Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

Satellite Systems Folder > Satellite Systems > Flashcards

Satellite Systems Flashcards

1
Q

Describe the (6) environmental factors acting on a satellite during launch. Which are the most severe?

A

1.Acceleration: Increases to the end of each working stage
2. Severe Acoustic/vibration environment
3. Mechanical Shocks: Explosive bolts, latches, stage ignition, etc
4. Thermal environment: Aerodynamic friction heats shroud
5. Ambinet atmospheric pressure: Permanently declines 4-5kPa/s
6. Electromagnetic interference. Can activate payload
Most severe are vibrations and shocks

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are the (9) principal effects of the space environment on a satellite?

A
  1. Vacuum
  2. EM radiation: Solar or Cosmic Rays, covers most of the spectrum
  3. Plasma (Solar wind, flare, Atmospheric Plasma in LEO)
  4. High energy particles/ions (cosmic/VA belts)
  5. Atmosphere (LEO experiences drag and erosion)
  6. Meteoroids/Micrometeoroids
  7. Man-made debris
  8. Gravitational Fields
  9. Magnetic Fields
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Describe the effect of a vacuum on satellite materials and systems. Outline negative and positive effects

A

Negative:
Materials sublime and outgas
Condensation of gases on cold surfaces is more serious
Traditional lubricants dont work as they evaporate quickly, low volatility required
Contact surfaces tend to cold-weld.
Positive:
Metals do not corrode
Fatigue life is improved

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Explain effect of “self-healing” of some metals in vacuum leading to improving material strength and fatigue life.

A

Microscopic cracks appear in a metals surfaces when it is stretched or bent. In atmosphere, this would be covered by oxides very fast, but in vacuum this can’t happen. Cracks opposite surfaces contact each other again and cold-weld restores materials integrity. On Earth, oxides form to prevent cold-weld, causing cracks to grow under alternating loads

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Describe the sources of high energy particles and describe the effect of ionizing radiation on satellite systems.

A
  1. Solar Flares
  2. Van Allen Belts (High energy particles trapped by the terrestrial MF) Essential for MEO
  3. Cosmic Rays
    Affects all materials, however semiconductors and biological tissue are especially sensitive to radiation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe the solar radiation mentioning all component of this radiation.

A

The Sun emits EM radiation across the spectrum, from high-energy x-rays to long-wavelength radio waves.
Peak of this emissions is visible portion of spectum, at 460nm.
Solar wind is plasma expelled at high velocity.
Sunspots emit X and Gamma rays

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Describe the structure of terrestrial magnetic field. How the solar wind affects its structure?

A

Earths magnetosphere has two sources, the current in Earths core and currents caused by different motions of charged particles in magnetosphere. The field can be modelled as a dipole structure, however magnetic field is not aligned with rotational axis. The solar wind also distorts Earths simple dipole field, creating a shock front and a magnetotail

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What hazard do meteoroids and material particles present to a satellite in orbit? What design is used to decrease the effect of meteoroid impact?

A

Impact of micrometeoroids causes a degradation of the surface thermal properties. Heavier particle can perforate and the component failure. Anexample of multilayer protect is a whipple bumper which uses a bumper to break-up particle and catch debris with Whipple bumper shield

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Describe solar flares and their effect on a satellite/spacecraft.

A

Solar flares are sudden releases of magnetic energy. Flare ejects clouds of electrons, ions, atoms, superheated plasma and EM radiation. These pose a threat to spacecraft electronics and any biological objects, such as humans. High-energy protons pass through human body, causing biochemical damage, which presents hazard during interplanetary travel.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Which of the environment factors are the most dangerous for the crew travelling to Mars?

A

X-ray radiation is the main risk in manned mission to Mars, the moon, or other planets. Other risks include fragments ejected from Mars during collision impacts

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Explain how you would calculate the area of a solar sail producing 1N thrust at Martian orbit . The sail is located perpendicular to the solar sail and reflect 100% of radiation.

