SATELLITE TRACKING, TELEMETRY AND COMMAND 1

ionosphere

troposphere

sungalaxy

system noise temperature in satellite communication

Receiving station architectureReceiving station architectureReceiver station diagram blockReceiver station diagram block LNA block and f irst conversionLNA block and f irst conversion

Receiving station architectureReceiving station architectureSecond conversion and IF2 amplifier blockSecond conversion and IF2 amplifier block

Noise

Noise is an electronic signal that gets added to a radio or information signal as it is transmitted from one place to another.

It is not the same as interference from other information signals.

Noise

Noise is the static you hear in the speaker when you tune any AM or FM receiver to any position between stations. It is also the “snow” or “confetti” that is visible on a TV screen.

The noise level in a system is proportional to temperature and bandwidth, the amount of current flowing in a component, the gain of the circuit, and the resistance of the circuit.

Signal-to-Noise Ratio

The signal-to-noise (S/N) ratio indicates the relative strengths of the signal and the noise in a communication system.

The stronger the signal and the weaker the noise, the higher the S/N ratio.

The S/N ratio is a power ratio.

External Noise

External noise comes from sources over which we have little or no control, such as: Industrial sources

motors, generators, manufactured equipment Atmospheric sources

The naturally occurring electrical disturbances in the earth’s atmosphere; atmospheric noise is also called static.

Space The sun radiates a wide range of signals in a broad noise

spectrum.

Internal Noise

Electronic components in a receiver such as resistors, diodes, and transistors are major sources of internal noise. Types of internal noise include: Thermal noise Semiconductor noise Intermodulation distortion

Expressing Noise Levels

The noise quality of a receiver can be expressed in the following terms: The noise factor is the ratio of the S/N power at the

input to the S/N power at the output. When the noise factor is expressed in decibels, it is

called the noise figure. Most of the noise produced in a device is thermal,

which is directly proportional to temperature. Therefore, the term noise temperature (TN) is used.

SINAD is the composite signal plus noise and distortion divided by noise and distortion contributed by the receiver.

Noise in Cascaded Stages

Noise has its greatest effect at the input to a receiver because that is the point at which the signal level is lowest.

The noise performance of a receiver is determined in the first stage of the receiver, usually an RF amplifier or mixer.

11

The Earth is Curved !• Radio waves above 30 MHz travel in straight lines• Ways must be found to get signals beyond horizon• Ionospheric reflection uses hf band, 2 – 30 MHz• Microwave link uses line of sight between towers

• Chain of repeaters can take the signal thousands of miles

• Satellite communications uses a repeater in the sky

• Single link via GEO satellite can reach round one third of the earth’s surface.

12

Earth

Ionospheric layers

Tx Rx

multipath

Fig. HF Radio Communication

13

Earth

Tx Rx

Fig. LOS Microwave Communications

14

Earth

Tx Rx

GEO satelliteAltitude 35,680 km

Fig. Satellite Communications

noise @ noise @ SatellitesSatellites

1.1. Thermal NoiseThermal Noise

2.2. Intermodulation noiseIntermodulation noise

3.3. CrosstalkCrosstalk

4.4. Impulse Noise Impulse Noise

Thermal Noise

Thermal noise due to agitation of electrons

Present in all electronic devices and transmission media

Cannot be eliminatedFunction of temperatureParticularly significant for satellite

communication

Thermal Noise

Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:

• N0 = noise power density in watts per 1 Hz of bandwidth

• k = Boltzmann's constant = 1.3803 x 10-23 J/K• T = temperature, in Kelvin's (absolute temperature)

( )W/Hz k0 TN =

Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz

(in watts):

or, in decibel-watts

TBN k=

BTN log10 log 10k log10 ++=

BT log10 log 10dBW 6.228 ++−=

Noise Terminology

Intermodulation noise – occurs if signals with different frequencies share the same mediumo Interference caused by a signal produced at a

frequency that is the sum or difference of original frequencies

Crosstalk – unwanted coupling between signal paths

Impulse noise – irregular pulses or noise spikeso Short duration and of relatively high amplitudeo Caused by external electromagnetic disturbances, or

faults and flaws in the communications systemo Primary source of error for digital data transmission

Comm. Subsystem—Design Typical System Noise Temperatures

Expression Eb/N0 Ratio of signal energy per bit to noise power

density per Hertz

The bit error rate for digital data is a function of Eb/N0

o Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected

o As bit rate R increases, transmitted signal power must increase to maintain required Eb/N0

