Examining Avionics In Space Engineering Essay

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Aircraft's avionics systems and Spacecraft avionics are quite similar in a sense they control most of the craft system if not all. For the spacecraft avionic system, they also have to efficiently and effectively operate in a much challenging environment. This paper will investigate spacecrafts and probes avionic system, power generation of spacecraft is another aspect that will be uncovered; how to supply and store power for the duration of the mission, which sometimes last for many years. Lastly, communication system of orbital spacecraft and long distance probes; in this topic we will identify data encryption method and the crafts antennae. Improvement has been made to make spacecraft component smaller, durable, efficient/accurate and effective. Actual and future avionics systems used in satellite and other spacecraft is the subject we want to investigate. Avionic systems for manned vehicle usually comprise a navigation system, a flight control system, an environment control system to supply breathing air to the crew and regulate the temperature inside the crew's cabin, a communication system and a cockpit or crew interface. Obviously, the systems required in a manned space mission will slightly differ to an unmanned mission or remotely operated flight.

1. Introduction.

Avionics system embodies all electronics and electrical system used in an airborne vehicle. Nowadays, we also found avionics in ships and ground vehicles, although it has its roots in the aerospace industry.

It uses advance systems that are detrimental to aircrafts and spacecrafts if they come to fail or malfunction. For spacecraft as the space shuttle, the avionic system controls most of the crafts systems. These systems found in the space shuttle are:

The guidance system;

The flight control system and navigation system which contain the Inertial measuring System;

The communication system, which contain Radar, Antennae and Transducer/Receiver etc…

The redundancy management system;

The tracking system;

Monitoring systems and sensors;

The electrical power distribution system etc...

Avionics systems used in space are inherently subject to increasing their lifetime; reduce their physical size, components radiation tolerance, increase the local storage and mass memory capacity and operation immunity of the system in case of failure.

Table Characteristics variable of spacecraft avionics (example)

Mission name

Local mass

Mass storage

CPU performance

(MIPS)

Radiation tolerance (KRAD'1'

11)

Mass

Pluto Express

8 MBytes

4 Gbits

>5

200

3 kg

Mars Pathfinder

128 MBytes

10-22

0.5

8.1 kg

The mission that the spacecraft is built for also whether it is a manned or a remotely operated spacecraft dictate the avionics required for that particular mission. The similarities found as a minimum avionics systems requirement for spacecraft and probes is what we are going to talk about. The avionics variable of spacecraft mission can be:

Managing the deployment of a parachute for controlled descent into the atmosphere;

Avionic system for final approach and landing manoeuvre of the space shuttle;

Data acquisition for scientific mission in the atmosphere or planet and relay back to earth test results of samples (telemetry);

Provision of escape facility for the crew;

Built in test equipment of avionics for safe operational/ fail safe system;

Fly-by-wire for stability augmentation of unstable airframe;

Autonomous operability of avionic system, which by the way reduces cost, involved with ground support.

As for aircraft construction, the avionic system of spacecraft adopted the backbone network for data processing. This allows all subsystem to be connected to one data processor and monitoring system for component such as actuator, power management, propulsion and so on.

An antenna is an object susceptible to RF energy. The propagation of electro-magnetic wave radiated by the antenna in the air is the result of an electrical current from the electronic circuit causing the electron of the current to oscillate and be discharged from the antenna. The electromagnetic wave is transmitted through air and space by the propagated electrical charges. In the antenna, a transducer/receiver converts the electro-magnetic wave into an alternating current and deciphers the information contained in the signal.

The design of the antenna will determine its characteristics. A radar dish focuses the transmitted signal in one direction while radio station antenna emits the signal in all direction. The dimension of the antenna restrict the frequency of the signal, this implies that the strength of the signal emitted/received depends on the physical size of the antenna.

Radar dish are directional, the signal received is focused on an antenna element by the parabolic reflector.

Figure Ground Radar Dish.

2. Spacecraft avionics.

A spacecraft is any machine used for spaceflight with a purpose assigned to it while a probe is a robotic craft used for scientific mission in space. They are autonomous and remotely monitored. Spacecraft are used for many applications such as:

Observation and meteorology;

Communication satellite;

Space tourism, travel and exploration.

2.1. Telemetry/telecommand.

For spatial application, it is a wireless system that allows the ground crew to monitor and command the spacecraft remotely as with UAVs.

The information collected by the Mil-std 1553 B bus controller or the AFDX network are received on earth via a antenna and command to the spacecraft are transmitted the same way.