A

SRP,e = 10^-5 Pa
SRP,m = SRP,e(R,e/R,m)^2
Area = F/(2SRP,m)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Describe Van Allen belts and their effects on Satellites

A

Layer of energetically charged particles trapped by terrestrial magnetic field. Electrons form outer belt whereas inner contains protons as well. Two belts go from 640-58000 km alt. Most particles originate from solar wind or cosmic rays. Magnetic field deflects or captures particles and protects atmospheres. Inner belt is more dangerous for satellites as protons cause more damage.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe the physical phenomenon lying in base of van Allen belt formation

A

The phenomenon of magnetic trapping of charged particles in the non-uniform magnetic field causes Van Allen belt formation. Particles spiral along MF lines, and as intensity of field increases, rotation velocity increases while translational velocity decreases. If MF intensity gets too high, translational component vanishes, particle cannot penetrate further into field, and is reflected. This point is known as mirror point.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Explain the South-Atlantic Anomaly.

A

An area where the VA belts come closest to Earths surface, to altitude of 200km. Causes increase flux of energetic particles in this region, exposes orbiting satellites to higher radiation levels.
Caused by non-concentricity of Earth and its dipole. Anomoly appears where MF is weakest

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Describe the problem with lubrication in space.

A

Usual lubrication as liquid substances usually evaporate fast in vacuum. Also large/fast change of temperatures and radiation can change the properties of standard lubricants.
Special solid material withstanding vacuum, fast change of temperature and radiation have to be
used, eg ceramic, special alloys: MoS2, Stellite

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Describe origin of atomic oxygen (O) and its effect on a satellite.

A

Intensive ultraviolet radiation lead to dissociation of neutral O2 molecules into atomic oxygen, LEO atmospheric 96% atomic Oxygen.
Atomic Oxygen is a extremely reactive, and travels at high speed in space. Causes erosion, formation of oxides, chemoluminiscent glow, general material degradation. Silver is especially vulnerable, along with lube Molybdenum Disulfide

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Explain how you would calculate expected time to collide the satellite of 5 m diameter by a meteoroid in the mass range 0.01-0.5 g, given equation of approximation of number of meteoroids in mass range, eg:
log10(𝑁 ≥ 𝑚) = –14.37–1.213*log10(𝑚) , 0.01<𝑚<0.5g

A

Calculate Area, modelled as a circle. Sub in min and max meteoroid mass, calculate number, N, within range by finding difference. Then use equation t = 1/(dN*A) to find time.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Describe the evolution of an elliptic orbit effected by atmospheric drag at its perigee.

A

Orbit evolution caused by the atmospheric drag: elliptic orbit → circular → spiral → burned.
Draw Diagram

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Describe the problem of zero weight in a spacecraft and methods of its solution.

A

Due to zero gravity, small effects can disturb satellite attitude normally not accounted for on Earth, causing an extremely small torque which leads to rotation, such as:
GF non-uniformity
MF non-uniformity
Outgassing
Solar and antenna radiation pressure

LEO can use GF to control attitude but any hgiher orbits the GF is too weak

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Describe the (10) existing satellite missions. Indicate types of orbit most preferable for the given mission.

A
  1. Communications: GEO for low latitudes; Molniya-Tundra orbits for high latitudes; Constellations of polar LEO satellites for global coverage
  2. Navigation (positioning): Inclined MEO for global coverage
  3. Earth observation and imaging: Polar LEO for global coverage
  4. Weather observation (meteorology sat): Polar LEO, or GEO
  5. Space observation including astronomy: LEO, HEO, GEO and L-points
  6. Scientific experiments: LEO, MEO, GEO with small inclination
  7. Space stations: LEO (below Van Allen belts)
  8. New technology testing and displaying: Various
  9. Military: Polar LEO for global coverage, but various
  10. Amateur radio: small satellite (LEO, but not only)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Derive equation of the gravitational potential per unit mass: E,p = − 𝜇/𝑟

A

W = int(F dr, r2, r1) = m * int(mu/r^2 dr, r2, r1)
W = [-m * (mu/r)]r2,r1 = (mu/r1 - mu/r2) * m
W = PE(r2) - PE(r1)
Therefore PE(r) = -mu/r * m +const
Per unit mass, pe(r) = -mu/r

