1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME
119
EVALUATION OF REMOTE SENSING SATELLITE GROUND STATION
PERFORMANCE IN PRBS LOCAL LOOP, TPG LOOP MODE FROM FAR
FIELD BORESIGHT AND VALIDATION OF SATELLITE DOWNLINK
CHAIN FOR INDIAN REMOTE SENSING SATELLITE SERIES
R Srinivas 1
, Dr. L. Nithyanandan 2
, P V V Subba Rao 3
1
National Remote Sensing Centre / Indian Space Research organisation
Department of Space, Govt of INDIA, NRSC, Hyderabad, INDIA
2
Electronics and Communications Engineering Department, Pondicherry
Engineering College, Pondicherry, UT, INDIA
3
National Remote Sensing Centre / Indian Space Research Organisation
Department of Space, Govt of INDIA, NRSC, Hyderabad, INDIA
ABSTRACT
The video data from IRS remote sensing satellites is transmitted in S and X-bands. To qualify
good data reception at ground station, various simulation tests are carried out at ground station
before tracking and acquiring the remote sensing satellite in real time. The satellite downlink chain
at ground station needs to be qualified and certified before tracking the live satellite to avoid loss of
data.This may be due to probable failure or malfunctioning of any system or sub-system in satellite
downlink receive chain. Hence local loop tests are performed which certifies the ground station data
chain right from Antenna feed to data ingest system. Also Bore sight tests are conducted from far
field place; simulating as if data is being transmitted by the satellite. This gives simulation of data
reception from satellite in offline mode. There are two kinds of checks to certify the satellite
downlink chains, one is quantitative check and the other is qualitative check. The Remote sensing
satellite data downlink from satellite is transmitted in X-band. In order to certify the entire link
performance right from remote sensing satellite to ground station ,Pseudorandom sequence is
transmitted from satellite instead of actual video data. This is also called the calibration pass. The
received PN data is monitored at the output of the demodulator. The Bit error rate (BER) is checked
for performance in order to certify the data downlink chain from satellite to ground station. Generally
for good video data quality the specified BER is 1.00 X 10-6
is required. With this kind of BER the
video data is free from line losses line losses, pixel dropouts etc. This paper discusses the concepts
of various tests performed at ground station before the actual arrival of satellite. This will be very
useful to identify the satellite downlink related system problems well in advance, so that corrective
action is taken and the satellite downlink chain is ready for real time pass.
INTERNATIONAL JOURNAL OF ELECTRONICS AND
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 4, July-August, 2013, pp. 119-125
© IAEME: www.iaeme.com/ijecet.asp
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2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME
120
Key words: Bit error rate BER, Indian remote Sensing IRS, Quadrature Phase shift keying QPSK ,
Satellite downlink, Test pattern generator TPG , Pseudorandom Binary sequence PRBS
I INTRODUCTION
The Remote satellite ground stations operates in L, S and X bands. S-band is used for satellite
tracking and acquisition of satellite [1]. The X-band is used for satellite video data downlink. The
line of site Visibility of satellite at zero degree for ground station is generally 3500Kms radius.
Before the arrival of the satellite when it is not in the visible range it is necessary to check & certify
the satellite data downlink chain and related base band systems etc. These local loop tests and bore
sight tests are required to be performed before actual satellite pass, since it will identify; if there is
any problem in the satellite downlink receive chain. Hence prior to real time pass the problem is
identified and solved so that we get error free video data from the satellite at ground station downlink
chain. There are different methods to certify the ground station performance. First is Local loop test
which comprises PN Sequence generation at different code lengths i.e. 27
-1, 215
-1, 220
-1, 223
-1[2].