2.2. Inertial Measurement Unit and attitude control system.

A Pleiades of motion sensor, accelerometer and laser gyroscopes provide information on the movement of the spacecraft for actuator and fly-by-wire system. This correctly aligns the spacecraft with the receiving antenna on earth for communication purpose and the sun to receive solar array for power generation.

2.2.1. Gyroscope.

A gyroscope is a device that exhibit gyroscopic inertia and precession. It exploits the momentum of its spinning disc. When spinning it inherently tends to resist linear input of force to its axes, therefore the force applied to the gimbals that enclose the spinning disc in the middle will not bulge instead the gimbals at 90 degrees to the first one will move this is precession.

Figure Gimballed Gyroscopes

A fibber optic gyroscope on the other hand has two beams emitted in opposite direction in its enclosure. These beams meet at the same time on the opposite side of the emitter. When the casing of the laser gyroscope is disturbed the beams will not meet at the same time but one beam will take more time to reach the other beam and they will meet at a different location. This difference is used to calculate the motion of the spacecraft by giving the deflection angle experienced by the spacecraft. There are laser gyros for the horizontal and vertical plane of the spacecraft. Laser gyroscopes are more accurate than their counterpart and preferred for spacecraft.

Figure Sagnac Effect on Laser Gyros

On the left, the beams meet at the same time, in the middle there is a phase difference and gyro pick up a rotation to the right, in the last figure the phase difference occur due to a rotation to the left.

2.2.2. Accelerometer.

Solid state accelerometer can exploit acoustic, vibration or quartz device to measure vehicle acceleration. In the surface acoustic wave accelerometer, a cantilever beam resonate at a particular frequency, the deflection of the beam caused by an accelerating force will change the frequency resonance of the beam which has a mass attached to the free end of the beam. The frequency resonance is relative to the acceleration experienced on the vehicle. Micro-machined silicon accelerometers work in the same way.

2.3. Propulsion system.

Electrical power is primarily necessary for the propulsion system. The propulsion system alone is the most power hungry subsystem of the spacecraft. The spacecraft store its own propellant or working fluid/gas to provide the thrust required through chemical reaction of the propellant, which is expended in a converging/diverging nozzle before being expelled from the electrical rocket or spacecraft for the acceleration of this body of mass in orbital station keeping or to overcome planet gravitational force for deep space exploration. An electrical charge will raise the gas/fluid temperature and pressure; this escape through the nozzle to match ambient atmospheric pressure and avoid pressure builds up inside the spacecraft. In doing so, the spacecraft will also change its momentum. Depending on the type of propellant and rocket design used for the propulsion of spacecraft, the power consumption for the propulsion system will be determined. Some spacecraft do not require a propulsion system.

2.4. Guidance navigation control.

This system determines the orbital position of the spacecraft and provides a mean of steering the craft to meet mission requirement.

2.5. Thermal Control System.

Due to the environment in which it is required to operate, the spacecraft must provide a mean of controlling the temperature inside the vehicle for its subsystem components such as fuel (hydrogen is the most preferred because of its low density but is an hypergolic propellant and must be kept in its liquid form below -85 0C also tank is pressurized by heating the propellant), electronics etc…

In another scenario, the mission may require the craft to land on the surface of a planet where the friction forces will raise material temperature and probably destroy the craft if provision for this has not been met. Thermal control provides heating and cooling of components.

2.6. Electrical and Power Generation System.

There is several ways to produce electrical power with solar panel being the easiest form when the spacecraft is near the sun. For deep space exploration, a radioisotope thermoelectric generator is the first choice. Solar sail is in development and can be used for space mission. The electrical power will be converted to either Dc or Ac power via a power conditioning equipment. This power will then be supply to the distribution unit connected the electrical bus, the bus distribute the electrical power to different subsystem. The batteries charge regulator connected to the bus store the electrical power and supply it when needed such as when the craft passes in shadow of the sun behind the planet.

Figure Launch Vehicle

Advanced lithium batteries provide longer power delivery and batteries life.

2.6.1. Solar panel.

Usually photovoltaic solar panels are used in spacecraft for power generation. The computability of power generated is based on total craft mass such as every watt/kwatt generated per kilogram mass. Photovoltaic solar cell rectangle are closely packed together to increase the power generated for the mass used by the panel since they are usually large to increase their surface area for greater power generation. The panels are pivoted to receive sunlight and to move away from it when batteries are full and power is not needed. Better solar cell is developed to reduce the added weight that comes with the panels, to get more power per unit area and to receive solar array at considerable distance from the sun. These are thin film photovoltaic cell, use composite material, solar concentrator and flexible blanket substrates in the construction of panels.

