Capturing Slow Scan Television transmitted from space
All signals intercepted for this article are posted on YouTube. The event has finished but you can carry out these same exercises by using the captured signals and processing them yourself using a decoder of your choice. The ISS usually has something switched on as it flies overhead, so feel free to check anyways. You’ll see something!
As part of our new publication Radio Hackers, we’ve talked about some of the things that can be done in space using existing infrastructure. While some of these adventures require a license, an advanced ground station, as well as a bit of experience to setup and capture we’ve tried to keep a strong focus on intermediate activities as well. We’ve styled it in this way so that people can follow at a level of their own choosing with budgetary and technical skills being less relevant with persistence and a willingness to have a go being the more important attributes.
In the spirit of this, today we’ll be looking at a reasonably simple exercise that anyone with some free time can follow along with. As always if you don’t have an SDR system in your possession, you can still use a receiver. Alternately, you can also do a direct decode using captures that we’ve uploaded for experimentation purposes.
While it sounds a little complex, it’s probably the simplest way for beginners to get started with this type of thing. By the end of it, you’ll have captured and decoded signals from a transmitter orbiting around 350 km above the earth while it moves at around 27,000 km/h. No small feat and a worthy achievement!
While looking at the internet recently, we noticed that the amateur radio payload on the International Space Station would be transmitting Slow Scan Television signals. While this format was old, and most notably used during the Apollo missions, it’s still relevant today due it is ability to transmit images quickly and easily.
While you’ll typically need experience for space communications, the 2 meter down link of these transmissions along with their ability to be easily decoded make them a great type of signal to intercept as you build your skill set. However there’s a few reasons why this is the case, so let’s look at those first.
Being a receive only activity means that no amateur license is required for this activity. It’s perfectly legal to intercept these signals, with no requirement to re transmit or emit a signal back.
Secondly, these transmissions will occur on the 2 meter amateur band, centered around 145 megahertz (145.800mhz). While orbital signals in the 2 meter band will be affected by Doppler shift, it’s less obvious making this one less complication to deal with while it’s happening.
Lastly, the ISS has a great radio payload on board. This means that typically, the signals will be strong and clear for most of the pass providing your view to the horizon is acceptable. Very High Frequency signals perform surprisingly well when there’s no interference within the path.
The requirements for this are pretty straightforward. You’ll need:
- A radio receiver that’s capable of working at 145 megahertz. An RTL-SDR is perfect for this.
- An antenna cut to this bandwidth. We’ll be releasing an article on antenna construction in the coming weeks, but if you’re looking for something simple in the meantime you’ll find plenty of simple designs on google. If you’re using the RTL-SDR dongle, the standard antenna will work, but you’ll probably have better luck working with something that’s a bit more efficient.
- A device to run your SDR on, as well as to decode your transmissions.
- A satellite pass predictor. You can either use an App, or a web based system like this one
For the purposes of this exercise we’ve used a system that comprises of an SDRplay RSP2pro receiver, with a Discone antenna that’s feeding to a Linux machine running SDRConnect software. There’s also a Gpredict install taking control of orbital predictions and automatic frequency adjustment.
The RSP2 is a wide band receiver that’s capable of receiving signals from 1kz all the way through to 2ghz. It’s got dedicated filtering built in, the ability to take three separate antennas and is far more sensitive that the RTL-SDR devices while rejecting out of band signals more effectively.
The SDRplay is however, a receive only device. This means that for Radio Hacking purposes it’s less suitable than equivalent systems. However it’s price point, along with it’s good sensitivity and wide receive capability means that for signals interception purposes, it’ll still find a good home on many peoples shelves.
The Next Steps:
Now we understand our equipment we can look to plan our intercept. For the purposes of this we’ll assume you already have a working SDR system or receiver that’s being fed by a suitable antenna.
To successfully catch our orbital SIGINT target, we’ll need to prepare by obtaining a few things. Firstly, we’ll need the orbital data so we know when and where to point our antennas. And secondly, we’ll need frequency information so we can set our receivers to the right position. Let’s get the frequency information first
As we’ve mentioned previously, the ISS has a pretty comprehensive amateur payload aboard, with multiple transmitters configured in different modes at different times. By the time of this article SSTV events had finished, but keep an eye out into the future as these tend to pop up more regularly than you’d think. Find the status information
We can see slow scan television transmissions down links on 145.800 mhz, while packet radio transmissions can be found nearby on 145.825 mhz. There’s also a voice repeater that down links on on 437.800 mhz but for the purposes of today’s exercise we’ll be focusing on the 145.800 down link. These transmissions use Frequency Modulation (FM) and as such, have a reasonably wide bandwidth.