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Given hp, ha, find time to move from perigee to furthest point from line of apiside if TA>180 degrees. (t - tp)

A

a = (hp+ha/2)+RE
T = 2pi*sqrt(a^3/mu)
T/4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Given ha and hp, find a,b,e,T and then True Anomoly and distance to centre of Earth at t-tp = 3

A

a = ((hp+RE)+(ha+RE))/2
e = (ra-rp)/(ra+rp)
b = asqrt(1-e^2)
T = 2pisqrt(a^3/mu)
n = 2pi/T
M = n(t-tp)
Use graph to find E
x = acos(E) - ae
y = bsin(E)
r = sqrt(x^2 + y^2)
TA = tan-1(y/x)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Given satellite coordinates x and y, find r and theta (TA)

A

r = sqrt(x^2 + y^2)
TA = tan-1(y/x)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Describe Geocentric Equatorial Inertial (GEI) coordinates
A frame of reference fixed relative to the stars. X¡ - and Y-axes lie in the Earth’s equatorial plane and the Earth spins about the Z-axis. The X¡ -axis is in a direction from Earth to the Sun at the vernal equinox (21 March). The direction thus indicated is termed the first point of Aries ¡. It did point towards the constellation of Aries some 2000 years ago, but at present it points to Aquarius. The Z-axis is along the Earth’s spin axis, in the northerly direction, i.e. at an angle ~23 degrees to the normal of the ecliptic plane.
26
Find position of satellite in (Cartesian) Geographic coordinates (CGeo) having GEI coordinates [x,y,z] km if the sidereal time is ST hour.
ST,d = (ST/24)×360 Z = Z,gei 𝑋 = cos ST × 𝑋,gei + sin ST × Y,gei 𝑌 = − sin ST × 𝑋,gei + cos ST × 𝑌,gei
27
List the (7) parameters which are necessary to determine special position of a satellite in the Cartesian Geographic (CGeo)
1. Longitude of ascending node 2. Orbit inclination 3. Argument of periapsis 4. Perigee altitude 5. Apogee altitude 6. Time since last passing perigee 7. Sidereal time
28
Explain the meaning of terms: (i) the true anomaly, (ii) the eccentric anomaly, (iii) the mean anomaly, (iv) the mean motion. Draw a sketch of an elliptic orbit clarifying the terms.
True anomaly - Angle between line of apsides and direction of satellite from focus Eccentric Anomaly - Angle between direction to perigee and direction to auxillary point r' Mean motion - The average angular velocity relative measured from centre of ellipsis. Mean Anomaly - Angle made by the mean motion multiplied by the time since last passing the perigee.
29
Explain the meaning of terms: (i) line of nodes, (ii) ascending node, (iii) right ascension of this node, (iv) inclination angle, (v) argument of periapsis. Draw a sketch of an elliptic orbit in space clarifying the terms.
(i) Line of nodes is where the orbital plane intersects the equatorial plane (ii) the ascending node is the point which the satellite intersects the equator moving from the southern into the northern hemisphere. (iii) The right ascension of this node is the angle of the direction to the ascending node referenced from the X,gam-axis (iv) The inclination angle 𝑖 is the angle between the orbital plane and the equatorial plane (v) The orientation within the plane is defined by the angle w, called the argument of periapsis, which is the angle measured in the direction of motion between the line of nodes and the vector pointing to the periapsis.
30
Explain the difference between the solar day and the sidereal day
A solar day is the time it takes for Earth to complete one rotation relative to the Sun, approximately 24 hours. A sidereal day is the time for one complete rotation relative to a distant star, about 23 hours, 56 minutes, and 4 seconds. The difference is due to Earth's orbital motion around the Sun.
31
Describe which forces should be balanced in a Lagrange point.
For a two-body system, the graviational components of force of both bodies and the centrifugal force from its angular velocity whilst orbiting
32
Describe how the Earth’s oblateness disturbs satellite orbit motion and explain why the inclination angle of 63.4o is the most appropriate for high elliptic orbits.
Earth’s oblateness results in rotation of the line of nodes. It leads to a rotation of the orbit within the orbital plane, the direction of which depends on the orbit inclination. The procession is zero at the inclination at 63.4 deg. This`frozen apogee’ condition gives a good effect for high-latitude communication satellites using a highly eccentric orbit.
33