2.6.2. Radioisotope Thermoelectric Generator.

Heat released by radioactive decay is converted into electricity by this machine. The radioactive fuel is in a container to which thermocouples are attached to convert the heat produced by the fuel into electrical power. The heat is drawn from heat sink around the container to the thermocouples. Although, radioactive contamination can occur if there is a fuel leak, it is the best solution for deep space exploration where solar array is week to be exploited for the generation of electricity. Solar sail is much safe but has not been used yet for both power generation and propulsion of spacecraft.

Quality requirement of radioactive fuel:

Generate large amount of power per mass and volume used.

Produce high energy radiation absorbed by the heat sink that can be converted into electricity.

It must have a long enough half life to produce energy at continuous rate for a relatively long amount of time.

Some radioactive fuel:

AmeriHYPERLINK "http://en.wikipedia.org/wiki/Americium-241"cium 241 (has a longer half life)

Plutonium IV oxide (does not require much for container shielding).

Polonium 210 (in development).

Promethium 147.

Cobalt 60 (in development).

Strontium 90.

Plutonium 238.

RutheniumHYPERLINK "http://en.wikipedia.org/wiki/Ruthenium-106" 106.

2.6.3. Solar Sail.

Solar sail exploit the photons emitted by the sun or other stars to produce propulsive power of the spacecraft. Ultra-thin mirror shaped like an umbrella capture these photonic pressure to slowly move in the opposite direction to the source of the light emitted. The other way is to capture solar wind, which is more powerful than the photonic pressure, but less predictable while the photon radiation is constant. The mirrors reflect light of the sun and exert a small force per square meter in the opposite direction.

Figure Solar Sail

2.7. Communication System.

Space probes usually employ x-band and UHF links to relay communication data from the probe to the planet orbiters that relay the information to earth satellites down to ground station antenna. The international telecommunication union assigned the x-band exclusively for space travel communication in the frequency band 7.9 to 8.4 GHz for sending modulated signals and 7.25 to 7.75 GHz frequency band for receiving signals. NASA Deep Space Network is the primary user of these frequency bands; it has three terrestrial listening/emitting stations positioned at approximately 120 degree longitudinal distance apart. The x-band is the microwave portion of the frequency spectrum. These stations allow constant monitoring deep space mission and the universe as earth rotates. They are positioned at Goldstone in California USA, near the city of Madrid in Spain and near Canberra in Australia. Each station has eight

Figure Frequency Spectrum (source Avionic Systems note)

Spacecraft uses radar technology for planet exploration by mapping the surface of the planet with the sound wave return as well as for communication purpose. Ground antennas on earth are large radar dish with high gain and parabolic reflector. The antenna can be position in any direction. The antenna sends command to robotic craft in deep space; receive data from spacecraft via the telemetry system (housekeeping data, scientific data etc…) and track spacecraft position and velocity. [5]

Spacecraft are fitted with both high and low gain parabolic dish antenna. The data transmission rate can be as high as 135 kilobytes per second. For instance, when the transmission power of the antenna of a spacecraft orbiting Jupiter is 20 watts, by the time it reach earth this power is significantly low.

2.8. Encryption Method.

Sometime transmitted message are encrypted to restrict unwanted people to access the data package.

SBMV protocol is one of the encryption methods that break down the information into small data packet which are then reproduced into hundred thousand slightly altered data to make it impossible to access the data.

There is different encryption method; by far the proposed encryption tool for satellite or spacecraft is the AES. The consultative committee for space data system is planning to implement the AES as the standard encryption algorithm for spacecraft. [6]

The data are compressed before encryption and being transmitted to reduce time taken to receive the data.

2.9. The Bus.

Some spacecraft use the Mil-std 1553 B data, which has all component system, and their subsystem connected to it. The bus comprises a bus controller, remote terminals, the wiring and the bus monitor. The bus controller coordinate the flow of data from remote terminal to remote terminal and bus controller, allow/restrict data transmission on the bus unless the request is permitted by the bus controller and evaluate status of remote terminals.

3. Conclusion.

As in many aerospace applications, weight and propulsion systems tend to put a limitation on the current design, although improvement in space travel is made on a daily basis where new concept has not been tested flight.

The improvement of antenna is of great significance in our modern day life. It helps new system to be implemented. The requirement imposed on space avionics must be met for safe operation and to benefit from the mission carried out. The research made for advanced avionics system enables the essential capabilities we need for deep space exploration. Especially the propulsion, power generation and communication systems. Space avionics are scalable and modular which mean when a component fail, it can be easily replaced with another plug and play device.

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