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
GPS Antennas(for precise positioning Nearly all antennas are patch antennas (conducting patch mounted in insulating ceramic) Rings are called oke-rings (used to suppress multi-path) 04/1603 12540Lec17 Simple Multipath A simple approach to treating multipath is with ray-optics. Approach should be valid for reflectors that greater than one wavelength from the antenna It is important to note that all real antennas have gain below the horizon(ie, zero elevation angle) and will therefore see reflections from the ground 04/1603 12540Lec17
04/16/03 12.540 Lec 17 11 GPS Antennas (for precise positioning) called choke-rings (used to suppress multi-path) Nearly all antennas are patch antennas 04/16/03 12.540 Lec 17 12 ray-optics. • Rings are (conducting patch mounted in insulating ceramic). Simple Multipath • A simple approach to treating multipath is with Approach should be valid for reflectors that greater than one wavelength from the antenna. • It is important to note that all real antennas have gain below the horizon (ie., zero elevation angle) and will therefore see reflections from the ground. 6
Surface reflections The amplitude of a reflected signal from a surface depends on incidence angle and refractive index of medium e perpendicular to plane of inciden Where n'is E refractive n cos L n sin I index of E parallel to plane of incidence reflecting E n,2-n sin i nedium E n'cos i+ Normal incidence reflection For normal incidence: the two cases reduce to Pe d E. n'tn E n r E.n'+ Reflection strength will depend on dielectric constants Air E=1; water 80; Dry Sand 3-5; saturated sand 20-30; shale 5-15; silt/clay 5-40; Granite 4-6; Ice 3-4 Reflected strength at least 30% of incident signal 04/1603 12540Lec17
† † 04/16/03 12.540 Lec 17 13 Surface reflections E perpendicular to plane of incidence Er Ei = n cosi - n' 2 -n 2 sin2 i ncosi + n' 2 -n 2 sin2 i E parallel to plane of incidence Er Ei = n' 2 cosi - n n' 2 -n 2 sin2 i n' 2 cosi + n n' 2 -n 2 sin2 i n = me refractive index of reflecting medium (m’=m) • The amplitude of a reflected signal from a surface depends on incidence angle and refractive index of medium Where n’ is 04/16/03 12.540 Lec 17 14 • For normal incidence: the two cases reduce to • Reflection strength will depend on dielectric constants: e=1; water 80; Dry Sand 3-5; saturated sand 20-30; shale 5-15; silt/clay 5-40; Granite 4-6; Ice 3-4 Er Ei = 2n n'+n Er Ei = n'-n n'+n Normal incidence reflection – Air – Reflected strength at least 30% of incident signal Perpendicular Parallel 7
Multipath characteristics The path length difference between the direct and reflected signal determines the nature of multipath When the reflector is close(dn1)multipath will be slowly varying When reflector is distant d/>>>1)multipath will vary rapidly and average to zero quickly A class of multipath is what happens when d/x<< This characteristic of antenna and is called phase center model (needed when antenna types are mixed in high-precision applications Receiving Antenna phase center models The specific characteristics of an antenna need to be calibrated either with Anechoic chamber measurements(absolute calibration) In-situ relative measurements(one-antenna relative to another) In-situ absolute calibration by antenna rotation In-situ multipath calibration using a directional antenna 04/1603 12540Lec17
04/16/03 12.540 Lec 17 15 • The path length difference between the direct and • When the reflector is close (d/l • When reflector is distant (d/ l rapidly and average to zero quickly. • l>1) multipath will vary A class of multipath is what happens when d/ in high-precision applications). 04/16/03 12.540 Lec 17 16 Receiving Antenna Phase center models – Anechoic chamber measurements (absolute calibration) – In-situ relative measurements (one-antenna relative to another) – – antenna • The specific characteristics of an antenna need to be calibrated either with: In-situ absolute calibration by antenna rotation In-situ multipath calibration using a directional 8
Phase center models First phase center models were made using data from a chamber in which l1 and l2 signals were transmitted and antenna rotated to measure phase difference between transmitted and received signal Signal strength also measured so that gain ot antenna can be measured (expect it to behave like sin (0)but with response for 0>90(back- plane gain) 04/1603 12540Lec17 Relative phase center models If an antenna with o phase center variation is available, then phase center of another antenna can be found by making differential measurements between antenna on monuments with known locations National Geodetic Survey(NGs) has largest setuphttp://www.ngs.noaagov:80/antcal/ 04/1603 12540Lec17
Phase center models • First phase center models were made using data from a chamber in which L1 and L2 signals were transmitted and antenna rotated to measure phase difference between transmitted and received signal. • Signal strength also measured so that gain of antenna can be measured (expect it to behave like sin2(q) but with response for q>90 (backplane gain). 04/16/03 12.540 Lec 17 17 Relative phase center models • If an antenna with 0 phase center variation is available, then phase center of another antenna can be found by making differential measurements between antenna on monuments with known locations. • National Geodetic Survey (NGS) has largest setup: http://www.ngs.noaa.gov:80/ANTCAL/ 04/16/03 12.540 Lec 17 18 9
NGS Calibration set-up Reference Antea Rb ose 04/1603 12540Lec17 Typical calibration results Two types of information given Phase center Position"relative to physical point on antenna(ARP--normally base of pre-amplifier) Elevation angle dependent deviations of phase 13"Micro Centered with Ground Plane NGS( 4)01/1012 15-:s1:2-4-2.7- 6=3.94.0 04/1603 12540Lec17
04/16/03 12.540 Lec 17 19 NGS Calibration set-up 04/16/03 12.540 Lec 17 20 Typical Calibration results • • TRM 36569.00+GP NGS ( 4) 01/10/12 .0 -.3 63.2 .0 .7 1.4 2.3 3.1 3.9 4.6 5.2 5.6 5.9 5.9 5.6 -.9 -.8 44.6 .0 -.9 -1.5 -1.9 -2.2 -2.4 -2.7 -3.0 -3.3 -3.6 -3.9 -4.0 -3.8 .. 4 MEASUREMENTS .3 1.3 .1 .0 .1 .1 .1 .1 .2 .2 .2 .2 .1 .1 .1 .4 .6 .3 .0 .3 .5 .6 .7 .6 .6 .5 .5 .5 .5 .5 • Two types of information given: “Phase center Position” relative to physical point on antenna (ARP--normally base of pre-amplifier) Elevation angle dependent deviations of phase: 13" Micro Centered with Ground Plane 5.0 .. RMS mm (1 sigma) .1 .. .5 .. 10