Calculate the distance from earth’s centre to centre of mass in the system earth-moon given mass of Earth, Moon, and average moon orbit radius
rC = (mE*0 + mM*rM)/(mE+mM) Ans should be within Earths crust if its Earth-Moon system
34
Describe factors disturbing the satellite orbit from that described in the framework of two body motion.
1. Earth's GF not being uniform 2. Atmosphere drag (LEO) 3. The Moon’s gravitational field 4. The Sun’s gravitational field 5. Sun radiation pressure (SRP)
35
Calculate propellant required to transfer from a GSO with incline orbit to a GEO orbit, given inclination angle, final mass and specific impulse.
w = 2pi/T where T is 86400s r = cuberoot(mu/w^2) v = rw dv = 2 * v * sin(i/2) mp = m * e^(dv/v -1)
36
Describe the terms energy consumption and propellant mass consumption of a thrustor. How they depend on the exhaust speed?
Energy consumption in a thruster refers to the power needed to operate, influenced by design efficiency. Propellant mass consumption is the rate at which propellant is expelled to generate thrust, affected by the exhaust speed. Higher exhaust speeds generally increase energy consumption but decrease propellant mass consumption, optimizing spacecraft efficiency.
37
Explain the term specific impulse and why it is an important characteristic for a thruster?
It quantifies how effectively a propulsion system utilizes propellant to generate thrust. Defined as the ratio of thrust produced to the rate of propellant consumption. Thrusters with a higher Isp require less propellant to achieve a given momentum change. Units are seconds
38
Describe cold gas thruster design
Engines based on releasing a gas stored under pressure in a reservoir through a nozzle, such as Freon, Propane or Nitrogen. They're simple, reliable, but have low thrust and Isp.
39
Describe Field-emission electric propulsion Thrusters (FEEP)
A propellant of liquid metal such as mercury, caesium, etc emit ions when under influence of electric field. Ions are accelerated by field and expelled at high velocity. Good for micronewton attitude control.
40
Describe design and operation of the Electrostatic (gridded ion) thruster
An electron gun is used to ionise neutral molecules in chamber, which is surronded by magnets. These positive ions accelerate between by high differential voltage grid, exiting the back of chamber and meeting an electron gun to neutralise the flow
41
What is the advantage of the Hall Effect thruster by comparing with the Gridded Ion thruster?
Hall effect thrusters generally have higher thrust and efficiency as it allows for the direct acceleration of the ions, and no grid is required
42
What is meant by attitude?
Attitude is the orientation of the satellite in space about its centre of mass
43
Describe the (2) main Attitude Control functions
1. Stability function: To maintain axis alignment 2. Steering function: Missioon may require adjustments to direction, ie observations
44
When is the steering function of the Attitude Control essential
a constant bias or a particular law of progression about one or more axes can be needed to fulfil mission and/or orbit requirements
45
Describe methods of measuring the attitude of a satellite
Sun sensors measure one or two angles between their mounting base and incident sunlight Earth Horizon Sensors detect Earths IR emissions in CO2 absorption band, at which Earth looks like Black Body at 255 K. By contrasting to deep space at 4 K. Star sensor: Image of portion of sky is compared to reference map. Terrestial MF detector: Measures direction of local MF, like a compass Radio-freq and Laser sensors: Both use ground based sources (radiowaves/laser) to determine attitude Interial Unit: Use accelerometers to detect translational motion or gyrometers to detect velocity Laser ring/fiber optic gyroscope: Intereference caused by difference in light paths travelling in opposite directions can be used to measure angular rate
46
Find period of oscillation of a cylindrical satellite, given altitude, length H and diameter, D.
D=2R (Ix-Iz)/Iy = (H^2 - 3R^2)/(H^2 + 3R^2) r = h+RE T = 2pi*sqrt[(r^3/3mu)*(Iy/Ix-Iz)]
47
Describe sources of the disturbing torques acting on a satellite
The external disturbance torques: 1. Solar radiation pressure; 2. Gravitational forces; 3. Terrestrial magnetic field interacting with satellite’s electric loops; 4. Atmosphere drag for LEO 5. Lack of alignment of thrust of actuator to centre of mass 6. Satellite antenna radiation
48
