Propagation of AM and FM radio signals up into the sky would behave differently than propagation along the surface of the Earth.

As a Gedankenexperiment, if you were on the ISS and had an AM/FM radio sitting at the ISS' cupola window, what would it sound like as you slowly tuned through one of the bands? Would it be packed full of stations competing at each frequency, or would it be nearly empty with only an unusually strong station making it through. Naturally AM and FM should be discussed separately.

If you were hoping to catch a particularly weak AM or FM station, would you arrange to listen when you passed overhead, or would a different pass be best?

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above: from the Radio Jove Project's exercise The Effects of Earth's Upper Atmosphere on Radio Signals.

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above: Vintage Singer 9-Transistor Radio, AM-FM Bands, Model HE-229, Made in Japan. From here.

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    $\begingroup$ The FM radio stations antennas are optimized to radiate parallel to the Earth surface but not vertical to it. The power of the transmitter should be used efficiently. Therefore the position overhead a radio station should be not the best one. $\endgroup$ – Uwe Aug 19 '17 at 20:44
  • $\begingroup$ @Uwe yep! If you could look image the Earth from space (at a million km for example) with a radio telescope tuned to the FM (or part of the TV) band, the brightest areas might be around the edge. But from LEO, $1/r^2$ is still very important. But for AM it's important to consider other effects as well - including the ionosphere. $\endgroup$ – uhoh Aug 20 '17 at 3:51
  • $\begingroup$ Not sure if there is an answer to this question... like the picture of the "Singer-9 Transistor Radio" though... how beautiful old equipment looks... but that is probably just me... $\endgroup$ – Edwin van Mierlo Feb 17 '18 at 14:26
  • $\begingroup$ @EdwinvanMierlo actually I've given this some thought and it's not that hard to answer. Line of sight path, typical broadcast antenna directionality, and known ionosphere transparency is all you would probably readily lead to an answer of "many" or even "hundreds" for the FM broadcast band. However getting the typical ~1 MHz AM broadcast band through the ionosphere is going to be extremely difficult. $\endgroup$ – uhoh Feb 17 '18 at 15:29

TL;DR: Based on the below information, and given 4 assumptions, the maximum number of stations one could legibly hear would be 42 or less.

Not sure if there is a factual answer to this question. But as the OP has indicated a "Gedankenexperiment" as the basis of the question. We may theorize, how many stations you can receive.

Lets concentrate on the FM broadcast band, which in many countries goes from 87 MHz to 108 MHz, some countries lower, and some countries higher. However, looking at the picture of the proposed radio receiver, I think this is the band we need to concentrate on. These frequencies are not reflected by the Ionosphere back to earth, so we are good to receive them.

It is mentioned in the comments, that the main broadcasting antenna's will have their main directivity pointed to their intended audience, which certainly is not up to the Ionosphere, but horizontal, parrallel to ground, or even in a downward angle.

However RF is a strange beast, and antenna's are far from ideal. Most antenna's will have something which is called "side lobes". And it is in theory possible that some of these side lobes are pointing more upwards, with a larger angle, therefore some of the signal is "going into space".

As the ISS is roughly 406 km above earth, the radius of earth is about 6371 km, it is simple to calculate the theoretical area which is "visible" from the ISS.

A = 50%/(1+R/d)

Which brings A to about 3%.

This brings us to:

  • how many stations in the 3% of the visible earth,
  • and how many of those stations have antenna's with side lobes which are radiating signal to the location and path of the ISS
  • and are strong enough to be received/demodulated above the noise floor.

From here it is guessing, as factual calculation is not possible.

  • At least some
  • Probably many
  • Possibly a lot

Now, some more information, there seems to be 44,000 radio stations in the world, well if you believe the source, as the deep link seems to be gone.

Lets take this 44000 and with this, there are AM and FM stations. With AM in decline, lets apply an 80/20 rule on this [assumption 1] and put the total number of FM stations to 35200.

Lets say that these staions are equally distributed on the surface of the earth [assumption 2], and 3% of that is visible. This will bring the number of visible stations to 1056 stations.

The afore mentioned side lobes are not all directly pointing to the ISS, it is a bit of hit-and-miss, so lets apply a 20/80 rule on those [assumption 3], which now brings the number of receivable stations to 211.

However, as this is FM: if two, or more, stations are on the same frequency, you will not be able to hear any (likely) or only the strongest. This needs to be seen in comparrisson to AM; if more than one station is on the same frequency then you can still hear all stations... just like you can hear multiple people talk simultaneously. (This is one of the reasons that Aviation traffic on 110 MHz - 135 MHz is still on AM, the flight controllers can hear more than one plane on the same frequency)

Now, the band we have chosen is 87 MHz to 108 MHz, and with a 200 KHz bandwidth for an FM station, this means that there are 105 possible "slots" for non-overlapping broadcasts.

These "slots" are actually the most limiting factor. The stations will not be evenly spread, many will have "popular sounding frequencies", and overlaps will happen. For this purpose lets apply another 20/80 rule to the equation [assumption 4]. Which will bring the number down from 211 to 42.

Suppose you take the radio receiver as indicated, and "tune" in to a station which is receivable. The fact that the ISS is traveling at an orbital speed of 27000 km/h, it would mean that the received signal is influenced by a doppler effect. You will have to tune higher when approaching the transmitter location, constantly lowering the frequency during your listening experience. It is debatable if the estimated 2.6 kHz drift doppler will introduce, is significant or not. The radio receiver used may even have an AFC circuit which may correct this.

How many stations could one hear with an AM/FM radio in front of the ISS' cupola window?

Based on the above information, and given 4 assumptions, the maximum number of stations one could legibly hear would be 42 or less.

