From: Paul F Austin <paustin@digital.net>
Newsgroups: rec.aviation.military
Subject: Re: Question on the effect of EMP (Electro Magnetic Pulse) on GPS
Date: Sun, 23 Aug 1998 17:42:56 GMT

Tarver Engineering wrote:

> Dwezil Dwarftosser wrote in message <35DBD6A5.DFA@usa.net>...
> ...
>
> >But that is similar to your assumption that EMP is a phenomena
> >*exclusively* related to a nuclear blast - which would make us
> >even in the number of errors posted.  However, your statement
> >above seems to completely ignore magnetic field theory, relying
> >once again on only the electrostatic half of the story.
>
> Electron cascade is in no way a static electric event.  The magnetic
> component of the field is large; these charges are moving in space.  Gamma
> transmission and capture moves these charges at the speed of light.
>
> There is a frequency of light that matches exactly the energy required to
> move an electron to a next Shell.  This energy will be given up as a new
> gamma transmission when the electron moves to the next lower shell and so on

There are a couple of misconceptions in this thread.

During detonation (within the first few hundred nanoseconds), the prompt _gamma_
radiation generated by the primary event is absorbed and thermalized in the
compressed and ionized material that made up the outer layers of the bomb. The
range of prompt _gamma_ radiation is no more than 100m from ground zero
(considerably less in practice). The radiation from the bomb is a more-or-less
black body spectrum whose temperature is a key part of the bomb design. The
spectral peak is in the soft X-ray region and it's those thermal X-rays that
ionize the atmosphere to generate the source region. Glasstone and others refer
to _gammas_ causing the source region disassociation but the size of the source
region is large enough (hundreds of miles at high altitude) that gammas _can't_
play a significant part. It's the long range of X-rays in rarified air that
makes the EMP from a high altitude (but not exoatmospheric) event a wide area
threat.

Secondly, the interaction of the X-ray photons with the atoms of the high
altitude air results on ionization, not electron excitation. It's the free
electrons in the source region that cause the large electric field that
generates the EMP. If excitation were the dominant mode of interaction, there
wouldn't be any appreciable current or EMP.

--
"A fanatic is one who can't change his mind
and won't change the subject"
-------------------------------------
Paul F Austin
paustin@digital.net

From: Paul F Austin <paustin@digital.net>
Newsgroups: rec.aviation.military
Subject: Re: Question on the effect of EMP (Electro Magnetic Pulse) on GPS
Date: Fri, 11 Sep 1998 23:42:19 -0400

Tarver Engineering wrote:

> John Weiss wrote in message <6rqmpg$5mc@q.seanet.com>...
> >Could you tell us where you got this information, and explain the
> >attenuation mechanism?  Gamma radiation is, in general, of higher energy and
> >of longer range than X-rays...
>
> At sufficient altitude, the Aptmosphere is already ionized.  Mr. Austin is
> passing fantasy off as fact, just as Mr. Copp has.


This arrived here very late but because John was so <erm> civil, I'll answer.

First, "at sufficient altitude" bullshit. The ionosphere plays virtually no
role in EMP. Too few molecules. The source region electric field peaks in the
high stratosphere during a high altitude EMP event.

Regarding the relative effect of fission gamma and thermal X-rays in
generating the source region, read up, John. For open literature, consult
George Messenger's book. He has a dandy table of effective fireball
temperature vs time, reproduced here for your edification:
Time(us)     Temp(MK)     Black body Peak (Kev)
0.1            7.5                  3.2
0.12           6                    2.5
0.15           5                    2.14
0.17           4                    1.7
0.2             3                    1.2
In the epoch after 0.1us, the radiance of the fireball generates _lots_ of
photons above 1Kev but well short of the 1Mev threshold for "gammas". Why's
that important? If prompt Gammas dominated EMP generation then the very long
range of fission gammas compared to soft X-rays would mean that low altitude
burst would generate the same long range EMP as high altitude bursts do. It
doesn't. Quantitatively, a 1Mev gamma has a mean free path at 10,000 foot of
740KM, compared to essentially 0m MFP for 1Kev X-ray.

At very high altitude, soft X-rays have long range. At lower altitudes their
ranges (in terms of mean free path) are very short. At high altitude, X-rays
can ionize gas over the hundred kilometer ranges needed to induce EMP over a
wide area. As to whether a soft X-ray can ionize gas molecules, a 1 Kev photon
has enough energy to ionize Nitrogen and Oxygen down to the inner electron
(881eV and 1172eV respectively).