Explain principal axes of a body
The principal axes of a body is the reference frame in which the principal moments of interia are based. If body is symmetric, the principal axes align with axes of symmetry.
49
Explain the three axis stabilisation of satellite attitude.
This is an attitude control system in which the body of the satellite maintains a fixed orientation with respect to the local coordinate system.
50
Explain how Gyroscopic stabilisation is used to limit the motion of a satellite about the pitch, roll and yaw axes?
Pitch control is realised by exchanging angular momentum between the wheel and the body of the vehicle by varying the rotation rate of the wheel. Roll and Yaw control is obtained by using an actuator which generates a torque about the proper axis.
51
Explain how you would calculate the time for a non-spinning satellite to change attitude by angle theta, given Td and I
t = sqrt[(2*dtheta*I)/Td]
52
Explain how you would find propellant mass required to maintain attitude control over lifespan, given dH, H, Td, lifespan length t, Thruster thrust F, thruster arm length L, flywheel velocity omega, and permissable deviation of roll,pitch and yaw
Td * t = total disturbing time Tc = 2 * F * L tc = Td * t/TC m = 2 * F/(isp * g0) (2 because two thrusters) Total propellant mass consumption: m,t = m * 2 * 3 * tc
53
Explain how you would calculate the time for a spinning satellite to change attitude by angle dtheta, given Td, S and I
H = I * S * (2pi/60) (60 to change rpm to s) omega = Td/H t = dtheta/omega
54
In a six-pair-thruster system, which thrusters are used to control which axis disturbances
Pitch: Thruster pairs 1 and 2 Roll: Thruster pair 4 and 6 Yaw: Thruster pair 3 and 5
55
Describe methods of passive attitude control and list advantages and disadvantages compared to active control methods.
Passive attitude control methods include gravity, spinning satellite and gyroscopes. They require no propellant consumption or electronic equipment, however they have low accuracy and a large evolution time
56
Describe methods of active attitude control and list advantages/disadvantages
Measurement then correction. Satellites attitude is measured using external references, and use onboard actuators to correct attitude. Actuator types: Angular momentum devices Thrusters Magnetic devices Solar Sails
57
List types of Angular Momentum Devices as Attitude Control Actuators
1. Reaction wheel 2. Momentum Wheel 3. CMG - Control momentum gyroscope By changing the rotational speed of these devices, they exert torque, allowing precise control over the spacecraft's orientation without expelling mass.
58
Describe three main elements that the spacecraft electric power system and the main functions of each element.
The spacecraft electric power systems consists of i. Primary energy source: converts fuel or solar energy into electric power ii. Secondary energy source: stores and subsequently delivers electric power to the satellite system and payload, when the primary system’s energy is not available. iii. Power control/distribution network: delivers the correct voltage and current levels to all spacecraft loads when required
59
List the types of primary energy sources used in a satellite. Describe their advantages and drawbacks and missions in which they are mainly used
Main primary electric energy sources are i. Solar arrays: are used in majority of modern satellites. They do not need fuel and can supply energy for many years. They need a large area (and mass) of the arrays to provide a significant amount of power. They can be damaged by radiation and have relatively low efficiency. ii. Fuel cells: are used mainly in manned spacecrafts in few days/weeks missions. They can provide a significant amount of power but they consume fuel. iii. Radioisotope thermoelectric generators (RTG): they adversely affect radiation of the satellite whilst in orbit. Also high temperature operation is required for efficient energy conversion. iv. Solar heat systems: have higher efficiency then solar cell arrays. Are under development for the International Space Station (ISS). v. Nuclear fission systems: can damage the environment. They were used in Soviet military satellites, considered to be used on the ISS. vi. Electrochemical batteries: were used in very early missions as a primary source. Also they are often used in planet landing modules
60
