G/T MEASUREMENTS MEASUREMENTS (Lecture Notes by A. Bhattacharya to CSSTEAP SATCOM course) INTROUCTION A communication communication link is said to be perfect only when the information transmitted from the source is received at the destination without any errors. In practice it is rarely so. This is because the different sources of noise, form the modulator of the transmitting end till the demodulator at the receiving end, reduce the Signal - to Noise ratio S!.N". This most important criteria depends on a number of factors# the $arrier-to-Noise ratio $!N" of the %& or I& signal in the receiver, the type of the modulation used to impress the base band signal onto the carriers, transmitter 'I%(, and the I& and base band channel bandwidths in the receiver. $onsider $onsidering ing the limited limited 'I%( available available on board board and having having chosen chosen modulati modulation on scheme the S!N depends on $!N. The $arrier-to-Noise ratio for received signal is given by P G C = t t N kB
*here (t+t
2
Gr T s
............. )"
transmitter 'I%(
[ 4π R / λ ] k +r Ts
λ 4π R
2
path loss oltmann/s constant bandwidth of the power measurement device in 0 +ain of the receiving Antenna Antenna Syste System m noise noise temper temperatu ature re of the recei receiver ver refer referred red to its
input. Thus, $!N 1 +r !T !Ts as all other terms are constant for a given satellite system. +r !T !Ts is usually shortened shortened to +!%atio, sometimes sometimes called 22&igure 22&igure of 3erit// of the receiving station as this factor determines determines 4uality of the signal received. +!T is defined as
G / T = 10 log
Antenna Antenna power powe r gain dB / k System noise temperature
MET!O O" MEASUREME ME ASUREMENT NT To allow comparison of signal 4ualities received at various stations it is must that +!T ratio be measured with high accuracy. The measurement of +!T can be done by number of methods. These are5 6irect 3ethod Indirect 3ethod - y measuring + 7T separately - 8sing satellites $omparison 3ethod #rect Metho$ The +!T measurement of medium and large earth stations is done by 6irect 3ethod, because the indirect method of +!T measurement of antenna is inconvenient and does not model model actual actual operatin operating g conditio conditions, ns, mismatch mismatch losses losses etc. Independe Independent nt
1
measurement of gain for a high aperture antenna re4uires microwave source at boresite or on satellite, placed in geo-synchronous orbit. The best and straightforward way is +!T measurement using radio stars. A suitable radio star is the one, which is well calibrated for its noise flu9 density in the measurement fre4uency range. The flu9 density variation of the noise source with time should be minimum for better measurement accuracy. The celestial bodies such as $assiopeia A, Taurus A, $ygnus A, :irgo A, 3oon and Sun are considered as sources of noise for this purpose. The choice of particular noise source depends upon the antenna sie of the earth station, because larger the ratio between antenna beam width and the angular e9tension least correction factor re4uired, as antenna will see the source only through its peak Theory Antenna pointed at clear sky Noise power output ( )kTs Ts system noise temperature. Antenna pointed at radio star Noise power output ( ; <.= & Ae > kTs & &lu9 density of radio star. Aeeffective aperture of antenna <.= is multiplied to take into consideration the random polariation.
1 Y =
P 2 P 1
=
1 FAe
or
2 kTs
Y − 1 =
or G
or
Ts
=
2
FA e
B + k Ts B = 1+
k Ts B
1 FAe 2
kTs
= Y − 1
1
2
F
Gλ
1
4π kTs
2
2
=
F λ
G
8π k Ts
#Y " 1!8π k λ 2
;"
2
Proce$ure The receiving antenna is pointed alternately at selected noise source and the cold sky to measure ? factor. The steps are given below. ). Arrange the set up as shown in fig. ). 6isable A+$ circuitry if any in the measurement path. Noise source
$old sky
3IB'%
Antenna
N& 3eter! (recision %eceiver
@NA! %eceiver
@ocal scillator "IG % &
;.
G/T MEASUREMENT B' USING RAIO SOURCES
@ocal oscillator is set C< 30 higher to the fre4uency at which measurement path are to be taken.
C. D.
Antenna is pointed towards the cold sky i.e. no noise source min the vicinity". Input level to precision test receiver is so adEusted that minimum )
=. F.
Note the attenuator setting Aid" and needle position. Antenna is then pointed towards selected noise source and ma9imied for reading on precision receiver with same position as it was in step D.
G.
Note the attenuator setting Af d".
H.
$alculate ? factor and ? using following formula. ? Af - Ai y )
.
$alculate +!T
)<.
%epeat the e9periment for other fre4uencies.
