Receiving Telemetry

Are you a beginner to radio or satellite communications? See below for how to get started!

Anyone with a UHF-band amateur radio can hear EQUiSat transmissions, but currently the only known way to decode transmissions is using the XDL Micro transceiver board. However, we’re actively working on building a software decoding method for transmissions, so it may be possible to decode transmissions received today in the future!

We’d love to hear from amateur radio enthusiasts who have heard EQUiSat even if the data is not decoded. Raw I/Q dumps from SDR software such as GQRX, Matlab, or SDR# are particularly helpful. You can submit these, demodulated audio samples (FM/AM/SSB), or anything else here. You can also just tweet us that you heard it. We’d very much appreciate your help in keeping up with EQUiSat!

Anything you can send us is helpful, even a screenshot of your SDR waterfall or just a message letting us know you heard it.

The appearance of EQUiSat transmissions in GQRX

The sound of EQUiSat’s telemetry burst (FM-demodulated). This is transmitted every 20 seconds.

The Details

Beacon Frequency: 435.55 MHz

Bandwidth: 12.5 kHz

Transmit Power: 0.5 W

Modulation: 4FSK @ 9600 baud

Callsign: WL9XZE

Current TLE:

tracking.brownspace.org/api/equisat_tle

 

Transmission Frequency

Every 20 seconds, a batch of four 213ms long transmissions 750ms apart. The satellite transmits constantly, but in low power mode the batches are reduced to two transmissions every 40 seconds (this is a great way to tell the satellite’s state without decoding the data).

Antenna Information

Half-wave dipole, with an estimated gain of 1.82 dBi

Packet Structure

255 byte packets separated into sections of preamble, “current data”, “historic data”, errors, and parity bytes for error correction.

See here for details on the format and here for a description of satellite signals.

Antenna Orientation

The satellite aligns itself with the earth’s magnetic field lines, which run in an arc from the South pole to the North pole. So, the antenna may be in any orientation around an axis in the same direction of the earth’s magnetic field lines. Note that these field lines (and thus EQUiSat’s flash panel) point more towards the earth as the satellite moves north.

Why can’t EQUiSat’s transmissions be decoded as conventional 4FSK?

Although the XDL Micro we use reports 4FSK modulation, it is a non-standard format, with FSK deviations of 2.4 kHz and 2.8 kHz from the center frequency.

We have configured the radio to use “transparent mode” such that the data is encoded transparently (without scrambling or encryption) onto the 4FSK protocol, but because the modulation format is non-standard, most 4FSK decoders cannot decode its transmissions.

We are currently investigating other options, including GNU Radio, to decode the XDL’s transmissions. We always appreciate help or suggestions, so please reach out!

Decoding EQUiSat as a beginner

 

1) Getting the hardware

There are many different types of radios and antennas out there to use, and some can be very expensive, but recently a new kind of radio called a Software Defined Radio (SDR) has become very popular. Though the basic versions of these new radios are often not as high quality as conventional “analog” radios, they are very versatile and inexpensive, and are good enough for receving EQUiSat!

The most popular SDRs are based off of the RTL-SDR chip (a modified TV tuner), and can be found on Amazon or other sites for as low as $25 for a complete kit. BSE recommends these for first time radio operators, because they don’t require an amateur radio license, work pretty well for hearing satellites, and are useful for many other radio tasks as well. We have also confirmed that these SDRs can hear EQUiSat with their stock antennas in the right conditions.

When buying an RTL-SDR, two good brands to look for are NooElec and RTL-SDR Blog, which both produce good starter kits (NooElec kit, RTL-SDR Blog kit). If you look for other models, make sure that the kit you buy includes an antenna of some kind with it!

NooElec SDR
RTL-SDR Blog SDR

2) Installing SDR software

Once you have an SDR, you’ll need to plug it into a computer and install the software. There are several types of software out there for your RTL-SDR. The most popular are SDR# (SDR Sharp) for Windows, CubicSDR for Mac, and GQRX for Linux.

Adafruit has a great tutorial on getting started with SDR# using an RTL-SDR (there’s also a section about using CubicSDR on a Mac). This is a perfect tutorial to follow to get aquainted with the software and tune to an FM radio station!

