Lecture 3, Earth Segment (http://www.satcom.co.uk/article.asp?article=5)

Introduction

Welcome to the third lecture in the RPC Satcom Tutorial series.

At any point in the lecture, you can skip to a latter section, or back to earlier sections using the guide on the left. 

If you want to check out the other Lectures in this series, you can find Lecture 1 (First Principals) here, and Lecture 2 (Space Segment) here.

Topics contained in this lecture:

  • Types and components of an Earth Station
  • General Construction
  • Antenna Theory
  • Radiation Patterns
  • Polarisation
  • Noise
  • Low-Noise Amplifiers
  • Power Amplifiers
  • Tracking

Earth Station Types

-International Gateway Earth Station
(8 - 32m Antenna)
e.g. BT's Goonhilly and Madley, UK

-National Trunk System Earth Stations
(8 - 32m Antenna)
e.g. Rio, operating to Brazilsat

-TV Distribution Earth Stations
(5 - 18m Antenna)
e.g. IDB Teleport, Newark, USA

-Small-dish Business System Stations
(2.5 - 5.5m Antenna)

-Very Small Aperture Terminals (VSAT)
(5.5 - 15m Hubs)
(1.2 - 3.5m VSATs)

-Television Receive Only Terminals (TVRO)
(0.45 - 3m Antenna)
Earth Station Components
An Earth terminal will always include an antenna.

Terminals may also include:
  • Low-noise amplifiers (LNAs) or low-noise down-converters
  • High-power amplifiers (HPAs)
  • Signal processing equipment (e.g. down-converters, up-converters, IF amplifiers, modems and codecs)
  • Transmission and signalling equipment at the interface between the terminal and the terrestrial network
  • Supervisory and control equipment
  • Enclosures to protect the equipment from the environment

Note, not everything is present in every terminal. For example, HPAs are not required for receive-only terminals.

General Construction

Requirements:
  • High gain in the direction of the satellite (because the free-space attenuation between the satellite and the earth station is around 200 dB)
  • Very low gain in all directions other than the satellite (so that the antenna does not cause nor receive too much interference or noise)
  • Good polarisation isolation (e.g. good rejection of vertically polarised signals when receiving horizontally polarised signals)
  • High efficiency
    Good pointing accuracy because high antenna gain means narrow bandwidth (e.g. an antenna with a diameter of 13m has a 3 dB beamwidth of around 0.1 at a frequency of 14 GHz)

 

An earth-station comprises:

  • A feed and reflector system.
  • A pedestal or mount which supports the reflector and feed and enables the antenna to be accurately pointed towards the satellite.


The feed and reflector have to be made very accurately in order to maximise the gain on-axis, minimise the sidelobes and ensure good polarisation isolation.

Large antennas (which have narrow beams) normally have tracking systems which keep the antenna pointing at the satellite even when the satellite moves. Even for tracking antennas, the reflector, feed and mount must be very rigid so as to ensure maximum beam stability, even during high winds.

The most commonly used mount for large earth stations is the elevation-over-azimuth (EL-AZ) mount. The azimuth axis is vertical and the elevation axis is horizontal and the EL-AZ mount gives full steerability.

Single Axis Mount.
If the beam of an hour-angle/declination mount is pointed at a distant star in the equatorial plane then the beam points parallel to the equator.

If the beam is pointed at a satellite in the GSO then the beam will point at an angle (d) to the celestial equator. This angle varies only slightly with satellite position. The satellite may be tracked over a limited arc of the GSO by rotating it about the hour-angle axis alone. By tilting the axis of rotation away from the hour-angle axis it is possible to improve the tracking.

Antenna Theory

An Isotropic Antenna is a (theoretical) antenna that radiates energy uniformly in all directions.

Thus if the power radiated by an isotropic antenna is Pt then the power flux density (PFD) at a distance s metres from the antenna (in free space) is:

PFD = Pt / (4*pi*s2) W/m2

Note that (4*pi*s2) which is the area of the sphere of radius s is called the "spreading area".

