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uplink-to-mars.txt
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uplink-to-mars.txt
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Abstract
This document describes a way to create a communication link to Mars
using TCP/IP protocol. Aim of this proposed solution is to lower the
latency in the worst case scenarios, such as when the Sun is blocking
communications or when a packet gets lost and has to be
retransmitted. Our solution to this problem is setting up a
satellite link relay system, which will save the packets it receives
until it receives ACK signal from the next satellite. If a packet
goes missing, it will be retransmit from the satellite rather than
from its origin.
Proposal [Page 1]
Relay system for Earth to Mars Digital Communication May 2020
Table of Contents
1. Introduction ....................................................3
2. The Latency Problem .............................................4
2.1. Satellite Relays .......................................4
2.2. Satellite Constellation ................................4
3. Data Integrity vs. Transfer Speed ...............................5
4. Summary .........................................................6
5. Sources .........................................................7
6. Feedback review .................................................8
Proposal [Page 2]
Relay system for Earth to Mars Digital Communication May 2020
1. Introduction
As colonization of Mars gets closer, there are still some problems
that must be solved before a such plan can happen. Currently NASA
has a long-term goal of sending humans to Mars by 2030s [1], so there
is not much time to lose.
One of the problems is how to use wireless data transmission as a
means of communications. The problem is that if a packet gets lost
into the space, the time cost of resending the packet from Mars
(or Earth) is way too long, making the communications between the
planets challenging.
Our solution to this problem is setting up a relay link system
between the planets, using satellites strategically positioned to
Lagrangian points and to the orbit of the Sun, Earth and Mars. The
relay links will have some memory, which will be used to cache TCP/IP
packets. This way, if a packet goes missing on its way to Earth
or Mars, it can be resent from the relay satellite, instead of
sending it from its origin planet, which would cause massive overall
latency.
The above solution will be discussed in more detail in the second
chapter. In the third chapter, we will discuss about data integrity
and the fourth chapter will have quick summary about this paper.
Proposal [Page 3]
Relay system for Earth to Mars Digital Communication May 2020
2. The Latency Problem
As both Earth and Mars occupy their own respective orbits of the Sun,
the distance between the two planets is by no means constant. The
theoretical minimum of the interplanetary distance is as little as 56
million kilometers, with the theoretical maximum being as large as
401 million km - over 7.3 times the minimum distance. This means,
that when using a lightspeed physical layer in Earth-Mars
communications, the transmission delay varies from roughly 3 minutes
to 22 minutes (of course, this would require communications through
the Sun, so the actual maximum delay is even larger). On average,
the interplanetary distance is 225 million km - causing an average
theoretical delay of 13 minutes. [2]
While obviously an inconvenience in itself, the transmission delay
is not a problem that can be solved with our current understanding of
physics. Instead the problem that this document proposes a
mitigation to is the overall latency realized as packets are lost and
retransmitted - the retransmission of a single package at the
worst-case scenario (that is the theoretical maximum interplanetary
distance) would cause an overall theoretical latency of some 45
minutes (again, the real-world value would be even larger).
2.1 Satellite Relays
Our proposed solution to the latency issue is setting up a relay link
system to the space, which would relay the packets between the
planets. When a connection from Earth to Mars (or vice versa) is
established, every link will open their own TCP session to the link
next to them. The link relay satellites must have enough memory and
logic so that they are able to create these sessions between them.
The motivation behind this satellite relay solution is to split the
total travelling distance to smaller chunks. In case of packet loss,
the lost packet would have to be transmitted only from the previous
relay satellite, instead of the planet of origin - reducing the
ultimate latency time. The relays should use some kind of algorithm
to determine the best link relays for the whole path at the current
situation. If the planets are close to each other, there should be
fewer links than for example when the planets are furthest away from
each other.
Now if a packet goes missing, noticing it will be much quicker,
because of the TCP sessions between the links. The links will cache
all packets that they receive for transmitting, and if a packet goes
missing they will simply retransmit it until they receive ACK from
the next relay. At this point, the packet may be removed from the
link's memory.
Getting this communication link up and running might be slower,
because it requires individual connections to bet set up for the link
satellites. This problem can be overcome, if the links between the
satellites always stay up.
2.2 Satellite Constellation
We propose that the satellite link relays will be placed to the
Lagrangian points of both Earth and Mars. In addition, there should
be some edge relays placed in the orbits of the said planets. By
placing the satellite link relays to Lagrangian points, we can make
sure that the Sun will never be able to block the communications
between the planets completely.
