Thursday, May 09, 2013

setiQuest GOES-11

The Allen Telescope Array (ATA) collected an 8.7 MHz wide chunk of spectrum from the NOAA GOES-11 weather satellite at 1692 MHz back in September 2010.  The GOES-11 satellite has since been decommissioned and replaced by GOES-15 but its glorious signal lives on in the setiQuest data archive.  This particular quadrature dataset has a duration of 532 seconds and is 8.9 GB in size.  The baudline signal analyzer is going to take a look at some of the signals in this GOES-11 dataset.

There are an incredible number of signals hidden in this dataset.  We're going to zoom in, browse around, measure, and identify a few of them.  For fun I'm going to take the approach of doing a blind analysis of the GOES-11 signal data.  This means that I'm not going to use the Internet to determine any signal features such as modulation types and baud rates.  No external help.  Just baudline.  Let's go ...

The average spectral plot above is a map of where we're going to zoom in and analyze.  The blue lines represent wideband regions that will be focused on in different sections of this analysis.  The red dots are representative of individual signals that will analyzed in depth.  We begin the analysis on the left side of the spectrum and will work our way to the right.

-2000 ... +100 kHz
Here is the chunk of spectrum from the first blue line.  Tip: use the size and position of baudline's lower scroll bar to determine the frequency resolution and position in relation to the main spectrogram/average map.  In this example notice how the size and position of the scroll bar match the size and position of the first blue line.

It looks like a lot is going on here but in fact there are only two unique signals.  Everything else is caused by mixing problems.  Let's try to sort things out by making some frequency measurements.

The two red dots represent the signal that we'll being analyzing in depth next.  The orange, yellow, and blue horizontal lines represent the distance between the constant features seen in the previous spectrogram.

orange = 294400 Hz
yellow = 258133 Hz
blue = 36267 Hz

Here are some interesting mathematical relationships:

orange = yellow + blue
yellow / orange = 7 / 8
orange + yellow = width of the main central lobe

The 7/8 ratio is very unusual as is the coupling of the two different signal types.  Now let's zoom into the time domain and check out the two signals marked by the red dots.

This level of signal resolution clearly shows six Frequency Shift Keying (FSK) modulated signals.  Notice that the mark and space frequencies on the left side are inverted from the right side.  Also the two FSK's on the outer edges are wider with roughly twice the frequency delta between mark and space.  To get a better view here is a zoomed in spectrogram extract of them:

Baudline's periodicity helper bars measure a bit rate of 9600 bits/second.  The multiple FSK signals have different frequency widths but they have the same bit stream.  This makes them the same base signal.  One of them has been stretched by an unknown process.

Above are the stacked average spectrum of the wider FSK and the narrower FSK signals.  The upper FSK has a measured mark/space delta of 20 kHz and the lower has a mark-space delta of 8 kHz.  This 20 / 8 = 2.5 ratio is far enough away from a factor of 2 that something else other than basic harmonic mixing is occurring.

What is stranger is that the spectral shapes look completely different.  How and why is a mystery.  They are the same signal.  Straight alias mixing should not be able to do this.  Here are some modulation measurements for both FSK signals:

modulation type: FSK
spectral width: 30 and 16 kHz
mark-space delta: 20 and 8 kHz
bits / symbol: 1
symbol rate: 9600 symbols / sec

Note that when the bits / symbol parameter equals 1, the symbol rate is equal to the baud rate.  So both of these FSK signals have a 9600 bit/sec baud rate.

-1000 kHz
Next let's look at the wide spectral hump that is in-between the FSK signals discussed previously.  Centered at -1000 kHz, the red dot in the following image highlights our target signal.

Sharp filters keep the FSK signals from impairing this signal of interest.  Carrier lock with baudline's new IQ display:

The two bright points are Phase Shift Keying (PSK) symbols that are 180° out of phase.  This signal is called Binary Phase Shift Keying (BPSK).  Here are some modulation measurements:

carrier frequency: 1692 + 0.999991 MHz
modulation type: BPSK
spectral width: 350 kHz at -6 dB
bits / symbol: 1
symbol rate: 293884.01 symbols / sec

For PSK modulations, the symbol rate being roughly equal to the spectral width is a good measurement sanity check.  An interesting observation is that the accurate BPSK symbol rate measurement is approximate to the less accurate orange delta Hz measurement bar that was discussed at the beginning of this section.  An unknown mixing process must be at work for the BPSK symbol rate to be connected to the frequency positions of the FSK images.  This is very unusual.

Next let's focus on the transient horizontal bursts that are overlaid on top of this BPSK signal.  There are more than eight bursts and they appear to have random durations and spacing.  The duration of the bursts range from less than a second to more than a minute.  It's interesting to note that the IQ constellation doesn't change when mousing over the bursts.  The gain fluctuation in the spectrogram frequency space is quite dramatic yet the gain, phase angle, and jitter all remain constant in the IQ space.  How is this possible?

