Space Avionics
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Table of Contents
Answer 1
Satellites employ transponders to receive signals from the globe and retransmit them to their orbiting platforms. When constructing satellites, bear in mind that the harsh environment of space, including radiation and severe temperatures, must be considered when considering how long a satellite can last. Due to the high expense and difficulties of satellite launches, spacecraft must be as light as possible. To achieve these criteria, satellites must be compact, light, and long-lasting (Li et al., 2017). They must be very reliable in the absence of any maintenance or repair options, with an uptime of at least 99.9 percent. A satellite’s most significant components are its antennas and transponders, which are in charge of receiving and retransmitting data, as well as its solar panels and rockets, which supply power. A satellite is a spacecraft that travels in a circular route around another object known as an Orbit. When it comes to internet applications, radio, television, and telecommunications, a communication satellite is nothing more than an orbiting microwave repeater station. A repeater is a device that amplifies and retransmits a signal. Consider this a transponder that modifies its broadcast frequency band based on what it receives (picture 1).
The frequency used to send the signal into space is referred to as the uplink, and the frequency used to return the signal to the transponder is referred to as the downlink.
Satellite propulsion systems are required for satellites to attain their proper orbital location and repair any flaws that may have developed. A satellite in geostationary orbit may drift up to one degree north to south or east to west every year due to the gravitational pull of the Moon and Sun. A satellite’s thrusters are fired repeatedly to fine-tune the spacecraft’s trajectory. Attitude control, on the other hand, refers to the process of adjusting the spacecraft’s thrusters to keep it in orbit. The amount of fuel needed to power these thrusters is directly proportional to the lifetime of a satellite. When a satellite’s fuel runs out, it stops working.
Picture 1- Concept of satellite communication
Source- (Liao et a.l, 2017)
Maintaining a satellite’s operational status is a necessity of its life cycle. Its electronics and communications systems rely on their power source. The spacecraft’s solar panels are powered mostly by the sun. When the Sun’s rays are blocked by the Earth, the spacecraft’s batteries kick in to supply electricity. Solar panels generate power when the sun shines, which may be used to recharge batteries. Temperatures may reach 150°C (238°F) during an orbit around the Earth. Aluminum and other materials are used to shield satellite components exposed to radiation. The major function of this system is to protect the satellite’s sensitive electronic, mechanical, and electrical components. The thermal system of a satellite runs cooling and/or heating systems as needed to protect sensitive components from damage caused by sudden and extreme changes in ambient temperature.
Satellite tracking telemetry and control technology enables two-way communication. The satellite’s position can be tracked and its propulsion, thermal, and other systems may be remotely controlled from a ground station. This device can also monitor the satellite’s temperature, voltage, and other vital parameters. Satellites weighing one kilogram (2.2 pounds) to 6,500 kg may be manufactured (14,000 pounds). Satellites can now carry more payloads than ever before because of advances in digital technology. Because Early Bird only had one transponder, it could only transmit one television channel at a time (Liao et al,2017). However, owing to digital compression technology, the Boeing 702 satellite series can transmit over 1,600 TV channels from a single satellite with up to 100 transponders and a total of 16 channels on each transponder.
Satellites may operate in low, medium, and geosynchronous (or geostationary) orbits (GEO). Satellites in low Earth orbit (LEO) orbit at altitudes ranging from 160 to 1,600 kilometers above the Earth’s surface. MEO satellites travel 10,000–20,000 kilometers (6,300–12,500 miles) every year. The Van Allen radiation belt prohibits electronic components from functioning between low and medium Earth orbits. Because it takes a GEO satellite 24 hours to complete one orbit, they remain in the same location(Wang et al., 2019). With at least 20 LEO and 10 MEO satellites, just three GEO satellites can cover the whole world. To connect with LEO and MEO satellites, tracking antennas on the ground are necessary to provide consistent contact between satellites.
There are various advantages to employing satellite communications, which include:
- Flexibility
- New circuits may be easily installed.
- Long-distance travel is not about money.
- Options for transmission
- Everything in this world has been covered.
- A user may seize control of the network.
- There are a few restrictions while using satellite phone service.
For example, the early expenditures of segmentation and launch are too expensive.
The Applications of Satellite Communication
- To be utilized in radio broadcasting.
- Internet applications include providing an Internet connection for data transmission, GPS applications, and online browsing
- To exchange phone calls.
- The R&D sector is only one of many instances.
- Military and navigational applications
Answer 2
A system may continue to function even if one or more of its components fail due to a trait known as fault tolerance. Because the system is designed to survive even little damage, it is less prone to failure than a poorly built one, which might have a substantial influence on its overall performance. In systems that must be highly accessible or even have the ability to save lives, a high level of fault tolerance is essential. “Grateful degradation” refers to a system’s capacity to operate even when parts of it fail. Systems built to be fault-tolerant may continue to work, although to a lesser extent, rather than shutting down altogether.
