|Lecture 3, Earth Segment (http://www.satcom.co.uk/article.asp?article=5)|
|Welcome to the third lecture in the RPC Satcom Tutorial
At any point in the lecture, you can skip to a latter section, or back to earlier sections using the guide on the left.
Topics contained in this lecture:
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:
Note, not everything is present in every terminal. For example, HPAs are not required for receive-only terminals.
An earth-station comprises:
|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:
Received Power Between Antennas:
The effective area Ae of an antenna is related to the physical area
of its aperture Aa by the expression:
We have already seen that:
|ITU-R Recommendation S.465 defines a radiation mask
for use in coordination and interference assessment:
For antennas with D >= 100*l:
For antennas with D < 100*l:
ITU-R Recommendation S.580 defines a radiation mask design objective for new earth stations:
For antennas with D > 150*l:
For antennas with 50*l < D <= 150*l installed before 1995:
For antennas with 50*l < D <= 150*l installed after 1995:
(Where qr is the maximum of 20° and (100*l/D)
The main influences on sidelobes are:
|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:
No power will be transferred if the 2 polarisation states have:
|Circular polarisation results from the combination of
two orthogonal equal-magnitude waves in quadrature.
Usually try to produce a true circularly polarised wave.
The polarisation state of any wave can be completely described by:
Emax / Emin is the voltage axial ratio (VAR) of the wave, or AR = 20 log (Emax /
|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.
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.
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).
|There are two main sources of noise in a satellite system:
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:
|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
(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.
|The noise power collected by an antenna comprises:
|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:
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?
|EIRP of emissions from earth stations (excluding mobile
- 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 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:
|Real GSO satellites do not remain exactly stationary in orbit.
Movement will occur east/west and north/south due to:
Stationkeeping on the satellite maintains the position within a small "box"
typically ±0.1° N-S and E-W.
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:
Disadvantages of Steptrack systems:
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.
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)