These are CCITT standards taken for different PN Sequence lengths. This PN data is up converted i.e
modulated and then down converted i.e demodulated at the ground station to evaluate the satellite
downlink performance. For all Indian Remote Sensing satellites Missions 215
-1 PN Sequence is
transmitted during calibration pass from satellite in X-band. At ground station the BER is measured
at the output of QPSK Demodulator/Bit synchroniser and signal conditioner [2]. During this process
pay load passes are terminated and PN Sequence data is scheduled in play back mode (PB). Hence
PN data of 215
-1 is taken to check local loop [3].Normally to check any digital channel PN data is
given at the input and BER is measured at the output i.e errors are monitored .The degradation of
BER will give bad BER which reflects the bad performance of the receive downlink chain. This is
called quantitative measurement where in the PN data bits are read in a window of one million bits
and displayed. Thus PN sequence data for e.g. RISAT-1 is simulated, up converted, down converted
and the satellite downlink chain output is checked for good BER performance. At the ground station
this PN sequence data is received by parabolic dish reflector, received signal is amplified to optimal
level by Low noise amplifier and this is down converted from X-band frequency of 8212.5 MHz to
2557.5 MHz [3]. The signal from antenna pedestal is transmitted using F.O. links to the main control
room. Again the 2557.5 MHz signal is once again down converted to an IF of 720MHz. The IF is fed
to QPSK Demodulator which separates data from the carrier. The data is fed to bit synchroniser &
signal conditioner. The bit synchroniser generates clock in synchronous with incoming data which is
a PN Sequence. The output of bit synchronisers are data & clock which is monitored for BER. If the
channel performance of BER is better than 1 error in one million bits i.e. 1x10-6
then the RF chain is
supposed to be good and good data quality is assured. For remote sensing satellite ground station
BER of 1x10-6
is specified and acceptable. The other method is qualitative check i.e. Test pattern
generator is simulated, up converted i.e modulated, down converted i.e demodulated and finally the
data is visually checked for quality of video data. The TPG grey scale pattern is seen at the output of
the data ingest system on moving window display. Here the visual qualification of video data is done
and any pixels dropouts, line drop outs, line breaks can be identified. The problem can be trouble
shooted and rectified before the actual arrival of satellite. Hence the probability of loosing data or
occurrence of errors will be minimised. Hence for any ground station both qualitative and
quantitative loop tests are be performed to increase the reliability of the ground stations.
II PRBS LOCAL LOOP TESTS
One of the method is using PRBS local loop check i.e. PN sequence of length 215
-1 is
simulated for both Right hand circular polarisation (RHCP) and Left Hand circular polarisation

3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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(LHCP)chains with data rate of 320 Mbps[3]. As a test case of Radar Imaging Satellite-1 bit rate of
320 Mbps and carrier frequency 8212.5 MHz is taken. The outputs of simulator or modulator are 720
MHz in RHCP and 720 MHz in LHCP with QPSK modulations. These signal outputs are fed to up
converter or frequency multiplier whose output is -90 dBm. At first up conversion 10 dB loss will
occur. The signal is amplified by 20 dB gain which results -80dBm with two QPSK modulated
carriers. These are transmitted through Fibre Optic cable cores in mono mode to Far field bore sight
control room. Here the signals are received by Fibre Optic receivers. The received signals are up
converted i.e frequency multiplied to X-band carrier frequency of 8212.5 MHz; which experiences
10 dB loss, and the signal is amplified by 30dB gain which results in -60 dBm .Hence the signal
from up converter is -60dBm. Again these signals are connected to bore sight antennas with low loss
cable which results 30 dB attenuation. Here the bore sight Antenna gain is 30 db, so the transmitted
output level from boresight will be -60dBm. This is received by 7.5 meter parabolic dish reflector
with cassegranian feed. The antenna and passive sub systems will result in the loss of 20 dB (septum
polariser, monopulse comparator and monoscan converter) hence the output is -80 dBm at the input
of Low noise amplifier, LNA will give a gain of 50 dB, so the output of LNA is -30 dBm, This
output is connected to DPC (divider, phase shifter and combiner). DPC gives the gain of 40 dB , so
signal of -10 dBm is given to first down converter i.e. frequency divider ; with Down Converter loss
of 10dB will occur , the output will be at -20 dBm . This is fed to Fibre Optic Transmitter in pedestal
room and is received by Fibre Optic Receiver in Centralised control room. This output is given to
second down converter whose output is 720 MHz; which results a loss of 10 dB, so the input to the
QPSK demodulator is Intermediate Frequency of 720 MHz with output level -30dBm .The dynamic
range of QPSK demodulator is -5 to -50 dBm So after demodulation and bit synchronisation the BER
is seen for both RHCP and LHCP chains. This kind of check results in quantitative analysis, where
the errors are totalised and seen in a window of one million bits. If the BER is better than 1 error in 1
million bits then the link is qualified as good. If there are more than one error then the satellite
downlink chain is said to be erroneous. Proper care and corrective action is to be taken to validate the
chain for best BER performance. Hence this validates the entire End-End downlink chain check
using PRBS link as shown in Figure1, Table 1 and Table 2.
