Rocket Science — Basics — Part 4— Satellites 🛰️📡
In this blog post, we will explore the fascinating topic of payloads in rockets, particularly focusing on satellites as examples.
So, What is a payload? — A payload refers to the part of a rocket or spacecraft that is designed to carry out specific mission objectives. This can include satellites, scientific instruments, communication equipment, or any specialized technology necessary for the mission’s success. Essentially, the payload is what the mission aims to deliver or deploy into space.
We will focus on satellites. The satellite can be hand held ones called as cubesats or it can be a navigation satellite, similar to size of a bus or it can be astronomical telescope for scientific purposes which are size of a football ground.
The first part of the satellite is the solar panels which is mandatory for generating electric power for the satellite to function. If there are no electric power for the satellite, it becomes a space debris i.e., a space waste. In the figure above, the blue panel on either side of the satellite is the solar panels. Usually, the solar panels will be closed until it reaches the orbit and once it reaches it unfolds the panels. The working of the solar panel is similar to that of solar panels in earth.
Now, You must have question like what if the satellite is in earth’s shadows for some period when it is hidden from sun? It will be obvious when satellite is in LEO (Low Earth Orbit) than other orbits 🤔.
The answer is, the satellites are equipped with rechargeable batteries that gets powered by the solar panels. The more the size of the panels, the more the power it can generate.
Solar flux — The amount of solar radiation per unit surface area. If the distance between the sun and the satellite panel is high, the solar flux will be low. This means the solar flux will be reduced to minimum or even zero after getting even close to mars and beyond. The solar panels works by the photovoltaic theory. When sunlight strikes a solar cell, it excites electrons in the material, causing them to flow and generate an electric current. Most of satellites contains a tracking system that allows them to orient themselves towards the Sun continuously.
In most satellites Gallium Arsenide is used as a solar panel material rather than silicon used in Earth in order to increase the efficiency.
Next, we will discuss about Batteries. There are two types of batteries. One is Primary battery and the other is the Secondary battery.
Primary battery — It is also called as One-shot battery meaning it is not rechargeable. It is a high energy density battery and is usually used in missiles and rockets.
Secondary battery — It is also called as rechargeable battery. It is a low energy density battery. This kind of battery is used in satellites.
So, from the figure above, we can see that the shaded region represents the earth’s shadow part i.e., no direct sunlight, the battery power will be discharged and during the non-shadow where the sun light is directly on the solar panels, the batteries will be recharged.
Next, we will discuss about Radioisotope Thermal Generator (RTG)
If you look at the above satellite, you can see there are no solar panels. This type of satellites is used for exploring other celestial objects which are farther away from sun.
RTGs convert the heat released by the radioactive decay of isotopes into electricity. Commonly used isotopes in RTGs include Plutonium-238 (Pu-238) and Strontium-90 (Sr-90), both of which produce heat as they decay over time. They are reliable because they provide a consistent power supply over long periods, with lifetimes often extending beyond 10 years due to the long half-lives of the isotopes used. They require minimal maintenance also when compared with solar panels.
For the fact, the above New Horizons satellite was launched into space to capture photos of Pluto🫡.
The next part, we will look into is Satellite Communication.
The main important part of the satellite is the Communication system. This system facilitates various applications, including television broadcasting, internet services, telephony, and military communications. The fundamental principle involves relaying radio frequency (RF) signals between ground stations and satellites in orbit.
For the fun fact, the microwave oven also uses the radio waves very similar to that of our wi-fi wavelength.
Space segment of the communication system use transponders that receive, amplify, and retransmit signals. Ground systems comprises of earth stations that send and receive signals to and from the satellite. These stations include antennas, transmitters, receivers, and processing equipment. Lastly, Control segment is responsible for monitoring and controlling the satellite’s operations, ensuring it remains in the correct orbit and functions optimally.
The earth station transmits a signal to the satellite at a specific frequency (uplink frequency). The satellite receives the signal, processes it (often changing its frequency), and then retransmits it back to Earth. The processed signal is sent back to another earth station at a different frequency (downlink frequency).
Advantages of satellite communication is it can cover wider geographic area, not prone to natural disasters and high band availability.
Disadvantages of satellite communication is latency (uplink and downlink time) is high, maintenance cost is high and some high frequency bandwidth may have signal degradation due to rain or atmospheric conditions.
Next, we will look into Imaging Camera.
Camera in satellite works very similar to the one that is present in our mobile phones. The camera can be pointed away from Earth to capture outer space or it can be pointed towards Earth to capture information like remote sensing.
Satellite cameras are equipped with lenses that focus light from the Earth’s surface onto a sensor. The quality and design of these lenses are crucial for capturing high-resolution images. The core of satellite cameras is the imaging sensor, which converts incoming light into electrical signals.
When the satellite is in orbit, the camera captures images as the satellite moves over the Earth. The speed of the satellite (approximately 7 km/sec) necessitates rapid image capture techniques.
Once an image is captured, it is processed onboard or transmitted to ground stations for further analysis.
That’s it for the basics of satellites.
In the next blog post, we will look into advanced orbital mechanism.
Thanks for the reading !!!
