I have been experimenting with meteor detection using radio forward scatter from the GRAVES transmitter and have developed an improved set of conditional actions for identifying and logging meteor events using Spectrum Lab. If you’re interested in trying this for yourself, then everything you need to get started is explained in a couple of articles by Paul Hyde of the BAA Radio Astronomy Group which were published by the Sky at Night Magazine. The articles and configuration files can be freely downloaded from the BAA web site.
I won’t repeat the contents of the articles, which comprehensively explain how to build an antenna and receiver equipment. Note that the original article recommends the use of the FunCube Pro+ SDR Dongle, which costs approximately £125 (GBP) at the time of writing. In fact, it is entirely possible to undertake this project using a much cheaper RTL2832U based SDR dongle.
I have used the NooElec NESDR SMArt dongle which can be obtained from the manufacturer and other major retailers such as Amazon for approximately £25 (GBP) at the time of writing. This reduces the cost of the entire project to perhaps £60 (GBP) to build an antenna and set up the dongle assuming you have a suitable computer available to run Spectrum Lab.
Improved Spectrum Lab Conditional Actions
The starter conditional actions available from the BAA are good for testing, but suffer from a number of deficiencies. In another presentation (pages 25 – 28), Paul Hyde explains that the basic conditional actions set run on a cyclic basis, typically 5 or 10 times per second depending on your Spectrum Lab configuration. The upshot is that the actions run in a different processor thread to the actual FFT process which monitors the data stream for meteor signals. By the time the conditional actions are triggered by the output of a single FFT, multiple further FFTs will have been executed, rendering the readings of signal, noise, peak frequency, etc. taken by the conditional actions invalid.
Instead he recommends switching off the cyclic processing of conditional actions and instead triggering them each time a new FFT is run. This overcomes the above problems and produces valid data. A simple example of a few lines is provided in the above article, which I have used as the basis to develop a much more comprehensive setup.
IMPORTANT: If you have an existing meteor scatter (or other) Spectrum Lab configuration you wish to keep, use the Spectrum Lab ‘File -> Save Settings As..’ option to save them to a .USR file before proceeding. Then make a copy of that file somewhere else as a backup. Spectrum Lab can be a little erratic and it is possible to overwrite an existing configuration unexpectedly simply by closing Spectrum Lab.
In order to use this configuration open the ZIP file and proceed as follows:
- Copy Retune.exe to your Spectrum Lab executable folder. By default, the configuration file expects this to be ‘C:\Spectrum\Retune.exe’. If you need to copy the executable file elsewhere, once you have loaded the configuration, you will need to modify the interpreter commands by right clicking the ‘Tune to GRAVES’ and ‘Tune to Test Beacon’ buttons and changing the path at the end of the interpreter command section.
- If you (wisely) do not trust the executable I have provided, you can created your own using the ‘Retune.ahk’ file. You will need a copy of AutoHotKey. Load the .ahk file and convert it to a stand-alone executable file and install as described above. The purpose of ‘Retune.exe’ is simply to programatically click the mouse in Spectrum Lab’s ‘vfo’ box and then programatically hit the Enter key. Spectrum Lab’s own interpreter will quite happily change the frequency display in the ‘vfo’ box, but there is no interpreter mechanism to force the SDR dongle to re-tune to that frequency so it is a misleading cosmetic change. The ‘Tune To’ buttons in the supplied configuration file call ‘Retune.exe’ to make the retune actually happen.
- Copy ‘Meteor_freqlist.txt’ to the ‘C:\Spectrum\frequencies’ folder (or the frequencies folder inside your spectrum lab installation folder if different). This will show the GRAVES and GB3VHF frequency markers on the spectrum display.
- Copy ‘MetScat_Comprehensive_v7.USR’ to an appropriate location where Spectrum Lab can load it, such as your documents folder. Start Spectrum Lab and use ‘File – > Load Settings From..’ to load this file and configure Spectrum Lab.
- At this point you will need to check your SDR and other settings to ensure your SDR dongle is still working and correctly configured. It is beyond the scope of this README to explain how that is done. As a bare minimum check under ‘Options -> Audio I/O’ to ensure the correct ‘ExtIO_***.dll’ file is set as the source.
- If you have an existing configuration that you wish to retain and only require the improved set of conditional actions, these are available in the ‘MetScat_Conditional_Actions_V7.txt’ file. These can be loaded in to the conditional actions tab using the ‘Load’ button without affecting any of the other settings. Note, there is no need to do this if you have loaded the full configuration at step 4 above.
If you open the conditional actions dialog via ‘File -> Conditional Actions…’ you will immediately note that the ‘Cyclic’ option has been disabled and the ‘on FFT’ option enabled. This causes the ‘new_spectrum’ condition to become true at line 18 each time a new FFT is processed. This line captures the key signal and noise data which is them monitored and used by the rest of the conditional actions.
The script is well documented with comments and uses meaningful variable names so that you can figure out what is going on. (As an aside, it’s the bane of my life that amateur software developers still use meaningless variable names like ‘a’, ‘b’, ‘c’, etc. making it painful to deduce how their code works!)
As the conditional actions model used by Spectrum Lab is headache inducing even for a seasoned professional software developer, the core of this script is a Finite State Machine with a number of other side actions queued or triggered using timers. Basically the ‘State’ variable tells you (and the script) what is going on, whether waiting for a meteor, monitoring one, logging it, etc. The current state is also displayed near the bottom of the programmable button set on the main waterfall display, hopefully simplifying debugging of any problems.
