DATA TRANSFER - Sending Data in a Beam to the Starlink Satellite
The final topic we’re going to dive in for this article is how information gets sent between Dishy and the Starlink satellite. For example, we’ve talked about high-frequency sinusoid-shaped electromagnetic waves, but that doesn’t look anything like binary and even less like a TV show.
So, what’s happening? Well, Dishy and the satellite indeed send a signal that looks like a wave from the diagram below.
If you are on this page for the first time on how the Starlink Satellites and Dish works? You have missed the previous parts that introduced what made-up these amazing devices, and the other one that explain Beam steering. You can click on the topics below to read them:
Starlink Dish and Satellite! How it Works? An Introduction
Starlink Dish and Satellite! How it Works? BEAM STEERING
However, they vary the amplitude and the phase of the transmitted signal and then assign or encode 6-bit binary values to each different combination or permutation of amplitude and phase. With 6 bits, there are 64 different values, and thus we need 64 different permutations of amplitude and phase. However, instead of listing all the permutations, it’s more easily visualized by arranging the 64 different values in a graph called a constellation diagram as shown.
6-bits values of 64 different permutations of amplitude and phase.
Innerworkings of 64-QAM
Let’s look at the point 101 000 from the graph below and draw a line from the origin to this point.
Graph of Constellation diagram
The distance from the origin is the amplitude of the signal, and the angle from the positive-X axis is the phase. It’s a bit like using polar coordinates. Thus, for Dishy to send these 6-bits, it transmits a signal with an amplitude of 87% and a phase shift of 305-degrees.
Then, if the next value being sent is 011 101, the signal switches to an 59% amplitude or brightness, and a 121-degree phase shift. After that it sends the next value with a different amplitude and phase shift. Each of these 6-bit groupings are called symbols and they last for only 10 or so nanoseconds before the next symbol is sent. Lots of times, the signal scrunched up like that however, because the frequency of the signal is just once every 83 picoseconds, or 12 Gigahertz, and since a symbol lasts 10 nanoseconds, it’s more accurate to have around 120 wavelengths per symbol before the next symbol is sent. Because we’re dealing on the order of pico and nanoseconds, that means that we can fit 90 million 6-bit groups or symbols, resulting in 540 million bits per second.
However, note that this data transfer is shared between download and upload. Since this particular antenna can’t transmit and receive data at the same time, about 74 milliseconds of every second is used to send data from Dishy to the Starlink satellite and 926 milliseconds is used to send data from the satellite down to Dishy. And, for the sake of reducing latency, these time slots get distributed throughout a single second instead of grouping them all together. This technique of sending 6-bit values using different variations of amplitude and phase is called 64-QAM or Quadrature Amplitude Modulation.
Now that we have a stream of millions of 6-bit symbols yielding hundreds of megabits of data per second, in order to turn it into a favorite TV show, the advanced video codec, or h.264 format is used.
That’s pretty much for how Starlink and Dishy send data to each other.
The TV dishes we mounted on top of our house or office to watch a news broadcast used a technology called parabolic reflector signal to focus the electromagnetic waves. These signals sent from broadcast satellites orbiting the Earth at an altitude of 35,000 kilometers away. TV satellite dishes only receive TV signals from space, they can’t be able to send data. The McFlatFace Dishy, employed optical inter-satellite links and phased array beam-forming, and digital processing technologies in the Ku and Ka microwave bands (super high frequency [SHF] to extremely high frequency [EHF]). Satellite internet works by beaming information through the vacuum of space, where it travels 47% faster than in fiber-optic cable. It can receive the signals from the satellites orbiting the earth at 550km and could also send signals back to the satellites at orbit.
The Starlink ground Dishy contains several components inside it that makes the satellites internet constellation connectivity possible with a very high speed of data, over several hundreds of megabits per second.
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REFERENCES
- Starlink (https://en.wikipedia.org/wiki/Starlink)Retrieved 25 July, 2023.
- How does Starlink Satellite Internet Work? https://www.youtube.com/watch?v=qs2QcycggWU&t=803s