Grating Lobes

The first precision microwave radars used parabolic dishes to focus the beam. The waves from the transmitter source bounce off the parabolic dish reflector and reflect back into a narrow beam with a horizontal wavefront:

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Look carefully at the waves around the edge. A parabolic antenna under uniform illumination leaks a little energy off to the side. These are called "side lobes".

Because of the "diffraction limit", larger dishes produce narrower (smaller angle) beams than small dishes (with larger angle beams). The upper plot shows the dish changing size, the lower plot shows the size of a ground spot far below a larger or smaller orbiting dish:

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Large dishes make smaller spots, but they are heavy. On the ground, they are distorted by weight and wind, and lose their shape. A parabolic dish is like a telescope mirror, it should keep its shape within a fraction of a wavelength or the distortions will spread the beam, making it fuzzy.

Also, big dishes take time to move around. Old style rotating dishes, and steered dishes, took many seconds to move from target to target. This was troublesome when detecting and tracking single airplanes; for ballistic missile early warning, a second is too long. As electronics became cheaper and budgets bigger, radars moved to a new system, phased array antennas.

Phased Arrays

Traditional phased arrays are two dimensional uniformly spaced grids of antennas in a flat plane. By changing the delays of the RF signal to from the antennas, the sum of the wavefronts from the antennas can be steered in different directions, electronically, very rapidly, while the planar antenna stays fixed.

Real phased arrays emit signals at angles to the perpendicular of the plane, but that is difficult to show on a two dimensional screen. So, I've made a slightly different kind of "two dimensional plane" phased array, so you can see how the transmitter antennas are phased to produce a beam. Server sky uses a three dimensional grid of thinsats to produce phased array beams, so this could be a slice of one of those arrays.

The individual antennas, shown here as little circles, are phased to emit signals that add up to a vertical downwards beam.

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If you look carefully, you can see some sidelobe energy.

By phasing the antenna signals differently, you can steer the beam at a 30 degree angle. Switchover time can be very quick, moving from one beam to the other at the speed of light across the array.

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The left side lobe is slightly more visible in this animation.

Server sky arrays are distorted by orbital mechanics; I call this Apogee Skew. This distorts the array into a rotating parallelipiped. This and the following plots show the ground illumination from a 5x5x5 three dimensional phased array. Typical arrays will range from 32x32x32 ( A 32768 thinsat, 98 kilogram array ) to 100 x 100 x 100 ( A 1 million thinsat, 3000 kg array ), but those would be much too large to illustrate here. The spacings between thinsats are on the order of meters, and increase for larger arrays, so they do not shade each other from the sun.

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The illuminated parallelogram in the middle is the main lobe. It is surrounded by thousands of other spots. These are called grating lobes. Because the spacings are much larger than a wavelength, there are many angles at which the waves add together. Besides the energy focused on the very small main lobe, the vast majority of the energy is scattered into the other lobes.

The signals from all the thinsats, added together, are producing "nulls" in the pattern, black lines ("loci", the plural of locus) representing very little energy at those angles. Those null loci move as the array rotates around its orbital track (one rotation per orbit). Imagine that you are looking at the primary beam sweep along the equator, while the grating lobes create interference for other ground sites.

The size and spacing of the grating lobes are inversely proportional to the spacing of the thinsats:

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If you add more thinsats, making the array bigger, the size of the grating lobes becomes smaller, but they do not vanish:

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However, there is a very important trick we can play with server sky, which is more difficult with ground arrays: we can vary the pitch in between elements, changing them over time, and make the spacings nonuniform, in a 3 dimensional array. The results are spectacular:

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The grating lobes are washed out! They do not go away, the same amount of radio energy is splattered away from the main lobe, but it is not concentrated. That means it will not interfere nearly as much with receivers in other locations. Using time coded spread spectrum and signal correlation techniques, it is possible to punch a signal through near the power noise floor. So until all the power from many different thinsat arrays, added together, exceed the power of a main lobe, we can still punch a signal through. For a steady beam at one ground spot, the interferers will NOT be random, and evolve fairly slowly, given the 4 hour orbits; we can anticipate the interference, change signal amplitudes, and move nulls from array X over the ground spot of array Y. Or just try again; when an array redirects its packets at a different ground spot, the interferers shift, too.

The phasing of the transmitters gets a little more complicated, but not much - we are still doing a 12 digit accurate trigonometric calculation, but dependent on the individual position of each thinsat (in X, Y, and Z) relative to the target, as opposed to making some global calculation and spacing it out element by element.

This is a demonstration only. The actual spacing in deployed arrays will be dithered from an exact grid, but probably not randomly. Array position design will evolve towards smarter spacing functions, resulting in more closely packed non-shading arrays. See the page ArrayFill for a discussion.