GT ECE 6390 - Satellite Communications and Navigation Systems Test

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Name:GTID:ECE 6390: Satellite Communications and Navigation SystemsTEST 1 (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 (8 points)(a) (1) (2)A drum-shaped Answer 1 satellite uses gyroscopic forces of its own rotation to controlattitude, while a box-shaped satellite typically uses internal Answer 2 .(b)The name of the first US-made satellite was Answer .(c)One drawback of using Answer in space is that they typically require small, radioactivewarming devices to keep active.2. Radar Detector: A police radar gun operates at a carrier frequency of 10.525 GHz. Ifevery commercial device manufactured in this application space uses a common 9.825 GHzlocal oscillator for first-stage mixing, what will the image frequency be for a radar detectordetector detector detector? (6 points)3. Antenna Gains: Explain the difference concisely between gain and directivity. (6 points)24. Look Angles: We learned that one way to achieve goo d overhead satellite coverage of ahigh-latitude Earth station was to incline the orbit, maximize the eccentricity, and make theperio d 1 sidereal day. Presuming the satellite can approach no closer than an altitude of 1000km to minimize atmospheric drag, calculate the maximum eccentricity that could be used inthis type of orbit. (20 points)5. Transponder RF Chain: In the space below, sketch the labeled RF block diagram fora simple bent-pipe transponder that converts an uplink signal with a 15 GHz carrier to adownlink signal that uses a 8 GHz carrier. (20 points)36. Link Budget for MEO Satellites: A satellite with 500 Watt transmit power uses anelectrically small antenna to broadcast to a mobile user on the surface of the earth. Themobile receiver also uses an electrically small antenna and requires -100 dBm received p owerfor proper data reception. If the gains of the transmit/receive antennas are both 0 dBi (andindependent of frequency) and the satellite is 10,000 km from the Earth’s surface, what is thehighest frequency that can be used in this link? (20 points)7. Antenna Coverage: A satellite in circular orbit is using an electrically small dipole withgain pattern G(θ) = 1.5 sin2θ (linear scale). If all points on the Earth visible to the satellitemust fall within the half-power beamwidth of this antenna, what is the minimum orbital radiusof the satellite? (20 points)4Cheat Sheetλf = c c = 3 × 108m/s ²o= 8.85 × 10−12F/m µo= 4π × 10−7H/m k =2πλ¨r = r˙θ2−GMPr2¨θ = −2 ˙r˙θrT2=4π2a3µµ = GMpG = 6.672 × 10−11Nm2/kg2ME= 5.974 × 1024kgb = ap1 − e2perigee = (1 − e)a apogee = (1 + e)aCircular Orbit: V =rµREarth Radius: Re= 6378 km Sidereal Day: Ts= 86, 164 sLogarithmic Link Budget: PR= PT+ GT+ GR− 20 log10µ4πλ¶− 20


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GT ECE 6390 - Satellite Communications and Navigation Systems Test

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