Describe design of Radioisotope thermoelectric generator (RTG), its advantages and disadvantages. In which mission it is preferable power source?
Consists of thermocouples connected to a heat sources. Radioactive decay produces heat, which is converted to electricity by thermocouples. Inefficient and waste heat can be a problem, but design is long-lasting so is used to power missions to deep-space
61
List the types of secondary electric power sources used in a satellite. At which phases of the spacecraft mission such the power sources are necessary?
Secondary energy sources include: Rechargeable Electrochemical batteries Rechargeable Fuel Cells Neccessary during: Launch and first minutes of flight Eclipses Manoeuvre and minutes after Landing phase
62
Explain how you would find the required area of a solar cell needed to provide P kilowatts of power, given mission lifetime t, BOL efficiency nu, efficency decay law, cover losses l, filling efficiency f, and find number of cells in cell dimension Ac
Find nu,EOL using law and t Pc=Ac * nu,EOL * (1-l) * Js n = P/Pc A = n * Ac (watch for m^2 vs cm^2)
63
Find number of elements and mass of battery required for longest eclipse given: Capacity C, DoD, lifetime t, discharge voltage Vd, discharge efficiency nu,d, specific energy E/m, altitude h, Power P and voltage V
Round up any N values Find orbital period, then calculate eclipse time te. te= T * ((sin-1(RE/r))/180) Energy required: Ee = P * te N = (V/Vd)+1 Energy of 1 cell: Ec = C * Vd * DoD * nu,d Energy of 1 section: Es = Ec * N Number of sections: ns = Ee/Es Total Cell number: n = N * ns Cell mass = C/(E/m) Mass of battery: m = n * mc
64
Write down the heat balance equation including the albedo and planetary radiation. Explain every term in this equation
Qs + Qa + Qp + Qi = Qsat Qs = As * alph * Js = Heat recieved from sun Qa = Aa * alph * J a = Planetary albedo, where Ja =Js *alpha * F where F is visibility factor Qp = Ap * alph,IR * Jp where Jp=237 *(RE/RE+h)^2. Planetary radiation Qi is internal heat sources Qsat = Asat * ep * omega * T^4. Heat radiated by satellite into space, where ep is emittance in IR range
65
Describe the Spectral radiance of a black body and a real body. How is the integral emittance of a real body defined?
Spectral irradiance is the radiance per unit frequency or wavelength. Integral emittance of a real body is the emittance over all wavelengths.
66
Explain why the absorptance a in the term for the planetary radiation absorption is equal to emittance s in the term for heat radiated by a satellite into the deep space.
Kirchoff's law states when a surface is in thermal equilibrium, its absorption is equal to its emittions. Meaning for s atellite in space, the absorbed planetary radiation equals the heat radiated into deep space.
67
Describe the 2nd Kirchoff’s law and explain why the integral absorptance of a body does not equal the integral emittance of the radiated body in most cases.
Kirchhoff's law of thermal radiation states that at thermal equilibrium, the absorptance of a body equals its emittance at a specific wavelength. However, in the general case, across a broad spectrum, or under non-equilibrium conditions, the integral absorptance may not equal the integral emittance due to factors like temperature variations and material properties
68
Explain the "visibility factor” for calculation of the albedo radiation
It is dependant on satellite’s altitude h and angle beta between sun rays direction and the local vertical
69
Explain how you would calculate average satellite temperature when orbit is perpendicular to solar rays, given: radius R, altitude h, absorptance alph, emittance ep, visibility factor F
T^4 = (Js/4 * om) * (1+alph * F) * (alpha/ep) +(Jp/4 * om) Jp = 237 * (RE/RE+h)^2 om = Stefan-Boltzmann Const
70
Describe methods of passive and active thermal control of a satellite
Passive: Surface finishes (coating) Insulation systems Conduction paths Phase change mterials Heat pipes Active: Heaters Variable Conductance heat pipes and diodes Mechanically pumped two-phase loops Liquid loops/central heating Refrigerator/Heat pumps Louvers and shutter Ablation
71
Describe design and operation of the heat pipe. How it can be used in the satellite?
Transfers heat in the form of latent heat of vaporization, using volatile working fluid circulated by capillary action in a porous wick structure, using thermal conductivity and phase transition.
72
Describe design and operation of heat pump. What is the purpose to its use in the satellite?
