15 January 2019

Every day satellites collect a wealth of information about Earth, but they must send it down to the ground before we can make use of it. Sometimes this data can be lost, damaged, or delayed, but our access to it could be improved using Delay Tolerant Networks (DTNs) – a new way of communicating with spacecraft.

Imagine the inconvenience of only being able to send a message to a friend when your phones are directly facing each other with a perfect connection. Fortunately, the internet allows us to circumnavigate this problem by passing data to in-between nodes, and if your friend’s phone isn’t connected to the internet, the data is stored until it can be transferred to them.

ESA’s Aeolus satellite sending data to a ground station in Sweden (artist’s impression)

Currently communication with Earth observation satellites does not benefit from this internet-like transmission and storage of information through in-between nodes. Earth observation satellites only send data down to Earth when they are directly overhead a ground station. When a satellite isn’t facing a ground station, data starts queuing up. There is no system that automatically sort the queue to prioritise the sending of urgent data – information captured about natural disasters,perhaps – from the ordinary. 

Delay Tolerant Networks offer a solution in the form of using relay spacecraft and other ground stations. These would act as intermediate nodes that would be able to hold on to data and pass it on as soon as the next ‘hop’ is available, ensuring its safe delivery by relaying it up to a spacecraft or down to a ground station at just the right time. DTNs provide a new way of transmitting information, creating the foundation for a ‘space internet’.

So far, DTNs have mostly been explored in the context of deep space – when distant planetary orbiters and rovers need to use intermediary nodes to communicate with Earth. But a team of researchers supported by ESA’s Discovery and Preparation Programme recently investigated the possible benefits of Delay Tolerant Networking for Earth observation.

The team, made up of representatives from GMV INSYEN, German Aerospace Center (DLR), Solenix Deutschland and the University of Bologna analysed how DTNs could improve our communication with Earth observation spacecraft.

Different types of Delay Tolerant Networks

Sebastian Martin, responsible for the project from ESA’s side, explains further, “DTNs allow information to be sent through a network that does not have a direct route from the starting point to the final receiver of information. In-between ‘nodes’ receive information and store it until they can send it on to a neighbour. In addition, DTNs can automatically schedule when to store information and when to forward it over different possible routes.”

The team began by investigating how one of the existing Copernicus Sentinel missions could benefit from DTN technology. They then modelled a futuristic scenario with a full system of DTN-enabled Earth observation satellites, ground stations and control centres. In both scenarios, they found that the data automatically reached their destination via optimal routes and that the network correctly handled data with different priorities.

They also looked into a second benefit of DTNs. When existing Earth observation satellites pass over ground stations on their orbits around Earth, a short amount of time at the beginning and the end of its pass is not used to transfer information. This is because there is a risk of data getting lost when the satellite is low over the horizon with a poor connection with the ground station. Using DTNs, it is possible to automatically check for lost data and get it resent. This means it is safer for satellites to attempt to send information as soon as the ground station is in sight, possibly resulting in more data being transferred during each pass of the satellite.

Data can also be lost for other reasons, like bad weather. Optical communications, for example, can be severely affected by cloud cover, so to avoid loss of data it is necessary to wait for clear skies to send information. DTN networks would support this data being sent at any time, as lost data would be automatically detected and resent.

A third advantage of DTNs is that data can be automatically prioritised. Michael Staub, who managed the project from GMV INSYEN, explains, “The DTNs that we created successfully sorted data depending on its priority, meaning that important observations – for example those made during natural disasters – would be sent as quickly as possible, even if other data joined the queue first.”

This study is one step further in our understanding of how DTNs could revolutionise space communications. Next, experts will investigate how DTNs can be implemented technically, what data types would profit most, the potential impact on operations and operators, and where new markets and users could benefit from this technology.


The OPS-SAT CubeSat will test new techniques in mission control and onboard systems

One option being considered for testing DTNs is ESA’s OPS-SAT CubeSat. The 30-centimetre high demonstrator will test out a large variety of technologies, including DTNs, to provide information useful for larger future missions. Eventually, the more spacecraft and ground stations using DTN technology, the more benefits the technology will bring.

“Through this study, we have shown that DTNs would be very beneficial for Earth observation scenarios,” concludes Staub. “The networks we propose will enable organisations and commercial entities to interoperate, including encouraging the sharing of each other’s facilities and resources.” With space exploration becoming more complex, current communication networks become increasingly inadequate. DTNs would facilitate the next generation of space missions.


B)Maintaining large-scale satellite constellations using logistics approach

Today, large-scale communication satellite constellations, also known as megaconstellations, have been more and more popular. OneWeb launched the first batch of satellites of an initial 650-satellite constellation in February 2019, and SpaceX also launched the first batch of its 12,000-satellite constellation in May 2019. On July 8, Amazon also filed an application with the FCC for its planned satellite constellation with 3,236 satellites. These satellite constellations are expected to be a game changer by realizing the worldwide satellite Internet service.

However, the unprecedently large scale of these megaconstellations also brings numerous challenges, some of which are hidden and not well-explored. Researchers at the University of Illinois at Urbana-Champaign identified a critical hidden challenge about replacing the broken satellites in megaconstellations and proposed a unique solution with inventory control methods.

“Maintaining these large-scale megaconstellations efficiently is far more complex than the traditional space systems. In fact, it has become more and more like a ground logistics problem that FedEx or UPS has been working on. So we tackled this megaconstellation maintenance problem leveraging the idea from ground logistics, which turns out to be not only unique and interesting but also very suitable in this context” said Koki Ho, assistant professor in the Department of Aerospace Engineering at U of I.