A real receiving antenna will collect power in an effective area Ae m2 and if it is at a distance s metres from the transmitting antenna then the power received (Pr) is:

Pr = Ae . PFD = Ae . Pt / (4*pi*s2) W

The relationship between the gain G of an antenna and its effective area (where l is the wavelength in metres) is:

Ae = (G* λ2) / (4*pi) m2

The effective area Ai of an isotropic antenna, which by definition has unit gain is therefore:

Ai = λ2 / (4*pi) m2

Free Space Attenuation:

The power Pr received by an isotropic antenna as a distance s from an isotropic transmitter is therefore:

Pr = Ai . PFD = Ai . Pt / (4*pi*s2) W 
   = Pt / L W 
Where: L = (4*pi*s2) / Ai = (4*pi*s2) / (λ2 / 4*pi)
i.e. L = (4*pi*s / λ2)

So, L, which is the ratio of the spreading area to the area of an isotropic antenna is the free-space attenuation between isotropic antennas and is often called the "path loss".

Received Power Between Antennas:

Now, if the transmitting and receiving antennas have gains Gt and Gr respectively then the power C received is:

C = (Pt . Gt) . Gr / L = EIRP . (Gr / L) W

Note two ways of finding the power received:

  • Using the PFD and effective area of the receiving antenna
  • Using the EIRP and gain of the receiving antenna

The effective area Ae of an antenna is related to the physical area of its aperture Aa by the expression:

Ae = h . Aa

where h is the efficiency of the antenna.

The efficiency is less than 100% because the antenna is not perfect and the main factors are:

  • Spillover past the subreflector and main reflector (rays A and C in the figure)

  • Blockage of the antenna aperture by the subreflector (B) and supports (not shown)

  • Losses due to profile and other manufacturing errors

  • Ohmic losses

  • Non-uniform amplitude and phase distribution in the aperture

We have already seen that:

Ae = (G*λ2) / (4*pi*m2)

So: G = (4*pi*Ae) / λ2 
        = (4*pi*h*Aa) / λ2 
i.e. G = h * (pi*D / λ)2
where D is the antenna diameter in metres. 

As gain increases, the beamwidth decreases.

The half-power (3 dB) beamwidth is given by:

HPBW = (N*λ) / D degrees

Where N is a constant dependant on the aperture illumination.
For an ideally illuminated aperture (i.e. each point in the aperture is illuminated with an RF signal of the same amplitude and phase)
N = 58 but this is not achievable in practice.

For an efficient real antenna, N is approximately 65.

Radiation Patterns

ITU-R Recommendation S.465 defines a radiation mask for use in coordination and interference assessment:

For antennas with D >= 100*l:

G = 32 - 25 log q dBi for 1 <= q < 48
  = -10 dBi  for 48 <= q <= 180

For antennas with D < 100*l:

G = 52 - 10 log(D/l) - 25 log q dBi  for 1 <= q < 48
   = -10 - 10 log(D/l) dBi  for 48 <= q <= 180

ITU-R Recommendation S.580 defines a radiation mask design objective for new earth stations:

For antennas with D > 150*l:

G = 29 - 25 log q dBi 1 <= q < qr

For antennas with 50*l < D <= 150*l installed before 1995:

G = 32 - 25 log q dBi for 1 <= q < qr

For antennas with 50*l < D <= 150*l installed after 1995:

G = 29 - 25 log q dBi 1 <= q < qr

(Where qr is the maximum of 20 and (100*l/D)

The main influences on sidelobes are:

  • Aperture illumination:
    - Sidelobes are an integral part of the radiation pattern of an aperture
    - The larger an aperture the more rapidly the sidelobes decrease
  • Scattering and blockage:
    - Any object which blocks part of the radiation from an aperture disturbs the wavefront and causes additional sidelobes
    - There is a lower practical limit to the subreflector size so for small antennas the front fed configuration may give better sidelobes
    - Struts supporting the feed or subreflector also cause blockage and scattering
  • Spillover:
    - Spillover past the subreflector makes a significant contribution to the near-in radiation pattern of an axisymmetric dual reflector antenna
    - Spillover also occurs past the main reflector but this is at a lower level
  • Reflector profile errors:
    - Sidelobes caused by reflector profile errors are influenced by two factors, the rms value of the errors and the correlation distance of the errors
    - A repeated pattern of errors in the formation pr assembly of a series of panels can cause a serious increase in the sidelobes.
    - Profile accuracies are usually better than 1 mm rms
Antenna Polarisation
An antenna has a polarisation state that can be described in the same way as that of a wave.

