[Leica] Re: More GPS quibbles

[lrzeitlin at optonline.net (Lawrence Zeitlin)]

On Feb 25, 2007, at 8:45 PM, Frank wrote:

> GPS weapons are used for their pin point accuracy and their ability  
> to follow terrain.  Inertial navigation is used as well, but it
> is inaccurate ( relative comment only).  GPS is absolute position,  
> INS is a relative position.
>
> If I were in a plane, trying to "go" somewhere, fast, stealthy, and  
> maybe not on a  direct route, I would want or need GPS.  INS
> will not do the job as well and specifically not as accurately.  If  
> in a bomber at 600+ MPH, I do not think celestial navigation is
> a real player.
>
> BTW, you all realize that our satellites ( including GPS  
> satellites) are all based upon celestial navigation themselves?   
> ( Look up
> star trackers....)
>
> GPS satellites are in specific orbits to cover an area of the  
> world.  You do not need to take them all out only those that are
> functional in an area of interest.  If I remember correctly, in any  
> area, there are 6 satellites in use.  The other 18 ( or
> whatever) do not "shine over" the area of interest.
>
> Trivia...... the original application for commercial use of GPS was  
> for shipping, to keep ships off sand bars.

This has nothing to do with photography. Anyone averse to discussing  
weapons technology should hit the delete key.

No one doubts GPS accuracy. My nephew is president of a company that  
uses GPS to provide maps of facilities for cities. After all the city  
fathers have to know where all the fireplugs are. With modern  
equipment the GPS fixes are far more accurate than the maps they are  
plotted on. As I said, my store bought GPS units can provide a  
location within three feet. His surveying quality gear can provide a  
location to within three inches.

That being said pinpoint accuracy is not always required for  
weaponry. Inertial navigation is preferred for critical military use  
because it is entirely self contained and not subject to external  
jamming or interference. Accuracy of inertial navigation is a  
function of time, not distance traveled. Missiles and bombers have  
relatively short flight times. I terminated my involvement with the  
military/industrial complex in the 1960s but at that time an  
inertially guided missile could hit a target on the other side of the  
world within a 300 foot center of probable error. Close enough for a  
nuclear weapon. That was with comparatively primitive technology. My  
four year old iBook is smarter and faster than all the computers on  
the Apollo moon rocket combined. I'm sure today's missiles can thread  
a needle.

GPS satellites are not in a fixed orbit relative to the earth. They  
orbit the earth twice a day and cover different areas of the earth's  
surface on each pass. Knocking out all satellites covering a given  
location would only buy you about 90 minutes of freedom from GPS  
guided weapons. GPS satellites last about 10 years and are constantly  
replaced. There are several spare satellites in orbit waiting to be  
turned on. On the other hand the signal transmitted by each satellite  
is relatively weak, only about 50 watts. Local area jamming is  
accomplished by transmitting an overriding signal which swamps the  
received GPS signal. The Navy exercises this option regularly near  
Naval bases during periods of ship movement. Small boat sailors learn  
not to rely on their GPS receivers near Newport News or San Diego  
during blackout periods. A technically sophisticated potential enemy  
could, and probably would, jam GPS signals over its military targets.  
Another reason why inertial guidance is preferred for strategic  
weapons. Also our own military can degrade the accuracy of GPS at  
will by jittering the time base signal (Selective Availability) to  
prevent enemies from using it to locate our targets. Selective  
availability was turned off in the '80s but could be turned on at any  
time. Most recently it was turned on for a period after 9/11. If that  
is the case, all those expensive navigational computers in BMWs and  
Mercedes cars will become useless. You will turn down the wrong streets.

Finally, satellite navigation systems were originally developed and  
used for the benefit of the Polaris missile launching submarines.  
These submarines would spend 90 days at a time under the polar ice  
waiting for the signal to launch missiles. I spent one such cruise on  
the George Washington, our first Polaris submarine. Boring, and I  
gained 20 pounds. If given the signal, the submarines were supposed  
to pop out from under the ice, get a quick navigational fix, arm  
their missiles, and launch. Although the missiles themselves were  
inertially guided, an inertial guidance system requires exact data  
about it's starting location to achieve the required accuracy. One of  
my last Cold War jobs was as a consultant to Admiral Rayburn in the  
Navy Special Projects Office. Rayburn was the honcho behind the  
Polaris system, which, incidentally was one of the few large scale  
military projects to come in ahead of schedule and under budget. Any  
commercial use of the GPS system came quite a bit later.

I'm waiting for the new Leica M9 to include a GPS chip so I don't  
forget the location of each picture. That would really be progress.

Larry Z