Tarver aspersed Kopp's (and my) comments as fantasy. I don't know Carlo but I
designed and built the most radiation hardened electronic artifact on Earth,
under contract to the US Army Strategic Defense Command (USASDC). The AHAT
guidance processor demonstrated radiation hardening techniques against EMP,
dose rate and total dose threats and was immune to upset against its design
threat.

I learned a lot on the subject, being the beneficiary of a colloquium held
every six weeks for three years in which the Army brought most of the nation's
experts on radiation effects to Melbourne to assure that our little device
would operate through the most severe radiation environments. Experts from
places like HDL, Physitron, JAYCOR and Mission Research reviewed the details
of our work. Oddly enough, Tarver Engineering wasn't present.

Since Peace broke out, the bottom has fallen out of the dose-rate hardened
electronics industry. Too bad. I was getting ready to go out to the Nevada
test site with my device and watch the earth move when underground testing was
halted. Damn.

--
"people of means-decent folk-should be given more votes
than drifters, whores, criminals, degenerates, atheists
and indecent folks-people without means."

Paul F Austin
paustin@digital.net

From: Paul F Austin <paustin@digital.net>
Newsgroups: rec.aviation.military
Subject: Re: Question on the effect of EMP (Electro Magnetic Pulse) on GPS
Date: Sun, 13 Sep 1998 00:26:24 -0400

Cecil Turner wrote:

> All right, very interesting.  But I'm a dumb jarhead--you guys went all
> doctoral on me and I never did get the answer to a very interesting and
> militarily useful question:  can you set off a nuke in the upper
> atmosphere and get a good EMP going outward (toward space), or is the
> effect limited to earthward propagation? thanks,

The short answer is "yes". The EMP pulse propagates outward as well as
inward. The low frequency portion of the pulse don't affect spacecraft as
much as terrestrial systems because the long wavelengths don't couple as
well with physically short spacecraft structures but where coupling is
efficient (VHF and up), it's a problem.

It's not an insuperable problem. In the first place, spacecraft systems
have stringent EMI/EMC requirements. Many of the same techniques used to
build EMI-tight boxes are usefull in providing EMP shielding. The field
strengths are higher which means more attenuation is required but many of
the techniques are the same. Where EMP threats require additional
attention is in the penetrations into the EMI-tight enclosure. EMI
filtering on power has the same effect on EMP energy coupled into power
buses. Low power signal and control conductors are protected by
pass-throughs circuits, usually either embedded in connectors or in
special "dog-houses" that provide an EM-tight enclosure outside the box
proper where filtering occurs. I don't know much about protecting high
power systems, since that's not what I deal with. Because EMP threats can
corrupt signalling between boxes, you have to pay attention to band
limiting circuits into and out of boxes and providing a robust signaling
protocol. The simplest example would be to make sure that an EMP pulse
isn't mistaken for a legitimate control pulse. Inter-box controls are
frequently done with pulses. You obviously have to distinguish between
the pulse you want and external erroneous pulses from other sources of
which EMP is one.

Back to GPS: the sats themselves have very stringent EMP protection
requirements. In addition, the system is designed to continue to operate
in a general war environment.  That means that for a defined, limited
period, the GPS constellation can continue to operate autonomously
without the ground control and updating that keeps the system operating.
I'm not an expert on GPS as a system but ground systems provide updated
ephemeris information and maintain time synchronization for the whole
constellation.  The GPS sats have on-board precision clocks (rubidium
oscillators) that can maintain system synchrony for long enough to fight
a cannonical nuclear war. Eventually the various clocks drift far enough
out of sync and the sats's orbits are perturbed enough that the system
won't work but there's a specified time over which that wont happen.

It would take a _lot_ to take down the entire GPS system. Popping a
single nuclear device won't do it. Local jamming is another story. It's
certainly possible overwhelm the signal of a distant satellite with a
local jammer. For that reason, users are designing in anti-jam techniques
into tactical terminals and the USAF is looking to make the next block of
GPS sats more jamming resistant.

I hope that's sufficiently un-doctoral. It's easy to go on about details
and miss what a user needs to know.

--
"people of means-decent folk-should be given more votes
than drifters, whores, criminals, degenerates, atheists
and indecent folks-people without means."

Paul F Austin
paustin@digital.net