The word is most often used to describe computer systems that, in the case of a partial system failure, will continue to function with lower throughput or increased reaction time. However, no hardware or software issues have resulted in the system coming to a halt as a whole. Automobiles with puncture-resistant tires, for example, or structures that can withstand wear and tear or damage from corrosion, manufacturing faults, or even crashes are examples from other fields. A system’s fault tolerance may be increased by designing it to withstand unusual occurrences and striving towards self-stability. Duplication may be desired if a system failure has catastrophic repercussions or if the expense of making it sufficiently dependable is too expensive. In the case of a catastrophic failure, the system must be able to restore to a safe state. In the event of a malfunctioning system, human intervention is possible, however, it is equivalent to roll-back recovery (Wang et al., 2020). The ability of a system to function in the presence of system failures is also referred to as fault tolerance. Even after a battery of testing, the system might still fail. It is almost impossible for any system to operate error-free at all times. As a result, systems are intended to continue operating effectively and producing anticipated results even if errors or breakdowns occur. Every system is made up of two fundamental components: hardware and software. One or both of them may fall short at the same time. As a result, fault-tolerance solutions for hardware and software vary.
Attempts made to survive hardware breakdowns
Hardware fault tolerance is substantially easier to build than software fault tolerance. Even if there is a problem with the system hardware, fault-tolerant operations may provide the desired outcomes. There are two primary methods for ensuring the integrity of computer hardware:
- It is abbreviated as “Build in Self-Test” (BIST for short). The BIST approach for hardware fault tolerance requires retesting the system at regular intervals. In the case of a failure, the system switches to a backup automatically. When anything goes wrong, the system essentially resets itself.
- TMR is an acronym for “Triple Modular Redundancy.” It is required to duplicate crucial components three times and utilize all three copies at the same time. An election was conducted to determine if all extraneous copies should be removed. At any one moment, just one mistake may be permitted.
The use of software fault tolerance solutions protects the program’s dependability in the event of a malfunction or failure. Software fault tolerance may be approached in three ways. For the first two alternatives, there exist hardware fault-tolerance approaches (Johnson et al., 2017).
- N-version programming is the process of producing N separate versions of the same piece of software by N different people or teams of developers. TMR, like N-version programming, is used in the hardware fault-tolerance approach TMR. As a result, the outputs of each N-version programming iteration will be different from those of any prior iteration. An n-version program is intended to detect all issues before they have a detrimental influence on your product.
- Recovery blocks, on the other hand, only produce redundant copies by other means. This is analogous to the n-version programming paradigm. There is no simultaneous running of all redundant copies in the recovery block. The recovery block strategy is only appropriate for jobs with a deadline that is longer than the calculation time.
This form of software fault-tolerance, unlike the other two, is based on checkpoints and rollbacks. Every time the system is utilized to compute anything, it is put to the test. When a processor or data has been hacked, this feature comes in handy.
Answer 3
The Real-Time Linux patch is a third-party kernel patch for Linux that allows the execution of hard real-time tasks in a multitasking environment together with normal, soft tasks. Real-Time Linux is used on top of 2.6 kernels. This Real-Time Linux kernel patch extends the standard 2.6 kernels to allow for hard real-time tasks called soft real-time tasks running inside the kernel while maintaining the existing Linux API which is required for portability. The Linux process scheduler, called the Completely Fair Scheduler (CFS), has been enhanced to support real-time task prioritization (Altuntas et al., 2021). The CFS implements nine levels of priority for each task with -20 being the highest priority and +19 the lowest. A kernel module provides a scheduling policy that takes into account real-time task priorities by setting hard static deadlines. The deadline of the task is computed at run time using a rate monotonic algorithm, which calculates the time lags that each task must respect to meet its deadline. The Real-Time Linux patch also provides system services to allow applications to access memory pages marked as real-time memory. Applications must detect if the target system includes this patch and use an internal memory allocator to allocate real-time pages. If an application is compiled using a kernel with no support for real-time tasks, the page fault handler will then temporarily remap the lowmem real-time memory pages into highmem (kernel space). The remapping of these pages is a potentially expensive operation, but it is done without impacting the performance of other tasks on the system. This patch provides for:
- Dynamic interrupt scaling allows real-time tasks to use all interrupts for which they have asked in a dynamically changing system.
- Timer frequency scaling to provide control over the interrupt frequency and thus the latency incurred by soft tasks.
- Pre-emption counts based prioritization that allows higher priority tasks with shorter deadlines to be scheduled before lower priority tasks with longer deadlines
- High-resolution timers, which can be used for both real-time and soft tasks
The software stack modification to support real-time operations and patching is the addition of a kernel module to become part of the operating system kernel. The function of this module would be to monitor and track the execution time deviation from a minimum and maximum threshold. It would also manage the pre-emptive task scheduling in a manner that achieves dynamic priority adjustment, and it would measure the execution time of soft tasks to allow for real-time interrupts when executing code at higher priorities. Upon patching the software stack on top of Real-Time Linux with this module, there is potential to achieve a 10%-time reduction. This module would allow for real-time interrupts to occur when required, thus interrupting the execution of tasks that might be running at lower priorities. These interrupts would be scheduled based on the dynamic priority adjustment achieved with this software stack modification, thus allowing for high-priority tasks to execute first and achieve their deadlines more often than not.
