Starlink Dish and Satellite! How it Works?<br /> BEAM STEERING

BEAM STEERING - Steering a Beam to Sweep Across the Sky

Now, let’s continue our exploration on how a powerful beam can be continuously swept across the sky, and then how it is fill with hundreds of megabits of data every second.

As a quick refresher from before, there’s an array of 1280 antennas and all of the antennas were fed with the same 12 Gigahertz signal in order to create a laser-like beam propagating perpendicular to Dishy. However, as mentioned earlier, we need to be able to angle this beam so that it points directly at the Starlink satellite zooming across the sky at 27,000 kilometers per hour. Using the motors as the one on the Dishy isn’t feasible because they would break within a month and aren’t accurate enough. So, the solution is to use what’s called phased array beam steering. Let’s go back to the previous two-antenna example. Before we were feeding the same signal to the two antennas, and thus the antennas were in phase with one another.

If you are on this page for the first time on how the Starlink Satellites and Dish works? You have missed the first part that introduced what made-up these amazing devices. You can click on the topic below to read the introduction:

Starlink Dish and Satellite! How it Works? AN INTRODUCTION

Understanding phase is critical, but let us quickly look at this brief explanation: changing the height or amplitude of the signal is done by changing the power sent to the antenna, thus making the signal stronger or weaker. The frequency is how many peaks and troughs, or wavelengths there are in one second, and changing the phase is shifting the signal left or right. Phase shifting is measured in degrees between 0 and 359, because, if we shift the signal 360 degrees, or one full wavelength, then we’re back at the beginning, exactly as if we were to loop around a circle. For example, the diagram below is a signal with a 45-degree phase shift, the other one with a 180-degree shift, and the last one with a 315-degree shift. Our naked eyes can’t see the differences in phase shifted visible light, however, high-tech circuitry such as what’s inside Dishy is really good at detecting and working with phase shifts.

Signal change sent to the Antenna

Starlink: Phase Array Beam Steering

Now, how does phase shifting used to angle the beam and have it point directly at the satellite?

The solution is to phase shift the signal sent to one antenna with respect to the other antenna and, as a result, the timing of the peaks and troughs emitted from one antenna is different from the other. These peaks and troughs propagate outwards, and the location of the constructive interference is now angled to the left with destructive interference everywhere else. If the phase of the antennas is change again, the zone of constructive interference is angled to the right. Therefore, by continuously changing the phase of the signals sent to the antennas, the sweeping zone of constructive interference can be created.

For instance, if we bring in six more antennas that we can only see a section of the peaks from each wave. Far away from the antennas, the waves join to form a wave front that is a planar wave. Kind of like ocean waves crashing on a shoreline. Just as before, by continuously changing the timing of when each wave peak is emitted by each antenna, we can change the angle at which the wave front is formed, essentially steering the beam in one direction or another. And, if we bring in more antennas in a two-dimensional array, we can now steer the beam in any direction within a one-hundred-degree field of view.

As we already know, there are 1280 antennas available in Dishy. In order to know the exact angle the beam needs to be pointed or steered, we use the GPS coordinates chip on the Dishy PCB, along with the orbital position of the Starlink satellite which is known in Dishy’s software. The software computes the exact set of 3D angles and the required phase shift for each of the antennas.

These phase shift results are then sent to the 20 larger chips called beamformers on the PCB, and each beamformer coordinates between 32 smaller chips called front end modules, each of which controls 2 antennas. Every few microseconds, these computations are recalculated and disseminated to all the microchips in order to perfectly aim the beam at the satellite. As a result, the beam can be steered anywhere in a 100-degree field of view.

Notes on Phased Array Beam Steering:

There are a few quick notes.

1. First, the main beam, also called the main lobe looks like the diagram below;
   Main lobe

   
   However, constructive and destructive interference isn’t perfect, and as a result there are additional side lobes of lesser power.
2. Second, Mr. McFlatface holds is a single phased array, however, on the Starlink satellite, there are in fact 4 phased array antennas. Two are used to communicate with multiple Dishys, and 2 are used to communicate with the ground stations to relay the internet traffic.
   Starlink ground station dishes



3. Thirdly, phased arrays are used in many applications, and interestingly they’re used on commercial airlines to allow for mid-flight internet.

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Starlink Dish and Satellite! How it Works? DATA TRANSFER

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