for the
accurate receivers. Military receivers get the key to the encryption and
civilian ones don't. The cheap receiver's accuracy dropped to 200 m, although
it's still apparently possible to get it down to 100 m.
A hundred meters isn't bad, but it isn't good either. Ships and
airplanes could still use it, but it doesn't seem quite accurate
enough to be useful for cars or hikers. There's also no guarantee that they
won't degrade it again. The receivers could be produced
for only a couple of hundred dollars and could be shrunk to handheld
units, but only if they are built in volume. Without better accuracy the
volume applications aren't there.
It makes you sick at heart. With a system like this no one need ever be
lost anywhere in the world. In rain or in snow, in Oklahoma or Antarctica, you
could always find out where you were. But the people with the purse strings
decided that it was more important to hurt the Russians than to help civilians.
If O'Neill wants to build his own system, then more power to
him. Anyone want to lay bets, though, that the FCC will be pressured not to
permit it?
John Redford
DEC-Hudson
--------REM%MIT-MC@sri-unix.UUCP (01/29/84)
From: Robert Elton Maas <REM @ MIT-MC> Let me step back and analyze this system from an abstract viewpoint. Why would anybody want to know their position (relative to a coodinate system fixed on the Earth; as we all know absolute position is either nonsense or useless as the Earth spins thru space) anyway?? To establish rendezvous with some desired target (a fixed object or somebody else who you're trying to meet), to prevent rendezvous with some undesired object (a hazardous area), or to estimate distance between the current location and a potential target to determine feasibility of rendezvous, or to find the set of all potential targets within some prescribed radius. Following a route on a computerized map is merely a combination of using the navigation system to get a direction vector to achieve approximate rendezvous with the next map target, and using local feedback to avoid local obstacles and to improve the proximity of rendezvous, this procedure being followed over and over for different intermediary rendezvous points along a route. Can anybody think of any other fundamental use for a locator system? In the woods/mountains, getting within a few hundred meters should be sufficient. When your two locators say you are "at the same location" but you still can't see each other, you shout or fire a gunshot or use a local radiobeacon or radio-describe landmarks. In a crowded city you need more accurate information, both because normal city noises tend to drown out your shouts and the like, and because gunshots tend to disturb the residents and the police. It would be nice if a single system could handle both country and city, but perhaps cellular radio in cities will do a better locator job anyway, as well as provide other services that are too compute-intensive for the satellite to handle (the satellite would have to handle a whole world's load whereas the corner radio-cell-transceiver would have to hande only one square block's worth of load, about 9 orders of magnitude less), so maybe a dual city/country system is inevitable and having the satellite accuracy be insufficient to handle crowded cities is a minor inconvenience?
flinn@seismo.UUCP (E. A. Flinn) (01/29/84)
Two potentially important uses for GPS are in civil surveying and in geophysical research. Texas Instruments and a small company in Massachusetts have developed and are already beginning to market backpack units which can determine position to a few centimeters in a geocentric coordinate system (i.e., the coordinate system defined by the GPS satellite orbits), although they do not work in real time. These units are not much more expensive than the usual doppler geoceiver rigs, which are accurate to a meter or so. NASA is also developing small GPS receivers for two purposes: (1) to locate altimeter satellites to a few centimeters (the altimeters have 2-cm accuracy and require similar knowledge of orbital position), and to monitor crustal deformation and movement of the tectonic plates. The NASA and competing units are now being field-tested in California, and there should be papers given at the next meeting of the American Geophysical Union on the results of this comparison study. For further information on geophysical applications of this kind of space technology, see an article I wrote for Science in June 1981, and the references cited there.
bill@utastro.UUCP (William H. Jefferys) (01/30/84)
Ther was an article in last week's or this week's Science which
said that the Eurpoean Space Agency (ESA) was planning to orbit
its own set of satellites, similar to NAVSTAR (I believe it was
called "NAVSAT"), which would provide the same accuracy, unencrypted,
that NAVSTAR provides to military users. If this is true, it may
not matter what the U. S. government feels about O'Neill's plan.
(Sorry, I don't have the issue handy so I can't supply more details).
--
Bill Jefferys 8-%
Astronomy Dept, University of Texas, Austin TX 78712 (USnail)
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utastro!bill@ut-ngp (ARPANET)kiessig@idi.UUCP (Rick Kiessig) (02/04/84)
It's worth noting that even if O'Neill's satellites provide
less accuracy than the GPS, the system he has proposed is TWO-WAY!
The ability to send messages to another person equipped with a
tranceiver is by itself a powerful tool.
--
Rick Kiessig
{decvax, ucbvax}!sun!idi!kiessig
{akgua, amd70, cbosgd, ihnp4, ios}!idi!kiessig