UNIT-II

Satellite Sub-Systems
The major subsystems on a satellite are
a) Attitude and Orbit Control System:
This subsystem consists of rocket motors that are used to move the satellite back to
the correct orbit when external forces cause it to drift off station and gas jets or inertial
devices that control the attitude of the satellite.
(b) Telemetry, Tracking, Command, and Monitoring (TTC&M):

 The telemetry system sends data derived from many sensors on the satellite, which
monitor the satellite's health, via a telemetry link to the controlling earth station.
 The tracking system is located at this earth station and provides information on the
range and the elevation and azimuth angles of the satellite.
 Repeated measurement of these three parameters permits computation of orbital
elements, from which changes in the orbit of the satellite can be detected.
 Based on telemetry data received from the satellite and orbital data obtained from
the tracking system, the control system is used to correct the position and attitude of
the satellite.
 It is also used to control the antenna pointing and communication system
configuration to suit current traffic requirements, and to operate switches on the
satellite.
(c) Power System:
All communications satellites derive their electrical power from solar cells. The power is
used by the communications system, mainly in its transmitters, and also by all other electrical
systems on the satellite.
(d) Communications Subsystems:
It composed of one or more antennas, which receive and transmit over wide
bandwidths at microwave frequencies, and a set of receivers and transmitters that amplify
and retransmit the incoming signals. The receiver-transmitter units are known as
transponders. There are two types of transponder in use on satellites:
• Linear or bent pipe transponder - amplifies the received signal and retransmits it at
a different frequency (lower).
• Baseband processing transponder - used only with digital signals, that converts the
received signal to baseband, processes it, and then retransmits as a digital signal.
(e) Satellite Antennas:
Most satellite antennas are designed to operate in a single frequency band, for example,
C band or Ku band. A satellite which uses multiple frequency bands usually has four or more
antennas. Four main types of antennas used on satellites:
• Wire antennas: monopoles and dipoles
• Horn antennas
• Reflector antennas
• Array antennas.
2.1 Attitude and Orbit Control System (AOCS)
• The attitude and orbit of a satellite must be controlled so that the satellites antennas
point toward the earth and the user knows where in the sky to look for the satellite.
• There are several forces acting on a satellite that tend to change the attitude and orbit
of a satellite
• Gravitational fields of the earth and the moon
• Irregularities in the earth’s gravitational field
• Solar pressure from the sun
• Variation in the earth’s magnetic field
• Solar pressure and the earth’s magnetic field generating eddy currents in satellite’s
metallic structure when it travels through the magnetic field, tend to cause rotation
of the satellite.
• These effects are cyclic and cause nutation (wobble) of the satellite.
• The attitude control system must damp out nutation and counter any torque or
movement
• The sun and the moon’s gravitational fields cause the orbit to change with the time
• The gravitational forces cause to change the orbit inclination at an initial rate of 0.860
per year
• The orbit control system must be able to move the satellite back into the equatorial
plane
Attitude Control System
• There are two ways to make a satellite stable when it is in orbit and weightless:
spinning the satellite and using momentum wheels.
• Spin stabilization
• 3-axis body stabilization
• The entire body of satellites can be rotated at 30 to 100 rpm to create a gyroscopic
force which provides stability of the spin axis and keeps it pointing in the same
direction. Such satellites are known as spinners (ex: Boeing 376)
• A satellite can be stabilized by one or more momentum wheels. This is called three-
axis stabilized satellite (ex: Boeing 701)
• The momentum wheel is usually a solid metal disk driven by an electrical motor.
• Increasing the speed of the momentum wheel causes the satellite to process in the
opposite direction, according the principle of conservation of the angular momentum.
• The satellite is spun up by operating small radial gas jets at an appropriate point in the
launch phase.
• There are two types- bipropellant thruster and arc jets (ion thrusters)
• Arc jets are mainly used for north-south station keeping
• In a three-axes stabilized satellite, one pair of gas jets is needed for each axis to
provide for rotation in both directions of pitch, roll and yaw

• Yaw axis-Directed towards earth center

• Pitch axis-Normal to the orbital plane
• Roll axis-tangent of the orbit
• The satellite must be stabilized with respect to the reference axes to maintain
accurate pointing of its antenna beams.
• When large narrow beam antennas are used, the satellite may have to be stabilized
within ±0.10 on earth axis.
• The reference for the attitude control system may be the outer edge of the earth’s
disk, as observed with infrared sensors, the sun, or one or more stars.
 • An infrared sensor on the spinning body of a satellite can be used to N-S control in
pointing toward the earth as shown in figure

