Receiving radio signals from polar orbiting weather satellites

In my last blog post I wrote ...

It's easier than ever to track National Oceanic and Atmospheric Administration (NOAA) polar orbiting weather satellites, ... Source Credit: Tracking polar orbiting weather satellites.

I used the n2yo.com Web site to get 10-day predictions for NOAA 18. I noticed a good pass on Tuesday, 05 September 2023 so I grabbed my Baofeng UV-5R radio and went outside to a clear place in order to listen for the satellite signal. I was delighted to hear the distinctive sound of the APT radio signal soon after I set up my "ground station."

Recording

I recorded the satellite pass by simply holding the Baofeng UV-5R radio speaker close to the internal microphones on my Apple iPad mini 6 -- a technique demonstrated in a YouTube video by "saveitforparts." I used the Shure "MOTIV Audio" app (for iOS) to record the pass as a 16-bit 44.1 kHz WAV file (mono).

The duration for the pass was 15 minutes. [Start: 12:08. Max: 12:16 (81 degrees, W). End: 12:23.]

First minute of the pass.

One minute centered around the midpoint of the pass.

More noise is to be expected at the beginning and ending of a weather satellite pass, when the spacecraft is near the horizon. The signal should be stronger and clearer when the satellite is nearly overhead, but as you can hear in the preceding WAV file there's still too much noise for a good image.

Decoding

It's easier than ever to decode weather satellite signals to create images. In this case I used "open-weather apt," the free online weather satellite image decoder.

"open-weather apt" seems to work well but only works with 16-bit 11,025 kHz WAV files, so I had to resample the 44.1 kHz WAV files I recorded using my iPad. I used a free audio editor called "Audacity." There are many "how to" videos on YouTube such as this one by Chris Murphy.

Here is my first attempt at direct reception of an APT radio signal.

05 September 2023 | NOAA 18

Not exactly perfect, but hey, experts say it's not bad for my first pass.

Tech tips for decoding using "open-weather apt"

The "open-weather apt" page includes a link to a sample WAV file that you can use to hear and see what a good pass is like.

Download the sample file and listen to it using an application such as QuickTime Player (for Apple and Windows). You can hear both the beeping and "metronome" sounds much more clearly than in my recording.

Then upload the WAV file in order to familiarize yourself with the decoding process. Here's one iteration of the decoded image.

The preceding composite image shows both the visible (left) and infrared (right) channels for the sample satellite pass. Just a little better than mine!

What are the take-aways?

It's easier than ever to receive APT radio signals from weather satellites, with one critical caveat: It's as challenging as ever to receive a strong, clear signal.

• It's clear (pun intended) I need a better antenna than the small "rubber ducky" antenna that comes with the Baofeng UV-5R. I'm considering upgrading to a "V dipole" antenna.
• Signal reception seemed to be better with the "rubber ducky" antenna oriented horizontally (parallel to the concrete pavement), rather than vertically as recommended by YouTuber "saveitforparts." This was especially true when the satellite was nearly overhead -- the dropout might be related to the "null" of vertical antennae.
• I setup my "ground station" on the top of a seven story parking garage located in exurban Washington, D.C. There was too much ambient noise for a clean recording by simply holding the radio speaker close to the microphones in my iPad mini 6, as suggested by YouTuber "saveitforparts."

Perhaps the biggest take-away is a good question. Why try to receive weather satellite images directly when so much high quality imagery is available on the Internet for free? I can't answer that question for you, but I can say for me the challenge is to see whether relatively inexpensive gear can be used to get reasonably good results. Although the quality of my first weather satellite image is disappointing, I was able to successfully track NOAA 18 and hear a radio signal from the satellite -- accomplishments that I can and will build upon.

Related Resource: APT and HRPT

There are two types of NOAA polar orbiting weather satellite imagery that amateur radio enthusiasts attempt to receive.

Direct Readout Services include the Automatic Picture Transmission (APT) for low resolution imagery and High Resolution Picture Transmission (HRPT) for higher-resolution imagery. Source Credit: NOAA Satellite Information Service, U.S. Department of Commerce.

Direct reception of HRPT radio signals is way beyond my level of expertise.

Copyright © 2023 Walter Sanford. All rights reserved.