Configuring Spectrum Lab for Meteor Scatter
Introduction
Observing meteor scatter using the GRAVES radar transmitter is one of the easiest ways to observe
astronomical phenomena in the radio part of the electromagnetic spectrum. The aerial is relatively
small and simple to build, receivers are readily available and the software needed to analyse the
data is freely downloadable from the internet. Meteor activity is very dynamic and although radio
observations have resulted in big advances in identifying shower activity, there is always the
possibility of unexpected outbursts occurring. The use of radio scatter makes it possible to observe
this activity, no matter what the time of day or the amount of cloud cover.
GRAVES transmits an enormous amount of power, producing strong meteor reflections that are
receivable over most of Europe. Nevertheless, as with all radio signals travelling long distances, it
should be noted that reception cannot be guaranteed at every location. High ground between the
receiver and the meteor trails can block the signals whilst local radio interference at the receiving
site can create problems when monitoring activity.
The following notes show how the FUNcube Dongle Pro Plus (FCDPP) receiver can be used in
conjunction with Spectrum Lab to observe and record meteor scatter events.
Spectrum Lab
Spectrum Lab is a freely downloadable application developed by Wolfgang Buescher that started out
as a software tool to look at the frequency spectrum of an audio signal. Development has continued
over the years, adding in support for a range of applications, from testing audio equipment,
capturing ‘Natural Radio’ signals created by thunderstorms, to listening to bat calls. It was even
filmed being used in the search for the missing Malaysian Airlines flight MH370. For amateur radio
astronomy Spectrum Lab can be used to observe the VLF radio spectrum up to 24 kHz, and often 48
kHz, depending upon the specification of the computer sound card. This means that it can be used
to detect and measure Sudden Ionospheric Disturbances (SIDs) created by solar flare emissions, but
it is particularly useful for analysing meteor scatter signals.
Spectrum Lab has a huge range of configuration options and finding your way around these is
daunting for a newcomer. However it is possible to load a configuration file which will set up the
system for a particular application. Three different configuration files are described here, one for
checking reception using a local FM radio transmitter, one for general meteor scatter reception, and
one which captures individual events in detail.
There is an extensive Help file available for Spectrum Lab and this has been converted to an Acrobat
file which can be downloaded from the internet.1 However, like the application itself, it can be a
struggle to find your way around this document given the vast amount of information available, so
below you will find a summary of the most important screens and the main parameters to get you
started. You can then use the Help file to find out further detail as needed, or to explore the full
capabilities of this very flexible application.
1
http://www.qsl.net/dl4yhf/speclab/SpecLab_Manual.pdf
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Configuring Spectrum Lab for Meteor Scatter
Using Configuration Files with Spectrum Lab
With the exception of certain
System Settings (see later for how
to change these) all the
configuration parameters used to
set up a particular mode of
operation can be saved in an .usr
file. You can then load one of
these files by using the ‘File’ and
‘Load settings from…’ options in
the Main toolbar.
Note that Spectrum Lab will save its current configuration when you exit or close down the
application, so it is worth keeping a ‘clean’ copy of the original configuration file in a separate
directory if you are going to make changes.
The following notes assume the use of a FUNcube Dongle Pro Plus receiver. Your computer will see
this as a standard external sound card, which is why installation is so easy. The Left audio channel
carries the frequencies up to about 90 kHz above the frequency that the receiver is tuned to, whilst
the Right channel carries the frequencies up to 90 kHz below this frequency.
FMradio.usr
It is always useful to be able to confirm that the system has been set up correctly before looking for
meteor reflections. By their nature these are transitory events and can be relatively rare in periods
of no meteor shower activity or if you are observing in the afternoon or evening. By contrast, FM
stereo signals are permanently available across the country and although the aerial is designed for
reception at 143 MHz it will still be adequate for receiving these services. You will need to tune to
an appropriate FM frequency for your region and a list of these can be downloaded from the
internet.2
This configuration file sets up Spectrum Lab to:
1. Process both of the audio channels coming from the FCDPP, using a sampling rate of 192,000
samples per second (192 kS/s).
2. Set the frequency range covered by Spectrum Lab’s Waterfall display to the widest possible,
to accommodate the full FM signal.
3. Set the Waterfall display for vertical scrolling.
2
A list of FM regional transmitters and their frequencies can be found at
downloads.bbc.co.uk/reception/pdfs/FMradiotrans.pdf
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Configuring Spectrum Lab for Meteor Scatter
You will need to use the FCD+
Frequency Control Program3 to set
the frequency to that of your local
FM service. Note that this needs to
be entered in kHz, so 98200 for a
frequency of 98.2 MHz
For meteor scatter work you should
always use the default settings as
shown here, and only change the
frequency setting.