Used to increase temperature of radiator, especially useful if area of radiator is limited. Works by transfering heat from a lower-temperature source to a higher-temperature sink using a refrigeration cycle.
73
Describe the reason to cool some sensors and devices mounted on satellite down to temperature close to absolute zero. Which factor restricts the lifetime such satellite to operate?
Cooling a sensor down to absolute zero reduces its signal to noise ratio
74
Explain the purpose of each segment of the generalised satellite communication system
The space segment contains one or several active and spare satellites organised into a constellation. The control segment consists of all facilities for the control and monitoring of the satellites, also named TTC (tracking, telemetry and command) stations, and for the management of the traffic and the associated resources on-board the satellite. * The ground segment consists of all the traffic earth stations. Depending on the type of service considered, these stations can be of different size, from a few centimetres to tens of metres.
75
In a satellite communication system describe what is meant by the terms downlink and uplink transmissionand what information they usually contain
Downlink transmission: must provide the ground control team with information about the functioning of the subsystems in the sat Uplink Transmission: must enable the ground controller to change the role of the spacecraft
76
Explain the term “Avionics” for a spacecraft or satellite. Indicate its two meanings used in space industry.
electronic part of Attitude and Orbit Control System (AOCS) But sometimes is used torefer to all onboard electronics
77
List the 10 on-board data handling (OBDH) functions.
Enabling flow of science data Receiving and distribution commands Performing telemetry protocols Time distribution Providing data storage Executing command and schedules Controlling payload and subsystems Monitoring spacecraft health Making autonomous decisions Performing data compressions
78
Describe basic forms of on-board data and how they're prepared before transmission
Data: 3 basic forms: (i) analogous (ii) digital bi-level (iii) digital serial Transmission Prep: Analog Data is converted to digital after filtering and sampling, 0-5/12V Digital bi-level are grouped into 8 or 16 bit words Digital data is acquired in single form
79
Why does the telemetry signal have to be modulated for transmission to the earth?
Signals transmitted by a communication system consist of frequencies too low for direct transmission as radio waves. The signal is imposed on a carrier wave of much higher frequency suitable for transmission.
80
What are the advantages of digital form of a signal compared to analogue one
The digital modulation is much more flexible. Only digitised information can be transported through a noisy channel without degradation and can be compressed to decrease the amount of bits needed to be transmitted.
81
What are main types of modulations?
Amplitude: A * (1+m * u) Frequency: w + dw * u Phase: psi,0 + beta * u
82
Describe all types of modulation of a digital signal. Draw the correspondent sketches.
Amplitude Shift keying - Switching amplitude of signal to indicate a switch from 1 to 0 Frequency Shift Keying - Switching frequency when signal changes from 1 to 0 or vice versa Phase reverse keying - when switching from 1 to 0, reverse the phase Quadrature phase-shift keying - Switch phase at quarter way through period
83
Describe the process of converting of an analogue signal to the digital form
First signal is sampled at high enough freq to satify Nyquist criterion Then each sampled value is quantised by dividing up the amplitude range into brackets, and alocating each bracket a level. These brackets correspond to a 3 bit number in binary, 0 to 7. It is then encoded by adding a parity bit.
84
State the Nyquist–Shannon sampling theorem. How it is applied for manipulation with a signal in satellite communication?
If a function contains no frequencies higher than fm hertz, its frequency can be fully captured by giving its ordinates at a series of points spaced fs = 2fm seconds apart.
85
Define the bandwidth of a signal
the difference between the upper and lower frequencies in a continuous band of frequencies
86
Define multiple access
A network with a multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity
87