TEST RESULTS &re4uency
Ai
Af
?
$
y
y measuring the ratio, y procedure for the measuring the ratio is given in section", of the noise powers at the receiver output, the ratio +!T can be determined using the formula ;"5 Correct#o actors a$ assess*et o errors The corrected value of +!T is given by +!Tc +!T > $) > $; > $C > $D .........C" $) is correction due to atmospheric absorption, for the angle of elevation above =o, is given by, $) <.
%adio star
Tab+e , & &lu9 density at D +0 &lu9 density at f Angular fD" *m-; 0-)" +0 ff" for C.G-D.; 6iameter band
$assiopea A
)
Taurus A
G)G s )<-;F
D" f ! D"-<.;=
C/ 9 D.=/
$ygnus A
DHH 9 )<-;F
D" f ! D" -).)
each K <.G/
3oon
:aries with time and phase
<.;=Go
Sun
:aries cycle
<.;FGo
with
for D" f ! D"-<.G=
sun-spot
D/
Po+ar#-at#o eect Taurus A is elliptically polaried and it is necessary to use the mean of two readings taken in two orthogonal directions. These precautions are not necessary for measurements using $assiopea A and $ygnus A.
4
I$#rect Metho$ +!T measurement of Small!mobile earth station is not possible by 6irect method because of the inaccuracy in the measurement of ? factor. In such cases, indirect method is adopted to evaluate the +!T of the earth stations. In This case, the gain of the antenna and receive system noise temperature are measured separately. The gain of the antenna is measured in Anechoic $hamber or Test %ange facility and the system noise temperature is measured as follows 5 PROCEURE L Arrange the set up as shown in fig. ;.
Antenna
Noise Source
@NA! %eceiver
N& 3eter! (recision %eceiver
6ummy @oad
"IG % . RECEIE S'STEM NOISE TEMPERATURE 0 G/T MEASUREMENT
L
3easurement should be done in open environment.
L
6isable any A+$ circuitry and tune the receiver to receive the signal.
L
$onnect the antenna to the receiver and measure the noise power (a.
L
$onnect the noise source to the antenna port of the receiver and measure the noise power ( n.
L
6isconnect noise source and connect the ambient load termination" at the antenna port and measure the noise"
L
$alculate the ? factor according to the formula 5 (n kTn > Tr ", (a kTa > Tr ", ?) (n ! (a
T n
+ T r
T a
+ T r
T o
+ T r
?; (o ! (a T a
+ T r
T o
+ T r
?C (o ! (n T n
+
T r
T r = # gives
(o kTo >Tr "
T o − T n %$
%$
−1
&
where,
L
Tn Temperature of noise source To Ambient temperature Ta Antenna noise temperature Tr %eceiver noise temperature &rom the above e4uations Y2
− Y = 1
− T n Ta + T r
To
or T a
L
+ T r =
T 0 Y 2
− T n − Y 1
System noise temperature TS is given by T 0
TS Ta > Tr L
Y 2
− T n − Y 1
%epeat the measurement at other fre4uencies of interest
TEST RESULTS &re4uenc y
(a
(n
(o
?)
?;
?C
Tr
Ta
Ts
The ratio of antenna gain, + di and system noise temperature, T S gives the +!T of the earth station.
G/T MEASUREMENT USING SATELLITE If 'I%( of the satellite is precisely known the +!T can be calculated from the received $!N of the beacon signal. The results are fairly correct.
TEST PROCEURE • •
Track the beacon signal of the satellite and optimie it by moving antenna in A!'l a9is.
•
3easure the $!N o at I& level.
•
+!T of the earth station can be calculated by using the e4uation 5
+!T $!N - Satellite 'I%( > (ath @oss > 3odulation @oss > k oltmann/s constant" > 3easuring device/s noise bandwidth correction factor.> other losses
'
G/ T MEASUREMENT B' COMPARISON MET!O The +!T of small or medium class earth stations can be computed by comparison method. In this method the +!T of the reference earth station must be precisely known. The satellite beacon signal is simultaneously received and the $!N o measurement is done at the earth station under test and at the reference earth station, whose +!T is known. The +!T value of the earth station under test is computed from the difference of the $!N o values received at these earth stations. +!Tt +!Tr - ∆$!N ∆$!N $!N<" r M$!N<" t
RE"ERENCES ).
%ecommended Test (rocedure for the Type Approval of IN3A%SAT - $ Ship 'arth Stations
;.
+!T 3easurement 8sing 3oon As A Noise Source INSAT - ) SS(-T% - <)HC y : +arg, 6 %authan
(
