Read the pictures, the aircraft, the meters and the sensors

Hexadecimal is a true answer to "what did that say" and not a useful one.
This is the work of turning the rest of what a receiver hears into
something a person can read, and most of it is pictures.

PICTURES

Three of the things on the air are images rather than sounds, and all three
arrive as the audio a scan already records:

  SSTV     14.230 and 144.5 MHz   Martin M1/M2, Scottie S1/S2/DX, Robot 36/72
  APT      137-138 MHz            the NOAA weather satellites
  HF fax   2-20 MHz, sideband     the marine weather charts

Each is written from its published specification, and the generators used to
test them are written from the same specification without reference to the
decoders -- so a picture that comes back matching the one that went in is
evidence about the format.  Every SSTV mode reproduces its published line
time exactly, which is worth failing a test over: a line a few milliseconds
long walks the picture off the screen inside ten lines.  Against synthetic
transmissions at 30 dB SNR, SSTV is 96-98% of pixels exact, APT correlates
at 0.97 and fax at 0.998; all three still read at 6-12 dB.

None of the three is guessed at, and that is what makes it safe to try them
on every recording.  SSTV needs its VIS header, APT needs both line syncs at
the right distance from each other, fax needs the phasing signal.  No false
pictures in 295 attempts over noise, tones, speech and swept whistles.

Two things had to be got right beyond the arithmetic.  A band-pass does not
switch between two tones, it slides between them, so every edge is measured
at the midpoint of the slide rather than at the first sample past a
threshold -- the earlier version was reading the coarse search stride back
as the edge and shifting Martin M1 sideways by a whole colour bar.  And a
picture now keeps its capture whatever the content check made of it: a
satellite is a steady tone with a wobble on it and SSTV is a whistle, so
both were being discarded as "no signal content" having already been
recognised.

PNG is written here rather than pulled in from Pillow.  A scanner that
cannot start because an imaging library is missing is worse than one that
cannot draw.

saunterbrowse marks a picture in the list, gives its path in full -- wrapped
rather than cut off, because half a path opens nothing -- and moves or
deletes the PNGs with the recording.  o prints the picture's path, not the
audio's.

GRIB is not a modulation and is not pretended to be one.  It is the format
weather models are published in and it travels by satellite link and by
e-mail; where a decoded byte stream begins with its magic number it is
named, and that is all.

AIRCRAFT

`bandsaunter adsb` parks the receiver on 1090 MHz and reads Mode S extended
squitter: address, callsign, altitude, position, speed.  A command of its
own because a megabit a second will not go through a channel twelve and a
half kilohertz wide.  Every frame carries a 24-bit checksum so there is no
threshold anywhere in it -- with one trap, which is that a frame of all
zeros satisfies that checksum and silence is exactly that.  Positions round
trip exactly through compact position reporting, and a pair straddling a
longitude-zone boundary is refused rather than resolved against two grids.

METERS AND SENSORS

Itron ERT utility meters on 900 MHz and AcuRite weather sensors on 433 MHz
are named rather than reported as hex, and neither is believed without its
own checksum -- BCH(255,239) for the meter, a checksum and four parity bits
for the sensor.  Both are implemented from published descriptions and
checked against frames built from the same descriptions, which proves the
framing and the arithmetic and is not the same as having held a meter.

HEX INTO WORDS

Everything else that decodes to bits now gets its fields named where the
shape is standard, its text read out where there is text, and its bytes laid
out in groups with the printable characters beside them.

The text search is where the care went, because printability is not
evidence.  Forty framings of each packet, and seven-bit values printable
three in four, meant a bar set on printability called 64% of random payloads
text.  Real text is nearly all one case where random letters are half and
half, two fifths vowels where random is a fifth, and mostly alphanumeric
where random draws punctuation one time in four.  Together: under 0.5%,
measured in the suite.

CALLSIGNS

The licensed address is recorded in full -- the street, not merely the town
-- and goes into the KML with everything else.  US amateur records are
public by law and carry it; holding it and not saying so is worse than
either showing it or not asking, and --no-lookup asks for none of it.

Also here: Morse is decoded again from the whole recording where the capture
was made in cw mode.  The first pass works from the classifier's buffer,
which holds a few seconds -- enough to say "this is Morse", not enough to
catch a callsign whole between two word gaps, so a beacon repeating every
eight seconds through an eight-second window was never identified.

And classify._psk_order took the logarithm of zero on a silent block.

1318 tests, up from 1161.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016PsWPTweCT6pwxKngvVxcg