Orbit Control
• A geostationary orbit must be circular at a correct altitude on the equatorial plane.
• If the orbit is not circular, velocity change is to be made along the orbit. For altitude
correction, Z -axis gas jet is used.
• The inclination of an orbit increases at about 0.850 per year. Satellite in inclined orbit
is shown in figure
• Most GEO satellites are specified to remain within in a box of ±0.050 and so, in
practice, corrections called a north-south station-keeping maneuver are made every
2 to 4 weeks to keep the error small.
 • It has become normal to split the E-W and N-S maneuvers so that at intervals of 2
weeks the E-W corrections are made first and then after 2 week, the N-S corrections
are made.

2.2 Telemetry, Tracking, Command, and Monitoring (TTC&M):

• The TTC&M system is essential to the successful operation of a communication
satellite.
• The main functions of satellite management are to control the orbit and attitude of
the satellite, monitor the status of all sensors and subsystems on the satellite, and
switch on or off sections of the communication system.
• On large geostationary satellites, some re-pointing of individual antennas is also
possible, under the command of the TT&C system. Tracking is performed primarily by
the earth station.
Telemetry and Monitoring System
• The monitoring system collects data from sensors within the satellite and sends these
data to the controlling earth station.
• There may be several hundred sensors located on the satellite to monitor pressure in
the fuel tanks, voltage and current in the power conditioning unit, current drawn by
each subsystem, and critical voltages and currents in the communications electronics.
• The sighting devices used to maintain attitude are also monitored via the telemetry
link.
• The faulty unit must then be disconnected and a spare brought in, via the command
system
 • Telemetry data are usually digitized and transmitted as phase shift keying (PSK) of a
low power telemetry carrier using time division techniques.
• A low data rate is normally used to allow the receiver at the earth station to have a
narrow bandwidth and thus maintain a high carrier to noise ratio.

