Apollo S-Band SDR Reboot (Patreon)

Many of you have asked how one would redo the Apollo link with modern electronics. And I believe one of the nicest way to do it would be using SDR (Software Defined Radio) technology. At least for the ground station where your electronics are not sensitive to radiation, and you can use the best and latest.

The SDR concept is straightforward: radio receiving or transmitting is nothing else than mathematical processing of very fast signals. Band filtering, frequency shifting, modulation and demodulation, these are all mathematical signal computations, which a radio does with electronic circuits. A traditional radio system is just one of the last standing analog computers, and it is on its way out.

Because, if you have enough computing oomph, and if you can do high bit rate digitizing and calculations that keeps up with speeds of say few 100 MHz, then all you need is an analog front end that shifts the microwave frequency down to this manageable intermediate frequency range, a fast, precise, and high dynamic range A/D, and then launch mounds of computer or FPGA computing power at it. It's essentially digital signal processing at RF frequencies. A professional-grade entry level SDR hardware platform is strong enough to do this for Apollo. The hardware, save the Tx power amp and receiver preamp, looks like this:

So we knew it should be possible to redo the Apollo S-Band stack with an SDR, but there is a catch. While the hardware gets simplified (actually it's not simple, but you just buy it!), all the heavy lifting is transferred to the software. And heavy lifting it is. Programming an SDR radio for something as complex as the Apollo stack is no task for a beginner. Nothing we'd have enough time to learn nor execute with the small team we have. We'd need the help of SDR gurus that can hit the road running.

Enter Balint Seeber and Austin Epps, through the comments section. They just casually dropped that they had done an entire S-Band SDR stack to reconnect to a forgotten NASA satellite launched in the 70s that was coming back close to earth. Then they connected their SDR hardware to the Arecibo antenna in 2014, successfully acquired and woke up the spacecraft, oriented it, and executed commands to fire its thrusters for a capture burn. They are local and willing to help us if we wish. Say again? You did WHAT?

Here is the satellite in question, which used a similar JPL S-Band system to Apollo, minus TV and voice of course. In their infinite wisdom, NASA had trashed all the transmission hardware and had no budget to make new one. So they let amateurs take over - because amateurs have way more money than NASA, right?

Here is Balint at Arecibo:

And below, with his diminutive SDR radio that NASA could not possibly afford. Actually they couldn't even afford to maintain Arecibo, as we found out recently.

And below is Balint in an Arecibo control room, with Austin in the background. Balint is doing the happy dance : they just re-awakened the spacecraft by sending a coded uplink command.

So they came in to my basement, and brought a similar SDR. The hardware is originally from an early SDR startup, but is now available from National Instruments.

Balint whipped up a transmitter and locking receiver in an afternoon, using Gnu Radio. This approach is sure flexible, but the programming takes real knowledge.

Wouldn't you know it, after a couple of tries, he sees the Apollo Transponder beacon signal, sends back his reference frequency and locks it.

A couple hours later, and we have subcarriers:

We transmitted tones both directions, then even some voice in emergency mode. But the real power of the approach will be the digital decoding of the telemetry. Which again is not for the faint hearted. This is the one they did for the ISEE-3:

But hey, it worked for ISEE-3.

More on their SDR feats in the next episodes...

Marc