Down
Conv
De
modulatorBSSC
PRBS -SimFO Link
Up Conv
PRBS Local Loop Check
D
C
BER
Reader
Bore site Tower
Ground Station AntennaTX antenna
Figure 1. Block Diagram of PRBS local Loop test

4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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Table 1
Data rate(Mbps)
Frequency(MHz)
System/
Sub-system
Input level
(dBm)
Loss (dB) Gain
(dB)
Output level
(dBm)
320 Mbps@
8212.5 MHz
105 Mbps @
8125 MHz
105 Mbps @
8300 MHz
RF
Simulator
- - -90
Up converter -90 10 20 -80
Fibre Optic
TX
-80 - - -80
Fibre Optic
Receiver at
Far Field bore
sight
-80 -80
up converter -80 10 30 -60
Low Loss
cable
Connectivity
to tower
-60 30 -90
Bore sight
antenna
-90 - 30 -60 (Radiated
by Bore site
tower)
Table 2
Data rate(Mbps)
Frequency(MHz)
Sub-system Input
level
(dBm)
Loss
(dB)
Gain(dB) Output level
(dBm)
320 Mbps@
8212.5 MHz
105 Mbps @
8125 MHz
105 Mbps @
8300 MHz
Parabolic dish
7.5 Meter
Antenna
-60 20 - -80
Low Noise
Amplifier
-80 - 50 -30
RF Cable -30 20 - -50
DPC
Divider, Phase
Shifter,
Combiner
-50 - 40 -10
First down
converter
-10 10 - -20
Fiber Optic
Transmitter
-20 - - -20
Fibre Optic
Receiver
-20 - - -20
Second Down
Converter
-20 10 -30
QPSK
Demodulator
-30 - - -
Note: The above tests are carried out with Input data rates of 320 Mbps and carrier Frequency of
8212.5 MHz in both RHCP and LHCP chains for RISAT-1 satellite.

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BER Evaluation
For good data quality the Satellite down link BER should be at least one error tolerable in one
million bits. i.e. BER = 1x10-6
.
Hence this confirms that the data is error free without any losses.
If the numbers of errors are exceeding more than one; then there is possibility of pixel drop
outs and line losses in the video data.
Hence, the Satellite down link data chain has been evaluated for BER and the total numbers
of errors are monitored.
Now that the BER has been monitored at QPSK demodulator end, simultaneously LVDS data
outputs are also given to front end hardware of the data ingest system.
III TPG (TEST PATTERN GENERATION) LOCAL LOOP CHECK
Down /
Conv
De ModBSSCFrame sync
Unit
Serial
to
Parallel
FC
INTERFACE
TPG
FO Link
Up Conv
SAN
Storage
TPG Local Loop Check
Simulator
TX antenna
Bore site Tower
Ground Station Antenna
Figure 2. Block Diagram of TPG Loop test
In this test instead of PRBS data the Test pattern which is grey scale pattern output is
connected to the input of the Simulator. Here a typical Format length of 19200 bits or 2400 bytes is
taken as shown in Fig 3. The same signal levels follow as explained in the Fig 1 , Table 1 and Table
2 .Here the QPSK Demodulator outputs are connected to frame synchroniser hardware which does
the frame synchronisation. As seen from Figure-3 below in the data format 50 bytes are dedicated to
Frame sync and Auxiliary data bits, and the other 2350 bytes are video data bytes [4].