 

3) Setting up the SDR software for EQUiSat

Once you’ve completed the tutorial and are familar with the software, you can set it up to listen for EQUiSat! The three main things you need to configure are:

  1. The frequency you’re tuned to. This should be set to EQUiSat’s frequency of 435.55 MHz or 435550000 Hz.
  2. The radio “demodulation mode” you’re set to. This controls how the software interprets the radio data to produce audio, and the best mode to hear EQUiSat in is “Wide Frequency Modulation” or WFM (side note: this is becuase the “wide’ mode allows you to be less worried about the exact frequency, specifically the variations from Doppler shift).
  3. The RF Gain under the “Settings” dialog. It should be set to maximum to maximize how sensitive the SDR and thus improve the amplitude or “size” of EQUiSat’s signal.

Once your software is all configured and your SDR dongle is plugged in, you can press the play button in the top left and start listening (make sure to do this before the pass starts!)

3) Finding and targeting a pass of EQUiSat

Satellites move so fast that there is only a small window of time, called a “pass,” where the satellite is flying over your head and can be heard on radio. These occur a few times a day, and are concentrated in a period of a few hours. For EQUiSat they will happen about every 93 minutes because that’s how long the satellite takes to orbit Earth. To get a good chance of hearing EQUiSat on your radio, you need to plan which pass you’ll listen during and be ready beforehand.

Where to find passes

You can find information on the next pass over your location using our tracking website or our mobile apps (iOS, Android), but sometimes it is more convenient to see all the passes available to you over the next few days. Until we add this feature, you can get a list of passes using the HeavensAbove website. To do this:

  1. Go to their location input page and search for your location. Hit “update” at the bottom once it’s found.
  2. Go to EQUiSat’s pass list page and choose a pass as described below

How to choose a pass

You’ll want to choose a pass with the largest value for “max elevation” or “max altitude.” This value represents how high the satellite will be at its highest point during the pass, measured in degrees above the horizon. The higher it is, the fewer ground obstructions (trees, buildings) and less atmosphere will be between you and EQUiSat, making the radio signal stronger.

BSE Tracking Site

HeavensAbove Pass List

4) Listening during a pass

Determing where to be while listening

Once you’ve picked a pass you’re going to listen during, you’re ready to set up and try and receive. Note that radio signals lose their strength when they go through anything (including air), so you’ll definitely want to be outside with a clear view of the sky when listening for satellites.

You’ll also want to make sure there are few obstacles between you and the satellite, and to do so you need to know approximately where the satellite will be in the sky when it’s going overhead. You can do this by looking at the pass details:

If you look at the rise and set “azimuth” of the pass, you can tell which direction the satellite will come from and which direction it will leave towards. These azimuth values are the number of degrees clockwise from due North, so if the satellite comes up at “270” azimuth and sets at “90” azimuth, it will rise from the West and set in the East. These numbers correspond to the numbers on your compass, so if you line up your compass needle with the North mark on the dial and look along the line corresponding to the azimuth number for the pass, you’re looking in that direction. HeavensAbove also gives you this azimuth number as an abbreviation like “NW” for North-West.

If you look at the maximum elevation number as well, you can tell how high the satellite will be in the sky when it’s right in the middle of its pass (remember that 90 degrees means the satellite is directly above your head and 0 degees means it is right at the horizon line).

So, once you have a general sense of what direction the satellite is coming from, you can decide the best place to stand such that there aren’t many big objects (trees, cars, buildings, mountains) between you and the satellite. An open field or balcony is always a safe bet.

What to do during the pass

Now that you’re in position, you can get your SDR software all set up and tuned to EQUiSat. Then, you wait for the pass “rise time” to come up, and after that you’ll want to keep your eyes on the screen watching for little bumps in the spectrum at 435.55 MHz and listening for quick beeps in the audio from your speakers. These are the sights and sounds of EQUiSat, so you need to look and listen very carefully. Keep in mind that the most likely time for you to hear or see EQUiSat’s transmissions is in the middle of the pass (it’s closest to you at that point).

If you do see or hear a burst with 4 beeps in it, you can start counting up to 20, at which point you should hear the next one. If you only hear 2 beeps, you’ll have to count to 40 because the satellite is in “low power mode” and isn’t transmitting as fast. You may only be able to hear two or three bursts during a pass, however.

Trying again

If you don’t hear or see anything the first time, keep trying! We have heard EQUiSat with setups like yours and it is possible! Sometimes it may be best to wait for a pass with a very high maximum elevation (more than 75 degrees) to be sure you can hear it. You can also try pointing your antenna differently during the pass (usually you just want to make sure the ends of it don’t point towards the satellite), or try moving away from big metal objects or trees and obstructions. Radio waves can be tricky to receive because a lot of things effect them, but with enough experimenting you’ll be able to hear EQUiSat and other satellites easily!