The activity of the script is controlled using the user-definable variables set in line 10. These are documented in the preceding lines but remember that you need to click the ‘”Init”‘ button to re-initialise everything before any changes become valid. The key ones that you may need to change are:
SNR_Threshold: This is set to 15 dB by default. You may need to increase it if you are getting too many false positive detections, or reduce it if you are missing meteors.
Waterfall_Length: The configuration marks the waterfall display once a second along the bottom of the screen. Count the number of seconds that fit on to your display and subtract 20%. This will trigger the screenshot of a meteor when it is close to, but not at, the left hand edge of the waterfall display maximising the chances of capturing the whole event.
Log_Path, Capture_Path, Sound_Path: Set the paths where you want to store your files. Note the use of double backslashes ‘\\’ as Spectrum Lab uses ‘\’ as the escape character in strings. NOTE: The folder must exist and be writeable by Spectrum Lab otherwise capturing / logging will fail silently until you fix the issue and restart Spectrum Lab.
The conditional actions script provides the following features:
- Captures screenshots of meteor events from the waterfall display. For events that are longer than the width of the waterfall, the script will attempt to capture multiple intermediate screenshots to ensure the whole event is recorded. The script also queues screen captures to capture each meteor event, whereas some scripts would miss subsequent events if one meteor is already scheduled for capture.
- Captures .wav sound files of meteor events, which can be enabled or disabled via the user defined variables in the script.
- Creates multiple log formats, logs can be opened by other applications (e.g. Colorgramme) whilst Spectrum Lab is active without file locking issues:
- RMOB.dat and RMOB-dur.dat formats for upload to RMOB.
- daily_log.csv and hourly_log.csv formats containing simple daily and hourly meteor counts.
- event_log.csv format containing comprehensive data for each captured meteor event. The comma separated columns are:
- Meteor Start Date (YYYY/MM/DD)
- Meteor Start Time (hh:mm:ss)
- Current daily meteor count (integer)
- Peak Meteor Signal (float, dB)
- Peak Meteor Noise (float, dB)
- Peak Meteor Signal to Noise Ratio (float, dB)
- Peak Meteor Frequency (integer, Hz)
- Meteor Duration (float, seconds)
The programmable buttons on the left of the main waterfall display provide the following functions:
- Last Event: Last event that occured (no function when clicked).
- Capture Now: Captures a screenshot of the waterfall display.
- Pause: Pauses the waterfall display and processing.
- Tune to GRAVES: Tunes to the GRAVES frequency (right-click to edit the frequency and path to Retune.exe if needed).
- Tune to Test Beacon: Tunes to the GB3VHF frequency. This is useful for some UK-based observers to test their equipment. If you are in range, you should see an alternating pattern of continuous single frequency, a dashed (morse code-like) single frequency and then series of dashes shifting over a small range of frequencies.
- Logging: Toggle logging of meteors, screenshot and sound captures on and off.
- Signal Gen: Toggle injection of a strong signal in to the FFT. Useful for testing script triggers, etc.
- Display Up / Down: Nudge the displayed frequency range up or down by a small amount. More accurate than using the standard Spectrum Lab display controls for small adjustments.
- Offset Up / Down: The tuning of the SDR dongle is affected by its temperature, and these controls change the ‘Radio Frequency Offset’ found on the ‘Spectrum (2)’ tab to compensate. I tune to GB3VHF, wait for the continuous tone and then adjust these buttons to line the blue frequency marker with the actual tone displayed on the waterfall.
- SNR: Displays the current peak SNR being read from the FFT for diagnostic purposes.
- DMC/HMC/Q: Displays the current daily meteor count, the current hourly meteor count and the number of screenshots currently queued for capture.
- vfo: Displays the current frequency tuned.
- SF: Displays the low and high frequency range on the waterfall display.
- State: Displays the current state of the Finite State Machine in the conditional actions.
- Time: Shows the current system time.
Bugs & Issues
- Occasionally ‘Retune.exe’ does not work when the ‘Tune’ buttons are clicked. If you suspect this is the case, manually click in the ‘vfo’ box and hit the enter key.
- The timestamps displayed on the waterfall may be significantly behind the PC’s clock. Whilst a short delay may be expected between capturing data and processing it for display, this drift can sometimes be in the order of minutes. It is not caused by the ‘Time’ slider controlling the waterfall display. It may be linked to having the conditional actions editor dialog open for long periods but this has not been proven. The simplest cure is to stop Spectrum Lab and restart it.
- Timestamps used to name screenshots and sound captures may not correspond exactly to the timestamps of events in the log files. The limitations of Spectrum Lab’s interpreter make it difficult to create a queue of filenames to be used for each capture so screenshots and sounds are named using the now() timestamp at the point they are created, not the actual time of the event which triggered them. (For the avoidance of doubt, timing of events in log files should be accurate if your PC clock is accurate).
- Timings for creation of RMOB log files are not defined anywhere I can find easily. The supplied conditional actions create RMOB log entries for the past hour at just before the hour ends, i.e. the count of meteors between say 23:00:00 and 23:59:59 will be timestamped for 23:00. Many, but not all, other observers seem to be using scripts which would stamp that entry 00:00 on the following day, which doesn’t make sense to me! It is easy enough to adjust the log trigger times if you prefer the other convention.