The challenge Ho described is to efficiently swap out a new satellite for one that breaks. For telecommunications companies, broken satellites mean interrupted communications and Internet service, which leads to disgruntled customers and loss of revenue.

“Deploying a large-scale constellation is one problem, but maintaining it is another possibly more challenging problem,” Ho said. “When the satellites break, providing a spare quickly is important so there is little gap in the service. Companies need continuous service to provide global coverage. In order to achieve that, we need to have sufficient spares in orbit. The question is: how many would be sufficient. Can we think of a smarter way to use as few satellites as possible to satisfy the gap requirement?”

In earlier satellite constellations, Ho said this was not a problem because the scale was small enough sophisticated methods to calculate the needed number of spares was not needed; just having a few spares per orbital plane was enough. But with a constellation made up of hundreds of satellites, the strategy won’t work. Also, new, small satellites are cheaper but have a relatively higher failure rate so many more spares are needed in each orbital plane, and that’s inefficient.

“Our idea is to use something called a multi-echelon inventory control method in the ground logistics and apply it to the orbital mechanic’s context,” Ho said. “In our solution, another orbit that is lower than the actual orbit, which we call the parking orbit; becomes an intermediate warehouse of the satellites. A small number of spare satellites are in the actual orbital plane for immediate replacement, while a larger inventory of replacement satellites is waiting in the parking orbit. The ones in the orbital plane cover an immediate need, the spares in the parking orbit can replenish the actual orbit.”

The research also takes advantage of the J2 effect of the orbital plane, which is caused by the Earth’s obliqueness, to deliver the spares. The Earth is not a perfect sphere, Ho explained, and because it’s not a perfect sphere, the orbital plane will shift.

“That orbital plane shift rate is different depending on the altitude,” Ho said. “So when we have a parking orbit that is at a lower altitude than the original constellation orbit, their orbital shift rates are different. The mathematical model we created takes into account that rate shift and which plane is closer to the satellite in need of replacing so that you will have continuous coverage of the Earth. The method looks at which orbital plane is the first one that will match with the plane that has a demand and also consider whether that plane actually has spares in it. If that plane doesn’t have spares, then we wait until the next plane,” Ho said.

Ho said this method also removes the costly urgency to launch a replacement satellite.

“With this warehouse strategy, when there is a failed satellite, there is already an inventory of stock available to replace it. When the stock goes below a threshold, you can launch more to the parking orbit. This takes advantage of the batch launch effect. It’s cheaper to send one rocket up with a bunch of satellites than launching each of them separately.”

Ho believes this new supply method solves a timely problem.

“People are talking a lot about these megaconstellations but they haven’t thought deeply enough about some of the new challenges they bring,” Ho said. “Using a unique warehouse approach provided an efficient solution to address this complex problem.”

Story Source:

Materials provided by University of Illinois College of EngineeringNote: Content may be edited for style and length.

Journal Reference:

  1. Pauline Jakob, Seiichi Shimizu, Shoji Yoshikawa, Koki Ho. Optimal Satellite Constellation Spare Strategy Using Multi-Echelon Inventory ControlJournal of Spacecraft and Rockets, 2019; 1 DOI: 10.2514/1.A34387


An artist's impression of what the Mayak satellite could look like once in orbit

A group of Russian engineers has raised over £17,000 to launch a reflective satellite which would be one of the brightest objects in the night sky.

The team behind the proposed ‘Mayak’ or ‘Lighthouse’ satellite have raised 1.8 million rubles (£17,500) at the time of writing, surpassing their initial goal of 1.5 million rubles.

These funds will go towards research and the development of prototypes, but the ultimate goal is to launch the satellite into orbit.

Once the small satellite is in space, it will unfurl a large, pyramid-shaped solar reflector, desigend to catch as many of the sun’s rays as possible. 

The team claim that it will turn the satellite into the brightest object in the night sky (after the Moon), and will serve to remind the world “who was the first in space” – that’s Russia, if you were wondering.

The enigneers have apparently been in talks with Russian space agency Roscosmos to place the Mayak satellite aboard a Soyuz-2 rocket which is scheduled to take off in mid-2016 – however, it’s not clear whether they’ll be able to meet this tough deadline.

Contributors to the crowdfunding campaign on Russian website Boomstarter can get a series of perks, depending on how much they give – those who donate 100,000 rubles (£970) get a Mayak-branded telescope, while a 300,000 ruble (£2,900) contribution is rewarded with an invite to Baikonur Cosmodrome in Kazakhstan to watch the launch live.

Although one of its aims is to remind the world of Russia’s dominance of space, the team also hope it will help get young people enthusiastic about science and space exploration.

By using an app, people will be able to track the orbit of the satellite and spot it as it passes overhead. It’s also hoped that the Mayak’s experimental braking equipment could be an example to other scientists working to solve the problem of space junk.



About sooteris kyritsis

Job title: (f)PHELLOW OF SOPHIA Profession: RESEARCHER Company: ANTHROOPISMOS Favorite quote: "ITS TIME FOR KOSMOPOLITANS(=HELLINES) TO FLY IN SPACE." Interested in: Activity Partners, Friends Fashion: Classic Humor: Friendly Places lived: EN THE HIGHLANDS OF KOSMOS THROUGH THE DARKNESS OF AMENTHE
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