Polar orbit

A polar orbit is an orbit that passes over the poles of a planet or very close to them, that is, the inclination of the orbit is close to 90 degrees. A satellite in polar orbit passes over each point on the planet as it rotates on its axis.

Types of polar orbits

Polar Synchronous Orbit

Determination of period T synchronous polar orbits performed at δ = 0 using the formula: T = 2 πk / N_k ω_W where:

T - orbital period

π - Pi number

k - orbital multiplicity

N_k - spacecraft orbit order

ω_З - angular velocity of the Earth's rotation

δ - angular displacement of the path

For orbits, whose period is a multiple of days, the calculation formula will be: T = 2 π / n_d ω _З where n_d is the number of days.

The period T and the height of circular polar synchronous orbits for some daily multiplicity n_d are shown in Table. Here h = r-R where:

h — is the height of the orbit at perigee

r — is the radius of the orbit of the spacecraft

R — is the mean radius of the Earth

Polar quasi-synchronous orbit

The period of quasi-synchronous polar orbits with | δ | = d is determined by the formula: T = 2 πk - δ / N k ω 3.

And for orbits of daily multiplicity by the formula: T = 2 πk - δ / n with ω_З

Utility

Polar orbits are mainly used for remote sensing from space to study and control the Earth's natural resources, study the dynamics of natural processes and phenomena, collect information on the state of territories on the planet's surface. For this reason it has utility both for military, reconnaissance satellites, and civil: scientific (such as the study of the atmosphere), agricultural, as well as for some meteorological satellites. The Iridium satellite constellation also uses a polar orbit to provide telecommunication services. Another application is the Cospas-Sarsat maritime rescue satellites.

The nature and duration of reconnaissance of the investigated areas of the Earth's surface, as well as the trajectory of the orbit are determined by the parameters of the spacecraft orbits. Such parameters of orbits as the period of revolution of the spacecraft, the eccentricity, the inclination of the orbit and others, to a large extent determine the quality of the information received by the satellites, the speed of reception and the transfer of stations. satellites to terrestrial ones. The lower the altitude of a spacecraft, the higher the quality of the information it receives and the less delay in transferring the collected material to Earth. The flight altitude of spacecraft can vary over time due to the resistance of the atmosphere. Therefore, during the flight of satellites, it is necessary to control them in order to maintain the basic parameters of the orbit.

Sun-synchronous orbit

Near polar-orbiting satellites commonly choose a sun-synchronous orbit, meaning that each successive orbital pass occurs at the same time of day. This can be particularly important for applications such as remote sensing of atmospheric temperature, where the most important thing to see may be changes in weather that are not related to changes in local time. In order to maintain the same local time at each step, the orbit time period must be kept as short as possible, this is accomplished by keeping the orbit lowest to Earth. However, very low orbits of a few hundred kilometers disintegrate rapidly due to friction from the atmosphere. Commonly used altitudes are between 700 km and 800 km, yielding an orbital period of approximately 100 minutes. The Sun-side half-orbit takes only 50 minutes, during which the local time of day does not vary much.

To maintain synchronous orbit with the Sun as the Earth revolves around the Sun throughout the year, the satellite's orbit must precess at the same rate. This is not possible if the satellite passed directly over the pole. Due to Earth's equatorial bulge, an orbit inclined at a slight angle is subject to a torque that causes precession; an angle of approximately 8 degrees from the pole produces the desired precession in a 100-minute orbit.