Explain the term: transmitting bit error (BER). Why the uplink BER should be essentially smaller than the downlink BER?
BER is when an error occurs during transmission and causes a corruption of bits. A small error in datafrom satellite is not important but could be disastrous in a mission critical command to the satellite. Uplink BER should aim to minimise possible values
88
Describe the types of antenna employed in satellite communications.
Horn, Helical (end-fire), Reflector (parabolic), Phased Array
89
Describe design and performance of the phase array antenna. What are the advantages of this type of antenna?
Uses a time delay of two or more neighouring antenna elements and the phase between them to send signals in the desired direction. Advantage: Can produce several beams simultaneously by a single antenna Disadvantage: Very complex design and equipment
90
Explain the term "free space propagation path loss"
loss of signal strength of EM wave that would result from a line-of-sight path through free space, with no obstacles nearby to cause reflection or diffraction
91
Explain the term "Effective Isotropic Radiated Power" or EIRP
the power that an omnidirectional antenna would need to radiate to give the same radiated power P as a directional antenna of gain G
92
Explain the term "Antenna gain"
Ratio of power radiated to power produced by isotropic antenna with same power source
93
Explain the term "Antenna Beam Width"
is the angle between the half-power points of the main lobe
94
Why is the free-space propagation path loss model used in satellite communications?
It is used due to its simplicity and applicability over long distances. The model assumes ideal conditions, making estimating baseline signal strengths easier
95
Explain the parabolic approximation of the antenna beam
Describes the radiation pattern of a parabolic antenna as a focused, narrow beam allowing simple analysis of the antenna's directional characteristics
96
Explain how you would calculate minimal Earth station recieved power given: Distance R, downlink freq fd, antenna beamwidth Theta, efficiency nu,T power Pt, Earth dish diameter D and dish efficiency nu,R, uplink freq fu
Calc wavelength Lam = c/fd LFS = (4pi*R/Lam)^2 dTheta = Theta*(fu/fd) Gt, max = nu,T*(70*pi/dTheta)^2 Gr,max = nu,R*(pi*D/Lam)^2 PR = (Pt*Gt*Gr) / LFS
97
Determine the C/N ratio in dB if the system bandwidth is B and the system noise temperature is T and comment on your answer
C = Pr (from previous Q) N = k * T * B C/N -> 10 * log(C/N) = C/N in dB Comment if C/N > 10, means sufficient for communication
98
Describe methods for tracking satellite’s distance (ranging)
1. Using GPS or similar navigation system or their satellite navigation or similar systems 2. Using TC/TM sub-carrier
99
Describe principals of satellite navigation (gps)
a system of satellites that provide geo-spatial positioning with global or local coverage. It allows devices to determine their location using time signals transmitted byline of sight radio. Each satellite continually transmits messages that include the 1. time the message was transmitted 2. precise orbital information 3. the general system health and rough orbits of all navigation satellites The receiver uses the messages to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites' locations are used with the possible aid of trilateration
100
Give a list of the sources of errors in position determination by the satellite navigation.
Sources of errors in position determination by the satellite navigation 1. Atmospheric delay of the sat’s signal 2. Signal multipath (reflection) 3. Receiver clock error 4. Orbital error 5. Low number of visible satellites 6. Satellite mutual position 7. Intentional degradation of signals
101
What is the method of trilateration?
Trilateration is the process of determining location of a point by measurement of its distances from points with known locations.
102
Describe how the conventional and small satellite are classified based on their mass and describe the design philosophy for small satellites versus conventional ones.
A small satellite is any under 500kg in mass. A convential mission requires less missions, at the expense of an increased cost and risk. Smaller satellites offer shorter development times, on smaller budgets and can fulfill many of the functions of their larger counterparts, at the expense of more missions
103

Satellite Systems Folder (4 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions