Low Earth orbit or LEO will be covered to a greater extent in this book than other types of space systems for a multitude of reasons. The most important to me is that as space becomes more reachable and feasible for varying organizations to operate within, that accessibility will begin at LEO first. Since LEO will be the most readily available portion of the space domain available to the widest potential operators, it will initially present the lion’s share of computing devices in space that is in need of appropriate cybersecurity implementations.
Exact definitions of what constitutes LEO vary from organization to organization. In a general sense, a space vehicle (SV) would be considered to exist within low Earth orbit if it did not pass beyond an altitude of around 2000 kilometers or very roughly 1200 miles above the Earth. The SV also has to maintain and recur that orbit and not return to the atmosphere immediately. For those of you versed in space operations, you may have slight corrections or opinions on this, but for the basis of understanding the unique aspects of SVs within this orbit, those assumptive measurements are more than adequate.
Further, I will be concentrating on the small satellites, also known as cubesats or smallsats as I discuss LEO SVs. I will be doing this because most LEO SVs are small satellites and because cubesats have proven to be a good way of standardizing this initial and burgeoning frontier of space operations. Cubesats get their name for being one unit or one “U” which is a 10cm by 10cm by 10cm cube. Small satellites or cubesats are often referred to by their size, such as 2U, 6U, and so on. A 2U cubesat closely resembles the size of a loaf of bread. For the rest of this book, I will refer to such SVs together as smallsats.
My focus on smallsats in LEO is not to make a statement that other types of SVs in LEO are impossible or improbable to exist. On the other hand, they provide a commonly used and relatively standardized form factor present in the LEO region of space around the planet and share characteristics that impact how general space challenges apply to them as well as why they create their own specific attributes and issues.
LEO, Smallsats, and the General Challenges of Space
As you would expect, having an extremely small form factor and flying at LEO present positive and negative adjustments to the general challenges of space system operations we already discussed. LEO itself and the smallsats that fly in it provide advantages and disadvantages to different mission sets that can be carried out by SVs. As will be shown with other orbits and vehicle types as well, there are also certain missions that can only be accomplished from specific orbits and that is due to their unique attributes as they apply to general space challenges and the attributes specific only to their orbit and intent.
Environmental Challenges
Due to flying closer to Earth, LEO SVs are more impacted by the Earth’s atmosphere more than a more distantly orbiting or non-orbital SV would. Additionally, the atmospheric influence and proximity to the Earth change the way other environmental challenges will impact the spacecraft.
Radiation
For instance, radiation is going to have less of an impact on LEO SVs than those that venture completely beyond the protective barriers of the Earth’s atmosphere and electromagnetic fields. This means that radiation absorbed throughout the life span of the SV will be less than would happen on an orbit that resided further from the planet. It also means that any singular radiation events such as solar flares will be at least somewhat muted by the time they penetrate the atmospheric and electromagnetic barriers and ultimately affect the SV.
What all this boils down to is that for LEO-orbiting SVs of any size, radiation hardening to protect from harmful bursts or accumulations is necessary to a lesser degree when considering the potential life spans of these vehicles. Risk acceptance decisions for such orbital regions are more likely to happen regarding increased radiation shielding instead of paying more for further radiation-hardened components. The byproduct of that means SVs in LEO can be of smaller form factors and weigh less since they often do not need to pack on additional radiation shielding. Of course, this is not always the case, and special payload missions or operational life spans intended to be longer than usual may still need to pursue preventative measures against radiation damage.
Temperature
Unlike radiation, temperature fluctuations are going to be more irregular for a vehicle orbiting close to Earth due to potential variations in atmospheric density. As a SV’s orbit is higher above the surface of the Earth, temperature fluctuations will be more easily predicted via orbital location in the vacuum of space. As such, preparing for and making risk decisions regarding temperature for LEO devices is not necessarily a straightforward endeavor.
Space Objects
Where the general challenges of radiation and temperature are less of an issue for LEO SVs, the challenge of space objects, specifically man-made ones, is exacerbated significantly. Since LEO is the most accessible and financially feasible region of space to conduct space system operations, there are many more space objects to avoid and in a much denser area. Even though SVs in this orbital region are more likely to fall into the atmosphere and burn up, the sheer prevalence of debris, junk, and dead as well as operating SVs means it must be a regular consideration.
