Types of Remote Sensing Satellites

What is Satellite?

The satellite better to say in the context of remote sensing, the artificial satellite is an object that has been intentionally placed into a revolving path or orbit, can operate by themselves, carrying with sensor to make star maps and maps of planetary surfaces, planetary observation, communication, navigation, weather monitoring, etc.

Types of the satellite:

There are two different types of satellites – Natural and Man-made. Examples of natural satellites are the Earth and Moon etc. The Earth rotates around the Sun and the Moon rotates around the Earth. A man-made satellite is a machine that is launched into space and orbits around a body in space. Examples of man-made satellites are the INSAT, SPOT, NOAA, IRS etc. But we have discuss here man-made satellite or remote sensing satellite.

Types of Remote sensing (RS) satellite:

Two types of remote sensing satellite as per orbital Height from the Earth and as per relative motion and orbital inclination.

1. As per orbital Height from the Earth:

1.    High Earth orbit: (>= 35,780km from the earth surface): Too many weather satellites and some communications satellites have a high Earth orbit.

    When a satellite reaches exactly 42,164 kilometers from the center of the Earth (about 36,000 kilometers from Earth’s surface), it enters a sort of “sweet spot” in which its orbit matches Earth’s rotation. Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south. This special, high Earth orbit is called geosynchronous.

2.    Medium Earth orbit: (2000 to 35780Km from the earth surface): In this group of satellites works for navigation and special purpose and designed to monitor a particular region.

Closer to the Earth, satellites in a medium Earth orbit move more quickly. Two medium Earth orbits are notable: the semi-synchronous orbit and the Molniya orbit. The semi-synchronous orbit is a near circular orbit (low eccentricity) 26,560 kilometers from the center of the Earth (about 20,200 kilometers above the surface). A satellite at this height takes 12 hours to complete an orbit. As the satellite moves, the Earth rotates underneath it. In 24-hours, the satellite crosses over the same two spots on the equator every day. This orbit is consistent and highly predictable. It is the orbit used by the Global Positioning System (GPS) satellites.

The second common medium Earth orbit is the Molniya orbit. Invented by the Russians, the Molniya orbit works well for observing high latitudes. A geostationary orbit is valuable for the constant view it provides, but satellites in a geostationary orbit are parked over the equator, so they don’t work well for far northern or southern locations, which are always on the edge of view for a geostationary satellite. The Molniya orbit offers a useful alternative.

3.    Low Earth orbit: (9180 to 2000km from the earth surface): Most scientific satellites are in this group. NASA’s Earth Observing System is an example of it.

Figure 1: All types of orbit

2. As per relative motion and orbital inclination: 

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

1. 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 2). 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 2: Geo-stationary Orbit (source NASA)

2. 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 3. This ensures similar illumination conditions when acquiring images over a particular area over a series of days.

Low Earth satellites get closer to Earth. So, the pull of gravity gets stronger. That is why the satellite of this group moves more quickly. For example, NASA’s Aqua satellite requires about 99 minutes to orbit the Earth at about 705 kilometers above the earth.

Inversely high earth satellite moves slower. A weather satellite about 36,000 kilometers from Earth’s surface takes about 24 hours to complete an orbit. In the same way at a distance of about 3,84,000 kilometers from the center of the Earth, the Moon completes a single orbit in 28 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 3: 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. 4). 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. 5). 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 4: Area Coverage on each Consecutive pass(source CCRS Website)

Figure 5: Complete Coverage of Earth Surface by Sun Synchronous Satellites (Source Earth Observatory)