The Fundamentals of Satellite

- How Satellites Work

A satellite is basically a self-contained communications system with the ability to receive signals from Earth and to retransmit those signals back with the use of a transponder—an integrated receiver and transmitter of radio signals. 

A satellite has to withstand the shock of being accelerated during launch up to the orbital velocity of 28,100 km (17,500 miles) an hour and a hostile space environment where it can be subject to radiation and extreme temperatures for its projected operational life, which can last up to 20 years. 

In addition, satellites have to be light, as the cost of launching a satellite is quite expensive and based on weight. To meet these challenges, satellites must be small and made of lightweight and durable materials. They must operate at a very high reliability of more than 99.9 percent in the vacuum of space with no prospect of maintenance or repair. 

Services that satellites can provide for disaster risk management and emergency response include weather forecasting, remote sensing, geolocation, navigation, television and telecommunications. 

Instruments on satellites orbiting the Earth are designed to cover specific wavelengths of the electromagnetic spectrum for image capture, atmospheric sounding, satellite communications, geolocation and navigation. 

Depending on the type of application or onboard instrumentation, satellites orbit the Earth in different orbits: A satellite in a geostationary orbit circles the Earth synchronously with the Earth's rotation above the equator (0° latitude). 

Its apparently fixed position at an altitude of more than 36,000 km above the equator makes it suitable for communications and regional climate observations in this particular region, with high temporal resolution but low spatial resolution. 

Earth observation satellites and those used for meteorological purposes are located in low Earth orbit, usually at an altitude of about 500-800 km, near polar inclinations. Due to their orbits, these satellites provide global coverage with relatively low temporal resolution, but moderate to very high spatial resolution. Because of the high cost of space transportation, constellations of communications or navigation satellites are also placed in low-Earth orbit. 

Earth observation satellites use optical or radar sensors to capture images of the Earth: Optical sensors used for Earth observation are designed to provide images in either panchromatic spectral format or multi-spectral format. 

Panchromatic refers to a black-and-white image reflected from the Earth's surface exposed to all visible light. Multispectral images typically include four bands of the electromagnetic spectrum: blue, green, red, and near-infrared.

- The Main Components of A Satellite

The main components of a satellite consist of the communications system, which includes the antennas and transponders that receive and retransmit signals, the power system, which includes the solar panels that provide power, and the propulsion system, which includes the rockets that propel the satellite. 

A satellite needs its own propulsion system to get itself to the right orbital location and to make occasional corrections to that position. A satellite in geostationary orbit can deviate up to a degree every year from north to south or east to west of its location because of the gravitational pull of the Moon and Sun. A satellite has thrusters that are fired occasionally to make adjustments in its position. 

The maintenance of a satellite’s orbital position is called “station keeping,” and the corrections made by using the satellite’s thrusters are called “attitude control.” 

A satellite’s life span is determined by the amount of fuel it has to power these thrusters. Once the fuel runs out, the satellite eventually drifts into space and out of operation, becoming space debris.

[More to come ...]