2.540 Principles of the Global Positioning System Lecture 17 Prof. Thomas Herring Summary Finish propagation medium with discussion of signal characteristics around gPs aI Basic operation of antenna Ray approximation to effects of multipath Phase center models for gPs ground antenna Phase center models for gPs satellite antennas Use of signal strength (SNR)to assess multipath
04/16/03 12.540 Lec 17 1 12.540 Principles of the Global Positioning System Lecture 17 Prof. Thomas Herring 04/16/03 12.540 Lec 17 2 – Basic operation of antenna – – Phase center models for GPS ground antennas – – Summary • Finish propagation medium with discussion of signal characteristics around GPS antennas Ray approximation to effects of multipath Phase center models for GPS satellite antennas Use of signal strength (SNR) to assess multipath 1
Basic antenna operation Receiving and transmitting antennas are identical: Time just flows in opposite directions Antenna problems are solved knowing the nt distribution J(x)in the antenna and using a vector potential JJJ(x)d'x' B=V×AE B Basic antenna theo Basic problem with using these equations is that the propagating EM field induces other currents to flow in the antenna that must be cluded in the integral Generally three distance ranges are treated with antennas for antenna size d <<h The near(static zone) d<<r<< The intermediate(induction zone d<<r-n The far radiation zone d<<<<r
† 04/16/03 12.540 Lec 17 3 Basic antenna operation directions. = 1 c J(x') e -x' x - x' d 3 x' B A E = i k B • Receiving and transmitting antennas are identical: Time just flows in opposite • Antenna problems are solved knowing the current distribution J(x’) in the antenna and using a vector potential A(x) ik x ÚÚÚ = — ¥ — ¥ 04/16/03 12.540 Lec 17 4 Basic Antenna theory l – The near (static zone) d<<r<<l – d<<r~l – The far radiation zone: d<<l<<r • Basic problem with using these equations is that the propagating EM field induces other currents to flow in the antenna that must be included in the integral. • Generally three distance ranges are treated with antennas for antenna size d << The intermediate (induction) zone 2
Simplest antenna Short center-fed dipole P is the radiated power ()e-=l(1-em from the ,如m0 antenna. with current Io center fed into antenna 04/1603 12540Lec17 Dipole antenna Notice that no power is transmitted in the direction of the antenna; maximum power is perpendicular to the antenna There is no p dependence to the power transmission The received strength follows the same pattern; Ne gain along the antenna, maximum gain perpendicular to it The first civilian gPs antennas were of this form. But how to mount the antenna? 04/1603 12540Lec17
† 04/16/03 12.540 Lec 17 5 Simplest antenna d/2 -d/2 q n f I (z )e - iwt = I 0 (1- 2 z d )e - iwt dP dW = I 0 32pc (kd ) 2 sin 2 q x y z radiated power from the antenna, with current I0 center fed into antenna • Short center-fed dipole P is the 04/16/03 12.540 Lec 17 6 Dipole antenna • Notice that no power is transmitted in the direction of the antenna; maximum power is perpendicular to the antenna • There is no f dependence to the power transmission. • The received strength follows the same pattern; No to it. • The first civilian GPS antennas were of this form. But how to mount the antenna? gain along the antenna, maximum gain perpendicular 3
Dipole antennas For GPS, you need to mount the dipole horizontally However, a simple dipole mounted this way will see reflections from the ground just as well as the direct signal from the satellite This is called multipath(multiple paths that the signal can travel to get to the antenna) How do you solve the ground reflection roblem Dipole over a ground plane To solve reflection from ground problem: You make your own, highly reflective ground If the ground plane is infinite, then antenna acts like a point source in the ground plane below the antenna Gain depends on h/ In zenith h=N4 give naximum gain
04/16/03 12.540 Lec 17 7 Dipole antennas horizontally problem? 04/16/03 12.540 Lec 17 8 Dipole over a ground plane h q Ground Plane Additional path 2h cosq If the ground plane is infinite, then antenna acts like a point source, in the ground plane below the antenna. Gain depends on h/l In zenith h=l maximum gain • For GPS, you need to mount the dipole • However, a simple dipole mounted this way will see reflections from the ground just as well as the direct signal from the satellite. • This is called multipath (multiple paths that the signal can travel to get to the antenna) • How do you solve the ground reflection • To solve reflection from ground problem: You make your own, highly reflective ground. /4 give 4
Polarization with dipole Since GPS signals are transmitted with right circular polarization, ideally an antenna should receive rcp radiation This can be done with dipoles by having two (horizontal) dipoles perpendicular to each other and adding the output with the correct 900 phase shift(sets rcP or LCP) Macrometer(early MIT GPS receiver) antenna worked this way. (Set height dipole was tricky to get L1 and L2 tracking) 12540Lec17 Other antenna styles Other styles of antenna commonly seen in GPS applications Helical antenna(wire around styrofoam coffee cup is good ). Early T14100 antenna was of this design Some hand-held receivers use this style( Garmin Microstrip patch antenna. very common now Patch mounted close to ground plane embedded in a dielectric Dorne-Margollian element 4-patchs mounted nside dome) embedded in choke rings. Standard tracking antenna 04/1603 12540Lec17
04/16/03 12.540 Lec 17 9 Polarization with dipole 90o 04/16/03 12.540 Lec 17 10 Other antenna styles – is good). Early T14100 antenna was of this design. GPS II/III) – Very common now. Patch mounted close to ground plane embedded in a dielectric. – inside dome) embedded in choke rings. Standard global GPS tracking antenna. • Since GPS signals are transmitted with rightcircular polarization, ideally an antenna should receive RCP radiation • This can be done with dipoles by having two (horizontal) dipoles perpendicular to each other and adding the output with the correct phase shift (sets RCP or LCP) • Macrometer (early MIT GPS receiver) antenna worked this way. (Set height dipole was tricky to get L1 and L2 tracking). • Other styles of antenna commonly seen in GPS applications: Helical antenna (wire around styrofoam coffee cup Some hand-held receivers use this style (Garmin Microstrip patch antenna. Dorne-Margollian element (4-patchs mounted 5