Tracking
• The tracking system at the control earth station provides information about the range,
elevation, and azimuth for a satellite.
• A number of techniques can be used to determine the current orbit of a satellite.
• Velocity and acceleration sensors - to establish change in orbit from the last
known position.
• Doppler shift of the telemetry carrier - to determine the rate at which range is
changing.
• With accurate angular measurements, range is used to determine the orbital
elements.
• Active determination of range can be achieved by transmitting a pulse, or sequence
of pulses, to the satellite and observing the time delay before the pulse is received
again.
• The propagation delay in the satellite transponder must be accurately known
 • If a sufficient number of earth stations observing the satellite, its position can be
established by triangulation from the earth station by simultaneous range
measurements.
• With precision equipment at the earth stations, the position of the satellite can be
determined within 10 m.
• Ranging tones are also used for range measurement.
Command
• A secure and effective command structure is vital to the successful launch and
operation of any communication satellite.
• The command system makes changes the position and attitude of the satellite,
controls antenna positioning and communication system configuration, and operates
switches at the satellite.
• During launch it is used to control the firing of the apogee kick motor (AKM) and to
spin up a spinner or extend the solar sails of a three-axis stabilization satellite.
• The command structure must have safeguards and the control code is converted into
a command word which is sent in a TDM frame.
• After checking for validity in the satellite, the command word is sent back to the
control earth station via the telemetry link where it is checked again.
• If it is received correctly, then an execute instruction is sent to the satellite so that the
command is executed.
2.3 Power Systems:
• All communications satellites obtain their electric power from solar cells which
convert incident sunlight into electrical energy.
• Three types of power systems
• Solar – most frequently used in commercial satellites
• Chemical – used for backup to power satellite during solar eclipses
• Nuclear – used for satellites leaving the Earth orbit (deeper space exploration)
• But because of the danger to people on the earth in case of launch fail and consequent
nuclear spread, communications satellites have not used nuclear generators.
• Solar radiation falling on a satellite at geostationary orbit has an intensity of
1.39 𝑘𝑊/𝑚2.
• The efficiency of solar cell is typically 20 to 25 % at the beginning of life (BOL), but falls
with time due to aging of cells and etching of the surface by micrometer impacts.
 • Sufficient power will be available at the end of life (EOL) of the satellite to supply all
the systems on board.
• Cylindrical solar arrays: A spin-stabilized satellite has a cylindrical body covered with
solar cells.
• Because the cylindrical shape, half of the solar cells are not illuminated at all, and at
the edges of the illuminated half, the low angle of incidence of sunlight results in little
electric power being generated.
• Recently a large communication satellite for direct broadcasting generates up to 6 kW
from solar power.
• A three-axis stabilized satellite can make better use of its solar cell areas than a
spinner, since solar sails can be rotated to maintain normal incidence of the sunlight
to the cells.
• Only one-third of the total area of solar cells is needed relative to a spinner, can have
more than 10 kW of power generated.
• In a three-axis stabilized satellite, solar sails must be rotated by an electric motor once
every 24 hours to keep the cells in full sunlight.
• The satellite must carry batteries provide power during launch and eclipses.
• On geostationary orbit, eclipses occur during two periods per year, around the spring
and fall equinoxes, with longest duration of about 70 min/day.
• To avoid the need for large, heavy batteries, a part of communication system load may
be shut down during eclipse, but this technique is rarely used when telephony or data
traffic is carried.
• However, TV broadcasting satellites will not carry sufficient battery capacity to supply
their high-power transmitters during eclipse, and must shut down during eclipse.
• By locating the satellite 200 (which is equivalent to 1 hour and 20 minutes) west of
the longitude of the service area, the eclipse will occur after 1 a.m. local time for the
service area, when shut down is more acceptable.
• Batteries are usually of the sealed nickel hydrogen type which have good reliability
and long life, and can safely discharge to 70 % of their capacity.
• Typical battery voltages are 20 to 50V with capacities of 20 to 100 ampere-hours.
2.4 Communication Subsystems:
• A communication satellite exists to provide a platform in a geostationary orbit for the
relaying of voice, video, and data communications. Its main function is to receive,
amplify and retransmit the signal to earth. Elements of Communication Subsystem is
shown in below figure
• All other subsystems on the satellite exist to support the communication subsystem,
although this may represent a small part of volume, weight, and cost of the satellite.
• Downlink is most critical part in the design of a satellite communication system.
• The satellite transmitter has limited power
• As link distance is about 36,000 km, the receiver power level is very small and rarely
exceeds 10−10 W even with a large aperture earth station antenna.
• For satisfactory performance, the signal power must exceed the power of the noise
generated in the receiver by 5 - 25 dB depending on the bandwidth of the transmitted
signal (which depends on the data rate) and modulation scheme used.
• With low power transmitters, narrow receiver bandwidths have to be used to
maintain the required SNRs.
• Higher power transmitters and satellites with directional antennas enable wider
bandwidths to be utilized, increasing the capacity of the satellite.
• Early communication satellites had transponders of 250 or 500 MHz bandwidth but
had low-gain antennas and transmitters of 1 or 2 W output power.
• When the full bandwidth was used, the earth station could not achieve an adequate
SNR because the system was power limited.
• In high-capacity satellites their available bandwidths have been reused by employing
several directional beams at the same frequency (spatial frequency reuse) and
orthogonal polarizations at the same frequency (polarization frequency reuse).
• Some satellites use multiple bands to obtain more bandwidth.