packaging/bandsaunter.1

@@ -1,5 +1,5 @@
 .\" Generated by packaging/make-man.py -- do not edit by hand.
-.TH BANDSAUNTER 1 "2026-08-29" "bandsaunter 2026-08-29_01" "User Commands"
+.TH BANDSAUNTER 1 "2026-08-29" "bandsaunter 2026-08-29_03" "User Commands"
 .SH NAME
 bandsaunter \- scan, record and identify radio signals with an RTL-SDR
 .SH SYNOPSIS
@@ -68,8 +68,14 @@ List attached receivers.
 .B profiles
 List saved profiles.
 .TP
+.B adsb
+Listen to aircraft on 1090 MHz. See
+.B AIRCRAFT
+below.
+.TP
 .B analyze
-Identify a signal in an already-recorded file, or decode Morse from it.
+Identify a signal in an already-recorded file, decode Morse from it, or write
+out the picture it turns out to be.
 .SH OPTIONS
 .TP
 .BI \-r " RANGE\fR, \fP" \-\-range " RANGE"
@@ -534,6 +540,15 @@ Setting name \fBclassify\fR, default \fByes\fR.
 Work out what each recording actually is, whether FM voice, AM, single sideband, Morse, a paging system or a digital voice mode, and write it into the log and the filename. Turning it off saves a little processing and leaves you to identify things by ear.
 .RE
 .TP
+.B --images / --no-images
+Decode pictures \[em] save SSTV, weather satellite and shortwave fax pictures as PNG.
+.br
+Setting name \fBdecode_images\fR, default \fByes\fR.
+.RS
+.PP
+Three of the things a receiver can hear are pictures: the weather satellites on 137 MHz, amateur slow-scan television, and the shortwave weather fax stations. All three are images sent as sound, so they arrive in the same recordings everything else does. Each is recognised by its own header rather than guessed at, so this costs a moment per recording and finds nothing where there is nothing. What it does find is written as a PNG beside the audio.
+.RE
+.TP
 .B --morse / --no-morse
 Decode CW to text \[em] decode keyed carriers as Morse.
 .br
@@ -1079,6 +1094,96 @@ A decode that does have repeats or a checksum behind it outranks the content
 check: a burst of keying demodulated as FM audio is a buzz, and the speech
 detector likes a buzz, but a frame whose own checksum came out right is not a
 statistic.
+.SH PICTURES
+Three of the things a receiver can hear are images rather than sounds. All
+three are analogue, all three encode brightness as a frequency, and all three
+arrive as the audio the scanner already records \[em] so they are looked for in
+every recording and written out as PNG beside it.
+.TP
+.B SSTV
+Slow-scan television, on 14.230 MHz and 144.5 MHz and wherever else amateurs
+send it. A transmission opens with a VIS header that says which mode follows,
+and that header is what is looked for: no header, no picture. Martin M1 and
+M2, Scottie S1, S2 and DX, and Robot 36 and 72 are decoded, in colour.
+.TP
+.B "APT"
+The NOAA weather satellites on 137 MHz, which spend a fifteen-minute pass
+sending one continuous picture. A 2400 Hz tone carries the brightness, two
+lines a second, 2080 words to a line, with both of the satellite's sensors in
+every line. The whole frame is written, and each sensor again on its own.
+.TP
+.B "HF fax"
+The weather charts the shortwave stations have sent for decades, in single
+sideband between 2 and 20 MHz. A transmission opens with a phasing signal \[em]
+twenty or so lines that are black but for a pulse at the start of each \[em] and
+that is what says where a line begins and how long one is.
+.PP
+None of the three is guessed at, which is what makes it safe to try them on
+every recording: each is recognised by a header or a phasing signal that
+nothing else on the air sends. A decoder without one draws static beautifully,
+and a directory of beautifully rendered static is worse than an empty one.
+.PP
+A picture keeps its capture whatever the content check made of it. A satellite
+is a steady tone with a wobble on it and an SSTV transmission is a whistle:
+neither is speech and neither has symbol structure, so both were being thrown
+away as "no signal content" having already been recognised.
+.PP
+Pictures take minutes rather than seconds \[em] two minutes for SSTV, fifteen for
+a satellite pass \[em] so
+.B \-\-record
+has to be long enough or what arrives is the top of one. A partial picture is
+kept and labelled as partial rather than discarded.
+.PP
+.BR saunterbrowse (1)
+marks these in the list and gives the path of the file.
+.PP
+GRIB, which is sometimes asked about in the same breath, is not a modulation:
+it is the binary format the weather models are published in, and it travels by
+satellite data link and by e-mail rather than as something a receiver can
+demodulate. Where a decoded byte stream begins with its magic number it is
+named as such; nothing here fetches or renders one.
+.SH AIRCRAFT
+.B bandsaunter adsb
+parks the receiver on 1090 MHz and reads the Mode S extended squitter that
+every airliner overhead broadcasts twice a second: the aircraft's address, its
+callsign, its altitude, its position and its speed, unencrypted, to nobody in
+particular.
+.PP
+It is a command of its own because ADS-B does not fit through the scanner. The
+signalling is a megabit a second, which needs at least two megasamples a second
+of raw receiver output; the scan path decimates everything to a channel twelve
+and a half kilohertz wide long before any decoder sees it.
+.PP
+Every frame carries a 24-bit checksum, so there is no threshold here and
+nothing to disbelieve: a frame either passes or is dropped. A position takes
+two frames \[em] the encoding sends a fraction of a zone, and one frame alone is
+ambiguous by hundreds of miles \[em] so an aircraft is placed once an even and an
+odd frame have both arrived, about a second apart.
+.PP
+.B \-\-kml
+writes what was heard as a map.
+.B \-\-frames
+prints each frame as it arrives instead of a running count. An aerial cut for
+1090 MHz makes the difference between hearing the airport and hearing the
+county; the whip supplied with a dongle is a quarter of the length it wants.
+.SH METERS AND SENSORS
+Two things on the ISM bands are worth naming rather than reporting as
+hexadecimal.
+.PP
+The Itron ERT modules fitted to electricity, gas and water meters across North
+America broadcast their reading every thirty seconds or so on 902-928 MHz, in
+the clear, so that a van can drive past and read a street. The message says
+which meter, what kind, what the register reads and whether the tamper
+switches have been tripped, and carries a sixteen-bit BCH check.
+.PP
+The AcuRite 433.92 MHz outdoor sensors sold with every consumer weather
+station send temperature, humidity, battery state and a channel letter every
+sixteen seconds, with a checksum and four parity bits.
+.PP
+Neither is guessed at: nothing is reported that has not satisfied its own
+checksum. Both are implemented from their published descriptions and checked
+against frames built from the same descriptions, which proves the framing and
+the arithmetic and is not the same as having held a meter.
 .SH THE MAP
 A callsign is looked up in the FCC's published licence data, which gives the
 licensee, the town, and coordinates. They arrive from three directions and