Since most SVs in LEO are smallsats, there are added complications due to the small form factor. Many smallsats do not have onboard propulsion and, if so, do in very small amounts. This means that the SVs in LEO are likely to have very slow maneuver capabilities like torque rods, or none at all. Due to this constraint, any maneuvers to avoid potential collisions must be orchestrated and conducted for potentially long periods of time. This may take significant portions of operational windows away from the total life span of the SV. It also means that due to the long lead time needed to actually avoid something via these mechanisms, collisions may not be predicted until it is too late to maneuver safely.
Gravity
Gravity is a two-way street for LEO SVs. On the one hand, the thrust needed to get to LEO and deploy a SV is much less than traveling further into space which means that scheduling and purchasing rides on launch vehicles are easier. Since it is an easier technological feat to enter LEO, more providers are available to get your SV there. Also, since there are more vendors and less fuel requirements, these rides are cheaper in general. Add to that the small form factor of many devices in LEO and the ride becomes even more easily attainable. A loaf of bread is a lot cheaper to get into space than a car.
On the other hand, since the SVs do not escape much of Earth’s gravity by only making it into LEO, they are more impacted by it. This means that entering orbit at the right speed and trajectory is very difficult because if done incorrectly, there is relatively little time or even ability for the SV to try to correct to a more sustainable orbit. Imagine a smallsat with only torque rods and flywheels, incorrectly deployed and in an orbit that will bring it burning up in the Earth’s atmosphere within 6 months. The attitude and position options available to the SV may not even have the energy to correct the SV into a longer-lasting orbit.
Even with successful deployment into the correct orbit, the effects of gravity at LEO combined with the drag from passing through the atmosphere acting on the SV mean that orbital life spans are going to be shorter in general than they would be much further from the planet’s gravity. Choices to use LEO with respect to gravity center around cost and needed operational life span.
Operational Challenges
General environmental challenges to LEO SVs are mostly impacted by proximity to Earth. General operational challenges are affected by that to a degree but are also impacted by the small form factor and operational life spans available to smallsat SVs.
Testing
Testing is a pretty standardized concept for SVs of all types. Things like radiation temperature and vibration are unavoidable necessities to prevent huge wastes of time, money, and effort due to launching a SV that becomes inoperable in space. The one benefit to smallsats, which as we covered are a typical SV for LEO, is the small form factor allows for easier efforts at finding test facilities. Irregularly shaped or large SV programs may have a much more difficult time finding a facility with a vacuum chamber or oven large enough to test the SV’s resilience to the elements of outer space.
Launch
I have already covered some of the benefits smallsats and SVs in LEO receive due to their form factor and the escape of gravity. One interesting thing about smallsats is they are oftentimes small enough to be deployed via the International Space Station (ISS) since they are small enough to fit in the air locks on board. Having the ability to ride-share on resupply missions to the ISS is an added perk to being small.
Deployment
In general, due to the growing standardization of single and multi-U SVs, there is less customization and fabrication needed for launch vehicles to be able to take and deploy smallsats in LEO. Additionally, smallsats are more easily deployed in groups. Some mission sets require a constellation of SVs orbiting the Earth. Having to deploy those vehicles on many separate launches can bring a level of complexity to the operation that may not be feasible, whereas being small means the same launch vehicle may be able to deploy multiple SVs of the space system at the same time and in the same orbital plane. Though certainly any dispensed members of a constellation must perform orbital maneuvering to achieve proper location within the orbital plane, doing so via a single launch is possible with the small form factor of LEO smallsats.
Stabilizing
As you may have guessed after reading the issues with space object avoidance in the “Space Objects” section, stabilizing for SVs in LEO can also be a greater challenge than faced by other sizes and locations of SVs. Small size and resources available to smallsats in LEO mean that if the SV is deployed and begins to tumble in a way that will degrade its mission, correction may be difficult or impossible. Even when not impossible, stabilization can become a very big issue when it will take a large portion of the overall intended operational life span of the SV. Also, there is the issue of being so close to Earth and not necessarily having a ton of time to course correct if the deployed trajectory of the SV will take it out of orbit.
Power
On board a SV power is the number one priority, it keeps the SV flying and the payload running. When you have small form factors, you have small batteries and small solar panels. When those are small, the SV’s ability to generate and store power becomes the largest constraint on operation. Any mission conducted by a smallsat in LEO must do so on a pretty small power budget. Any issues that require power to correct, such as stabilization, mean that power could limit or prevent correction. There are other issues with power budgets being small as well; any issue with a solar panel or the deployment of that panel means the overall mission could be extremely degraded.