• Input pass band: It rejects the out of band signals that includes transmission from
  Same satellite
 Adjacent satellite
 Any out of band signal received from service area
 Typical bandwidth-500MHz
• Low Noise Amplifier(LNA):The first amplifier provides about 20 dB gain to the very
weak signal and adds low noise that is called LNA
• More gain of the 30dB or more are provided in in the subsequent an amplifying stages
to the input requirement of down converter.
• Important specification of RF amplifier are
 Noise Figure
 Receiver sensitivity or minimum detectable signal level
 Gain
 Dynamic Range
• Down converter: It is nonlinear device which mixes input signal with locally generated
signal to produce required downlink frequency.
• To reduce unwanted harmonics a BPF is put after the Mixer.
• In some cases down conversion to IF at lower frequency is also done and then the
signal is up converted, this is called double conversion type repeater.
• Local Oscillator (LO): The local oscillator base frequency is of the order of 10 to 100
MHz.
• This is multiplied and amplified to generate the required LO frequency for mixing.
Stability of LO is typically 1 PPM.
• Important parameters for LO are,
 LO stability
 Oscillator phase noise (short term random fluctuations in frequency of phase)
• Input Multiplexer: Input sub-band formation is done through a set of BPF called input
de multiplexer. These filters should have high adjacent channel rejection and low
amplitude and phase ripple over the pass band.
• Power Amplifier: When power requirement of more than 20 W Travelling wave tube
TWTA is used as HPA. It introduces nonlinearity. Linearizers are used but that increases
the complexity, weight and cost.
• SSPA are used when lower than 20 W is required. SSPA is less efficient compared to
TWTA but it needs less space, weight and lower voltage operation.
 • Output DE Multiplexer: All HPA outputs are combined through another bank of BPF,
this is called output DE multiplexer. This is high power and low loss filter. Output of
OMUX is connected to transmit antenna.
Repeaters
In telecommunications, a repeater is an electronic device that receives a signal and
retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer
distances or be received on the other side of an obstruction.
• The total bandwidth (up to 500MHz-1.5GHz) is too large to be accommodated by a
single amplifier.
• On the other hand, no carrier occupies such a bandwidth on its own
• Hence the total repeater bandwidth is split into sub-bands (a few tens of MHz each).
Each sub-band is amplified by a transponder
• Signals (known as carriers) transmitted from an earth station are received at the
satellite by either a zone beam or a spot beam antenna.
• Zone beams receive signals from transmitters anywhere within coverage zone,
whereas spot beams have limited coverage.
• The received signal is often taken to two low-noise amplifiers (LNAs) and is
recombined at their output to provide redundancy for traffic.
• Redundancy is provided wherever the failure of a component will cause the loss of a
significant part of the communication capacity of the satellite.
Functions of repeaters
• Receive signal from service area (Receive antenna)
• Amplify only the required receive band (Filter & LNA)
• Convert to downlink band (Mixer, LO,filter, amplifier)
• Amplify and remove the spurious (power Amplifiers, filters)
• Transmit service area (Transmit antenna)
Transponders
• The transponder is essentially a receiver which receives the signal transmitted from
the earth by the uplink, amplifies it and retransmits it with the downlink, with a
different frequency. Thus the word “Transponder” is formed by combining the two
words- TRANSmitter and resPONDER.
• Most satellites have anything between 10 to 30 transponders of different bandwidth
on board.
• Transponders can be either active or passive.
 • A passive transponder allows a computer or robot to identify an object. Magnetic
labels, such as those on credit cards and store items, are common examples. A passive
transponder must be used with an active sensor that decodes and transcribes the data
the transponder contains.
• The overall specification of transponder is
• Channel bandwidth
• Adjacent channel rejection
• Group delay
Classification of Transponders:
a. Single conversion transponder (bent type) for 6/4 GHz band

• The output power amplifier is usually a solid state power amplifier (SSPA) unless a very
high output power (>50 W) is required, when a traveling wave tube amplifier (TWTA)
would be used.
• The local oscillator is at 2225 MHz to provide the appropriate shift in frequency from
the 6-GHz uplink frequency to the 4-GHz downlink frequency, and the band-pass filter
after the mixer removes unwanted frequencies resulting from the down-conversion
operation.
• The attenuator can be controlled via the uplink command system to set the gain of
the transponder. Redundancy is provided for the high-power amplifiers (HPA) in each
transponder by including a spare
• TWT or solid-state amplifier (SSPA) that can be switched into circuit if the primary
power amplifier fails. The lifetime of HPAs is limited, and they represent the least
reliable component in most transponders.
• Providing a spare HPA in each transponder greatly increases the probability that the
satellite will reach the end of its working life with all its transponders still operational.
• Transponders can also be arranged so that there are spare transponders available in
the event of a total failure. The arrangement is known as M for N redundancy.
b. Double conversion transponder (bent type) for 14/11 GHz band
• It is easier to make filters, amplifiers, and equalizers at an intermediate frequency (IF)
such as 1100-MHz than at 14 or 11 GHz, so the incoming 14-GHz carrier is translated
to an IF of around 1 GHz.
• The amplification and filtering are performed at 1 GHz and a relatively high level
carrier is translated back to 11 GHz for amplification by the HPA.
• Stringent requirements are placed on the filters used in transponders, since they must
provide good rejection of unwanted frequencies, such as intermodulation products,
and also have very low amplitude and phase ripple in their pass bands.
• Frequently a filter will be followed by an equalizer that smoothes out amplitude and
phase variations in the pass band.
• Phase variation across the pass band produces group delay distortion which is
particularly troublesome with wideband FM signals and high-speed phase shift keyed
data transmissions.

