Arduino radio communications

Question

I have an Arduino and I am building a space satellite, just for fun. I'm not going to launch it obviously.

I have built all the sensors and everything works. I was looking into communication because if a satellite is in space it needs to send down the data.

Is it possible to have radio communication between two Arduino or an Arduino and something else that are 100km apart, it doesn't need to particle, I'm just wondering if it is possible and how it works. Is it possible without going out to buy shields?

Answer 1

There are three basic factors that control long distance communication:

1. Transmitter Power
2. Receiver Sensitivity
3. Signal Directionality

The more transmission power you have the "louder" your signal is at the receiver.

The more sensitive your receiver the "quieter" the signals it can pick up.

The more directional (focussed) your signal is the more arrives at the receiver and the less gets sent off elsewhere.

So you would think get all three of those and you're laughing, but it's not that simple:

• High power transmission requires high-power power source
• Good focus requires heavy and bulky antennae.

Neither of those are very good on a satellite, since you want it to be as small and light as possible and use as little power as possible.

So in general you have a medium to low power transmitter on the satellite with reasonable directionality, and put all the bulk on earth. That is why satellite dishes are so big. They give good focus, and the large size captures a lot of the signal making them more sensitive. You can also use very precise electronics to amplify the signal more.

There is, of course, nothing like that in a shield format, so you would have to create your own. The transmitter isn't so hard, since you don't want to have anything too meaty - the hard part comes when you want to make the base station. If you can afford enough power to transmit a strong enough signal and have it with enough directionality you can use a satellite TV dish (not a little Sky dish, one of the bigger solid round ones - Sky satellites actually have quite powerful transmitters and the massive solar panels to go with it; not really practical for a small home-built satellite). Getting the receiver and LNB (Low Noise Booster) tuned properly as well is vital - unless you're transmitting at the same frequency as the satellite the dish is intended to receive (which you mustn't or there'll be a lot of annoyed people around - there are specific frequency ranges you are allowed to use) then you will need to ensure the antenna is exactly the right length and that the LNB amplifies and filters the right frequency range.

Once you have got over all those hurdles the receiver is just a simple receiver.

The actual transmitter and receiver circuits themselves can be something fairly off-the-shelf (the lower the frequency the better since that goes further than higher frequencies) - it's what goes between - the antenna and amplifier - that makes it much more special and long range.

Answer 2

Majenko covered several aspects of radio communications between a satellite and a ground station. An additional factor to consider is that the azimuth or altitude of a low-altitude satellite changes rapidly as it passes over a ground station. For example, a satellite 100 km up takes about 86 minutes per orbit and travels at about 7.84 Km/s, and probably would be in view for less than a minute per pass. [See eg Orbit of a satellite Calculator at casio.com.] Unless a phased array is used, and assuming communication is highly directional, the big dish on the ground will need to swing at up to 4.5° per second (about 270° per minute) as it tracks the satellite passing over. The much smaller antenna on the spacecraft should rotate at the same rate, the opposite direction.

Note, an alternative to radio communications is a tight-beam laser system. Rather than using big, bulky microwave or UHF antennas to achieve directionality, you would use a small telescope (a few cm of aperture) on the satellite and a larger telescope (as much aperture as you can afford) at the ground station. Infrared or visible light lasers at each end of the link would be modulated to transmit at a high data rate. Accurate tracking and aiming might end up being important research projects for this approach.