(PaulH) wrote
Thanks to all of you for sharing your experience. I do have a new
stormscope installed but haven't yet used it enough to know to what
extent I can trust it. A couple of recent flights when Tstorms were
within range show a possible 30 degree azimuth error (compared with
where I thought the activity was via radar before takeoff), so I plan
to bring it back to the shop next week.

Actually, you can do a decent job of checking azimuth error using a
spark plug tester, or really any small unshielded engine with
electronic ignition. Put it about 5 feet from the antenna, and run
it. The sparks will show up.

An electric power saw or power drill can also work, but it will need
to be closer - say 2 ft - which limits the accuracy of your
determination. Still, a 30 degree azimuth error is pretty obvious.

Keep in mind that the antenna is mounted to the fuselage, and thus the
azimuth information you get is relative to your heading, not your
track. This can be a significant difference in a slow airplane,
though 30 degrees of crab would be very unusual.

Despite what you have been told by the less than knowledgeable, the
Stormscope can usually be used to estimate range as well as azimuth,
though not so accurately as RADAR.

First off, it is incorrect to say that the Stormscope (or competing
brand - my experience is that the Strikefinder is so similar as to
make no difference in operation) looks simply at intensity. There is
more to it. It also looks at spectral spread using digital signal
processing. I won't go into the physics of it unless someone insists,
but the general principle is that a strong distant storm shows up as a
longer duration (broader) peak than a close weak one, ON AVERAGE.
Thus if you fly a constant heading and let the dots collect, the
average position of the dots is a fair estimate of range. Any
individual dot is suspect.

Since you probably don't care about stuff over 100 miles away (that
weather will likely change by the time you get there) you should only
be using the 200 mile range (if your device even has it - some don't)
as a general indicator that something is out there. Turn to a less
sensitive range (50 or 100 miles) for avoidance. This rejects the
weakest peaks out of hand, and the stronger the peaks are the better
the range estimate.

In any case, you need only use that method for activity at your 12
o'clock. For stuff off to the side, you can do a little math. You
might recall those wingtip bearing change problems from your
instrument written. Nobody actually uses that method to establish
position anymore, but it's actually quite useful for Stormscope
interpretation.

Let's say you've been watching a cell (which shows up as strikes along
a radial line) and in the time you've been watching it (say 10
minutes), it's gone from being at your 2 o'clock to your 2:30 o'clock.
That's 15 degrees in 10 minutes, or 1.5 degrees a minute. Now, let's
say you're cruising 90 kts - that's about 1.5 nm a minute. So at what
distance is 1 degree of bearing change equal to a nautical mile?
Approximately 60 nm. So that cell is about 60 nm away (that number is
about as accurate as your estimate of time, speed, and angular drift),
and you can accept that turn of 90 degrees to your right for the next
10 minutes with no worries. This is really all basic geometry.

Now all of this goes out the window when you're dealing with lines
rather than individual cells. As a general rule, spherics is not a
way to penetrate lines - it is a way to avoid scattered cells.
However, if you know something of the geometry of the lines and
clusters you are dealing with (maybe you looked at Nexrad before
takeoff or have it in the cockpit) you can often get a good enough
picture to slip between lines and/or clusters.

Michael