If noist/sampling/Nyquist errors cause bit detection errors in the DF
pare of the frame, then we may not be able to work out the correct
length of the message. We have to guess whether the bit should be a 0 or
a 1.
In such circumstances we assume the message length is long(112 bits)
However, if we start to get encoding errors after bit 56 the we attempt
to change our original guess at the bit, and invert it. If this change
of guess would have resulted in a short message, and if the short
message would have been error free then we can recover.
Change (and hopefully improve) the Message bit decoder.
When decoding bits, a "0" is encoded as (m[j]<m[j+1]), and a "1" is
(m[j]>m[j+1]) . However, there is a problem if mpj[ == m[j+1] because we
can't decide what it's supposed to be.
Antirez's original code defaults to a '1', and then lets the bit error
detection code sort it out. I *think* it's better to default to '0'
because it's more likely that noise added to the signal will produce a
spurious '1' rathar than anything subtracting from the signal to produce
a spurious '0'
Also, Antirez''s code only looks for errors in the first bit of the
message. I don't know why this is.
There is a potential problem in deciding the message length if there are
any errors in the first 5 bits decoded, because this defines the message
type (the DF-??), and it's the DF that determines how many bits the
message shall contain.
If there is an error in the first 5 bits, then we could ignore the DF,
and continue decoding a long format (112 bits). However, for weak
signals, if the message is a short one (56 bits) this results in the
sigStrength decaying to the point where it's level drops below squelch,
so we discard a possibly decodeable 56bit
However, if we assume it's a short message, and only decode 56 bits, and
it's really a long message we won't have decoded all the bits.
Not sure what to do about this
Three changes in this one
1) Change the checksum testing for DF-11
2) Recode the Checksum generation routine to use pointers.
3) Tidy up the appearance of some print debug statments
Change the I/Q lookup table for better detection. Changes fully
described in the source dump1090.c at line 347 onwards. This change
results in about 30% more frames being detected at weak signal input
levels.
Also a bug fix from the last commit - C doesn't support the min()
function.
Original code loops through the analogue array m[] detecting data bits
and putting them into the bits[] array. It then loops through all the
bits[] creating the msg[] byte array. It then loops through the analogue
array m[] again calculating the signal strength.
Change this so that everything is done in one loop so we can go straight
from analogue samples to bytes, calculating the signal strength on the
fly.
Also use the results of the signal strength calculation to populate the
message records mm.signalLevel variable.
Create a pointer, pPayload, which points to the start of the data bits
in the analogue sample buffer m[]. So pPayload =
&m[j+MODES_PREAMBLE_SAMPLES] Then use this
pointer to perform the data bit detection tests. It should save a few
cpu cycles per test because accessing pPayload[2] should be quicker than
m[2+j+MODES_PREAMBLE_SAMPLES].
Also change the way phase correction works. the original code saves the
original data (from m[pPayload] to aux[], and then phase corrects m[],
and then restores aux[] back to m[] afterwards. Change this so that m[]
is copied to aux[] and then phase correction is carried out in aux[],
and the pPayload pointer points to aux[]. This then avoids the
requirement to copy aux[] back to m[] afterwards, which saves a fair few
CPU cycles.
Create a preamble pointer in the message detector loop
Create a pointer, pPreamble, which points to the start of the preamble
in the analogue sample buffer m[]. So pPreamble = &m[p] Then use this
pointer to perform the preamble detection tests. It should save a few
cpu cycles per test because accessing pPointer[2] should be quicker than
m[p+2].
Also move the decision on whether to try OutOfPhase correction to the
end of the first pass, rather than automatically going into phase
correction if the first pass fails. This saves two memcpy's if the
decision in the second pass is to not do phase correction.
Create a pointer, pPreamble, which points to the start of the preamble
in the analogue sample buffer m[]. So pPreamble = &m[p] Then use this
pointer to perform the preamble detection tests. It should save a few
cpu cycles per test because accessing pPointer[2] should be quicker than
m[p+2].
Also move the decision on whether to try OutOfPhase correction to the
end of the first pass, rather than automatically going into phase
correction if the first pass fails. This saves two memcpy's if the
decision in the second pass is to not do phase correction.
Change the following so that M$ compilers and debuggers complain less
1) change all long long data types to uint64_t.
2) Typecast all malloc function returns to the correct pointer types.
3) Explicitly typecast all float to int conversions.
4) Remove inline variable declaration. Allowed in C++, but not in C.
Change the following so that M$ compilers and debuggers complain less
1) change all long long data types to uint64_t.
2) Typecast all malloc function returns to the correct pointer types.
3) Explicitly typecast all float to int conversions.
4) Remove inline variable declaration. Allowed in C++, but not in C.
Apparently, the Beast output timestamp has a frequency of 12 Mhz.
Therefore, I've updated the timestamp counter to simulate a 12 Mhz
frequency.
Also incorporate terribls latest updates
Increase the speed of the I/Q to magnitude calculation lookup by
expanding the table to 65536 entries (256*256*2 bytes). At runtime, this
allows us to pick up raw I/Q bytes as a 16 bit value and index into the
magnitude table to get a 16 bit result. This removes the need for
subtracting 127, and then correcting for -ve numbers, so should be
faster, at the expense of a larger data table.
Change the maglut lookup table from 129*129 to 256*256
Initialise the maglut buffer accordingly
Change the data->maglut lookup to use the new maglut buffer
Change the I/Q data buffer pointet to a uint16_t *
I messed up merging the Squawk display in interactive mode into my
master.
However, the original source posted by terribl causes a print line
length greater than 80 characters. This in turn causes the lines to
spill over on a terminal display. I have therefore re-formatted some of
the output so that it fits within 80 characters.
Increase the speed of the I/Q to magnitude calculation lookup by
expanding the table to 65536 entries (256*256*2 bytes). At runtime, this
allows us to pick up raw I/Q bytes as a 16 bit value and index into the
magnitude table to get a 16 bit result. This removes the need for
subtracting 127, and then correcting for -ve numbers, so should be
faster, at the expense of a larger data table.
Change the maglut lookup table from 129*129 to 256*256
Initialise the maglut buffer accordingly
Change the data->maglut lookup to use the new maglut buffer
To try to decode messages with a fundamentally flawed preamble is mostly
a useless waste of CPU time.
The new aggressive mode still detects a sensible percentage of
additional messages because of the error tolerance and two-bits fixes
but does not waste your CPU time.
The phase correction was applied only to a subset of bits! Because of an
offset error.
The detection code layout was modified a bit to make it simpler to
implement more corrections in the future. However only phase correction
is performed currently. Slope correction, or to compensate for the cycloid
effect are two possible improvements.
* Better preamble detection to skip most of the messages we'll likely
not be able to decode.
* A Phase correction algorithm that improves the recognition compared
to the previous algorithm used.
* Javascript output in debug mode, and a debug.html file that can be
used in order to see graphically undecoded samples.
* Ability to detect cross-read messages, that are, messages that happen
to start and end across two different reads from the device or file.
* A few bugx fixed.
* README improved.
This commit address two issues with the implementation of CPR decoding:
Fix#1: The two functions N() and DLon() used to have the latitude
parameter as an integer (!), basically truncating the fractional part
before calling the NL() function to perform the lookup.
This resulted into random strange movements of aircrafts, especially
jumps or shifted positions.
Fix#2: Use milliseconds for CPR odd/even timestamps. Dump1090 already
tried to use the most recent packet received among the odd and even
packets currently available, however to do this correctly millisecond
resolution should be used, otherwise many times the odd and even packet
appear to have the same time in seconds and we don't always use the
latest received.
