Study on Effects of Yaw and Co-Location on Cross-Polarized Signal Variation
R. Thaiprayoon and N. Homsup
Department of Electrical Engineering
Kasetsart University, Jatujak, Bangkok 10900, Thailand
E-mail address: rungnapt@cscoms.com, fengnth@ku.ac.th
Abstract
This research concerns the study and test design to
find the cause(s) of the variation in the cross-polarized
signal level measured by a receiving antenna. Such
variation in turn causes interference and degrades the
performance of the satellite communication links utilizing
same frequency but orthogonal polarization. Specifically,
the study is focused on Thaicom-2 and Thaicom-3
satellites which use frequency reuse in orthogonal
polarization fashion. The concept and hypotheses for
possible causes are based on the basic characteristics of
the satellite, attitude control, station keeping, co-location
strategy, Faraday rotation and antenna cross-polarization
pattern.
characteristics of the satellite, attitude control, station
keeping, co-location strategy, and antenna crosspolarization pattern. We also found that the measurements
suggest the strong effect of Faraday rotation.
2. Satellite Yaw Error Angle
To a satellite attitude control, the yaw angle is defined
to be the angle that rotates about the axis pointing to Earth,
which is a reference plane of polarization (Figure 1). If the
reference plane rotates, the levels of co-polarized signal
and cross-polarized signal will be affected in opposite
direction. However, the cross-polarized signal increases
more rapidly than the co-polarized signal decreases.
1. Introduction
To efficiently utilize the same bandwidth, the Thaicom
satellites employ frequency reuse by using orthogonal
linear polarizations to avoid interference of the signals
occupying the same frequency.
Before accessing the satellite, an uplink station is
required not to generate any interference signals into the
opposite polarization. However, the real-world experience
shows that although there is adjustment of the polarization
of an antenna, which is done manually during Uplink
Access Test, to have largest difference in the level of copolarization signal to that of the cross-polarization signal,
called “Cross Polarization Isolation”. The interference
from the cross-polarized signal still exists and could
seriously interfere the other signals. In another word, the
adjustment at one time is not enough. The antenna needs
continuous adjustment for the polarization to keep the
Cross Polarization Isolation at the acceptable value.
From actual monitoring of the cross-polarized signals,
the cross-polarized signals vary during a day. The
magnitude of the variation is in 8 to 12 dB range. This
range can tremendously degrade the quality of the
signal(s) in the opposite polarization especially if the
quality of the signal(s) is already right at the edge between
acceptable and unacceptable. Therefore, it is valuable to
study the causes of such variation in the cross-polarized
signal. Possible causes studied are based on the
Figure 1 Yaw, Pitch, and Roll definitions [1].
In general, the polarization of the receiving antenna
will not be the same as the polarization of the incident
wave. The amount of power received by the antenna from
the incoming signal will not be at maximum because of the
polarization loss. The polarization loss can be taken into
account by introducing a polarization loss factor (PLF). It
is defined as [2]
CoPLF  cos p
2
CrossPLF  sin  p
(1)
2
(2)
where p is the polarization angle between the planes of
the incoming wave and the antenna co-polarized axis.
Note that the Yaw error in dual-spin stabilized satellite
behaves like a sine curve with a day period while the yaw
error in a three-axis stabilized satellite does not but still
repeats every day. Figure 2 shows the Yaw error for both
types of satellites [3].
Yaw Attitude : NM
0 .0 1
0.8
0 .0 0 9
Three-axis
0 .0 0 8
0 .0 0 7
0 .0 0 6
0 .0 0 5
0.4
0 .0 0 4
0 .0 0 3
0.2
0 .0 0 2
0 .0 0 1
0
0
-0 .0 0 1
-0 .0 0 2
-0.2
-0 .0 0 3
-0 .0 0 4
Dual-spin
-0.4
-0 .0 0 5
-0 .0 0 6
-0 .0 0 7
-0.6
Yaw Angle of Dual Spin Satellite (Deg)
Yaw Angle of 3-Aix Stabilzed Satellite
(Deg)
0.6
-0 .0 0 8
-0 .0 0 9
-0.8
0:00
-0 .0 1
1:00
2:00
3:00
4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00
20:00
21:00
22:00
23:00
0:00
Time hh:mm (GMT)
Yaw T3
Colinearity Region
Yaw T3
Colinearity Region
Yaw T2
Yaw T2
Figure 2 Yaw error angles of dual-spin stabilized and
three-axis stabilized satellites in a day.
