Types of Satellite orbits and Kepler’s laws

 Satellite Orbits:

•  The path followed by the satellite is called orbit.

•  The satellite moves as per Kepler’s law.

 Kepler’s law:

1. 1st Law: The path followed by each planet is an ellipse with sun at one FOCI.

2. 2nd Law: The line joining to the planet to sun sweeps out equal areas in equal times.

3. 3rd Law: The square of the period of the planet is proportional to the cube of the semi major axis.

Satellite Orbital Characteristics:

•  Altitude

•  Inclination angle

•  Period

•  Repeat Cycle

•  Swath

•  Ascending pass & Descending pass

•  Perigee

•  Apogee

Altitude: 

It is the distance (in Km) from the satellite to the mean surface level of the earth. The satellite altitude influences the spatial resolution to a large extent.

Inclination angle:

The angle (in degrees) between the orbit and the equator. The inclination angle of the orbit determines the field of view of the sensor and which latitudes can be observed. If the inclination angle is 60° then the satellite flies over the earth between the latitudes 60° South and 60° North, it cannot observe parts of the earth above 60° latitude.

Period:

It is the time (in minutes) required to complete one full orbit. A polar satellite orbiting at an altitude of 800km has a period of 90mins.

Repeat Cycle:

It is the time (in days) between two successive identical orbits.

Swath:

As a satellite revolves around the Earth, the sensor sees a certain portion of the Earth's surface. The area is known as swath. The swath for satellite images is very large between tens and hundreds of kilometers wide.

Ascending pass and Descending pass:

The near polar satellites travel northward on one side of the earth (ascending pass) and towards South Pole on the second half of the orbit (descending pass). The ascending pass is on the shadowed side while the descending pass is on the sunlit side. Optical sensors image the surface on a descending pass, while active sensors and emitted thermal and microwave radiation can also image the surface on ascending pass.

Perigee:

It is the point in the orbit where an earth satellite is closest to the earth.

Apogee:

It is the point in the orbit where an earth satellite is farthest from the earth.

Types of Satellite orbits: 

Satellite orbits are designed according to the capability and objective of the sensors they carry. Depending on their altitude, orientation and rotation relative to the earth satellites can be categorized as:

1. Geostationary
2. Polar orbiting and Sun-synchronous

Geostationary satellites:

An equatorial west to east satellite orbiting the earth at an altitude of 35000 km, the altitude at which it makes one revolution in 24 hours, synchronous with the earth's rotation. These platforms are covering the same place and give continuous near hemispheric coverage over the same area day and night. These satellites are put in equatorial plane orbiting from west to east. Its coverage is limited to 70oN to 70oS latitudes and one satellite can view one-third globe (Fig 1). These are mainly used for communication and meteorological applications viz. GOES METEOSAT, INTELSAT, and INSAT satellites.

• Altitude ~ 36,000 km,
• Orbit inclination ~ 0°
• Period of orbit = 24 hours
• Global coverage requires several geostationary satellite in orbits at different latitudes
• Good for repetitive observations, poor for spatially detailed data
• Large distortions at high latitudes
• W-E satellite orbiting Earth

• Mainly used for communication and meteorological applications – GOES, METEOSAT, INSAT etc.

Figure 1: Geo-stationary Orbit (source NASA)

Sun-synchronous satellites:

An earth satellite orbit in which the orbital plane is near polar and the altitude is such that the satellite passes over all places on earth having the same latitude twice in each orbit at the same local sun-time. Fig 2. This ensures similar illumination conditions when acquiring images over a particular area over a series of days.

• Altitude ~700-800 km
• Orbit inclination ~ 98.7º
• Orbital period ~90 minutes
• Sun-synchronous, near-polar, nearcircular
• Satellite orbit is fixed in space (basically north-south ): Earth rotates beneath it (west-east)
• Cross the equator (N-S) at ~10.30am local time
• Satellite Orbital plane is near polar and the altitude is such that the satellite passes each place at same local sun-time.

• Cover entire globe – LANDSAT, SPOT, NOAA, IRS etc.

Figure 2: Sun synchronous orbit

The satellite's orbit (North –South) and the rotation of the Earth (west to east) work together to allow complete coverage of the Earth's surface, after it has completed one complete cycle of orbits. As the satellite orbits the Earth from pole to pole, its east-west position would not change if the Earth did not rotate. However, as seen from the Earth, it seems that the satellite is shifting westward because the Earth is rotating (from west to east) beneath it. This apparent movement allows the satellite swath to cover a new area with each pass (Fig. 3). The satellite's orbit and the rotation of the Earth work together to allow complete coverage of the Earth's surface, after it has completed one complete cycle of orbits (Fig. 4). Through these satellites the entire globe is covered on regular basis and gives repetitive coverage on periodic basis. All the remote sensing resource satellites may be grouped in this category. Few of these satellites are LANDSAT series, SPOT series, IRS series, NOAA, SEASAT, TIROS, HCMM, SKYLAB, SPACE SHUTTLE etc.

 

Figure 3: Area Coverage on each Consecutive pass(source CCRS Website)

Figure 4: Complete Coverage of Earth Surface by Sun Synchronous Satellites (Source Earth Observatory)