6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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Hence from 50 bytes or 400 bits , the frame sync code length is 127 bits for all Indian
Remote sensing satellites. Since the PN Sequence of 27
- 1 Frame sync code is known the same is
detected and frame sync lock is derived[5] . Since the data from the satellite is in serial fashion;
every frame length contains frame sync code along with video data. Hence Frame synchronisation is
done for every frame length and data is stripped from serial data to parallel data as shown in Fig 2.
The Parallel data is time tagged and data ingest is done on computer systems, Raid or Storage area
networks. Real time data ingest is done and simultaneously an interrupt is generated and video TPG
data is displayed on screen monitor. Here the quality of video can be seen. If the satellite downlink
chain is bad then this will be reflected as black pixel dropouts, white pixel dropouts or line dropouts
and line breaks etc. Hence the problematic system is identified and rectified in the satellite downlink
chain. This kind of link check is qualitative analysis where visually data certification of data quality
is done [6]. The qualitative analysis of errors is superior to quantitative analysis since better
monitoring can be done visually as each and every line is seen. Finally the data is stored in SAN
storage for further processing. Different IRS satellites have different format lengths and different
carrier frequencies [6]. Hence TPG patterns for the corresponding frame lengths are selected,
simulated , up converted , down converted and data ingest is done as shown in fig 2 . The selectable
frequencies are 8125MHz, 8300MHz, and 8212.5MHz etc with data rates varying from 105 Mbps to
320Mbps.
The test pattern is generated for respective Indian Remote Sensing satellites depending upon
format lengths [7]. In the slot allotted for video data, test pattern is embedded and thus actual
simulation of the exact format length can be tested and verified. This enables the link to be checked
in OFFLINE mode in the absence of the satellite. The above tests with respect to Figure 1 and Figure
2 are very important since they give clarity and condition of the satellite downlink chain.
IV CONCLUSION
With the help of PRBS local loop test and TPG loop test any remote sensing satellite ground
station can be checked, certified and validated for the satellite downlink chain performance. This is
very much required since the functionality of systems and sub-systems need to be checked and
certified before the real time satellite data acquisition. These tests will help in identifying the
problems in the satellite downlink chain and in case of any anomaly or improper functioning of any
system or sub-system in the Satellite downlink chain it can be troubleshooted systematically. Hence
isolating problems in the satellite downlink chain becomes easy and handy. Now in the absence of
the satellite the above said checks are sufficient to qualify the ground station receive chain. This
methodology can be implemented for validating any remote sensing ground station operating in S
and X bands, but still there is no so provision till now to certify the link between satellites to ground
station in real time except for calibration pass.
V ACKNOWLEDGEMENTS
I would like to thank to Mrs G Umadevi, Head BSD for the guidance, encouragement and
support. Also I would like to express my gratitude to Sri. M.Satyanarayana , Group Director, SDRS
and Sri D S Jain , Deputy Director , NRSC for the timely help and providing logistic support.

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VI REFERENCES
Books
[1] John G. Proakis, Digital Communications (4th Edition), McGraw-Hill, 2000.
[2] Gruenberg E.L “Handbook of Telemetry and remote Measurement”, Mc Graw hill Book
company, 1967.
[3] Bernard Sklar, Digital Communications: Fundamentals and Applications (2ndEdition), Prentice
Hall, 2001.
[4] Digital framing techniques for satellite channels, D.E.Doods, L.R.Button, A.G.Wacker, SCC-
june1983.
[5] Heegard, A. J. King, S. Lovely, and T. J. Kolze, “Synchronization and error detection in a
packetized data stream,” Dec. 30, 1997.
Journal paper
[6] R. B. Ward, “Acquisition of pseudo noise signal by sequential estimation,”IEEE Transactions
Communications. Tech.,Vol1. COM-13, pp. 475–483, Dec.1965.
Proceedings Paper
[7] Srinivas R , Nityanandan L, Umadevi G, Rao PVVS, Kumar P N ; Design and Implementation
of S-Band Multi-mission satellite positioning data Simulator for IRS satellites; Applied
electromagnetic conference 10.1109/AEMC.2011.6256919