With power storage being limited by small batteries, it is also more likely that the SV will have to enter modes of operation where all it is doing is facing solar panels to the sun to charge. When such operations become necessary at multiple unexpected points or for long durations, the mission of the SV may be impossible to conduct with any sort of needed efficiency. Since the SV also needs power to communicate to ground stations, if the SV is constantly in power saving and charging mode, it may not be able to receive communications from the ground on how to correct to an orbit or attitude or position in space that might allow it to operate more efficiently. This means that if a component on board is also being a large power drain and needs to be updated to regulate power consumption, the power needed to communicate this to the space vehicle may be unavailable or undependable.
Unique Aspects of LEO and Smallsats
We have already covered how the orbits and form factors of LEO smallsats work to both the advantage and the disadvantage of the space system in regard to challenges of space operations. Next we will cover the specifics of LEO and smallsats that are unique in comparison to other types of SVs and orbits.
Communications
One aspect of LEO that we have yet to cover in detail is how it impacts communications windows. Since the SV is so close to Earth, it must travel at an excessively high rate of speed to continue to fall around the Earth and not into it. This means that it will orbit the Earth very quickly. This depends on the altitude within the LEO range the SV operates at, but orbiting the Earth every 90 minutes is a good example timeframe to go off of. If the SV is passing around the Earth in 90 minutes, then the time it takes to pass the horizon relative to its ground station and then be gone over the opposite horizon is a matter of minutes.
This too depends on whether the pass will happen almost directly above the ground station or closer to the horizon. It is also important to understand that many of the orbits around the Earth will not be within view of a ground station at all since the orbits progress across the face of the Earth and the SV is so close. Though the SV may circle the Earth 18 times a day, it is possible that as little as one of those is going to have a viable communication window between the ground station and the SV.
There is the added benefit that since the SV is so close to the Earth, it does not need to expend as much energy to get a communication signal to the ground. While this is helpful, the small form factor of smallsats means their antennas are smaller and the power available to send signals is also smaller. Pair that with the fact that communications windows may be over in a matter of several minutes, and there are serious constraints on how much communications are actually achievable with the SV. This is less an issue for the bus portion flying the SV but more impactful on the payload and its mission.
If we go back to the example of imagery, let’s say that the SV has taken ten pictures while it was unable to communicate with our ground station. If the operators were trying a new more detailed resolution, the resulting images may actually be too big to download in a single pass over the ground station. In such a scenario hopefully, there has been engineering up front to account for the need to download chunks of files and reassemble them on the ground over the course of multiple passes.
If we can only try and get the whole picture at once otherwise it fails, then we may never be able to see the payload data. Also, at this point I will throw out consideration for hard drive management on board the SV. Hopefully protocols have been put in place for what happens when the payload hard drive fills up with images because they can’t be offloaded. Among this data movement and bandwidth concern of having short communications windows also falls concerns for being able to retask the satellite, if the bus or the payload were to achieve different flight or mission requirements.
Orbit Altitudes
Ground Footprint
Where communication issues largely stem from the ground stations’ ability to see the satellite at LEO, there are payload and communication issues as well with how much of the Earth the SV can see. If you have a camera payload with the mission of taking pictures of relevant spots on the Earth, the availability of those mission windows is dependent on how much of the Earth the SV can see. If it is really close, it will not have many options as far as slewing or turning the SV’s camera to face the important part of the Earth because it simply won’t be within the horizons the SV has.
Where communication problems were compounded by the speed of the SV and the Earth’s horizon, the mission windows for, say, a camera may be just as impeded. The camera payload of a LEO smallsat may actually only be able to take pictures over targeted areas on the Earth every so many passes. The number could be every few passes or many more and must be considered as the SV is tasked and data offloaded by the ground station. With short communication and mission windows at varying times each day, a lot of planning needs to go into orchestrating a successful mission.
Using the space system requires that; tasking is created, sent to the SV, the tasking is executed, and the SV passes that mission data back down to the ground station at a later pass. If each of those activities had ten passes between them, it could mean a significant delay before an image that was tasked to the SV to be taken and make it back down to the space system operators.
Persistence
There is the concept of persistence in space operations. True persistence means the ability to always task and always execute mission and is largely unrealistic for LEO SVs. Imagining how many ground stations and satellites you would need could be extremely unrealistic. True persistence is not a viable option, but identifying what level of persistence is necessary for the success of the space system mission will drive development and design requirements for the system. Being able to task and take a picture of a specific point on Earth once a day requires far fewer SVs and ground stations than, say, doing so every 30 minutes. Another factor in persistence is the mission target. Being able to take a picture of the same point on Earth is one thing, being able to take a picture of anywhere in a certain area on the Earth becomes harder the larger the area.
