One of my goal in my life is to spread knowledge and education to those who desire it. Since I specialize is Satellite Systems Engineering, I would like to help others who wish to follow similar path.
Now time is a limited resource. I cannot endlessly write articles on the topic and hope someone finds it useful. Therefore I have decided I will resume google analytics on this site, to find out what my audience is most interested in. Also, if you have any suggestion for improvement, leave them in the comments.
By the end of 2021, I wish to participate in forum promoting science and technology. I don’t know yet if the appropriate platform is already there. If not, I’ll also need to built it. Wish me luck.
[NOTE: For easy reading, you may want to read the PDF version of this document]
Launching Bangabandhu Satellite – 1 has created another milestone in the science and technology sector of our country. The event is truly worth celebrating. But what do you mean by a “Geostationary Satellite”? What makes it different than ‘other’ satellite? I would like to present you some of the key technical points of a geostationary satellite, from my notebook.
A geostationary satellite is a manmade artificial satellite, whose period of revolution is identical to diurnal motion of earth. In this way the position of satellite appears fixed with respect to earth. [Fun fact, geostationary satellite was first imagined by a British science fiction writer Arthur C. Clarke in 1945.] Since the relative position of satellite does not
change every day, tracking the satellite is possible without motorized antenna. Simply speaking, you can set up a receiver on your roof and forget about it. Had it been in any other orbit, you’d need an active tracking system. Geostationary orbit are used as a communication relay node most of the time. Typical example, a satellite phone. As depicted in the picture on the right, you’ll notice, it is quite big, unlike your smartphone. Company like Iridium argues that the large size of terminal and latency in communication is not worth it. Based on their argument, they have developed Sat Phone network on LEO.
Satellites basically stay afloat by orbiting the earth really fast. If you take Newton’s gravitational law and centrifugal force; you can roughly calculate satellite speed and altitude. Therefore, you need your launch vehicle [rocket, such as Falcon 9] to take your satellite to appropriate height, then travel parallel to surface to earth at your calculated, required speed. This is easier said than done. If you look back to any rocket launch, you’ll observe the rocket is not going straight up. Few seconds after the lunch, the rocket ascends in angle. This is called the pitching maneuver. Going straight up to 36,000 km is not an energy efficient approach. Once the satellite is out of earth’s atmosphere, we can use earth gravitational force to our benefit. The whole process can be divided into 3 steps.
Surface to parking orbit:
Rocket propels the satellite out of atmosphere into a temporary orbit, called parking orbit. Satellite is still orbiting the earth, but this is not the target orbit. Think of this orbit as a resting place. A break in between your long journey.
Parking orbit to GTO:
Using less energy (comparatively), circular parking orbit can become a highly elliptical orbit. Such kind of highly elliptical orbit can traverse from very high altitude to a very low altitude. This highly elliptical orbit is also a temporary orbit. We call it Geo Transfer Orbit or GTO.
GTO to Geostationary orbit:
Once the satellite reaches desired height, apogee kick motor [a kind of rocket, but attached with the satellite] activates and corrects the orbit from a high elliptical orbit to a circular orbit.
This 3 step process may seem lengthy, but this is the most efficient way to insert a satellite in GEO. Also, I have intentionally chose to exclude several other technical details in these steps.
Getting a satellite into orbit does not mean the satellite can start operating immediately. No matter how well a satellite has been build, a satellite engineer will never bet on his work. There are so many forces at play, you can never predict what will go wrong. USA put its first satellite in space in 1958. Yet, in 1999 NASA scientist lost a $125 million Mars orbiter due to a math mistake. JAXA lost a $360 million in ASTRO-H. Every satellite goes through an initial screening process. Remember, we can’t see the satellites with our naked eyes. If we command the satellite to ‘turn right’ for example, how do we verify the satellite has actually done what we asked for? Satellite engineers have a check list of tests they perform on a newly launched satellite. Passing these test, satellite can be considered calibrated.
