5.10. Types of Orbits

A space-borne remote sensing platform is placed in an orbit in which it moves continuously. From geometrical characteristics point of view, orbits of the space-borne platform can be circular, elliptic, parabolic or hyperbolic. But in practice, elliptical orbits are used.

Space orbits are paths that satellites follow as they move around the Earth. These orbits can be classified based on their shape into circular, elliptical, parabolic, or hyperbolic. However, most satellites utilize elliptical orbits due to practical advantages, such as fuel efficiency and the ability to cover specific areas of Earth more effectively.

Think of a satellite's orbit like a racetrack. Just as some tracks are oval while others might be circular or have unique shapes, satellites can follow different paths around the Earth. Most races happen in ovals because they allow for consistent speed and access to spectators — similar to why most satellites prefer elliptical paths to efficiently gather data.

Geo-synchronous or Geo-stationary satellite moves in an orbit so that it covers one revolution in the same time as the Earth to rotate once about its polar axis. The satellites revolve in the same direction as that of the Earth (west to east) at an angular velocity equal to the Earth’s rotation rate.

Geo-synchronous satellites are placed at a high altitude of approximately 36,000 km above the equator. This allows them to rotate around the Earth at the same rate that the Earth spins. As a result, these satellites appear to stay fixed over one point on the Earth's surface, which is useful for communications and weather monitoring.

Imagine spinning a toy on a table. If you want the toy to stay in one spot, you have to keep spinning it at just the right speed. Similarly, geo-synchronous satellites must 'match' the Earth's rotation speed to remain over a specific location, like a favorite park you always want to observe.

The geo-synchronous satellites, like INSAT, MeteoSAT, GOES, GMS etc., are used for communication and meteorological purposes. Satellites in the geo-synchronous orbit are located at any particular longitude to get a continuous view of that particular region.

These satellites provide a constant view of the same area, allowing for consistent data capture critical for weather forecasts, television broadcasts, and other communications. Their ability to observe the same region consistently helps in providing accurate and timely information.

Consider a security camera fixed in one position. It continuously observes the same area, allowing the owner to monitor activity over time. Similarly, geo-synchronous satellites continuously observe particular regions on Earth, making them indispensable for real-time applications such as weather forecasting.

Sun-synchronous or Polar satellites move in low orbits (approximately 700-900 km) above the equator. The orbital period typically varies from 90-103 minutes, covering several orbits per day.

Sun-synchronous satellites orbit the Earth at a lower altitude and are designed to pass over the same geographic area at the same local solar time, ensuring that the angle of sunlight is consistent. This allows for uniform lighting conditions which improve image quality.

Imagine taking a photo of a flower each day at sunrise. Each time, the light is similar, allowing for a fair comparison of how it blooms. Sun-synchronous satellites do just this but for large areas of Earth, capturing the way landscapes change under consistent lighting.

Due to the rotation of the Earth on its own axis, each time the satellite moves in the orbit, it observes a new area below it. The satellite's orbit period and the rotation of the Earth together are synchronized to allow complete coverage of the Earth's surface.

The collaboration between the satellite's orbit and the Earth's rotation ensures extensive coverage of the planet's surface, allowing these satellites to offer detailed imagery from a few days to weeks. The revisit period is important as it indicates how often a satellite can capture images of the same area.

Think about how a teacher checks in on students during a school year. If the teacher returns to each class at regular intervals, they can see how students are progressing. Similarly, sun-synchronous satellites revisit areas of the Earth regularly to observe changes over time.

These are many polar satellites with steerable sensors, which can view off-nadir areas before and after the satellite passes over a ground in an orbit.

The ability to adjust the sensors allows sun-synchronous satellites to capture images of areas not directly below them, improving their data collection capabilities and allowing for increased observation frequency. This flexibility is crucial in monitoring dynamic events like natural disasters.

Think of a camera on a tripod that has a movable lens. If it can rotate even slightly, it can capture pictures of a broader area rather than just straight ahead. This is similar to how polar satellites can adjust their sensors to monitor various parts of a region efficiently.

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