The current Linux system behaves this way when executing tasks in the order {(1,8), (1,8), (2,7), (1,12), (2,5)}, interrupt based on dynamic priority adjustment occurs when the real-time tasks reach a pre-emption count threshold (Avtin, et al., 2021).
- Task 1 with priority 8 is pre-empted by task 2 with priority 7, so task 2 still runs until its deadline of 12 – this would be the current behavior on Linux (Fuchs, and Murillo, 2021).
- Task 1 with priority 8 pre-empts task 2 with priority 7 and finishes at 13 past the same deadline as task 2 because it had to complete 8 units of work instead of 7.
- Task 2 with priority 7 finishes task 1 with priority 8, so its deadline at 5 past is met and it can terminate.
- Task 2 with priority 7 pre-empts task 3 with priority 3 and terminates at 4 past its same deadline of 7 because it had to complete 9 units of work instead of 8.
- Task 3 with priority 3 finishes task 2 with priority 7 and can terminate at 11 past its deadline of 4, but there is no task it pre-empts so it just tails until its same deadline expires.
- Interrupt based on dynamic priority adjustment occurs when the real-time tasks reach a pre-emption count threshold, so task 1 with priority 8 is running at 15 past its deadline of 8.
- Task 1 with priority 8 pre-empts task 2 with priority 7 and finishes at 13 past the same deadline as task 2 because it has to complete 5 units of work instead of 4 (Muri, et al., 2021).
Answer 4
The payload system architecture drawing includes several subsystems that are composed of assemblies. These assemblies are designed for required reliability when considering possible failure modes. All these subsystems must be taken into account to build a payload system that meets payload demands regardless of mission risk. Although they have different failure modes, most components have common basic failure mechanisms being electromechanical or thermal failure mechanisms (Benedikt et al., 2021). Therefore, they can be analyzed by calculating MTBF with corresponding standard deviations for each subsystem according to its reliability goal and requirement specifications, so that an overall payload system MTBF can be derived from all individual and manned capsules into orbit.” The payload is the part of our rocket we need to bring to safety. It contains all the equipment we want in space: a satellite, a telescope, an emitter. However, payloads are pretty fragile, so payloads need to be protected. This is why payload systems are designed in layers that are separated by the payload’s environment. We want our payloads to remain functional even if one of these layers fails because it means it has more chances to survive its stay in space. A payload system is made of one or more payloads and their environment. Each payload has its subsystem, which itself has several assemblies (Phelan, and Smith, 2021). A payload system is mainly composed of:
- The payload’s structure contains all the payload assemblies (in blue). This layer contains all these assemblies that must absorb accelerations and structural loads. It also protects them from external agents such as space debris (Leon et al., 2021).
- A shield, protecting against electromagnetic waves and energetic particles coming from outside sources like solar storms.
- Thermal insulation insulates the payload to protect it from heating or cooling down too much during different cycles, depending on how far it is from the Sun, or if it is in shadow.
- The payload’s subsystems, contain all payload components providing its basic functions as communications and receiving energy, as well as command and control links between payload and ground station (Pellish and Hodson, 2021).
- The payload’s environment contains the payload platform, which provides all power and data links from payload components to its subsystems. This layer also protects payload components against space debris with a mesh of titanium cable, as well as from electromagnetic waves and particles coming from outside sources.
Finally, the payload’s subsystems are connected by flexible pipes that provide hydraulic and pneumatic controls that manage pressure levels inside or between payload components. This design is a result of studying possible failure modes for a payload system during launch, orbital operations, and re-entry phases, as well as looking at how best they can be avoided or mitigated. To have more reliability in our payload systems, we can take into account payload components whose assemblies have similar failure modes when designing payload systems (Leon, et al., 2021).
Drawing the payload system architecture diagram showing subsystems and their relationships with dotted lines is essential to evaluate different requirements for individual components of payload systems enough information is needed to do so this includes component failure modes, MTBF calculations, or standard deviations of components depending on their required specifications (Karavay and Mikhailov, 2021). A payload system is a system that delivers payloads such as satellites, space probes, and the Moon rover to a target in outer space payload systems are composed of subsystems that are designed with different design features these subsystems are made up of assemblies that have common basic failure mechanisms being electromechanical or thermal failure mechanisms, for this reason, payload components can be analyzed by MTBF calculations according to their requirement specifications (Raj et al., 2021).
To derive an overall payload system reliability goal from payload demands, subsystem reliability goals must be taken into account these include manned capsules into orbit for spacecraft two standard deviations in line with its required reliability to reduce error sources in analysis in the final calculation phase the subsystem MTBF values are added together along with the payload demands to derive payload system reliability goals. From the payload system architecture diagram, subsystems can be seen which are made up of assemblies with common failure mechanisms because of this payload components can be analyzed by MTBF calculations according to their requirement specifications these include manned capsules into orbit for spacecraft two standard deviations to reduce error sources and give more accurate analysis and calculation phase and finally, payload system reliability goals can be derived along with payload demands (Majid et al., 2021).
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