The proportion of power in a wave that is transferred to an antenna depends on their relative polarisations.

For the general case (wave and antenna both elliptically polarised), power will only be transferred from the wave to the antenna if the 2 polarisation states have:
 - The same sense of rotation
 - Equal VARs and
 - Equal tilt angles

No power will be transferred if the 2 polarisation states have:
 - Opposite senses of rotation
 - Equal VARs and
 - The major axes of the ellipses are orthogonal

Elliptical Polarisation

Circular polarisation results from the combination of two orthogonal equal-magnitude waves in quadrature.

Usually try to produce a true circularly polarised wave.
Imperfections in the equipment often result in the wave being elliptically polarised

The polarisation state of any wave can be completely described by:

  • The amplitudes of the major and minor axes of the polarisation ellipse, Emax and Emin 

  • The tilt angle, T

  • The sense of polarisation, i.e. left-handed or right-handed

Emax / Emin is the voltage axial ratio (VAR) of the wave, or AR = 20 log (Emax / Emin) dB

Dual Polarisation
When no power is transferred from a wave to an antenna they are said to be orthogonally polarised.

By using a dual-polarised feed an antenna can transmit two orthogonally polarised waves on the same frequency.

Another antenna can then receive the two orthogonally polarised waves and separate them by means of an electrically identical dual polarised feed.

In theory this can be done without any interference between the two signals. However, in practice there will always be some interference because the orthogonally polarised waves and/or the receiving antenna will not be perfectly orthogonal because of imperfections in the antenna and feeds or changes of polarisation which occur during transmission as a result of the signal passing through the atmosphere

XPI and XPD
Cross-polar Isolation (XPI):

A measure of the strength of a cross-polar transmitted signal that is received by an antenna as a ratio to the strength of the co-polar signal that is received.
i.e. 20 log (E11 / E21)

Cross-polar Discrimination (XPD):

A measure of the strength of a co-polar transmitted signal that is received cross-polar by an antenna as a ratio to the strength of the co-polar signal that is received.
i.e. 20 log (E11 / E12)

Note that to measure XPI, two highly orthogonal waves must be transmitted, to measure XPD only a single polarisation needs to be transmitted (see figures).


Noise
There are two main sources of noise in a satellite system:
  • Noise arising in the satellite and earth-station equipment, and
  • Noise collected by the satellite and earth-station antennas

The antenna receives noise from the sky and from the earth.

Sky noise comprises cosmic (galactic) noise and noise resulting from absorption and re-radiation of energy by water and oxygen molecules in the atmosphere

Noise power radiated by the Earth is collected by the sidelobes of the earth-station antenna and the main beam of the satellite antenna.

Noise power N is related to an equivalent noise temperature by the expression:

N = k . T . B watts

Where:

K is Boltzmann's constant (1.38 x 10-23 J/K)
B is the bandwidth in which the noise is measured (in Hz)

 

G/T
The ratio between the receive gain of an earth-station and its noise temperature is a measure of the "quality" with which it is able to receive signals

(Also known as the "Figure of Merit")

G/T ranges from around 37 dB/K for high-gain low-noise FSS antennas to about -23 dB/K for low-gain high-noise mobile terminals.

Noise Temperature
The noise power collected by an antenna comprises:
  • Atmospheric attenuation noise caused by absorption and re-radiation of signal energy by water and oxygen molecules in the atmosphere; this noise increases rapidly with decreasing antenna elevation angle because the signal has to travel further through the atmosphere; it also increases when it is raining.
  • Noise radiated by the Earth - collected through the antenna sidelobes.
  • Cosmic (galactic) noise - only contributes a few K to the noise temperature.
  • Ohmic losses - i.e. losses due to the resistance of the feed system and the antenna reflectors.
  • Attenuation between the antenna and the LNA - contributes about 7K for every 0.1 dB of loss, so it is essential to put the LNA very close to the antenna.
Low-Noise Amplifiers
The information carrying capacity of any radio system is proportional to the ratio:

C / T = (carrier power / system noise temperature)

It is therefore necessary to make the system noise temperature as small as possible to maximise the information capacity.