To answer that question let's zoom into the time axis with a smaller FFT size.

There is significant structure here. Click the image for more detail.  The middle vertical line is one of the FSK artifacts.  Baudline's periodicity bars show that these horizontal burst groups have a 56 ms periodicity.  Each burst is 48 ms in duration followed by a 8 ms off time.  Zooming deeper into the spectrogram shows a highly repetitive spectral pattern.  An autocorrelation plot verifies this.

The primary lag in red represents a periodicity of 0.3845 ms.  Note that the rightmost red line has been folded by aliasing.  The secondary lag in yellow is 1.7358 ms with 0.3845 ms sidebands.  The calculation 1.7358 ms / 0.3845 = 4.514 is an unusual ratio.  Using the BPSK symbol rate, the two lags translate to 113.00 (red) and 510.12 (yellow) symbols.  The sidebands and the strange ratio suggests two possible explanations: that the secondary lag is being time modulated by the primary lag or that each transient burst consists of a repeating bit pattern that has two periodicities.  How to construct either option is not obvious.  There must be a simpler explanation.

Since GOES-11 is a weather satellite these bursts may signify blank or repeating image lines.  An idle pattern or training sequence is another option but the complexity of the signal makes this unlikely.  Demodulating the bits would answer this question definitively but that is a project for another blog post.

+1800 ... +2900 kHz
Now we're going to jump to some signals of interest on the right side of the main spectrum (the 2nd blue line).

First we'll look at the strong vertical signal on the left side and then we'll dive in and take a look at some of the many signals shown in the right half.

+2000 kHz
The average spectrum of the strong vertical signal on the left.

Notice the middle strong tone and the mirror image spectral humps.  This what happens when the transmission carrier in not suppressed.  The delta frequency between the strong carrier tone and one of the spikes on the hump is 2 kHz.  The spectral width of the one of the main humps is 4 kHz but the harmonics of this modulation stretch out to about 500 kHz wide.  This is not an effective use of the available bandwidth.

The IQ display can be used to determine the modulation type.

Notice how the two symbols are offset from the origin.  This is caused by the non-suppressed carrier.  With some decimation, down mixing, carrier suppression, and selecting either the upper or lower sidebands (USB or LSB) the IQ plot becomes:

Here are some modulation measurements:

carrier frequency: 1692 + 1.998614 MHz
modulation type: BPSK
spectral width: 4 kHz of SSB
bits / symbol: 1
symbol rate: 4000.025 symbols / sec

+2225 ... +2750 kHz
Here is a spectrogram of the many signals section (the 3rd blue line):

It is full of signals.  Thousands and thousands of signals.  Let's start on the left and zoom into several of the interesting sections.

I call this next section centered at +2250 kHz "snow" because that is what it looks like.  The deeper you zoom into both axes the more signals you see.  Very high signal density. It is like a whole world unto itself.

Click the image above to see the fine details.  Even at high zoom levels the signals are not well defined.  The blips look like very short duration tone bursts, some look like fuzzy snowflakes.  It is like some mysterious process has shrunk down and squeezed together what you are about to see in the following sections.

There are many signals of various durations, sizes, and shapes.  The signals look like short modulated packet bursts of data.  Most of the signals appear to be lined up in frequency slots.  This is known as Frequency Division Multiplexing (FDM).  Many signals appear to begin transmission at a common 30 second time slot and to a lesser extent at 10 second intervals.  This behavior doesn't apply to all of the signals but there is a great deal of organization.

The image above is a different section that is zoomed in a bit.  Some of the signals have strong sidebands and some don't which suggests that there are multiple transmitters involved.

+2422 kHz
From the right side of the previous image, here is a deeper zoom into the frequency axis.

Four modulated packets are visible.  Each packet has a strong central carrier that is the source of the upper and lower sideband symmetry.  On closer inspection the top two packets have a slight drift to the right and to the left.  This could be due to Doppler or it could be due to a non-stable and slowly drifting transmitter clock.

Slicing through the middle of the spectrogram is a fast moving object that is being Doppler drifted.  The slope is straight at the top (overhead) and then becomes curved as it recedes (away).  This behavior follows the expected Doppler flyby curve shape.  The measured drift rate for the top "overhead" section is 90.6 Hz / 118.1 seconds = 0.767 Hz/sec.  The lower "away" section is slightly curved which makes it difficult to measure accurately.  An approximate measurement of the lower drift rate is 83.3 Hz / 161.7 seconds = 0.515 Hz/sec.  This slow moving signal looks to be AM modulated with a carrier-to-sideband delta of 10 Hz.  If this was a modulated On Off Key (OOK) data signal then the baud rate would be 10 bits/sec.

Here is the lower sideband IQ plot of the last modulated packet.

