Can a 172 flying at 200 feet above water across Lake Superior get through
undetected by radar or AWACS? Are there holes in radar coverage?

With the transponder off, and at low altitude, I suspect a 172 could get
across the more remote portions of Lake Superior without being seen on ATC
radar. I'm not sure how much the controllers really notice tracks that
don't have a transponder data block. Maybe there wouldn't even be a track
visible on their scopes, maybe the return would be painted but not noticed
by the computer, and maybe it would be noticed by the computer but ignored
by a controller who had plenty of other things to worry about. You are
writing a novel here, and it doesn't need to be 100% accurate on things like
this. If I read this in a novel, I could suspend any disbelief, at least in
relation to ATC radar.

AWACS would be a whole different ballgame. If an AWACS was on station and
specifically tasked with looking for this 172 (or just covering the Lake
Superior area), I strongly suspect that it would find it. The 172 is boxy,
with all kinds of right angles and things sticking off of it to produce a
radar return. It's radar cross section is pretty large for such a small
plane, especially from above.

Some of what you descrive would depend on where the AWACS was stationed. If
it were flying an orbit over Michigan, for example, then flying low wouldn't
help very much. The idea behind flying low is typically to either get below
the radar horizon, get lost in ground clutter, or both. An AWACS flying
nice and high would be able to see the entire surface of Lake Superior, so
you wouldn't be able to get underneath the horizon. And a flat lake surface
wouldn't produce much in the way of ground clutter to hide in. More to the
point, the AWACS was specically designed to look for low targets, and it's
radar is unlikely to get confused easily.

If I was reading about a 172 that was able to sneak past an AWACS, I don't
think I really could suspend my disbelief. Maybe if the book was written by
someone who knew the AWACS systems really well, but not otherwise.

The one caveat to this might be if the AWACS was specifically looking for
only high-speed targets, and the 172 was flying very, very, slowly. I don't
know anything about the AWACS radar specifically, but most systems can be
set to ignore radar tracks that are beneath a certain speed. This way, the
system operators aren't presented with a bunch of returns coming off of cars
on an interstate highway or watercraft on the lake. If the AWACS was on
station looking for something fast, and the 172 was in slow flight, then
maybe (and this is a big maybe) the radar return from the 172 -- while the
computer would see it and recognize it for what it was -- would never show
up on the scopes because the computer would judge it to be too slow to be of
any concern. Tom Clancy used this trick in his book "Debt of Honor". A
helicopter flys directly over a train, matching it's speed. The airborne
radar detects it, but the radar crew adjusts the system filters to ignore
it, since they are sure it is a harmless train.

That is somewhat believable for a helicopter (which can fly as slow as it
wants) over land, and especially over a land with very fast trains like
Japan. But a 172 over Lake Superior wouldn't have many things it could
pretend to be. It could never go slow enough to pretend it was a freighter
or other commercial vessel on the lake. It probably could go slower than a
speedboat, but I personally wouldn't want to be on that boat running at 50
knots across the shipping channels of a choppy Lake Superior.

Good luck with your novel!