Radiosondes (RS-41/DMF-17)

Radiosondes are weather balloon computers, launched twice daily at 12:00 and 00:00 UTC by the National Weather Service for the purpose of gathering detailed high altitude weather data. Our purpose, however, is to collect these computers and reprogram them to create an extremely low cost COTS GPS Telemetry computer. While these are still in the realm of experimental trackers, early tests with L1 flights have proven extremely successful. While in flight, the GPS is limited by a <4 g limit (due to limitations inside the GPS chips themselves), they have a proven track record for ground recovery. When recovering these computers from the wild, they can be picked up and decoded at distances up to 1 mile. Combined with a purpose built high performance radio protocol Horus V2, the potential for the system is great.

The first official launch test was performed in the Spring of 2024, as a part of a Grim Reefer launch detection/sensor verification test in a L1 (See L1 Test Flight.xlsx, link to drive). The test showed agreement between the RRC3, Grim Reefer, and RS-41 Radiosonde with altitude (barometric vs GPS), and the final coordinates of the rocket where received at ~100ft AGL. No extra signal hunting post landing was done due to the visual tracking of the rocket, but the signal was only lost due to the URRG hill getting in-between the base station car and the landing rocket.

They where first implemented in the SA cup 2024 payload as a redundant GPS to the BRB. Testing indicated good performance, with tests done out to .7 miles between the airBnB and the local desert hill. Both BRB and Sonde where present and decoding. During ground tests in Rochester, the Sonde consistently slightly outperformed the BRB, and was able to decode more often, thanks to using Horus V2 as apposed to APRS. Due to very sad ground equipment failures, neither system was decoded during or after flight, and the payload was found using luck based methods. Evidence suggests that both systems functioned properly during flight (Both systems where powered on and happy when recovered, one sonde packet was decoded during accent with null data (expected), MCC reports “seeing something” that looked like Horus on their radio waveform), but overall the system did not work.

RS-41

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General info:

The RS-41 is a radiosonde manufactured by Valasia. These sondes are launched from the Buffalo airport, and when the winds are correct, land in the farmland surrounding Rochester. These sondes are less common than the newer DMF-17, but have much more work done around them in the HAB community. One of the special things about the RS-41 compared to the DMF-17 are the exposed pins in the form of the XDATA port at the top of the board. This allows for reprograming of the STM 32 to change the firmware to allow for transmission on the 70cm Ham band. This port also allows for the inclusion of various I2C sensors, which can be powered off the board, and their data can be transmitted back with the GPS data. This port can also provide the GPS data over serial, allowing for a as yet unrealized possibility of logging the GPS data locally. The antenna can be replaced with a SMA port by soldering the center pin to where the antenna is originally soldered, and soldering one of the ground pins to the vacant pad on the opposite side of the PCB.

The RS-41 dates back to 2014, with widespread deployment around 2016. It is expected to soon be replaced by the successor model, RS-41e. This model is designed to be much more biodegradable, but early reports seem to imply there was a major board revision, potentially slowing down the development of custom firmware.

Specs:

Power: Two AA batteries @ 3v in nominal, In practice can range from 4.2v to as low as .8v when bypassing some circuitry.

GPS: UBlox G6010-ST (typically locks between 5-9 sats)

Transmit: 70cm/433Mhz band @100mW

Dimensions: 120mm x 37mm (excluding XData port)

For more detail, this Github page has all of the parts, plus schematics of most of the circuits.

DMF-17

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General Info:

The DMF-17 is Graw’s radiosonde. This version is (probably) cheaper than the RS-41, and thus is much more common. Our primary source of DMF-17 boards is from the Albany launch site, which some of our members are able to collect while at home. It functions mostly the same as the RS-41, with a few major differences. It has a much newer GPS than the RS-41. This has yet to be tested in a rocket, but it is expected to have better GPS performance than the RS-41. It also has a much nicer set of pads to solder an SMA connector to. One of the major downsides of the DMF-17 is a lack of pins for external connection. It must be reprogramed using a set of empty pads in the center of the board, and lacks any of the external communication features of the RS-41.

the DMF-17 Seems to have begun production around 2020, and started widespread deployment around 2022.

Specs:

Power: Two CR123A batteries @ 6v in nominal, In practice can range from 6v-4v.

GPS: UBlox MAX M8C-0-10

Transmit: 70cm/433Mhz band @100mW

Dimensions: 80mm x 47mm

For more detail, this wiki page has the most centralized data I have found on the DMF-17

Firmware:

RS41ng, as well as the entire Project Horus Github provides the secret sauce that makes the conversion from weather instrument to general use GPS telemetry transmitter possible. RS41ng is the firmware that converts the RS-41 and DMF-17 into their Ham friendly forms, and contain a huge amount of information about what you can do with these boards. Project Horus is the overall project that RS41ng lives in, and it provides an entire ecosystem of dedicated HAB software and firmware that enables the modern system of recovery and reuse.