The value of T on the downlink of a satellite system depends primarily on the noise temperature of the earth-station antenna and the amplifier following it.

A satellite antenna looks at the earth (at a temperature of around 290K) so there is little point in spending a lot of money to fit it with a low-noise amplifier. However, an earth-station antenna looks at the sky and its noise temperature is usually much lower than 290K.

As an example, the noise temperature of an earth-station antenna working at 4 GHz varies from about 20K at high elevation angles to around 45K at an elevation angle of 5 (when the sky is clear).

Earth terminals equipped with large antennas used to use cryogenic parametric amplifiers (paramps).

Cryogenic means "at a very low temperature" and cryogenic paramps were cooled to around 20K (i.e. -253 C) by using refrigerating plant circulating gaseous helium. Cryogenic paramps are expensive and require a lot of skilled maintenance effort.

Higher satellite powers have made them unnecessary for most satellite systems and they are rarely used nowadays.

Types of LNA in common use today include:
  • Uncooled Field-Effect Transistor (FET) amplifiers which have a noise temperature of 55 to 75K at 4 GHz or around 200K at 11 GHz
  • Amplifiers cooled by thermoelectric diodes which have a noise temperature of 35K to 45K at 4 GHz and around 120K at 11 GHz

In the above example, an antenna of gain 52 dBi and noise temperature 35K is connected to an LNA of gain 50 dB and noise temperature 80K via waveguide and hence to a receiver via co-axial cable. What is the system noise temperature Te?

First some definitions:
  Tr = (NF - 1) x 290 K (1)
  Tp = [(1 - G) / G] x 290 K (2)
  To = G . Ti K (3)

Where (all components assumed to be at 290K):
  Tr = noise temperature NT corresponding to a noise figure NF
  Tp = noise temperature of a passive network (e.g. waveguide) of gain G
  To = output noise temperature of a noiseless network of gain G

Thus:
  T1 = [(1 - 0.955) / 0.955] x 290 = 13.7K
  T2 = 80 
  T3 = [(1 - 0.25) / 0.25] x 290 = 870K
  T4 = (15.85 - 1) x 290 = 4307K

Now:
  Te = Ta.G1 + T1.G1 + T2 + T3/G2 + T4/(G2.G3)

Where:
  Te = noise temperature of the system (earth-station) referred to the input of the LNA
  Ta = the noise temperature of the antenna at its output terminals and G1, G2, G3 and T1, T2, T3, T4 are the gains and input noise temperatures of the corresponding networks as given in the previous figure

Hence:
  Te = 35 x 0.955 + 13.7 x 0.955 + 80 + 870 / 105 + 4307 / (105 x 0.25) = 33 + 13 + 80 + 0.17
i.e.
  Te = 126 K

Power Amplifiers
EIRP of emissions from earth stations (excluding mobile stations) ranges
- around 20 dBW for low-rate data applications
- to nearly 90 dBW for some TV and large multi-channel telephony applications

EIRPs are achieved using antennas with gains ranging from around 20 dBi to 66 dBi.

Corresponding powers necessary to deliver to the antenna ranges from around 1 watt to several hundred watts. BUT maximum power capability of the transmitters in some earth stations is several hundred kilowatts. It is necessary to use the transmitters very inefficiently when they are amplifying more than one carrier

Losses in the networks required to combine the outputs of a number of transmitters.

Main types of PA in earth-stations are:

  • Travelling wave tube (TWT) amplifiers
  • Klystron amplifiers
  • Solid-state (SS) power amplifiers

Travelling wave tube amplifiers (TWTA) have bandwidths of 500 MHz and more and powers from a few watts to many kilowatts.

Klystrons have bandwidths of 40 to 80 MHz and are tuneable over 500 MHz or more. Powers from several hundred watts to many kilowatts. Cheaper, easier to set up, operate and maintain than TWTAs.

Solid-State Power Amplifiers are comparatively cheap and reliable. Power is relatively limited compared with TWTAs and Klystrons.

