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Evaluation of remote sensing satellite ground station performance in prbs local

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  • 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 Journal Impact Factor (2013): 5.8896 (Calculated by GISI) www.jifactor.com IJECET © I A E M E
  • 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 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME 121 (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 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME 122 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.
  • 5. 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 123 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 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME 124 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.
  • 7. 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 125 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