TR

S

N

RS

N

Ebk

/

00

==

Other Impairments

Atmospheric absorption – water vapor and oxygen contribute to attenuation

Multipath – obstacles reflect signals so that multiple copies with varying delays are received

Refraction – bending of radio waves as they propagate through the atmosphere

Multipath Propagation Reflection - occurs when signal encounters a

surface that is large relative to the wavelength of the signal

Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

Scattering – occurs when incoming signal hits an object whose size is in the order of the wavelength of the signal or less

R= Reflection D= Diffraction S= Scattering

Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phaseso If phases add destructively, the signal level

relative to noise declines, making detection more difficult

Intersymbol interference (ISI)o One or more delayed copies of a pulse may

arrive at the same time as the primary pulse for a subsequent bit

FadingTime variation of received signal power

caused by changes in the transmission medium or path(s)

In a fixed environment:o Changes in atmospheric conditions

In a mobile environment:o Multipath propagation

Types of FadingFast fadingSlow fadingFlat fadingSelective fadingRayleigh fadingRacian fading

Transmit beam

θτ θρ

Receive beam

Error Compensation Mechanisms

1. Forward error correction2. Adaptive equalization3. Diversity techniques

1.Forward Error Correction Transmitter adds error-correcting code to

data blocko Code is a function of the data bits

Receiver calculates error-correcting code from incoming data bitso If calculated code matches incoming code, no error

occurredo If error-correcting codes don’t match, receiver attempts

to determine bits in error and correct

2.Adaptive Equalization

Can be applied to transmissions that carry analog or digital informationo Analog voice or videoo Digital data, digitized voice or video

Used to combat intersymbol interference Involves gathering dispersed symbol energy

back into its original time interval Techniques

o Lumped analog circuitso Sophisticated digital signal processing algorithms

3.Diversity Techniques Space diversity:

o Use multiple nearby antennas and combine received signals to obtain the desired signal

o Use collocated multiple directional antennas Frequency diversity:

o Spreading out signal over a larger frequency bandwidtho Spread spectrum

Time diversity:o Noise often occurs in burstso Spreading the data out over time spreads the errors and

hence allows FEC techniques to work wello TDMo Interleaving

GLIMPSES

ECHO 1

TELSTAR

SYNCOM 2

Major problems for satellites

1. Positioning in orbit

2. Stability

3. Power

4. Communications

5. Harsh environment

1.Positioning• This can be achieved by several methods

• One method is to use small rocket motors

• These use fuel - over half of the weight of most satellites is made up of fuel

• Often it is the fuel availability which determines the lifetime of a satellite

• Commercial life of a satellite typically 10-15 years

2.Stability• It is vital that satellites are stabilised

– to ensure that solar panels are aligned properly– to ensure that communications antennae are

aligned properly

• Early satellites used spin stabilisation– Either this required an inefficient omni-

directional aerial

– Or antennae were precisely counter-rotated in order to provide stable communications

Stability (2)• Modern satellites use reaction wheel

stabilisation - a form of gyroscopic stabilisation Other methods of stabilisation are also possible

• including:– eddy current stabilisation– (forces act on the satellite as it moves through

the earth’s magnetic field)

Reaction wheel stabilisation

• Heavy wheels which rotate at high speed - often in groups of 4.

• 3 are orthogonal, and the 4th (spare) is a backup at an angle to the others

• Driven by electric motors - as they speed up or slow down the satellite rotates

• If the speed of the wheels is inappropriate, rocket motors must be used to stabilise the satellite - which uses fuel

3.Power• Modern satellites use a variety of power

means

• Solar panels are now quite efficient, so solar power is used to generate electricity

• Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite

• Nuclear power has been used - but not recommended

5.Harsh Environment

• Satellite components need to be specially “hardened”

• Circuits which work on the ground will fail very rapidly in space

• Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully

Alignment• There are a number of components which

need alignment– Solar panels

– Antennae

• These have to point at different parts of the sky at different times, so the problem is not trivial

Antenna alignment

• A parabolic dish can be used which is pointing in the correct general direction

• Different feeder “horns” can be used to direct outgoing beams more precisely

• Similarly for incoming beams

• A modern satellite should be capable of at least 50 differently directed beams