The most important characteristics of HPAs, apart from frequency, power, bandwidth and linearity are:

  • Gain: 
    The gain of TWTAs and klystrons ranges from about 35 to 50 dB and Intermediate Power Amplifiers (IPAs) are often needed between the modulators and power amplifiers.
  • Variation of group delay with frequency: 
    This is another cause of intermodulation.
  • AM/PM conversion: 
    This causes intelligible crosstalk and intermodulation noise.
  • Noise and spurious outputs: 
    All amplifiers generate noise; the noise figure (NF) of microwave power amplifiers is usually about 30 dB but special low-noise tubes are available as pre-amplifiers; Pas may also generate spurious tones and may modulate output signals as a result of ripple on power supplies.
Tracking
Real GSO satellites do not remain exactly stationary in orbit. Movement will occur east/west and north/south due to:
  • Small errors in placing them in orbit
  • Disturbing forces in orbit
    - Earth's gravity field is not uniform
    - Gravitational forces of sun and moon
    - Solar pressure

Stationkeeping on the satellite maintains the position within a small "box" typically 0.1 N-S and E-W.

Small antennas have a 3 dB beamwidth larger than the stationkeeping box so no difficulties arise.

Large antennas have narrow beams smaller than the stationkeeping box so require a tracking and steering system to remain pointing at the satellite.

In the early experimental days of satellite communications earth stations were manually steered by operators using joysticks. Manual steering is not practical for commercial systems and so automatic systems of two basic types are used:

  • Programme track:
    - Uses calculated theoretical pointing information
    - Pointing data calculated from satellite orbital elements from tracking stations
    - Data needs to be updated frequently to remain accurate
  • Autotrack:
    - Uses a signal from the satellite and a feedback control loop to track in real time
    - Not dependant on predictions or orbital elements
    Common types: Steptrack and Monopulse

Tracking errors result in loss of pointing accuracy and hence loss of earth-station gain in the direction of the satellite. The aim is to keep the loss to a fraction of a dB. Reduction in gain (dG) relative to the maximum (on-axis) gain of an antenna of half-power beamwidth (HPBW) for a small angular offset (dq) is given approximately by:

dG = 12 (dq / HPBW)2 dB

The aim is therefore to keep pointing error (dq) to about 0.1 x HPBW. 

For a large diameter antenna (~400*l) the HPBW is ~0.15. Desirable pointing error is thus < ~0.015 This is generally achieved by using an automatic tracking and pointing system

Smaller antennas (~ 25 to 100*l) the HPBW is ~ 0.5 2.5 and these do not need continuous tracking but may need to be manually redirected from time to time.

Steptrack systems make frequent small changes in the pointing direction of the antenna in both axes. 

Does the signal increase or decrease? If it increases then the next step is in the same direction, if decreases then the next step is in the opposite direction

Advantages of Steptrack systems: 

  • relatively cheap 
  • simple to implement

Disadvantages of Steptrack systems:

  • Stepping results in loss of gain
  • Easily confused by level changes (e.g. as a result of rain fade)
  • "Wear and tear" on steering equipment

Monopulse systems. Multimode systems originated from developments in RADAR technology. Antenna feed is designed so that higher order modes are generated in the feed when the signal source is off the centre of the antenna beam. Signal processing of the higher order modes generates pointing error signals used to steer the antenna

Advantages of Monopulse systems include very good accuracy.

Disadvantages include the relatively expensive feeds and recievers.


Both steptrack and monopulse systems need a beacon signal radiated by the satellite.

Improved steptrack (e.g. smoothed steptrack - SST) aims to improve the performance of steptrack. This combines the elements of programme track and steptrack. It uses normal steptrack approach to build a "model" of the satellite track. Once an accurate model is established SST keeps it up to date (under normal circumstances) by sampling every 30 minutes or so. SST thus ignores any random variations which might cause tracking errors. If the beacon signal is lost altogether (e.g. a beacon transmitter or receiver fails) then SST can continue to predict the track using the programme track model. If SST detects a serious departure from the model (e.g. because of an orbit correction) then the learning process is restarted. Accuracy is claimed to be close to that of monopulse.

Copyright 2002 Satcom Online (http://www.satcom.co.uk)
20/08/2017  14:16:19