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Is it possible for a sub GHZ frequency (Say 300 Mhz) can be used for space communication for small satellites such as a cubesat? What are the advantages and disadvantages for this.

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Most cubesats communicate in a combination of VHF and UHF for both uplink and downlink.

Advantages:

  • Better link budget (lower free-space attenuation).
  • Cheaper ground station (radio equipment, cables, antennas).
  • Lots of support worldwide in amateur bands (144 and 430 MHz bands).
  • No need for directional antennas (and fine attitude control) on the spacecraft.

Disadvantages:

  • Limited bandwidth (in the order of 10 kbps maximum).

Only a few cubesats have S or X band downlinks, and always as high-bandwidth extension to the main TT&C downlink.

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    $\begingroup$ Why would VHF have lower free-space attenuation? I think all radio communications obey the inverse-square law, regardless of frequency. (Perhaps excepting ELF communications, where the wavelength is a significant fraction of the Earth's circumference, so it's no longer accurate to model it as a point source) $\endgroup$ – Phil Frost - W8II Mar 11 '15 at 15:23
  • $\begingroup$ My friends where telling me that the lower frequency would be hard to penetrate threw the ionosphere, so if i use a low frequency such as a 300Mhz and my antina has a gain of 5.53 Dbm with a transmitting power of 100Mw would it be possible to receive signal from an altitude of 250Km in near earth orbit? $\endgroup$ – Sujay sreedhar Mar 11 '15 at 15:58
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    $\begingroup$ Free-space path loss is proportional to $f^2$ en.wikipedia.org/wiki/Free-space_path_loss $\endgroup$ – Juancho Mar 11 '15 at 18:52
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    $\begingroup$ A common misconception -- the problem is that a lower frequency also requires a larger antenna for the same gain. See en.wikipedia.org/wiki/Free-space_path_loss#Frequency_dependency and Is free space path loss dependent on frequency? $\endgroup$ – Phil Frost - W8II Mar 12 '15 at 12:05
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Certainly possible. 300 MHz is not a very low frequency for satellites. There are a great many amateur satellites operating on the 2m band, around 145.8 MHz.

Going much lower, there are allocations for satellites on the 10 meter band, from 29.3 to 29.51 MHz. There are fewer amateur satellites operating here, but they do exist.

In fact, early in space exploration history, HF was used regularly for communications. For example, Yuri Gagarin in Vostok 1 sent several reports via HF.

HF for space communication has the same advantages and disadvantages as it does for terrestrial communication, more or less.

The big advantage: HF supports skywave propagation, so you may get propagation beyond line of light. There are layers of the ionosphere that are high enough to still be above some low Earth orbits. For example, Vostok 1's orbit varied between 168 and 327 km; the F layer is around 300 km.

Also, lower frequencies require less sophisticated technology, generally. Of course this is becoming less of a concern in modern times since microwave radios are commodity items now.

The big disadvantage of HF is that antennas are larger and frequency allocations are more expensive and less available. As an example, the 70cm amateur allocation in the US goes from 420 to 450 MHz and is 30 MHz wide. You could fit the entire HF spectrum in that. Commercial availability of spectrum is similarly rarer at lower frequencies.

Additionally, high-speed communications are more difficult at HF due to the higher fractional bandwidth required. As an extreme example, ELF communications can take several minutes to send just a few characters.

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It's really a trade-off between transfer speeds and available transmit power. Higher your transmit power, the higher you can set your downlink carrier. You also get better transfer speeds because you can encode more information in a higher carrier frequency, using FSK or whatever. Some applications employ a sub-GHz downlink using LoRa modulation, not compromising on data rates.

Most amateur satellites and CubeSats are constrained by a lack of power to them, unlike traditional satellites that have some sort of solar array or power generation to power high frequency downlinks. Assuming you device a way to power it, you'd still have to clear through legals to use these GHz bands like X. Forcing most amateur radio (HAM) into the sub-GHz band as a result.

LoRa however, though my decent understanding of the concept, operates over a similar spectrum of FSK but employs chrip spread spectrum or CSS for short, using a chirp signal to encode information. The parameters of this chirp dictate the spectrum (through spread factor) and data rate. The people who make LoRa applications market it as the low-power counterpart to DSSS and other spread spectrum modulation techniques. It's got an advantage over traditional FSK in the fact that it's frequency and bandwidth can be scaled, allowing a significant increase in data rate - more spread spectrum = more data per unit time, with the same sub-GHz frequencies.

In my applications, I usually settle for something like 433 MHz, FSK/LoRa. Transceivers are readily available at this bandwidth and so are amplifiers and matched antennas. I personally feel like this gives me the perfect benefit of a decent data rate with conservative power requirements, and acceptable attenuation unlike the K bands that can't stand a rainy day 😁

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    $\begingroup$ Huh? Why is a higher power necessary for a higher downlink frequency? And doesn't FSPL increase with frequency? And how does spread-spectrum increase data rate? $\endgroup$ – Phil Frost - W8II Aug 1 '17 at 16:12
  • $\begingroup$ @PhilFrost-W8II oops! Sorry, it does increase free-space attenuation, I'd mistyped it. About the power, it's not necessary but I meant it as a requirement to improve reception. Spread spectrum uses a large bandwidth allowing an increase in data rate - kind of like CDMA I guess where you'd encode multiple streams of data and layer them upon another in the frequency domain. The receiver would then demultiplex the individual streams and translate them into their time domain $\endgroup$ – Shreyas R Aug 2 '17 at 18:41
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Back in the day (long ago) we used to regularly tune in a weather satellite at 136 MHz.

So it is certainly possible.

There is no technological barrier other than size and power requirements. Not to mention that stringing a wire (tether) behind a satellite is nowhere near as simple as unwinding wires behind aircraft. There is a lot of physics involved.

Here is an interesting story of when they tried out the concept:

NASA Unveils A Satellite On a Tether 12 Miles Long

One of the engineers thought it might serve as an ELF antenna to communicate with submarines.

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