Around the world meteorological weather balloons are launched twice daily. Once launched they continuously transmit weather telemetry to a ground station using something called a radiosonde. The RTL-SDR combined with a decoding program can be used to intercept this telemetry and display the weather data on your own computer. You will be able to see real time graphs and data of air temperature, humidity, pressure as well as the location and height of the balloon as it makes its ascent and descent.
Note that radiosonde decoding may be difficult in many places in the USA. Recently the USA switched to Vaisala radiosondes, only some of which are decodable. Unfortunately, they appear to be using the more difficult to receive frequency of 1680 MHz rather than the more common 403 - 406 MHz range used by the rest of the world. It is possible that some stations use the lower frequency band however. The decoding software SondeMonitor is able to decode Digital RS92SGP, Analogue RS92AGP, Analogue RS80, Analogue 92KL, Graw DFM-06, MeteoModem M2K2/M10 and Meteolabor SRS-C34 radiosonde protocols.
This tutorial is also applicable to other software defined radios such as the Funcube dongle, HackRF, BladeRF or even hardware radios with discriminator taps, but the RTL-SDR is the cheapest option that will work.
RADIOSONDE RECEIVING TUTORIAL
RADIOSONDE ANTENNA TUTOR IAL
In Europe and the Pacific, most radiosondes transmit at a frequency that is between 403 and 406 MHz. In the USA and Asia they most commonly transmit in the 1680 MHz band between 1668.4 and 1700 MHz. You will need an antenna capable of receiving those frequencies. Antennas good for the 403 to 406 MHz band could be a quarter wave groundplane, dipole, j-pole, turnstile or a QFH antenna. Circularly polarized antennas like the QFH tend to work better. If you have good signal some of the previously mentioned antennas may also work well for the 1680 MHz band if sized for that frequency. A Yagi antenna works especially well for the 1680 MHz band if you require more gain from a weak signal and are willing to track the radiosonde by hand or other means. See the antenna guide for more information about these antennas.
An LNA and preselector may also aid reception. See the LNA section and preselector section for more information. SOFTWARE TUTORIAL
In order to receive and decode weather balloon radiosondes you will need to ready five items: An antenna capable of picking up signals near 403 to 406 MHz (or 1680 MHz). A decoding program called SondeMonitor which is able to decode radiosonde telemetry. The knowledge of when and optionally where weather balloon radiosondes are launched in your area.
The knowledge of what type of radiosonde protocol is used in your area (this can sometimes be guessed with trial and error). An audio piping method. See Appendix A: Audio Piping for more information if you don’t have one set up.
Weather balloons are usually launched 45 minutes before the official observation time of 0000 UTC and 1200 UTC, though times may be different in some countries. You will need to either Google or check with your local meteorological agency for a more exact time of when they are launched in your area. You will also need to check with your local meteorological agency on the type of radiosonde that they are using. They are often happy to oblige this information.
One of the most commonly used radiosonde signals is the Vaisala RS92SGP digital signal. A waterfall example of this signal is shown below. It has a bandwidth of around 5 kHz.
To decode weather balloon radiosondes follow these steps:
1. Download and install SondeMonitor. SondeMonitor is paid software with a 25 euro price tag, but comes with a 21 day trial. Download SondeMonitor from http://www.coaa.co.uk/sondemonitor.htm .
2. When a weather balloon is due to be launched, open SDR# or another radio receiver GUI program, set your audio piping method under the Audio Output drop down box and tune to a frequency between 403 and 406 MHz (or 1680 MHz). Scan around and look for a radiosonde signal, which should be a narrowband signal of around 5 kHz bandwidth. If you find one tune to it and adjust the RF gain settings for best performance. Set the receive mode to NFM and filter audio to OFF. Adjust the filter bandwidth so that it just covers the signal.
3. Open SondeMonitor and go to Options -> Audio -> Audio Source and choose the audio piping method you are using.
4. Select the radiosonde decoding protocol used in your area. These can be selected with these icons which are to the right of the start and stop icons. Hovering over the icons will show which protocol they represent. If you don’t know what protocol your area uses, you can try each one one at a time and hope that one works
5. Now start the decoding by clicking the green start circle . A telemetry data window will pop up. Next, click on the raw signal icon . Adjust the audio volume using the audio gain in SDR# or Windows volume settings so that the audio graph is loud enough to be visible, but not too loud as to cause clipping. Clipping is when the signal is too loud and the waveform begins to look squarish. An ideal volume is shown below.
6. At this point, if your signal reception is good and you have selected the correct decoding method, the telemetry window will show data and have four green circles. Note that SondeMonitor requires some initial time to calibrate. The amount of the calibration process completed can be seen in the red bar in the lower left corner of the main window, which will slowly turn green as calibration completes. The calibration status can also be tracked in the telemetry window. If calibration does not complete, or takes a very long time, it means your signal strength is not good enough.
7. Click on the Graph 2 icon to see telemetry graphs. You can click the Autoscale icon to ensure that all the data is displayed on the screen.
GPS RADIOSONDE LOCATION TRACKING
For most launches, the weather balloon’s live location and altitude can be tracked live in Google Earth. However, some radiosonde protocols such as the RS92SGP do not transmit latitude and longitude GPS data directly. Instead, they transmit raw GPS data which must be converted into longitude and latitude on the receiving end (your computer). To do this, the radiosonde’s starting coordinates, UTC launch date and UTC time and an up to date GPS almanac are required. The almanac is a data file that stores information about the GPS satellite locations.
The MeteoModem M2K2/M10, Graw DFM-06 and Meteolabor SRS-C34 are radiosondes which transmit already decoded GPS location data. The following steps are only for the other radiosondes which transmit raw GPS data, such as the RS92SGP.
1. Download an up to date Almanac from the US Coast Guard Nav Center at http://www.navcen.uscg.gov/?pageName=gpsAlmanacs . You will want to get the current SEM Almanac with the .al3 extension.
2. In SondeMonitor, go to Tools -> GPS Arm . Enter the Latitude and Longitude of the weather balloon’s launch location. You will also need to enter the UTC date of the flight and the approximate UTC time at mid flight. Make sure the date and time is in UTC time. Weather balloons last about 2 to 4 hours, so just estimate a mid flight time. Select Almanac as your Orbital data. The other options can be left as default.
3. Once you press OK, a file selection dialog box will pop up. Browse to the folder where your current .al3 Almanac file is stored in and open it. Now, if you have set the Almanac, start coordinates and date and time set up correctly, the GPS Residual in the SondeMonitor telemetry status window will be to within a couple of hundred meters. The smaller the residual, the more accurate the weather balloon location is. If you have gotten any parameters wrong, the residual will be very large.
5. Now open your SondeMonitor installation folder in Windows Explorer. Find the file google_sonde.kml and open this file with Google Earth. You should now be able to see the weather balloon’s launch location and current live location in Google Earth.
ADDITIONAL LINKS AND REFERENCES: Video Demo: https://www.youtube.com/watch?v=Mz2id1cBmjs