Mission Persistence
The persistence of the mission is specific to, in continuance of our camera example, being able to take pictures. I have to identify how often and how large of an area I need to conduct that mission over to feed into how many SVs and on what orbits would be necessary to do so.
Communications
Communications persistence is always being able to talk to a satellite. In our current example, it does not make too much sense to have persistent communications as so far we have discussed only SVs that work on their own once tasked. In the next section, we will get into the concepts of mesh networks for SVs. Such concepts require not only some level of determined mission persistence requirement but will also require that the SVs are able to communicate in a similarly defined window to make best use of the mesh space system by tasking in a timely manner and receiving the mission data in a timely manner.
LEO Mesh Space Systems
Mesh systems are pretty self-explanatory; to achieve best case and efficient persistence, it is necessary to not only have multiple SVs and multiple ground stations within the system but to have those SVs able to communicate with each other and the ground stations able to do so as well. With enough SVs and ground stations networked together, it is much easier to be able to task any satellite from anywhere to take an image as long as one SV is over any ground station at the time of tasking. With enough SVs to close the loop around the Earth, that tasking can be communicated across the mesh to the next satellite most likely to be over the area needing a picture taken.
There are a lot of technological issues at hand in creating a mesh. How will the satellites communicate with each other? How will they route traffic across the mesh? I will not get into ways this is being addressed or attempts at doing so, but they themselves present a huge challenge for space system operation. The more satellites and ground stations, the more persistent the mission execution and tasking, but also the space system becomes more expensive and perhaps even loses the cost benefit all together of being a smallsat-comprised system operating at LEO. Those questions we will dig into in the next chapter when we discuss other types of SVs.
The Challenge of the Mesh
The real issue with the mesh is not achieving adequate persistence or getting the vehicles into space. The real challenge is understanding how the mesh will actually work and how complex payload and flight tasking could be. Let’s take a relatively straightforward fictional example and say that with 50 satellites and 5 ground stations, I figure I will have a satellite over the place on Earth I need a picture taken at least every 30 minutes, and I will be able to communicate with at least 1 of those satellites every 15 minutes. That would be some pretty great persistence.
The challenge comes in when you have multiple users, with varying levels of priority all trying to task those SVs for pictures over the area of concern. How that tasking gets routed across the mesh and prioritized is itself a large problem of logic. Throw into it that, at any given moment, some of the SVs may be charging their batteries via solar panels and can’t take pictures at that time. There might be a situation where a specific SV has been receiving most of the tasking due to its orbital position enabling it to take the best picture. To spread the tasking load or get a picture quicker it might become acceptable instead to take a worse angle or poorer resolution picture from one of the other SVs. How do I prioritize the shifting of tasking to slightly less optimal satellites, if they are available due to resources? These and others are all hard, operational problems that need to be addressed by any space system looking to leverage mesh type operations.
The challenge that mesh systems bring to the table that I really want to focus on is they make cybersecurity risk decisions incredibly difficult. First, you would have to figure out how to do all the other things I just covered in a satisfactory manner. Then, we would have to figure out the impact of, say, passing around a large patch across the mesh to each SV and installing and restarting each as it goes. Now around the complexities of mission tasking and flight of the mesh system, I have to know how the patch will be routed around the mesh.
I also need to know the time the SV takes as it installs and restarts around mission tasking and try to do it at points where various satellites are not around the mission area and less likely to be busy. Figuring out the amount of impact to the mesh compared to its overall operational window as a mesh is needed to appropriately make risk decisions about whether to accept the risk of cybersecurity issues or to address them via something like a patch. Figuring out the cost and benefit of doing either with regard to a mesh space system is quite a daunting task, but one that is likely to be necessary as the complexity of LEO space systems as well as others continues to evolve.
The Anomaly
Rough Outline of South Atlantic Anomaly
Conclusion
In this chapter we discussed in detail the operation of smallsats in LEO. LEO and small form factors present their own advantages and disadvantages. These systems bring with them added functionality and hindered operations and must address a plethora of issues and challenges environmentally, operationally, and from the design and execution perspective. Understanding these challenges for LEO smallsats and creating ways of implementing cybersecurity around them will be a tough but necessary task as LEO is currently the most populated and easily entered area for space systems. Addressing the cybersecurity needs of LEO space systems is the most immediate problem and will translate in many ways to the continuously evolving space domain and its other types of space systems.