Others have also concluded way before us that placing the satellities
to Langrangian points might be a viable solution to this problem.
For instance, Rahman, M., Islam, M. and Huq, R., have concluded in
their paper that that placing the satellites to the said points is a
great idea. The paper also discusses in detail about the satellite
constellation and how many satellites are required. [3]
Proposal [Page 4]
Relay system for Earth to Mars Digital Communication May 2020
3. Data Integrity vs. Transfer Speed
This document will not go into the actual technological components of
interplanetary receiver/transmitter implementations, however it is
important to know the main points to consider when making
compromises between data transfer speed and sacrificing channel
capacity for forward error correction purposes.
The main difference between relevant resources on Earth and
interplanetary communication is that massive latency is usually
an issue on Earth, while due to regulatory reasons, bandwidth is much
more of a scarce resource. The opposite could be said about
planet-to-planet communications; while there is a lot of bandwidth
available (for now), the latency, more specifically the multiplied
latency in case of packet loss, is a major factor to consider.
Taking these resource factors into account, development should
prioritize data integrity over transfer speed. However, similar to
terrestrial wireless communications, different channel codes should
be developed to provide varying goodput values, as the channel will
without doubt vary over time, as the interplanetary distance changes,
along with the strength of solar noise. As the communication will be
split into shorter distances via the use of relay satellites, the
channel coding can also be altered for example at every link, be it
either planet-satellite or satellite-satellite.
Proposal [Page 5]
Relay system for Earth to Mars Digital Communication May 2020
4. Summary
This proposal for an Earth-Mars communications system uses an array
of satellites which will simultaneously act as relay points and
memory caches. The caching decreases the total latency experienced
in actual planet-to-planet communications, as the need to retransmit
data from the planet of origin is eliminated, should it become
corrupted between two relay points. The satellites are to be
positioned so that interplanetary communications are possible even
when the two planets are at the opposite sides of the sun. Further
development work should prioritize data integrity over transfer
speed, as avoiding any need of resending packets is a key part of the
need for this kind of system.
Proposal [Page 6]
Relay system for Earth to Mars Digital Communication May 2020
5. Sources
[1] Mars 2020 Mission Contributions to NASA's Mars Exploration
Program Science Goal,
https://mars.nasa.gov/mars2020/mission/science/goals/
Read: 11.05.2020
[2] How Far Away is Mars? | Distance to Mars
space.com,
https://www.space.com/16875-how-far-away-is-mars.html
Read: 11.05.2020
[3] Rahman, M., Islam, M. and Huq, R., Deep Space Communication and
Exploration of Solar System through Inter-Lagrangian Data Relay
Satellite Constellation.
Department of Mathematics and Physics, North South University,
Dhaka, Bangladesh
https://icubesat.files.wordpress.com/2019/05/b.2.2.201905241449-paper.pdf
Read: 07.06.2020
Proposal [Page 7]
Relay system for Earth to Mars Digital Communication May 2020
6. Feedback review
From the feedback we received we think the number 2 was most
useful. It had clear structure and good points about the subject,
though we didn't really alter our work. The reason for this was
that we simply don't really have room for more techical details
because of the 10 000 character limit given to this task. Lack of
time was also an issue, because we are both working full time now.
The mention about forgetting assigning IP addresses was ignored,
because it was not part of the task.
Feedback number 1 had some feedback about our grammar and those
issues have been fixed now. Number 1 was also little bit confusing
so we didn't really dig deep into it ether.
Feedback number 3 questioned our academic honesty, which of course
is not nice a thing to do. Our honesty was questioned, because our
proposal had the idea of placing the satellites to Lagrangian
points. In our defence, we must say that the idea is not very
unique and can be easily formated in the following thought
process.
1. If we want to communicate to Mars at all times, even when it is
behind the Sun, we must have some kind of link system
2. As the links, we can use satellites and build a "router" system
3. How can we make sure, that there always is a satellite that can
be communicated with from both Earth and Mars?
4. Answer: Langrange points
Now because this truly is not a very unique thing to discover, we
should have probably looked into if someone else had already
written and published scientific research about placing the
satellites to the Langrange points. We were in a hurry though,
and forgot to do that. But in our opinion, by no means is coming
up with the idea of placing the satellites to Langrange points
academic dishonesty. We added a chapter to 2.2 to discuss, that
our idea was not unique and that others have came to the same
conclusion way before us. It was not our intention to say that we
were the only ones that have came up with this idea.
Proposal [Page 8]