Let’s discuss the orbital information next. Your software package of choice should be pretty straightforward to get up and running, so we’ll look at the essential pieces of information we need rather than a direct guide.
There’s three essential pieces of information here that are relevant to anything in orbit. We’ll need to know where it’s coming from (AOS), where it’s going to (LOS) and precisely where, and how high above the horizon from us it will be (TCA). These are pretty important concepts for space communications, so we’ll break them down a bit further. For the purposes of our imaginary pass, it’ll come from due south and leave at due north. This makes things simple to follow.
AOS: Will give us the approach vector for the satellite. This will usually be in degrees, so given our satellite comes from due south we’d expect to see it at 180 deg
TCA: Is time of closest approach to the calculated area during the orbital pass. In our imaginary pass, this would be when the satellite is directly overhead. We’ll also receive this in degrees, so this would be 90 deg elevation.
LOS: This is when the satellite disappears over the horizon away from us. Like AOS, it’s a calculated bearing so for our pass, its LOS would be at 00 deg.
Given our satellite orbits the earth in a circular (ish) direction, it’s important to grasp one last thing. As our imaginary satellite moves through AOS, hits TCA and then disappears through LOS it will arc across the sky, rising and falling relative to your position through the orbit. We’ll need to account for this third degree of movement as this will often impact signal strength and readability. While it’s less important for omni directional antennas that come with a cheap SDR dongle, the best results come from using directional antennas and azimuth calculations are an important part of getting good results from these. You’ll also find lower elevation passes off near the horizon to have a weaker signal, meaning capturing a signal effectively will require more work at your end.
The Capture:
As we work on our capture, we’ll need to keep a few things simple and organised as our target prepares to move overhead.
Firstly, we’ll need to look at our down link frequency, by setting our receiver to 145.800 (FM MODE) and then backing off the squelch, so weaker signals are able to break through the noise. Watch your computer volume here, as we’ll have an increase in audio level when the ISS is in range, even if there’s no clean signal. This is called the noise floor, and as you become more experienced at intercepting signals you’ll start to notice the presence of this more prior to signal transmissions.
We’ll also record our pass, as it gives us the chance to focus on a capturing a clean signal. You can either use a mobile phone, or plugin that operates with your SDR. Our chosen program, SDR Connect has the ability to record built directly in to the software.
We’d recommend popping over to our YouTube Channel prior to the pass, as you’ll be able to hear the transmission prior to the event. You’ll also understand what a clear pass should sound like, which should hopefully make it a little easier when you’re tracking the ISS your self.
Once you’ve captured your signal, we’ll need to look at the decode next.
How this will work will depend on what system you chose to use. On computer we can use FLDIGI or MMSSTV, while on Android & IPhone, there’s Robot SSTV Decoder. Using the decoder is pretty simple as they’ll work using the omni directional microphone on your phone.
So simply hold your phone up to the speaker as the signal is playing and you’ll start to see the frame decoding, line by line.
If you aren’t participating in the exercise but still want to see the decoded image then you’re in luck as we’ve included a copy of the image for those that aren’t playing along at home. For those that are though, your image should look pretty similar to this.
We also included a second capture that wasn’t as clear as the first. This has been included as well, as it clearly shows what occurs as the station moves through its orbital pass. The green lines within this image is “noise” where the emitted signal was tapering off. As we’re capturing audio, the goal is to reduce this noise level to nothing but signal, giving us a clear Signal to Noise ratio and a nice clean decode from our captured signal.
In Closing:
As we come to the close of today’s article, it’s important to keep a few things in check.
Firstly, like anything you’ll find that calculating orbits, setting up and locating the satellite within the sky is directly linked to experience. While your first attempt may feel rushed, with arms and antennas pointing everywhere, it’s essential to realize that it won’t always be like this.
As you develop your skills and adjust to capturing signals from orbit rather than the ground, you’ll find that your able to track the satellite and keep your antennas oriented much more easily. You’ll also find that your ears start to recognize the noise floor as the satellite moves through AOS and is acquired by your station.
All these little things will make it easier for you to find your signal, and more importantly explore other signals and devices within the space communications bandwidth. There’s a whole range of bandwidth allocated to space communications, meaning that there’s a whole world of orbital signals for the budding Radio Hacker to explore.
In our next installment of Radio Hackers, we’ll look at capturing and decoding digital telemetry signals from space based assets. So keep your SDR units ready, and your antennas pointed skyward.
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