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Automatic Packet Reporting System (APRS) 101

  • 02 May 2025
  • Text by: Karthik Raj, SEDS NTU, Avionics Lead, Singapore Rocketry Club

SEDS NTU Project New Dawn and the Singapore Rocketry Club are in the process of implementing an APRS-based tracking system to track our High-Altitude Balloon (HAB) and rocket.

Previously, in How To Track A Flying Object In Real Time?, we looked at how APRS works. This article will go over component selection and present a high-level overview of our APRS system that will fly on the HAB and eventually on the rocket.

APRS 101. Photo: SEDS-NTU

This communication system consists of 4 main components:

1. The APRS transmitter (on the HAB / rocket).
2. Transmitter antenna
3. Receiver antenna
4. Receiver (on the ground)

Note that this system will be simplex i.e. one way communication only.

We started off with the APRS transmitter – The transmitter is where the APRS packet begins its journey. We did a deep dive on the Commercial Off the Shelf (COTS) APRS Tracker Market, and this is what we found:

Table 1: List of Transmitter. Photo: SEDS-NTU

Our club ended up choosing the LightAPRS module from QRP Labs for several reasons. The LightAPRS module is priced at a very modest S$150, which was well within our budget. The module could easily be programmed through the Arduino IDE, which we are well-versed in. LightAPRS had also been tested and used by amateurs all over the world to track their homemade contraptions.

Next on the list was the transmitter antenna. We initially used a 50cm wire antenna soldered to the antenna pad on the LightAPRS. This did not work out well. After configuring the LightAPRS with the proper firmware and placing the module under the open sky with no obstructions from buildings or trees in order to get a line of sight to multiple GNSS satellites, we still failed to receive any APRS packets. 

One fine day, during a last-ditch effort, the handheld transceiver we were using to receive APRS packets suddenly crackled to life. We heard an oddly satisfying yet annoying sound that we had never heard before on 144.39 MHz. That was our first APRS packet! How did it work? We had propped up the LightAPRS module on a roller chair for testing. It turned out that the wire antenna was loose, and that particular orientation made the antenna contact the solder pad on the LightAPRS just the right amount to get out a few APRS packets. A few minutes later, the wire antenna fell off. Lesson learnt: Do not solder a wire antenna directly to a solder pad. Solder it on an SMA connector instead.

The LightAPRS. Photo: SEDS-NTU

It was high time we ditched the wire antenna for a more suitable transmitter antenna. Listed below are some of the suitable APRS transmitter antennas that we considered:

Table 2: List of APRS transmitter antennas. Photo: SEDS-NTU

Antennas come in many different shapes and sizes. Antenna design and simulation is a rabbit hole in itself. 

Let’s go over some of the basics. The lower the frequency → the longer the wavelength → the longer the antenna. Conversely, the higher the frequency → the shorter the wavelength → the shorter the antenna. Most antennas we found above are monopole antennas. Theoretically, the length of a monopole antenna has to be about ¼ the wavelength. The APRS downlink is on 144.39 MHz → Wavelength = ~2m, hence the required monopole antenna length = 50cm. This is simply too long to fit on a HAB or rocket! Interestingly, we were able to find antennas that were much shorter than 50cm but supported the APRS frequency 144.39 MHz. How? Simply put, these shorter antennas trade off efficiency for length. Our antenna of choice for this project, the 17cm Siretta Delta 12 antenna, was certainly able to be used for transmitting APRS packets at 144.39MHz; However, it was not as efficient as a 50cm antenna like the Taoglas FW.80.SMA.M. Will it provide enough range? I guess we will have to find that out experimentally!

Antenna length is just one part of the puzzle. The HAB/rocket might be flying in different orientations. So, the transmitter antenna should ideally transmit in all directions so that the receiver on the ground can capture the APRS packet no matter the orientation of the transmitting antenna. For this to work, the transmitter antenna must be omnidirectional, i.e., have low gain. The Siretta Delta 12 antenna we chose has a gain of only 3dBi, which is very omnidirectional. The antenna has a “donut” shaped radiation pattern. In order to get most of the radiated energy out, the antenna was aligned to the roll axis of the HAB/rocket (pointed towards the tip of the nose cone).

LightAPRS and the Siretta Delta 12 antenna. Photo: SEDS-NTU

Now, we had the LightAPRS hooked up to a Siretta Delta 12 antenna ready for field testing. We used a Baofeng handheld transceiver connected to a monopole receiver antenna for our initial range tests. We were able to pick up signals from the APRS transmitter up to about 1km. Beyond this, the signal was just too weak. Solution? Increase the gain i.e., use a directional antenna on the receiver side. This will greatly increase the received signal power. By far, the most popular directional antenna used by amateurs is the Yagi-Uda antenna, commonly referred to as the Yagi antenna. The Yagi antenna radiation pattern is very directional. The Yagi antenna focuses its main beam in a particular direction, unlike the monopole antenna, which radiates equally in all directions. This essentially means that the Yagi antenna has to be pointed directly at the HAB/rocket. If pointed accurately, the Yagi antenna’s high gain will be multiplied by the received power and result in a very strong received signal for post-processing and decoding the APRS packet.

Using a Yagi antenna on the receiver increased the range; we were able to receive APRS packets to about 2.5km. Better than before, although we still expected a longer range. We were hoping for 10km minimum. On top of that, throughout these tests, we “received” 100% of APRS packets but were only able to “decode” 10% to 20% of the APRS packets. In other words, the Baofeng handheld transceiver was able to receive the APRS packet (APRS tone was audible); however, once the received APRS packet was sent down the signal processing chain into the mobile phone for post-processing via APRSdroid, no data was decoded. This meant that the full APRS packet was not received. We were stuck again!

TLDR: Issue 1 – Communication range lower than expected (actual 2.5km, expected 10km).        Issue 2 – Only 20% of the received APRS packets could be decoded to retrieve the GPS coordinates.

An example of a Yagi Antenna. Photo: Efrem Efre, Pexels

Eventually, we narrowed down the root causes of these issues to the front-end processing on the Baofeng transceiver – a cheap handheld like this might corrupt the received signal and result in increased losses. A better receiver was required! Moreover, APRSdroid had a lot of subtle issues. A better software TNC was needed! This is when we decided to switch to a Software Defined Radio (SDR) for front-end processing and use Direwolf as the software TNC. This is the final setup:

1. GPS signal received by LightAPRS
2. LightAPRS generates an APRS packet encoded with GPS coordinates of HAB/rocket, altitude, onboard battery level, and internal temperature data.
3. APRS packet is sent to the ground via a low-gain omnidirectional monopole antenna
4. A high-gain directional Yagi antenna receives the APRS packet on the ground
5. Received signal is sent to an SDR for front-end processing.
6. SDR converts the received signal into an In-phase Quadrature (IQ) packet that is sent to a computer for real-time post-processing.
7. A computer running Direwolf software TNC decodes the APRS packet to retrieve the location and sensor data from the HAB/rocket.

With the above setup, we were able to receive and decode 100% of the APRS packets received over short ranges up to 1km. This system will be stress tested further over longer ranges before flying on Project New Dawn’s HAB launch in the second half of 2025 and eventually fly on the Singapore Rocketry Club’s next rocket, Sunbird, in 2026!

The development of this long-range APRS communications downlink was a team effort by SEDS NTU Project New Dawn Electron and Singapore Rocketry Club Avionics. I would like to thank everyone involved for the hours put in at the lab and long nights spent range testing at Pandan Reservoir.

Stay tuned for more exciting updates from the stratosphere in the months ahead! 

Over and out.

A Space Themed Illustration. Photo: Kindel Media, Pexels

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