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    $\begingroup$ This is really great! Yes, you've captured the spirit of the question very nicely, but done a more detailed analysis than most would. It sounds like the listening experience wouldn't be too pleasant, both with pile-up and the ISS moving so quickly in and out of range as well as in and out of lobes and side lobes. But as a thought experiment, still quite fun. $\endgroup$ – uhoh Feb 19 '18 at 12:18
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    $\begingroup$ @uhoh :-) hence the "or less" in my conclusion ! $\endgroup$ – Edwin van Mierlo Feb 19 '18 at 12:19
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    $\begingroup$ @uhoh fair-enough, still it is a valid item to take into consideration. With a modern PLL (Phase Locked Loop) detector not even worth mentioning. However with the receiver pictured, you do need a steady hand ! :-) $\endgroup$ – Edwin van Mierlo Feb 19 '18 at 13:30
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    $\begingroup$ Not sure how old the Singer-9 is, I could not find any circuit-diagrams nor any release data. It seems to be from end-1950's or start 1960's. And for such a basic radio; I would not know if it encompasses an PLL detector. the PLL technology is older, but in order to keep prices affordable radio's from that era may or may not have this. I will leave my answer as is for now. $\endgroup$ – Edwin van Mierlo Feb 19 '18 at 13:44
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    $\begingroup$ A FM receiver using only 9 transistors and no integrated circuit at all could not use a PLL. Without any IC, using a PLL in a consumer electronic radio receiver was economically impossible. You need a complex circuit to generate all frequency channels within the FM band locked to a very stable XTAL oscillator. $\endgroup$ – Uwe Feb 19 '18 at 15:06

Since most of the answers discuss FM broadcast signals, let's look at AM broadcast signals as well, and discuss propagation throughout the radio spectrum. I am speaking as an Amateur Radio hobbyist, and junior-level Electrical Engineering student.

We have to remember that AM and FM are merely methods of modulation. However, these methods of modulation are used in broadcasting at profoundly different frequencies. When the AM Broadcast band (500kHz to 1600kHz) was standardized, few could conceive of transmitting or receiving signals several orders of magnitude higher in frequency in what is now known as the FM Broadcast band (97MHz-107MHz).

Signals at these very different frequencies behave in profoundly different ways. These differences are often taken advantage of by radio hobbyists to try to get a signal to different places.

At Medium Frequencies (MF) (roughly 400kHz - 2MHz) signals will almost certainly either be absorbed by the D layer of the ionosphere, or be reflected by the E layer. Thus we can conclude that you would likely not hear any AM stations at the ISS.

High Frequency (HF) waves (roughly 2MHz-30MHz) are sometimes reflected by the F layer of the ionosphere. This depends on the level of ionization caused by the sun. Currently we are at a very low sun spot cycle, so it is quite likely that you could hear some HF signals at the ISS.

At Very High Frequencies (VHF) (roughly 30MHz-300MHz) which contain the FM broadcast band, signals will most certainly penetrate the ionosphere, and could be heard by the ISS. The 10-meter amateur radio band (29MHz) is sometimes used for satellite communication which would require penetration of the ionosphere and much more. VHF signals do sometimes skip however, during times of extreme ionization, or when they encounter anomalies such as an Aurora.

In general, the higher the frequency, the more likely it is to be heard by vessels in space. This is why microwave frequencies were employed by Apollo for communications at 2.2GHz.

  • $\begingroup$ Thanks for your post. That the AM broadcast band won't likely make it through the ionosphere was already mentioned in this answer but it's good to have an expanded reminder. $\endgroup$ – uhoh Oct 2 '18 at 10:50

The power of FM transmitters and the sensitivity of FM recivers are optimized for a range of some 10 km for local transmitters and up to 200 km for transmitters on a mountain. But if a wave goes horizontal from a transmitters antenna straight to the ISS in 400 km above the surface it has to travel a very long distance. Using the formula of Pythagoras gives a distance of 2292 km, that is a loss of about 21 db when compared to 200 km. Too much for a simple FM receiver without a high gain directional antenna and without a low noise preamplifier.

If the same frequency is used several times in the circle with 2292 km radius, receiptiom may be disturbed.

  • $\begingroup$ Since the main lobe of the antenna will have some finite angular width vertically, and there will definitely be side-lobes above that, and these angles will result in much shorter distances, this should probably be taken into account before declaring reception impossible. $\endgroup$ – uhoh Feb 20 '18 at 23:52
  • $\begingroup$ Actually the factors that limits the range of FM broadcast reception is the curvature of the Earth and the increased RF absorption and scatter of ground objects (trees, etc.) Both FM and AM stations can have radiated power of order 100 kW. AM ground waves go much farther than FM ground waves due to better propagation. FM receiver sensitivity is measured in dBf (dB femtowatt) for power, or fractions of a μV at the input... $\endgroup$ – uhoh Feb 21 '18 at 0:44
  • $\begingroup$ ...so I think this answer might not bear up on further quantitative analysis, but let's see if we can do the numbers (I seem to have been wrong last time!) $\endgroup$ – uhoh Feb 21 '18 at 0:44

The FM radio band is divided into 101 channels at 200 kHz intervals. In the USA, the FCC recommends that transmitters in the same area have frequencies at least 4 channels apart, leaving room for 25 transmitters in the same area. The next area over shifts this scheme by 1 channel, etc. to minimize interference.

In recent years, this has changed to give transmitters more freedom as long as they don't interfere with others. And other countries have different rules.

In the FM band, transmission is line of sight, so you can see far more stations from the ISS than on the ground. This increases the chance of collisions where two stations transmit on the same channel.

So on the ISS you can potentially receive 100 stations, in practice it'll be a lot less due to interference.


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