On board processing Transponders

• A considerable increase in the communication capacity of a satellite is achieved by
combining on-board processing with switched-beam technology.
• A switched-beam satellite generates a narrow transmit beam for each earth station,
and then transmits sequentially to each one using time division multiplexing (TDM) of
the signals.
 • The narrow beam has to cover only one earth station, allowing the satellite transmit
antenna to have a very high gain compared to a zone-coverage antenna.
• On board processing may also be used to switch between the uplink access technique
(e.g., MF-TDMA) and the downlink access technique (e.g., TDM) so that small earth
stations may access each other directly via the satellite.

2.5 Satellite Antennas

Basic Antenna Types and Relationships
Four main types of antennas are used on satellites. These are
1. Wire antennas: monopoles and dipoles.
2. Horn antennas.
3. Reflector antennas.
4. Array antennas.
Wire antennas
• These are used primarily at VHF and UHF to provide communications for the TTC&M
systems.
• They are positioned with great care on the body of the satellite in an attempt to
provide omnidirectional coverage.
• Most satellites measure only a few wavelengths at VHF frequencies, which makes it
difficult to get the required antenna patterns, and there tend to be some orientations
of the satellite in which the sensitivity of the TTC&M system is reduced by nulls in the
antenna pattern.
• An antenna pattern is a plot of the field strength in the far field of the antenna when
the antenna is driven by a transmitter. It is usually measured in decibels (dB) below
the maximum field strength. The gain of an antenna is a measure of the antenna's
capability to direct energy in one direction rather than all around.
 • The pattern is frequently specified by its 3-dB beamwidth, the angle between the
directions in which the radiated (or received) field falls to half the power in the
direction of maximum field strength.
• However a satellite antenna is used to provide coverage of a certain area, or zone on
the earth's surface, and it is more useful to have contours of antenna gain.
• When computing the signal power received by an earth station from the satellite, it is
important to know where the station lies relative to satellite transmit antenna contour
pattern, so that the exact EIRP can be calculated.
• If the pattern is not known, it may be possible to estimate the antenna gain in a given
direction if the antenna boresight or beam axis direction and its beamwidth are
known.
Horn antennas
• These are used at microwave frequencies when relatively wide beams are required,
as for global coverage.
• A horn is a flared section of waveguide that provides an aperture several wavelengths
wide and a good match between the waveguide impedance and free space.
• Horns are also used as feeds for reflectors, either singly or in clusters. Horns and
reflectors are examples of aperture antennas that launch a wave into free space from
a waveguide
• It is difficult to obtain gains much greater than 23 dB or beamwidths narrower than
about 10° with horn antennas. For higher gains or narrow beamwidths a reflector
antenna or array must be used
Reflector antennas
• These are usually illuminated by one or more horns and provide a larger aperture than
can be achieved with a horn alone.
 • For maximum gain, it is necessary to generate a plane wave in the aperture of the
reflector. This is achieved by choosing a reflector profile that has equal path lengths
from the feed to the aperture, so that all the energy radiated by the feed and reflected
by the reflector reaches the aperture with the same phase angle and creates a uniform
phase front.
• One reflector shape that achieves this with a point source of radiation is the
paraboloid, with a feed placed at its focus. The paraboloid is the basic shape for most
reflector antennas, and is commonly used for earth station antennas.
Array Antennas
• Array antennas are used in satellites to form multiple beams from single aperture.
• A group of small antennas properly spaced and fed with a different phase in order to
produce radiation in a particular direction are called as array antennas.
• Phased array antennas are the array antennas in which radiations are varied by
changing the phase of the elements rather than their position.
• An electronic steering device can be used to vary the radiations.
• The gain of an aperture antenna is given by
4𝜋𝐴
𝐺 = 𝜂𝐴 𝜆2

If the aperture is circular,

𝜋𝐷 2
𝐺 = 𝜂𝐴 ( 𝜆 )

Where A is area of the antenna aperture, λ is wavelength, 𝜂𝐴 is aperture efficiency

and D is the diameter of the circular apertures.
• 𝜂𝐴 is 65 to 80 % for horn antennas, 50 to 65 % for reflector antennas with single feed,
and lower value for reflector antennas with shaped beams.
• The 3 dB beam width for an antenna with dimension D is given as
𝜃3 𝑑𝐵 = 75𝜆/D