At the time of writing, Spectrum
Lab does not support listening to
FM signals using the FCDPP so you
will need to rely on the Waterfall
screen to confirm that all is well.
There are other applications that
can be used to listen to FM stereo
using the FUNcube Dongle, for
instance SDR Sharp (SDR#).4
MetScat_starter.usr
This configuration is for those who are setting up their system for the first time, but it can also
provide a basis for the long-term monitoring of meteor activity. It provides the following functions:
1. Process one of the audio channels coming from the FCDPP, using a sampling rate of 192,000
samples per second (192 kS/s). Only one channel is used as meteor reflections occupy a
relatively small range of frequencies and we might as well save the computer’s CPU time for
better things.
2. Set the frequency range covered by Spectrum Lab’s waterfall display to between 0 and 4
kHz. This assumes that the receiver is tuned to 143.048 MHz (143048 kHz) i.e. 2 kHz below
the frequency of the GRAVES transmitter. See the following text for more detail.
3. Set the waterfall display for vertical scrolling with approximately 90 seconds displayed on
screen and with 15 second time markers.
4. Provide a ‘Test Generator’ function which is controlled from one of the programmable
buttons on the left hand of the screen. You can use this to generate an artificial meteor
reflection to confirm the system set up.
3
4
Downloadable from www.funcubedongle.com/?page_id=1225
Downloadable from http://sdrsharp.com/
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Configuring Spectrum Lab for Meteor Scatter
5. Include a Conditional Actions script which will automatically detect meteor pings and record
their characteristics for later analysis.
6. Route the audio from the FCD through Spectrum Lab’s Left Channel amplifier, and then to
the computer’s own sound system. You should be able to use the computer’s own volume
control setting to adjust the sound level.
If you need to adjust the volume further, select the Components window from the top tool bar, click
on the Left Filter Output Amplifier to turn it blue (showing that the slider control now controls it)
and adjust the slider control on the right hand side of the screen.
Left
channel
amplifier
Volume
control
Spectrum Lab should now be able to monitor general meteor activity. Some fine adjustment of the
Waterfall Brightness (B) and Contrast (C) settings in the Colour Palette window may be needed to
suit your particular system and local noise conditions.
This configuration looks at a relatively wide frequency window because there is usually a small
discrepancy between the requested frequency setting for the FUNcube Dongle receiver and the
actual value, due to manufacturing tolerances. When setting up the system we want to be sure of
looking at a sufficiently wide range of frequencies to accommodate this. You need to observe a few
meteor events to establish the offset value for your particular device. The offset is typically 200 –
400 Hz, so you might see meteor reflections centred around 2,300 Hz, rather than 2,000 Hz. This
makes no difference for the operation of this basic set up but it will need to be taken into account if
you want to zoom in on individual events.
This and the following configuration will save screen captures of longer meteor events to the
Screenshots folder in the Spectrum Lab application folder. Shorter events are not captured to avoid
using up too much disk space. Details of every event, including the short ones, are saved in a daily
Events Log file in the Spectrum Lab application folder.
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Configuring Spectrum Lab for Meteor Scatter
MetScat_detail.usr
Once you have become familiar with using the Starter profile you can zoom into individual meteor
events and observe the behaviour of the trail as the plasma disperses with time. It is no good trying
to do this until you are sure of the frequency at which you will find the reflections and that the
system is registering them. You can think of the Starter profile as looking though a ‘Finder’ telescope
to centre on the target location, whereas this profile provides the high magnification version.
The Detail profile provides the following functions:
1. Accept one of the audio channels coming from the FCD.
2. 'Decimate' these incoming samples by a factor of 8, i.e. throw 7/8ths of them away. This
restricts the observable frequency range to 12 kHz, but this is more than sufficient for this
application and decimation reduces the loading on the computer cpu.
3. Set the frequency range covered by the Waterfall display to cover between 1,500 and 2,500
Hz (1.5 to 2.5 kHz). It may be necessary to modify these frequencies to take account of
individual receiver offsets. The receiver used here produces an output 340 Hz higher than
the requested frequency, so the GRAVES transmissions actually appear at 2,340 Hz and the
Waterfall window is set to cover 1,900 to 2,900 Hz.
4. Set the waterfall display for horizontal scrolling, to maximise the amount of space available
to show the detail. The scroll rate should result in the screen displaying a duration of 35
seconds and with 5 second time markers.
5. Provide a ‘Test Generator’ function which is controlled from one of the programmable
buttons on the left hand of the screen.
6. Include a Conditional Actions script which will automatically detect meteor pings and record
their characteristics for later analysis, as in the previous configuration.
The frequency scale for the Waterfall
window is adjusted by first setting fc
to the required centre frequency and
then sp to give the required span.
In this case the Waterfall covers 1900
to 2900 Hz.
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Configuring Spectrum Lab for Meteor Scatter
Changing the standard configuration settings
The settings in the standard configuration files provide a known starting point for investigating
meteor scatter. Once you are familiar with the way that Spectrum Lab works you can start to modify
these settings to suit your own requirements. Remember that Spectrum Lab will update its
configuration file each time it shuts down, so it is always worth saving a copy of the original file in a
separate folder so that you can get back to a known starting point if needed.
The following provides a brief introduction to the relevant (for meteor scatter) tabs in the main
toolbar located at the top of the screen. See the extensive notes under Spectrum Lab’s own Help
tab for information on the functions not described here.
File
Directories lets you define where individual files and images are saved.
Load Settings and Save Settings allow you to use a Configuration file (.usr) to set up Spectrum Lab to
a particular configuration.
Periodic Actions enables you to schedule various actions to occur regularly. For instance it could be
used to save a continuous record of screen shots to help confirm that the system is capturing every
event and to identify false counts due to local interference or reflections from satellites.
Conditional Actions is perhaps the most important feature used in Meteor Scatter work and is
covered in a separate section below.
Screen capture allows you to set the
quality of screen images saved to disk
and to change the information saved
with each image. This is important if
you intend to do long term work.
Suitable information would be the
date of the screen capture and the
frequency to which the receiver is
tuned, since you need to know this to
calculate the true frequency of the
event, and possibly the name of the
Configuration file currently being
used.
There is also a useful option to capture a snapshot of the screen if you see something of immediate
interest, and this function is replicated on the second button on the left hand side on the main
Spectrum Lab screen.
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Configuring Spectrum Lab for Meteor Scatter
Start/Stop
Stop and Start sound thread will pause the operation of the application without closing it down. If
you are going to carry out major reconfiguration work it is better to stop the sound thread first,
otherwise the application can crash. Changing the odd configuration parameter can usually be done
with the sound thread still running.
Options
The first four screens under the Options tab contain the parameters most used for meteor scatter
observations. The other screens can be ignored for this application.
Audio settings I/O device selection
The FUNcube Dongle Pro Plus
receiver appears as an external
sound card, which is why you do
not need any special drivers to
use it. Provided that you have
installed the FCDPP before
starting the Spectrum Lab
application the device should
appear in the pull down list of
Input Devices.
The FCD Pro Plus will only work
correctly with the Soundcard
Sample Rate set to 192000
samples per second.
Note that the Stereo input box is usually not checked for meteor scatter work. We are observing
quite a quite narrow band of frequencies here so we only need to use the Left channel of the FCDPP
output.
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Configuring Spectrum Lab for Meteor Scatter
FFT settings (Fast Fourier Transform)
The two key settings here are the
Decimate input and FFT input size.
Decimate input is used to throw away a
proportion of the input samples coming
from the FUNcube Dongle. This reduces
the workload for the computer's CPU,
allowing you to run the application on a
low-specification machine or use
capacity for looking at the detail in a
meteor trail.
Reducing the sampling rate reduces the
frequency bandwidth coming out of the
FFT calculation. An undecimated stream of 192,000 samples per second will support an FFT
calculation across a bandwidth of slightly less than 96 kHz. A decimation factor of 8 reduces the
sample stream to 24,000 samples per second, supporting a bandwidth of just under 12 kHz. We only
require a bandwidth of about 6 kHz so this is still more than enough for meteor scatter work.
The FFT input size, or length, determines the number of samples used in each FFT calculation. The
more samples you use, the better the frequency resolution in terms of the number of Hertz covered
by each 'bin' (screen pixel). However you lose a corresponding amount of resolution on the time
axis. You can see the interplay between time and frequency resolutions in the text box underneath
these parameter settings.
Spectrum Display Settings Part 1
The most useful fields here are the
Waterfall Scroll Interval where
increasing the value slows down the
scroll rate, and the Waterfall Time Grid
which controls the interval between
the time markers printed onto the
screen.
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Configuring Spectrum Lab for Meteor Scatter
Spectrum Display Settings Part 2
The Amplitude Range sets the range
covered by the Colour Palette slide
bars on the left of the main Waterfall
display. Once you have found the
normal operating range for your
system it is better to adjust these
limits to allow finer control through
the slider on the main screen.
A typical range for the FCDPP is shown
here, but if you are using a traditional
receiver connected via the computer’s
Line In socket a range of -30 to -80
dBFS (decibels relative to the Full
Scale limit) may be more appropriate.
System settings
The .usr configuration file does not
control settings that affect the use of
computer memory or other ‘system’
functions. These can be manually
changed in the Options – System
Settings window. The most relevant
feature here is the use of Local Time
or UTC. The Event Log data is saved
as UTC, but the time displayed onscreen will follow the Timezone
offset unless the checkbox for
getting the zone information from
the system is unchecked.
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Configuring Spectrum Lab for Meteor Scatter
Conditional Actions
The main feature that distinguishes Spectrum Lab from any other application of this type is the
Command Interpreter which allows you to modify its operation to support different investigations.
This is most clearly seen in the Conditional Actions window. The Spectrum Lab 'Help' manual
provides detailed information about the functionality available from this feature.
The Conditional Actions function is used to monitor activity and save screenshots and data about
individual events. The CA script included in the Starter and Detail configuration files triggers a
screen capture when it registers a signal within the range of 1000 to 3000 Hz that is more than 17
decibels (dB) higher than the background noise. This reference against background noise helps
prevent false triggers due to bursts of wideband interference.
The operation can be described as follows.
The CA script consists of a number of 'If' tests. If the test is true, then certain actions are taken. If
the test fails, the script moves on to the next test. Once all the tests have been completed they are
repeated after a configurable delay, in this case, 100 milliseconds.
Line
Operation
1
When Spectrum Lab is first launched this creates a number of variables and assigns
starting values to them
2
Every time you go around the loop, measure the background noise level (A) and the
amplitude (Sig) and frequency (Freq) of the strongest signal between the Low and High
frequency limits
3
Generate a number of time stamps for this precise moment in time
4
If Sig is more than 17 dB above the background noise level (A), increment counter C and
start a 2 second timer (timer0). The counter will be incremented and timer0 restarted
whilst Sig remains more than 17dB above the background noise
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Line
Operation
5
In the first time around the loop (C=1), move the timestamp and other values into fixed
registers. These record the values at the start of the event and remain unchanged in
subsequent loops
6
If the signal is still above the trigger level on the third time around the loop, set another
timer running (timer1)
7
Timer0 will only fire when the test in line 2 has failed for more than 2 seconds. This marks
the end of the event and it is given an event number which is printed on the screen. At the
same time the event_log file is updated to record the parameters measured at the start of
the event.
8
30 seconds after the start of the event, Timer 1 will fire and trigger a screen capture. This
gives time for the event to move across the screen before the snapshot is taken.
9 to
11
At the end of each hour the meteor event counter is reset
The trigger level of 17 dB may need adjusting to match local conditions. Too small a value will risk
false triggers from phenomena such as local interference and Tropospheric Scatter signals. Too large
a value will miss the smaller meteor events, but there will always be some lower limit to detectable
events and if you keep the settings the same you should be able to compare day-to-day activity.
To enable Conditional Actions, check the 'Active' box in the top left hand corner of the Conditional
Actions screen under the File tab in the Main Menu bar.
00:55:06,31,-72.2,-109.9,2400.9,6
00:58:13,32,-92.2,-109.6,2413.1,2
00:59:00,33,-98.5,-110.5,1518.8,1
01:01:11,1,-88.6,-110.1,2404,6
01:02:08,2,-92,-110.7,2399.7,2
01:04:54,3,-100.1,-111,1618.9,1
As well as saving screen shots of longer events, the
above script saves a Comma Separated Variable (CSV)
file containing details of every event as a daily file in
the Spectrum Lab folder. The file label will be in the
form “event_logYYYYMMDD.txt” and can be opened
using Notepad or similar simple text editor. It can
also be imported into a spreadsheet application for
further analysis.
The format for each entry is:
 Event start time in UTC
 Event reference, reset to zero at the start of each hour
 Signal Amplitude(dBFS) at the start of the event
 Noise amplitude (dBFS) at the start of the event
 Frequency (in Hz) of the maximum signal amplitude at the start of the event
 Duration of the event in terms of the loop counter, C. The precise loop time will be a
function of the evaluation time entered in the CA screen and your own system configuration
but can be calibrated using the test Signal Generator function.
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The above CA script is a very simplified version of that developed by Simon Dawes of Crayford
Manor House Astronomical Society (CMHAS)5. Simon's page contains a comprehensive description
of how his own CA script works and is a good reference for appreciating the potential of the CA
function. The script also includes provision for generating activity reports to send to the Radio
Meteor Observing Bulletin6 should you want to compare your observations with those from other
contributors.
Paul Hyde
7th May 2014
5
6
See http://cmhas.wikispaces.com/RadioMeteor
See http://www.rmob.org/livedata/main.php for information about RMOB
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