

16 bit signed int overflow brings air traffic control to standstill - madaxe_again
http://www.bbc.co.uk/news/technology-27292440

======
dalke
That BBC article says nothing about signed int overflow. I believe the title
of this link is pure conjecture by the submitter.

In any case, max signed 16-bit integer is 32767. It's unsigned 16-bit integer
to get to 65536, so this title is wrong.

This topic has come up a couple of times already here on HN, including
[https://news.ycombinator.com/item?id=7691583](https://news.ycombinator.com/item?id=7691583)
and
[https://news.ycombinator.com/item?id=7694121](https://news.ycombinator.com/item?id=7694121)
. The report is that the U-2 was flying at 60K feet, not 65K which might
trigger the problem.

One commenter pointed to a page which point to
[http://www.barryschiff.com/high_flight.htm](http://www.barryschiff.com/high_flight.htm)
which says "The U-2's Mode C transponder, however, can indicate no higher than
FL600."

Mode C can go slightly higher, but "commercially available Mode C transponders
cannot currently report an altitude greater than 62,000 feet" says
[https://www.faa.gov/about/office_org/headquarters_offices/as...](https://www.faa.gov/about/office_org/headquarters_offices/ast/licenses_permits/sub_orbital_rockets/newregs/media/EP_FR.pdf)
.

So of the limited information we do know, the U-2 should not have triggered an
overflow, assuming that such a bug existed.

BTW, according to [http://www.airsport-
corp.com/modec.htm](http://www.airsport-corp.com/modec.htm) , the altitude
encoder data has a higher range: -1200 to 126,700 ft. One of the reason for
negative altitude is of course places below sea level, but another is that
altitude when measured via barometric pressure depends on the weather. High
pressure days give a lower measured altitude.

------
kotnik
"The agency said it had now added more flight-processing memory to the
computer system."

int32_t to the rescue!

------
jameskilton
Would love to see the source that describes the problem in more detail but if
the "16-bit signed" is true...

Why would altitude ever be negative?

Just a thought.

~~~
zimpenfish
Low level flight over the Dead Sea (422m below sea level) could be negative, I
guess.

~~~
Turing_Machine
Furnace Creek Airport in Death Valley is at -211 feet (-64 m).

[http://www.nps.gov/deva/planyourvisit/airports.htm](http://www.nps.gov/deva/planyourvisit/airports.htm)

