science

Polar mount

Polar mount is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Polar mount rather than just read about it. In short: A polar mount is a movable mount for satellite dishes that allows the dish to be pointed at many geostationary satellites by slewing around one axis. It works by having its slewing axis parallel, or almost parallel, to the Earth's polar axis so that the attached dish can follow, approximately, the geostationary orbit, which lies in the plane of the Earth's equator.

Key takeaways

Polar mount belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polar mount to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polar mount from memory before moving on to harder problems.

Reference excerpt

A polar mount is a movable mount for satellite dishes that allows the dish to be pointed at many geostationary satellites by slewing around one axis. It works by having its slewing axis parallel, or almost parallel, to the Earth's polar axis so that the attached dish can follow, approximately, the geostationary orbit, which lies in the plane of the Earth's equator.

Description Polar mounts are popular with home television receive-only (TVRO) satellite systems where they can be used to access the TV signals from many different geostationary satellites. They are also used in other types of installations such as TV, cable, and telecommunication Earth stations although those applications usually use more sophisticated altazimuth or fix angle dedicated mounts. Polar mounts can use a simplified one axis design because geostationary satellite are fixed in the sky relative to the observing dish and their equatorial orbits puts them all in a common line that can be accessed by swinging the satellite dish along a single arc approximately 90 degrees from the mount's polar axis. This also allows them to use a single positioner to move the antenna in the form of a "jackscrew" or horizon-to-horizon gear drive. Polar mounts work in a similar way to astronomical equatorial mounts in that they point at objects at fixed hour angles that follow the astronomical right ascension axis. Like equatorial mounts, polar mounts require polar alignment. They differ from equatorial mounts in that the objects (satellites) they point at are fixed in position and usually require no tracking, just accurate fixed aiming.

Adjustments When observed from the equator, geostationary satellites follow exactly the imaginary line of the Earth's equatorial plane on the celestial sphere (i.e. they follow the celestial equator). But when observed from other latitudes the fact that geostationary satellites are at a fixed altitude of 35,786 km (22,236 mi) above the Earth's equator, and vary in distance from the satellite dish due to the dish's position in latitude and longitude, means polar mounts need further adjustments to allow one axis slewing:

Declination angle The declination angle or just "declination", from the astronomical term declination for the vertical value (north/south) on the celestial sphere, is a "tipping down" of the dish on the mount to allow it to observe geostationary satellites. When observed from any latitude other than the equator the observer is actually looking "down" on the satellite making it look as if it is just below the celestial equator, an angle from the celestial equator that increases with latitude. Polar mounts have mechanisms that allow the dish to be tipped down in a permanently fixed angle to match the declination angle. Mounts may also have a variable declination control to allow them to point at geosynchronous satellites in inclined orbits since those satellites have a constantly changing declination. (Adding such a declination axis to a polar mount results in an equatorial mount). Declination offset Because satellites toward the Eastern and Western sky are further away from the observing antenna, there is a change in the declination angle: towards the eastern and western limits the satellites get closer to the celestial equator because they are further out along the lines of perspective. To aim at this apparent shift in the arc of geostationary satellites polar mounts incorporate a slight offset in the angle of their polar axis towards the equator, called a declination offset, to more closely follow this arc. Slewing around a fixed axis which is not parallel with the Earth's rotation axis causes the dish to aim at a track in the equatorial plane which is (unless the dish is on the equator) an ellipse rather than a circle. Since the geostationary orbit is circular, the mount does not aim precisely at satellites at all longitudes. These slight differences in tracking have negligible effect on home C band and Ku band TVRO dishes since they have relatively wide-beam designs.

Worked examples

Example 1 — a first encounter with Polar mount

Start with the simplest possible case. Write down what Polar mount claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polar mount before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polar mount ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polar mount

In research: Polar mount appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polar mount in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polar mount is common in secondary-school and first-year university syllabi. It links to neighbouring topics Satellite broadcasting, so understanding it makes those chapters shorter.
In everyday life: Look for Polar mount outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Polar mount in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polar mount means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polar mount out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polar mount in simple terms?

Why does Polar mount matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polar mount?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polar mount.