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Peter Vogel

How to combine low-tech, high-tech, and space tech

Voices March 25, 2019

The International Space Station, from its orbit at about 400 km above the earth, occasionally transmits special images through audio tones. Photo: NASA

Want to see the International Space Station?

That is a fairly straightforward activity, even in city lights. I’ve written about it several times in this space over the past decade or more. I will summarize how you can go out and spot the ISS at the end of this column.

However, in recent years a somewhat novel activity involving the space station has sprung up. At one time, this activity was strictly in the purview of ham radio operators, people who have a license to work with radio equipment in novel and interesting ways.

Although the activity I am about to describe was indeed developed by ham radio enthusiasts, it is feasible for anyone with a suitable radio and a cellphone. Yes, the cellphone and its related technologies have even altered the more than 100-year-old field of ham radio.

Quite a few astronauts and cosmonauts who serve on the ISS are also ham radio operators. Through the auspices of an international group known as Amateur Radio on the ISS (ARISS), there is a functional ham radio station on the ISS. The ISS crew may make use of the station in free time, it may be used for ISS-to-school contacts, or it may find use for broadcasting special messages in a format known as SSTV.

SSTV, or slow-scan television, involves the transmission of images with a special protocol, or signal, resulting in images that may take as much as two minutes to render. 

Now, normally, to receive a television image, a fairly complex receiver is required. However SSTV images are fairly low resolution, quite small, and are encoded in a series of audible tones transmitted over a VHF (very high frequency) signal, in this case from an antenna on the ISS some 400 km overhead.

So strong is the signal from the ham radio station on the ISS, that it can basically be picked up whenever the space station is above the horizon.

Recently, ARISS conducted a special two-day transmission of special commemorative space program images, using equipment that was recently installed aboard the ISS in the Russian module. However, there was a technical problem on the first day so the two-day event was extended to four days across successive weekends.

I was determined to receive all twelve images that were being sent. My equipment consisted of a handheld ham radio transceiver (I am a licensed ham radio operator) and a cellphone. For those who aren’t ham radio operators, all that is needed is a radio capable of receiving VHF signals. Specifically, the ISS transmits on a frequency of 145.800 MHz.

In addition to a radio receiver, the only other equipment needed is a cellphone. In my case that would be an Android phone, but everything works pretty much the same way on an iOS device.

Radio, cellphone; alright, you are ready to intercept an SSTV image from the ISS. Just one problem. How will we know when to go outside for an ISS crossing?

That’s where a number of services mentioned in this column over the years come in handy. Most reliable of all is the site heavens-above.com. Once supplied with your location, this website can produce a table of ISS crossing times. For the recent ARISS experiment there were around a dozen opportunities each of the two weekends.

Some of those crossings were ideal, taking as much as ten minutes, others just getting barely above the horizon for a minute or two. Since a full image takes two minutes to transmit, anything shorter for an ISS crossing can only produce a partial image.

Furthermore, the protocol for image transmission from the ISS calls for a two minute gap of signal silence between images. In short, the process for pulling in an image does have some challenges.

Where does the cellphone fit in to all this? Thanks to the power of modern smartphones, they can be equipped with a small software application that can decode the audible tones that comprise the SSTV image. In the case of my Android phone, the free application is called Robot36.

Basically, the drill runs as follows. At one of the times the ISS is to cross, I head outside with my radio and phone. If an SSTV image is being transmitted, the tones (similar to those of dialup internet service from days gone by) are immediately apparent from the radio. On the phone, I have the Robot36 app already started. I keep the radio in proximity to the phone’s microphone.

On the phone’s screen, an image begins to form, one slowly scanning line at a time, hence the name "slow-scan television." Robot36 is able to assemble the image and, once complete, automatically save it as well. On an optimal path, the procedure may result in two or three complete images.

I was happy to be able to get all twelve images that were part of this particular experiment. My images weren’t perfect. Noise from passing vehicles interfered with some of the image decoding. I could improve this by connecting the radio directly to the phone with a cable.

While a single image in two minutes appears decidedly low-tech, it is very satisfying to bring together a number of technologies and have an image from an orbiting space station appear on a phone.

Perhaps you will give it a try one day. In the meantime, use heavens-above.com, or any of a number of ISS phone apps, to alert you as to when the ISS might be visible.

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