• For antenna with 60% efficiency the gain is

𝐺 ≈ 33000/(𝜃3 𝑑𝐵 )2

Factors on Selection of Antennas

• Structure: An antenna structure to be chosen for use in satellites is the basic concern
in almost all satellite systems. Space erectable type antenna is the most commonly
used antenna structure for satellites. Rigid (fixed) type of antennas is generally used
at the time of launching satellites.
 • Type of Beam: Proper selection of beam ensures effective coverage of earth's entire
area and also increases the communication capacity of satellites. Multiple beam
antennas are adopted in satellites such as spot beam, global or elliptical shaped and
steerable.
• Types of Feed: Multiple feed antennas are used in satellites to supply contours of
different types. Various types of feeds in use are dipole feed, horn feed, flared horn
and helical feeds.
• Type of Polarization: Multiple polarization techniques are employed in satellites to
improve the communication capacity of satellite system. The two types of
polarizations in use are linear polarization and circular polarization.
• Directive Gain: It is defined as the ratio of radiation intensity in a given direction to
the radiation intensity of a reference antenna. Since satellites require high gain values,
the directive gain required is also high.
• Bandwidth: The bandwidth required by a satellite is solely based on the number of
transponders supported by a satellite. The range of bandwidth required for
transmission varies depending on the types of services offered i.e., TV. voice, internet,
radio by the satellite.
• Power to be fed: The power requirement of satellite antennas is less. Therefore, in
order to maintain the system's reliability and to prevent system failures, low power is
fed to the transponder.
• Efficiency: To ensure proper operation of satellites efficiency has to be maintained at
a high level. Efficiency is varies between 0 and 1. For a parabolic reflector efficiency
55 percent.
• Environmental Requirements: Keeping in mind the variations in environmental
conditions, antennas with zero thermal expansion have been designed using
composite materials. Antennas that can survive and stay stable in extreme
environmental conditions varying between – 165°C and 120°C are only selected.
2.6 Equipment Reliability and Space Qualification:
Once a satellite is in geostationary orbit there is little possibility of repairing components that
fail or adding more fuel for station keeping. To guarantee the reliability, two separate approaches are
used: space qualification of every part of satellite to ensure its long life expectancy and redundancy
of the most critical components to provide continued operation when one component fails.

Space qualification

• A satellite on the geostationary orbit has in harsh environments: total vacuum, 1.4 kW/m2 of
heat and light radiation from the sun, and temperature falling toward absolute zero in
shadow.

• Electronic equipment must be housed within the satellite and heated or cooled so that its
temperature stays within 0o to 75o C.
 • This requires a thermal control system that manages heat flow throughout a GEO satellite as
the sun moves around once every 24 h.

• Thermal problems are equally severe for a LEO satellite that moves from sunlight to shadow
every 100 min.

• Quality control or quality assurance is vital in building any equipment that is reliable.

When a satellite is designed, three prototype models are often built and tested:

 The mechanical model contains all the structural and mechanical parts that will be included
in the satellite and is tested to ensure that all moving parts operate correctly in a vacuum,
over a wide temperature range. It is also subjected to vibration and shock testing to simulate
vibration levels and G forces likely to be encountered on launch.

 The thermal model contains all the electronics packages and other components that must be
maintained at the correct temperature. Often, the thermal, vacuum, and vibration tests of the
entire satellite will be combined in a thermal vacuum chamber for what is known in the
industry as a shake and bake test. The antennas are usually included on the thermal model to
check for distortion of reflectors and displacement or bending of support structures. In orbit,
an antenna may cycle in temperature from above 100°C to below 100°C as the sun moves
around the satellite.

 The electrical model contains all the electronic parts of the satellite and is tested for correct
electrical performance under total vacuum and a wide range of temperatures. The antennas
of the electrical model must provide the correct beamwidth, gain, and polarization properties.

Reliability

• We need to be able to calculate the reliability of a satellite subsystem for two reasons: we
want to know what the probability is that the subsystem will still be working after a given time
period, and we need to provide redundant components or subsystems where the probability
of a failure is too great to be accepted.

• Components for satellite are selected only after extensive testing.