3. Satellite Co-Location
Figure 3 Polarization Discrimination
With orbital slots for placing geo-satellites are limited,
it is necessary to operate a group of satellites sharing the
same orbital slot, which is so called co-location. The idea
of co-location is to maintain more than one satellite in the
same tolerance window.
Thaicom2 and 3 satellites are collocated at the same
orbital slot at longitude 78.5 degrees East. Thaicom2
frequencies are reused with opposite polarization in
Thaicom3. That means these two satellites are able to get
the significant effect from the cross-polarized signal of
each other. One suspected factor of this study is the
separation between the co-located satellites. The colocation strategy for these satellites, inclination and
eccentricity separation, is to place them in the different
orbital plane. Their orbital planes are differently inclined
from the equatorial plane. Therefore, this should be one
factor to affect the behavior of the cross-polarized signal.
5. Faraday Rotation
4. Cross Polarized Antenna Pattern
The cross-polarization of a source is becoming of
increasing interest to satellite communication antenna
designers. The case of an antenna transmitting, or
receiving, a linearly-polarized field, the cross-polarized
component is the field at right angles to this co-polarized
component. For example, if the co-polarized component is
vertical, then the cross-polarized component is horizontal
Of importance in a dual polarization frequency reuse
satellite communication system is the polarization
discrimination, previously cited as the isolation, between
the copular and cross-polarized signals, especially in the
antenna main beam region as illustrated in Figure 3.[7]
The Faraday rotation causes the linearly polarized
signal to rotate its polarization plane as it passes through
the Ionosphere about 100 km to over 400 km above the
earth. Thus, its effect is to rotate the polarization plane of
the signal as similar to the effect from satellite rotating its
Yaw angle.
The rotating polarization angle from Faraday rotation,
 , is
  (C / f 2 )  TEC
where
radians
(3)
C is a constant equal to 2.36  10 4 Bav and TEC
is the Total Electron Content [4].
The TEC profile for a location, although varies from
year to year, does not change much over years. It reaches
its bottom around before sunrise and its maximum in the
afternoon [5].
6. Relation to Cross-Polarized Signal
The calculated cross-polarized signal based on the three
parameters is
(Tx _ xpolpattern(  )) 2  
Xpol (dB)  Copol (dB)  10 log  2

sin ( FR   Yaw   )

(4)
where,
Tx-xpol-pattern() is the transmit cross-polarized pattern
for LM4 at  , the separation angle between Thaicom
2&3. Note that XPD of the antenna is Tx-xpolpattern( =
0).
 FR
= the angle of the polarized plane rotated by Faraday
rotation,
 yaw = Yaw error angle,
 = the angle caused by Faraday rotation at the time the
antenna’s polarization was adjusted.  is estimated.
The “+/-“ in front of
 yaw
is unknown because we do
not know the direction of the angle of the polarization
plane rotated by the Faraday rotation relative to that
rotated by the yaw error. If both rotations are in the same
direction (say counter-clockwise), the sign is “+”. The sign
was selected based on how well the calculation fits the
measured data.
7. Data Measurement and Results
The test and measurement was performed at the
Thaicom Satellite Control Station in Nonthaburi, near
Bangkok, Thailand during October 2001 to February 2003.
The measurement was done automatically using the
HPVEE program [6].
Figure 5 Test data collected during the same period for the
test signal and the telemetry. The measured data is
compared with the calculation.
However, it was perplexing why the cross-polarized
signal from Thaicom-1 varied as in Figure 5. As an effort
to find the cause, we added the effect of Faraday rotation
by using the TEC data during the same days but year 1996
(Figure 6) to represent the TEC data in year 2002.
7.1 Effect of Yaw of Dual-Spin Stabilized Satellite
The cross-polarization of the receive antenna was
adjusted at the zero yaw error angle on Feb 19, 2002. The
zero yaw error is defined as the position where the
satellite’s yaw is in the middle of its yaw variation
window. Figure 4 shows the test data collected during the
same period for the test signal and the telemetry. Both
signals are varied at about 6 to 8 dB. The calculated crosspolarized test signal is shown in Figure 5. The peak-topeak amplitude is about 0.05 to 0.15 dB.
Although the calculated cross-polarized signal resulted
from the satellite’s yaw error and the measured crosspolarized signal have more or less same peak time, the
amplitude of variation from calculation could not match
that of the measured signal. Thus, we conclude that the
satellite yaw angle variation alone partially explain the
variation in the cross-polarized signal.
