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GT ECE 6390 - ECE 6390 TEST 2

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Name:GTID:ECE 6390: Satellite Communications and Navigation SystemsTEST 2 (Fall 2008)• Please read all instructions before continuing with the test.• This is a closed notes, closed book, closed friend, open mind test. On your desk you shouldonly have writing instruments and a calculator.• Show all work. (It helps me to give partial credit.) Work all problems in the spaces below theproblem statement. If you need more room, use the back of the page. DO NOT use or attachextra sheets of paper for work.• Work intelligently – read through the exam and do the easiest problems first. Save the hardones for last.• All necessary mathematical formulas are included either in the problem statements or the lastpage of this test.• You have 80 minutes to complete this examination. When the proctor announces a “last call”for examination papers, he will leave the room in 5 minutes. The fact that the proctor doesnot have your examination in hand will not stop him.• I will not grade your examination if you fail to 1) put your name and GTID number in theupper left-hand blanks on this page or 2) sign the blank below acknowledging the terms of thistest and the honor code policy.• Have a nice day!Pledge Signature:I acknowledge the above terms for taking this examination. I have neither given nor received unau-thorized help on this test. I have followed the Georgia Tech honor code in preparing and submittingthe test.11. Short Answer Section (16 points)(a) (1) (2) (3)Name 3 different contributions to GPS location estimate errors that can be correctedusing a wide-area augmentation system.(b)The numerical factor that describes how well a spread spectrum receiver can remove noiseand interference from a received signal is called Answer .(c)When carrier frequency increases, dish diffraction losses tend to drop, but Answer lossesincrease.(d)A radio receiver can lower the amount of physical thermal noise taken in by the antennaby reducing Answer .(e)An Answer is a type of pseudo-noise signal that can be generated with a train of shiftregisters connected with feedback.(f)True or False: Geostationary antennas often transmit back to Earth with half-wave dipoleantennas.2. Dish Antennas (8 points): What are 2 drawbacks of using a dish antenna on a spacecraftas opposed to an electrically small antenna?23. Rain Fade (25 points): An earth station is trying to receive a signal from a Geostationarysatellite transmitting at 20 GHz with a look angle of 30-degrees of elevation in a K-typeclimatological region where average rain altitude is 2 km above the terrain level. Using oursimplified class model, estimate how much margin (worst case scenario in dB) must be designedinto the RF link budget to keep the system operating with 99.99% reliability.4. Doppler Shift (20 points): There is a satellite-to-satellite link in GPS used for synchro-nizing all of the spacecraft with the master atomic clock maintained by the Air Force. If thislink were to operate at precisely 4.000000000 GHz, estimate a rough upper-bound for themaximum possible Doppler shift in the carrier frequency. Recall that GPS satellites orbitat an altitude of 20,200 km.35. GPS (31 points): You are a cell site technician who one day notices that a particular 850MHz base station transmitter is experiencing slight non-linearities and transmitting a smallamount of power into the GPS 1575 MHz band (L1). A typical 3D GPS receiver in the vicin-ity of this cell tower receives -160 dBW of power from a single GPS satellite, resulting in atotal (C/(N + I))spread= −20 dB and 20m of positioning standard deviation error under idealconditions. Answer the following questions based on this scenario.(a) What will the location accuracy be if there is -134 dBW of additional in-band interferencefrom the cellular transmitter (15 points)?(b) For the scenario discussed in the previous question, would increasing the gain of the om-nidirectional GPS receiver antenna help? Why or why not? (6 points)(c) Estimate the averaging interval for this typical GPS receiver based on the informationprovided. Report your answer in terms of bits (bit fractions are OK in this answer). (10points)4Cheat Sheetσr= c TcsNsatTb¡CN¢despreadTintc = 3.0 × 108m/sGPS Stats: Rb= 50 bits/sec Rc= 1.023 Mchips/secA = γRLeff(dB) γR= kRα(dB/km) Leff=hrsin θELµCN¶despread= MµCN¶spreadµCN¶spread=CPN+ (Q − 1)C≈1Q − 1PN= kT B k = 1.3807 × 10−23J K−1Doppler Formula: f = fc−vRλcos θR+vTλcos θTV =rµRT2=4π2a3µµ = GMpG = 6.672 × 10−11Nm2/kg2ME= 5.974 ×


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GT ECE 6390 - ECE 6390 TEST 2

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