• The reliability of a component can be expressed in terms of the probability of a failure after
time t , 𝑃𝐹 (𝑡)

• For most electronic equipment’s, probability of a failure is higher at the beginning of life - the
burn-in period -than at some alter time.

• As the component ages, failure becomes more likely, leading to the bathtub curve shown in
figure

• The initial period of reduced reliability can be eliminated by a burn-in period before a
component is installed in the satellite.

• Semiconductors and integrated circuits that are required to have high reliability are subject to
burn-in periods from 100 to 1000 hours, often at a high temperature and excess voltages to
induce failures in any suspected devices.
• The reliability of a device or subsystem is defined as
𝑁𝑠 (𝑡) 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑢𝑟𝑣𝑖𝑣𝑖𝑛𝑔 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠 𝑎𝑡 𝑡𝑖𝑚𝑒 𝑡
𝑅(𝑡) = 𝑁0
=
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠 𝑎𝑡 𝑠𝑡𝑎𝑟𝑡 𝑜𝑓 𝑡𝑒𝑠𝑡 𝑝𝑒𝑟𝑖𝑜𝑑

• The number of components that failed in time t is given by

𝑁𝑓 (𝑡) = 𝑁0 − 𝑁𝑠 (𝑡)

• Mean time before failure (MTBF) is the probability of any one of the components failing
𝑁0
1
𝑀𝑇𝐵𝐹 = 𝑚 = 𝑁0
∑ 𝑡𝑖
𝑖=1

• The average failure rate (λ) is the reciprocal of MTBF

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑎𝑖𝑙𝑢𝑟𝑒𝑠 𝑖𝑛 𝑎𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
λ=
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑢𝑟𝑣𝑖𝑣𝑖𝑛𝑔 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠

1 𝛻𝑁𝑓 1 𝑑𝑁𝑓 1
λ=𝑁 = =
𝑠 𝑑𝑡 𝑁𝑠 𝑑𝑡 𝑀𝑇𝐵𝐹
• Failure rate is also known as the average failure rate per 109 h. The rate of failure is defined
as the negative of rate of survival, so
1 𝑑𝑁𝑠
𝜆=− 𝑁𝑠 𝑑𝑡

• Using reliability equation, the above can be written as

1 𝑑(𝑁0 𝑅) 1 𝑑(𝑅)
𝜆=− 𝑁0 𝑅 𝑑𝑡
=− 𝑅 𝑑𝑡

• The reliability is the solution of the above equation and is defined as

𝑅 = 𝑒 −𝜆𝑡
• The reliability of a device decreases exponentially with time and zero reliability after infinite
time (certain failure)
 • The end of useful life is usually taken to be the time at which R falls to 0.37 (1/e), which is
when
1
𝑡𝑙 = 𝜆 = 𝑚

• The probability of a device failing has an exponential relationship to the MTBF and is
represented by the right hand end of the bathtub curve

Redundancy

• In a satellite, many devices, each with a different MTTF, are used, and failure of one device
may cause catastrophic failure of a complete subsystem.

• If redundant devices are incorporated, the subsystem can continue to function correctly.

• There are three different situations for which subsystem reliability is computed:

• Series connection - used in solar cells arrays

• Parallel connection - used to provide redundancy of the high power amplifiers in

satellite transponders

• Switched connection - often used to provide parallel paths with multiple

transponders

• The switched connection arrangement is also referred to as ring redundancy since any
component can be switched in for any other.
Problem: The earth subtends an angle of 170 when viewed from GEO. What are the dimension
and gain of the horn antenna that can provide global coverage at 4 GHz.
Solution:
3 dB Beam Width 𝜃3 𝑑𝐵 = 75 𝜆 / D

Where 𝜃3 𝑑𝐵 = 170, 𝜆 = c / f = 3 x 108 / 4G = 0.075 m

Dimeter of Antenna, D = 75 * 0.075 / 17 = 0.33 m = 1 ft

Gain of the Antenna 𝐺 ≈ 33000/(𝜃3 𝑑𝐵 )2

G = 33000 / 172 = 114.18 W= 20.57 dB

Problem: What dimension must a reflector antenna have to illuminate half of the coverage
area with a circular beam 30 in diameter at 11 GHz. Find the gain?
Solution:
Wavelength 𝜆 =c / f = 0.027 m
Diameter of the Antenna D = 75 * 0.027 / 3 = 0.675 m
Gain of the Antenna G = 33000 / 32 = 35 dB