This jittery constellation is BPSK.  It looks different than the previous BPSK signal seen above.  The extra jitter and other signal instabilities suggests a lower quality modulator / transmitter source.

carrier frequency: 1692 + 2.421614 MHz
modulation type: BPSK
spectral width: 200 Hz of SSB
bits / symbol: 1
symbol rate: 200 symbols / sec

+2505.6 kHz
Here is the spectrogram display of a nearby frequency.

Five packets are visible.  Each packet begins with a brief tone and is then followed by what looks like a wider bandwidth modulation.  Notice the modulation sidebands.  Here is the waveform display of one of the packets.

Notice the constant envelope with many different types of phase shifts.  Next, let's look at the IQ display.

This modulation is 8-PSK.  Each of the eight symbols has it's own distinct phase (360° / 8 = 45°).  In this modulation scheme a symbol encodes 3 bits of information (2^3 = 8).

carrier frequency: 1692 + 2.505610 MHz
modulation type: 8-PSK
spectral width: 160 Hz at -6 dB
bits / symbol: 3
symbol rate: 150 symbols / sec
baud rate: 450 bits / sec

+2704 kHz
What is this mysterious looking stuff next to the wide vertical line?

This particular signal is featured in the found a DNA strand comment on the setiQuest forum.  We will be avoiding any erroneous time aliasing distortion here.  Let's zoom into the time and frequency axes to get a better look.

This looks like a harmonic power ramp up with the frequency carrier stabilizing after a half second.  There are some faint noise-like data bursts interspersed in the middle.

Let's zoom into the time axis some more.

This signal is a sine carrier that is FM modulated by a 71.4 Hz sine wave.  The delta of the upper and lower modulation frequencies is about 1040 Hz.

This signal looks and sounds a lot like the very strange "flying saucer" signal that was discovered a couple years ago in the setiQuest data.  The measured modulation parameters are slightly different but I wouldn't be surprised if it was generated by a similar mechanism, perhaps by spinning.  Note that this signal's whimsical nickname is a reference to classic B-movie scifi sound effects.  There is zero reason to believe that this signal is from a real flying saucer.  UFO alert canceled.

+2725.7 kHz
This next signal looked like a constant tone until zooming way into the frequency axis.  The spectral resolution of this image is 0.521 Hz/bin.

The tone dips are -15 Hz to the left.  The repeating dips have a periodicity measurement of 61.4 seconds.  The repetition timing is exact like clockwork.  The whole structure is slowly drifting to the right at a 5.73 Hz / 523.3 seconds = +0.0109 Hz/sec rate.  What it is and why it is doing this are unknown.

+3350 kHz
This spectrogram from the far right of the dataset's spectrum is an example of a Doppler flyby.

The top third of the curve is fairly straight and it represents an overhead position.  The measured Doppler drift rate is 22200 Hz / 109.3 seconds = 203.1 Hz/sec. That is fast.

The fainter middle section is curved and represents a transition.

The lower third section still has a slight curve but it is beginning to straighten out.  This motion represents that the object has transitioned to a receding (away) position.  The measured Doppler drift rate is 5067 Hz / 102.9 seconds = 49.24 Hz/sec.

No modulation is visible.  This object is likely a fast moving Low Earth Orbit (LEO) satellite.

Note that with judicious scaling, the basic shape of the Doppler curve above roughly matches the curve seen in the 8-PSK packets image.  The scaling is off by a factor of 265X but the overhead and away transitions occur at about the same times.  The two Doppler signals may be related by some extreme mixing artifacts.

The datasets in the setiQuest archive use signed 8-bit quadrature samples.  Below is the histogram plot of the I channel.  The Q channel looks the same.

 The holes between every other bin is normal when 8-bit samples are viewed in 9-bit space (bins=512).

The large number of spikes reveals a rounding problem.  The alternating cadence is a spike every 2nd and then 3rd sample.  Somewhere in the ADC --> beamformer --> data collection chain the higher bit samples are being poorly quantized down to 8-bits.  This sort of error causes alias distortion.

The spike at 0 is interesting as is the drop at the next sample value of 1.  This signifies a truncation round to zero problem. It also will cause a DC offset.

Only about 7 of the 8 bits are being used.  There is plenty of sample headroom and clipping is not a potential problem. Not using 1 bit of resolution reduces SINAD by 6 dB.  Note that SINAD is related to SNR.

For an informative and amusing look at other histogram problems take a look at the Quantization Shop of Horrors.

This setiQuest GOES-11 dataset is a smorgasbord of signals.  We saw FSK, BPSK, offset-BPSK, 8-PSK, and the "flying saucer" modulations with a drifting LEO thrown in for good measure.  The mirror aliasing throughout the entire spectrum is troublesome.  Either the ATA's AGC or the 8-bit quantization is likely to blame.

Data licensed through SETI.
Software licensed through SigBlips.