The firmware has lots of documentation, and is mostly self explanatory. It has instructions for reprograming, explains what many of the options are, and is overall a wonderful resource. There are a few things that require special attention due to our slightly unconventional use case:

Dynamic Platform Model: This should be set to 8, which corresponds to the airborne model, with a max acceleration total of <4g. This is the maximum this GPS chip supports, and is supposed to help with bad data, and a physical limitations of how fast the GPS can calculate its position. If it is accelerating faster than this, then it assumes it is bad data, and reads out zeros for all values. This is expected behavior during accent, typically it becomes gentle enough for a packet with good data close to apogee. Note that this is not a CoCom limit, this is just a physical limitation of the lower end chips present in these sondes.

Altitude of LED shutdown (not sure what its real name is but this is basically it):

This should be set to above 1.5km ASL minimum for any sonde going to the SA Cup. If it is below this, then the sonde will not turn its light on as the elevation at Spaceport America is ~1440m ASL.

Recovery:

Sondehub is the website which tracks and predicts the landing location of weather balloons all over the globe, and is the main tool enabling the easy recovery options. It has a few offshoots, including Sondehub Amature (Focusing on amature HABs not related to weather), a HAB burst calculator (used to calculate how much lifting gas in your balloon, and when you can expect it to pop), and Sondehub Predict (used to predict the path of your HAB, but can also be used as a drift predictor to estimate where a payload/rocket may have landed while taking winds at every altitude into account). This suite of tools provides everything you need on the planning side of a HAB, or aiding the recovery of a weather balloon.

Recoverys usually happen in a few steps.

1. Using the prediction tools of Sondehub, look a few days out and see if there are any predictions that land in your general search area (I usually limit it to a 30-40 minute drive).

2. As the day gets closer, continue watching the prediction. They can change a lot over the full week time range. Assuming your prediction stays within your search area, watch for the launch of the balloon.

3. When the Balloon launches, the prediction will start updating with the actual position of the balloon taken into account. Wait until the balloon pops before you go for anything, as this can dramatically shift the predicted landing location.

4. It is recommended to wait until landing to leave for the balloon, to not waste time chasing something that lands in a forest. Once it lands, leave and set your GPS for the predicted landing location of the sonde. Make sure to enable Chase mode on Sondehub to indicate to other ballooners that you are actively searching for the balloon. Once you arrive, the next step depends on what type of sonde you are chasing.

5. Decoding sondes;

   1. RS-41 - Plug in your antenna to the SDR and SDR to computer. RS-41’s can be decoded directly in SDR Angel, and this makes them very simple to decode. Open SDR Angel, Select the RTL-SDR from the sample list, and tune to 403.8 MHz (Sonde frequency from Buffalo). Then press the tiny button with four circles in a triangle shape with lines between them and a plus button, which is right next to RTL-SDR in the samples window. Search this list for “Radiosonde Demodulator”, and add it to the workspace. This will create a colored overlay on the spectrum, which represents the bandwidth that the decoder is looking on. If you are in range of the sonde, you will see a regular, once a second rectangular shape on the waveform (See image bellow). Move the highlighted area over the brightest part of the shape in the waveform (you can edit the bandwidth if necessary using the “BW” slider), and if everything is right, you will start seeing decoded packets in the grey area bellow the settings for the Radiosonde Demodulator window.

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      SDR Angle Correctly decoding a RS-41

   2. DMF-17 - Plug in your antenna to the SDR and SDR to computer. The DMF-17 is not yet supported by the radiosonde demodulator in SDR Angle, so we have to do it the more difficult way by exporting audio. This is possible to do in SDR Angle, but as someone who is cursed to making every radio thing difficult, I have never achieved this. The primary workflow used is using SDR Sharp. This software is similar to SDR angel, with a few different features. First open SDR Sharp and select the hamburger menu in the top left. Press RTL-SDR USB in the “Source” menu. You also need to click on the “Radio” button in that menu, which will add a radio window to the screen. Make sure that you select NFM in the radio menu, as this is how the DMF-17 (and RS-41) transmit. (FINISH)

6. Once you have the final coordinates of the sonde, drive to its location. If it is on public land, or just off the road in someone's yard, it is usually ok to just run over and grab it. If it is well into someone's property, you should always seek permission. If it is normal human hours, knocking on someone's door and asking usually goes well. If it is well into the night, it is recommended to leave a note in their mailbox explaining what you are doing, where the balloon is, and leaving your number for them to call.

7. Once you have the sonde, you should open it up and disconnect the batteries to stop it from transmitting. After it is disarmed, congratulations! You have ethically sourced another GPS tracker for RIT Launch.