Utilization of BS-1:
A satellite is as good as its design and it’ll do whatever it was designed to do. We can’t demand any absurd performance from satellites. A communication satellite will never give you weather update for example. If you want weather update, you should build a weather satellite. Bangabandhu satellite – 1 is communication satellite. It has 40 transponder in total. That means there are 40 discrete channel we can use. To put it simply, image it as 40 pipes. Whatever you transmit through one of them, will be broadcasted back to earth. It is up to us what we chose to be broadcasted. It may be TV channels. It may be telephone. Please keep in mind, like every other device, the channels have their maximum capacity. It’ll be impractical to demand 4K streaming from a 2G cell phone data connection.
As mentioned earlier, satellite motion is not so simple. It has the gravitational pull effect from sun and other celestial bodies. It has plasma interaction. The satellite will surely but slowly drift away from its slot. Satellite operators constantly monitor the drift and condition of their satellite. Upon requirement, they can command thruster to fire, use cold gas thruster to fix attitude. Space has nearly negligible friction. So if you start spinning, you’ll spin for a very very long time. The only way to move is to use thruster or means such as reaction wheels. Also, there is no gas station in space. This brings us to mission lifetime.
Mission lifetime is the time frame we expect a satellite to work. Say for example, mission lifetime of any satellite is 5 years. The satellite maybe working even after 5 years. But it doesn’t count as mission lifetime. Similarly, it may even live shorter than 5 years. Nobody knows. While planning satellite missions, we, satellite engineers, will assign a feasible number for mission lifetime. Beyond that mission lifetime, we will have future plans. Sending another satellite for example.
Fuel: The first major limited resource for geostationary satellite is fuel. You can only carry a limited amount of fuel onboard. If you carry more fuel with your satellite, than it means, your satellite is very heavy. To lunch a heavy satellite, you need more fuel. Therefore, the launcher will charge you more. You need to find an optimum point as an engineer. If your fuel is less, your satellite will die quickly. If you carry too much, it’ll be too expensive.
On-board electronics: Think of your computer. If you buy a very expensive, top of the shelf computer today, how long do you think it’ll be relevant? Within 6 months, your very expensive computer turns into a slow irritating PC. Similar condition also apply for satellite. Fun fact, this the reason I’m so excited about Lean Satellites.
Geostationary satellites are generally designed for 15 years of mission life. Solar panels of a satellite gradually lose their power due to space radiation. Batteries of satellites lose their kick due to use over long period of time, just like your cell phone. And the fuel required form station keeping usually finish.
Replacing Geostationary Satellite:
After your satellite is expired, it is time to replace it. Otherwise, you cannot utilize your geo slot. However, at the end of mission, your satellite probably do not have enough fuel to journey back to earth. For geostationary satellite, we push them further to a graveyard orbit. This is where geostationary satellites go to die! Now you can put your new geo satellite in the previous place. Ideally, you’ll place a new satellite much earlier than your current expiry date. This allows you to calibrate ahead of time and have a backup satellite ready in case you need it.
In my opinion, satellite engineering is a great way to learn about system engineering. We previously had no way to come close to it. Now, we have that access. Imagine trying to build yourself a wooden table, when you have never seen a hammer. When you have seen all the tools that are available to your disposal, you’ll have more confidence in your future builds. Therefore, I refuse to put a numerical value to a satellite project. It is simply an opportunity cost. In the same way, I believe true utilization of BS-1 is hard to express through metrics at this point in time.
For further questions, please feel free to drop me an email.
 Over here, were considering classical two body [earth and satellite] problem. When you consider gravitational pull from moon, sun and other celestial body, the problem become too complex. Often, simpler calculation is often enough for rough estimation.
 Cold gas thruster is a common means of attitude correction. It uses pressurized gas in tank. By releasing it systematically, it can maintain its attitude.
 Reaction wheels are heavy wheels specifically positioned to correct the facing of satellite. It uses the gyroscopic effect to correct attitude. Such imagine yourself in your rotating office chair. How do you start/stop spinning without touching anything? Same principle.