Figure 4 Yaw error angle, Delta Yaw angle, and calculated
cross-polarized signal level when the polarization was
adjusted at zero Yaw error angle
Figure 6 Average TEC profile for Bangkok and Chiang
Mai in Feb 1996 [5].
We found that the estimated rotating polarization angle
from the Faraday rotation does match quite well with the
measured data in term of peak time and the look of the
curve (Figure 7). Both calculations based on “+” and “-”
signs match quite well with the measured data. This
matching helps to confirm that the Faraday rotation should
have an effect on the cross-polarized signal variation at CBand. Note that the small difference in the two calculated
signals is due to the Yaw error.
Figure 7 Measured cross-polarization variation of
Thaicom-1’s telemetry and calculated variation from
Faraday rotation
7.2 Effect of Yaw of Three-Axis Stabilized
Satellite
The result of the test to verify the effect of the yaw
error angle to the variation of the cross-polarized signal in
the environment of Thaicom-3 satellite is shown in Figure
8. Again the yaw error alone cannot explain the measured
variation. The effect of the Faraday rotation should be
included in order to explain the variation as shown in
Figure 9.
Since the antenna pattern for co-polarized signal is very
symmetrical in azimuth and elevation angle, the composite
angle or "angle distance" at a particular azimuth angle
(Az) and elevation angle (El) can be calculated as:
Composite angle 
Az'2  El 2
(5)
Since the earth is spherical, the azimuth angle in the
composite angle must be scaled down by [1]
Az '
CorrectionFactor 
Az
'
where Az is calculated from
 Az ' 
 Az 
  sin   cos( El )
sin 
 2 
 2 
(6)
(7)
Figure 8 Test data collected during the same period for the
test signal and the telemetry. The measured data is
compared with the calculation based on yaw error angle.
Again, the estimated polarization angle rotation was
based on the TEC data in year 1996, not in 2003 the time
of the measurement. This surprising match strongly
suggests that the Faraday rotation should play a major role
in the variation of the cross-polarized signal.
Note that the yaw error angle in the shaded stripes is
ignored because the yaw error angles in these periods are
not real due to the co-linearity of the satellite and the sun.
Figure 10 Relative Latitude and Longitude of the
Thaicom-2 seen by the LM4 antenna which is tracking
Thaicom-3 satellite during 6-9 December 2002.
The calculated co-polarized signal is derived from the
co-polarized receive antenna pattern at the composite
angle. The calculated co-polarized signal matches well
with the measured signal (Figure 11).
For cross-polarized signal, the horizontally polarized
signal was uplinked from LM4, which was tracking to
Thaicom-3. Its cross-polarized signal, which was vertically
polarized, went to Thaicom-2 and received by LM1
antenna that was tracking Thaicom-1 satellite (see
illustration in Figure 12).
Figure 9 Measured and calculated cross-polarized
telemetry signals from Thaicom-3 based on Faraday
rotation and yaw error angle.
7.3 Effect of Yaw and Satellite Movement
Since the LM4 antenna is tracking Thaicom-3 satellite,
it sees Thaicom-2 satellite moves as shown in Figure 10.
The position in latitude and longitude of Thaicom-2
satellite seen by LM4 antenna can be translated into the
azimuth (Az) and elevation (El) reference plane of the
LM4 antenna by using standard Az/El equations as in [1].
Figure 11 Comparison between the test data and the
calculated data of co-polarized signal variation due to the
satellite movement
2
Telemetry
1
Variation in Sig1&1_2 depend on Uplink
cross pol. pattern &distance between
T2-T3
THAICOM2
V
H
Variation in Sig 2_3 depends on
downlink Co pattern & distance
between T3-T2
H
V
Var 1_3 = Var 1_2+Var 2_3
Up
lin
k
k
lin
wn
Do
THAICOM3
V pol
X-signal
V'
H pol
H
Co-signal
T2-11m TrckAnt.
LM1
T3-11m TrckAnt.
LM4
1_2
V pol
T3-11m TrckAnt.
LM4
H pol
1_3 2_3
Figure 12 Signal path and polarizations for the crosspolarized signal used in the test configuration.
Because the test results in the previous section strongly
suggest that the Faraday rotation plays a major role in the
rotation of the polarization angle, the variation of the
received cross polarized signal at LM1 should have
reflected the combined effects of the following three
factors: (1) The uplink cross-polarized pattern due to the
separation of Thaicom-2 from Thaicom-3, which was the
direction the LM4 antenna was pointing to, (2) Yaw Error
of Thaicom-2, and (3) Faraday Rotation on the uplink path
through the Ionosphere.
