From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Solar Bursts May Threaten GPS
Date: Thu, 05 Apr 2007 12:50:34 -0400
Message-ID: <uv8a13diiljhhfj9eq5lcu5da06kntduc9@4ax.com>

On Thu, 5 Apr 2007 13:22:20 -0400, "Steve Wolf" <news@w8iz.com> wrote:

>How is a satellite that is "off", with a receiver listening intently for
>commands, protected against solar radiation that would otherwise do damage?
>Is the protection through redundancy?  Is the spare satellite a spare, which
>we believe and hope will work after some other is damaged?  Or is being
>"off" significant.

I hang out on a mailing list where several of the GPS system operators
and a few design engineers hang out.  This "news" has been laughed at
extensively.

GPS satellites, being a valuable military resource are highly
radiation hardened, more so than regular commercial satellites.  At
most, a solar burst will cause a momentary safe shutdown.

There are several issues concerning a radiation burst.  The
semiconductors are hardened to several millions of RADs total
integrated dose plus the most sensitive (memory and processors) are
shielded.  The concern isn't from primary radiation damage but from
ionization-induced secondary effects.  Ionization upsets of memory
bits or latches in the processor chip could send the software off to
never-land.  These processors are extensively protected from such
events but there is the rare chance of something happening.  If it
went off and, say, fired a thruster, the satellite could be lost.  So
they power down as much of the system as they can on the leading edge
of the radiation burst.

High powered electronics such as microwave transmitters, waveguides,
antennae arrays and so on are subject to electrical breakdown (arcing)
if an ionization path is established by a dense external ion field. In
addition to the ionization effects, protons absorb electrons and
become hydrogen attoms.  Alpha particles become helium.  The tiny bit
of atmosphere that this can create in sealed units can degrade the
voltage withstand of the components in the unit.  This a particularly
important problem where high vacuum is the dielectric. This build-up
happens over time in hermetically sealed units but the bursts speed
things along. Thus, this sort of stuff is powered down or reduced in
power during the event.

Sealed assemblies where the high vacuum is essential contain getters
that absorb tramp gas atoms.  The gas production from an intense burst
could temporarily overwhelm the getter, increasing the absolute
pressure and making flashovers a possibility.

The cesium beam tube in the atomic clocks is affected both by
radiation and by the electric field associated with the radiation. A
radiation burst could affect the time-keeping which is the basis of
GPS functionality. The new satellites use rubidium beam oscillators
because they are less sensitive to radiation upsets and because the
phase noise is lower and the short term stability is better.  Since
the satellites are synced to ground atomic clocks anyway, the one
advantage that Cs has - excellent long term stability - is not
required.

In a rare instance where the satellite is oriented just so relative to
the incident stream of ions, the ion stream could be preferentially
absorbed by one polarity of a power bus and cause an over-voltage. The
bus would trip, of course, but it's highly preferable to do an orderly
shutdown of devices on the bus, thus another reason for powering down
during a radiation incident.

Not trying to play satellite expert, just reciting some stuff I've
listened to intently over the last few days plus some stuff I've
learned in radiation-hardening instrumentation for terrestrial nuclear
applications.

BTW, as I understand it, the practice of parking new satellites as
spares is past.  After the newly launched satellite is proved
functional it is put into service.  Many times these replace
end-of-life units that are spared out but not turned off.  This is
fuzzy memory and I'm not motivated to look it up so if I didn't get it
quite right, sue me.

The system was designed for 36 satellites.  There was some concern
awhile back that when they activated the 37th satellite, some very old
receivers would suffer a roll-over fault.  Turned out not to be a
problem just like the epoch rollover event wasn't a problem.

John