Antenna speed: slow and fast tracking( explanation of a parameter in comparison table)

Question

I researched the LEO, MEO and Geo differences and found a website that provides a comparison table

In the table, there is a parameter " antenna speed". Can someone explain what kind of antennas they are? On board a satellite or on the ground?

Answer 1

I think the "antenna speed" row would apply to both the on-the-ground and on-orbit antennas.

There are two basic categories of antennas; dishes and phased array antennas.

When a dish is used it is either stationary (typically the case for GEO satellites) or mounted on a two-axis gimbal system. These gimbal systems can move the dish quickly enough to track the satellite if they are on the ground or track a nearby ground station if they are on the satellite. What they can't do very well is flit around quickly between multiple targets. Also, if they are on the satellite, a single gimballed dish can't transition between successive ground stations without causing brief interruptions in connectivity.

A phased array antenna can flit around quickly to avoid significant interruptions in connectivity and can even do things like place multiple beams down on the ground simultaneously.

The table's distinction between "slow tracking" for MEO and "fast-tracking" for LEO isn't very relevant. Gimballed systems are fast enough to handle either. What matters more is whether you need to upgrade from gimballed dishes to more sophisticated phased array antennas for a given link. That decision is influenced by other factors far more than it is influenced by orbit altitude - at least until you get to GEO.

Answer 2

That row only tells you how long a satellite in that orbit remains in view from a single ground location. Therefore, any antenna you want to point from one to the other must be able to track fast enough to handle that motion. As phil1008 said, if you can track one satellite, you can probably track them all. The biggest change, in complexity and expense, is going from a fixed antenna to one that can move.

The GEO column oversimplifies the true situation. It applies only to perfectly geostationary satellites, but does not adequately represent inclined geosynchronous satellites, like the ones described in Are there any satellites in geosynchronous but not geostationary orbits?

For most ground locations that are ever in view of an inclined GEO, the satellite is always in view, but it moves across the sky, usually tracing a figure-eight shape called an analemma. Depending on how big your antenna is, how much link budget (see Satellite-Ground link and Intersatellite link and How to compute each of these link budget terms?) you have, and how much inclination the satellite has, you may have to track it. Because larger antennas have narrower beams, and thus suffer more from the same angular pointing error, the bigger your antenna is, the more likely it will need to track even when pointed at a GEO.

At zero inclination, a GEO never sees latitudes poleward of 81 degrees, both south and north. A GEO with an inclination of 9 degrees or more will see both poles every day, but only for a short time, and not both at once. The more it sees of one hemisphere, the less it sees of the other; which one it sees more of reverses every 11 hours and 58 minutes. This means it generally doesn't make much sense to point antennas near the poles to or from GEOs, which is the reason for the Molniya orbit.