How well the calculation base on Equation (4) matches
with the measurement is shown in Figure 13. Again, the
data for TEC during the test time in 2002 is estimated
from the data in year 1996. The calculated data matches
quite well with the measured data in term of the peaks
occurred at about the same time as the measured data. The
selection of the parameter  was done by visual
inspection with the constraint that its value should be
within the maximum and minimum angle of the variation.
The two calculated signals based on “+” (additive) and “-”
(subtractive) of the combination between yaw error angle
and the Faraday rotated angle are very similar. This is
again due to the small yaw error angle.
Figure 13 Comparison between the test data and the
calculated data of cross-polarized signal variation due to
the satellite movement, yaw error and Faraday rotation.
8. Conclusion
From the testing and measurement, it can be seen that
the variation of the cross-polarized signal is mainly caused
by two effects: (1) satellite movement of the two
orthogonally-polarized satellites and the cross-polarization
pattern of the earth station antenna and (2) the Faraday
rotation of the polarization plane of the signal when it
passes through the ionized layer of the Ionosphere. The
yaw error angle of the satellite which was one of the two
original suspecting factors turns out to have a small effect
on the variation of the cross-polarized signal.
Although the effect of the Faraday rotation was not
originally considered in the study, it turns out from the test
data that it could play a major role in describing the
rotation of the polarization angle of the cross-polarized
signal in uplink and downlink paths. Many literatures
confirm that it does have a significant effect in the C-band
environment (3-4 GHz) which is the environment in our
tests. Its effect though could be ignored for the Ku-band
environment (12-14 GHz) since the rotating polarization
angle inversely depends on the square of the frequency.
The result of this thesis study can be used to improve
the link quality and improve the cross-polarization
interference of one satellite link to the others by
minimizing the amount of the variation in the crosspolarized signal.
Specifically, there are two main benefits of the study:
(1) Find the best time to do the antenna
azimuth/elevation pointing and polarization
adjustment
(2) Calculate the minimum XPD the satellite can
expect to experience.
Equation (4) suggests that the adjustment to minimize
the cross-polarized variation could be done independently
between the polarization angle which is the feed angular
position of the antenna and the Azimuth and Elevation
angles of the antenna. The polarization angle should
mainly depend on the Faraday rotation effect while the
Az/El angles depend on the relative position of the crosspolarized satellite to the antenna. This independence helps
to separate the antenna adjustment and minimize the crosspolarized variation at different time.
Regarding the Faraday rotation effect, the antenna
polarization should be adjusted at the time when the Total
Electron Content (TEC) is around its mean. This ensures
that the variation of the polarization angle is minimized.
Regarding the satellite movement effect, there are two
cases to consider: tracking and non-tracking antenna.
Case 1: Tracking Antenna. The azimuth and elevation
angles of the antenna are tracked automatically according
to the satellite movement of the co-polarized satellite.
Thus, there is no need to find the best time for Az/El
pointing. However, the polarization of the antenna is not
adjusted by the tracking mechanism and it is adjusted
manually by the installer or operator. Thus, the best time
to adjust the polarization should follow the rule for the
Faraday rotation above.
Case 2: Non-Tracking Antenna. The Az/El of the
antenna and the polarization angle of the antenna feed
must be adjusted to its best position manually. The rule to
adjust the Az/El is to achieve the best position where the
variation of the cross-polarizes signal is minimized. The
time for Az/El adjustment depends on the cross-polarized
pattern. For simple case like the case in Figure 3 where the
cross-polarized pattern with peak at the boresight and,
within the satellite movement in the station keeping box,
rolls off monotonically as the angle is further away from
the boresight, the best position of the antenna is to have
boresight at the center of the satellite station keeping box.
This can be done by adjusting the Az/El of the antenna
when the co-polarized satellite is at the middle of the
satellite station keeping box.
9. Acknowledgement
The authors would like to acknowledge Shin Satellite
Plc. for the test location and equipment support.
10. References
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[2]
[3]
[4]
[5]
[6]
[7]
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Kan Laopipattana. Study of Ionospheric Electron
Content by Using GPS Satellites Signals. M.S.
Thesis, King Mongkut’s Institute of Technology
Ladkrabang, 1997.
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Company,
Controlling
Instruments with HP VEE, 2nd ed, USA, 1998.
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