Receiving Telemetry

During its lifetime, EQUISat allowed radio enthusiasts of all skill levels to interact and learn from it. 

Anyone with a UHF-band amateur radio could hear EQUiSat transmissions. Amateur radio enthusiasts could even decode the satellite’s telemetry using one of several methods:

The first two apps have the option to submit your data to our telemetry database at, and we’d really appreciate you forwarding us the telemetry you get to help us keep up with EQUiSat!

The appearance of EQUiSat transmissions in GQRX

The sound of EQUiSat’s telemetry burst (FM-demodulated) when the satellite was right next to the receiver. This is transmitted every 20s in normal mode.

The sound of EQUiSat’s telemetry burst from space. This is what you’d hear amidst the static using a handheld radio. The two beeps (as opposed to four) indicate this is a low power message.

The Details

Beacon Frequency: 435.55 MHz

Bandwidth: 12.5 kHz

Transmit Power: 0.5 W

Modulation: 4FSK @ 9600 baud

Callsign: WL9XZE

Current TLE:

CW Beacon: disabled since launch

Software Decoder: gr-equisat_decoder

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.

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

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 “Narrow Frequency Modulation” or NFM, but you may want to widen the bandwidth by dragging the edge of the gray section surrounding the red frequency tuning line, or messing with the “Filter width” setting in the “Receiver Options” pane.
  3. The RF Gain under the “Settings” dialog. It should be set to maximum to maximize how sensitive the SDR is and thus improve the amplitude or “size” of EQUiSat’s signal.
  4. I/Q Recording; this is a way for you to “record” the radio data that you’re seeing, so you can play it back later. You’ll want to record the whole pass if you’re going to try and decode it later. To record I/Q in GQRX, click the “Record and play I/Q data” icon in GQRX’s toolbar, navigate to a directory that has plenty of space left (I/Q recordings are very big, usually a few GB), and then hit the “Rec” button once the pass starts. In SDR#, look under the “Recording” section and check “Baseband” before hitting “Record.”

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!). If you’re taking an I/Q recording, make sure to start recording now as well!

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!