Student Innovation - How to Watch Netflix on MarsFollow article
My name is Likhit Macharla, I am a 4th year aeronautical engineering student at the University of Glasgow. This article outlines the research I completed for a project I completed last year on Deep Space Optical Communication (DSOC). I will also showcase some of the work I did as a finalist for NASA’s RASC-AL competition where our team were tasked to design a short stay crewed mars mission.
The Need for Speed
Since space communication systems have existed, the demand for high frequency, high-speed data transfer has continually grown. In the 70s during the Apollo era, NASA employed S band frequency communications operating at around 2GHz. When international television became popular, the C-Band was introduced at 4GHz. Most deep space operations now use X band with 9GHz and now, SpaceX launch Starlink satellites capable of Ka band at 24Ghz. This trend of increasing frequency will always continue, and as the next era of human space travel to Mars begins, the need for reliable, fast and efficient data transmission is just as essential as ever. Laser communication is the next step from Ka band with the opportunity to increase efficiency by 10-100 times. DSOC is the use of laser communication for high frequency, high data rate communications beyond Earth’s atmosphere.
How Does it Work?
Red Martians Invade Venus Using X-Ray Guns. My physics teacher taught me this mnemonic when I was 15 to learn the electromagnetic spectrum from low to high frequency. It turns out that understanding this is enough to comprehend the concept of laser communications. Increasing the frequency of the wave is directly proportional to the energy transmitted. This is important to us because it leads to a higher data rate - ‘data rate’ being a measure of how much information you can propagate over one second. The range of frequencies used for communications today sit primarily in the radio but also in the microwave band of the electromagnetic spectrum. DSOC utilises laser communications which sit within the infrared band of the electromagnetic spectrum, equivalent to 10 million times the frequency of high-frequency microwaves.
The beauty of laser communications lies in the atmospheric windows to Earth. Our atmosphere on Earth is perfect because it absorbs frequencies of the electromagnetic spectrum that can be harmful to us including ultraviolet, x-ray and gamma waves. For our species, this is great as it means we do not die, but for communications, it limits us. Our atmosphere allows radio, microwave, visible and (most importantly for us) some infrared waves.
To recap, by utilising a specific range of frequencies in the infrared spectrum, engineers can take advantage of communicating a very high-frequency wave with a lot of energy to achieve extremely high data rates. From now on we will also talk in terms of wavelength as the numbers are less ugly. For DSOC, 1530 nm to 1560 nm is the dominant spectral range.
Designing a Martian DSOC Network
During my NASA RASC-AL project, my goal was to design a communications network utilising DSOC and keep constant connectivity to Mars. The main factor I had to consider was the effect of weather. In my research, I completed an analysis of how different weather scenarios can affect communications across a range of wavelengths. I found that hazy and foggy weather was the worst conditions for optical communications, yet for RF communications the worst was actually rain or snow.
Another big problem I came across with laser communications was beam divergence. Beam divergence is a measure of how wide a beam of a specific wavelength spreads across a certain distance. Beam width determines how accurate the pointing capabilities of the transmitters need to be. Lasers have low beam divergence, meaning the light does not spread out easily. RF (Radio Frequency) communications do not incur these issues because they are higher in wavelength and so have a higher beam divergence. I wrote some code for beam divergence and found that for RF comms a satellite from Mars to Earth has beam widths approximately 100 times the Earth’s diameter. Laser communications signals have beam widths of less than 10% of the Earth’s diameter. To combat these issues, I designed a unique hybrid relay satellite, that took advantage of both these technologies meaning communications could operate in all-weather scenarios on both Earth and Mars. The satellite has three laser communication packages as well as a 1.5-meter-high gain antenna operating at Ka-Band. The lasers receivers have 20cm aperture and will be the primary source of communication to reach the high data transmission rates.
Three laser transmitters are used as this minimises the attitude control input, speed of transfer and adds robustness. This means the spacecraft could receive a transmission from the Martian surface and relay it instantaneously. This also reduces the need to store the data for long periods of time. In the below picture you can see the relay satellite utilising multiple transceivers at once.
There is also the phenomenon known as conjunction. This is the Sun’s blockage around Mars. This happens around every 780 days and can occur for weeks at a time and would totally cut off any link possibilities. The solution was to have a constellation of three relay satellites. This means that at least one of the three satellites will have both Earth and Mars in view at a time, so conjunction is no longer an issue. The satellites around Mars would be 120 degrees apart at 17,000km. Satellites at this altitude above Mars are in a unique orbit called an areostationary orbit. This means the satellites have an orbital period equal to one Martian day, thus from the ground, the satellite appears to be in a fixed position in the sky. Hence, having three satellites in this orbit would confirm global coverage of the planet. The constellation gives flexibility to NASA as it allows them to continue direct radio contact to the astronauts on the ground, even if they are on the opposite side of Mars.
This is because the relay satellites would be able to transfer the signal around the planet. This kind of manoeuvre is possible with the relay satellites working in conjunction to position, point and aim themselves.
What’s holding us back? + Next Steps
The issue is the technology isn’t ready yet for implementing in deep space. There must be significant improvements in technology development for optical communications. One of the main aspects of my research was an optical communications link budget analysis. Below are some of the results from the effect of altering ground receiver diameter and photon sensitivity on data rate.
These showed that the opportunity for extremely high data rates are possible without the need for increased mass, power or volume of the satellites themselves. The key is to improve the ground receiving capabilities of the receiver whilst keeping the complexity of the satellites transceivers as little as possible. LADEE was a satellite deployed to Lunar orbit and also the first satellite to demonstrate laser communications. The ground receiver used for this experiment had an approximately 20 photons/bit receiver sensitivity and receiver diameter of 1meter to achieve a downlink data rate of 38.55 Mbps. To consistently use DSOC in the future, the infrastructure for highly sensitive and large aperture receivers need to be built on Earth. Further analysis is required to be completed on improving space communication links such as putting a cluster of satellites on stable locations in between Earth and Mars or in high altitude orbits on Earth.
Thank you for reading! If you have any constructive criticism or questions feel free to contact me with my student email at email@example.com
You can find all my code here: https://github.com/LikhitMacharla/DSOC.git
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