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? ? 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? ? 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 I 0 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? 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? 04/16/03 12.540 Lec 17 9 Polarization with dipole 90 o 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 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 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? 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? ? ? 04/16/03 12.540 Lec 17 13 Surface reflections E perpendicular to plane of incidence E r E i = ncosi - n' 2 -n 2 sin 2 i ncosi + n' 2 -n 2 sin 2 i E parallel to plane of incidence E r E i = n' 2 cosi - n n' 2 -n 2 sin 2 i n' 2 cosi + n n' 2 -n 2 sin 2 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 E r E i = 2n n'+n E r E i = n'-n n'+n Normal incidence reflection – Air – Reflected strength at least 30% of incident signal Perpendicular Parallel 7? 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. This characteristic of antenna and is called phase center model (needed when antenna types are mixed Multipath characteristics reflected signal determines the nature of multipath. ~1) multipath will be slowly varying >>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 of antenna can be measured (expect it to behave like sin 2 (q) but with response for q>90 (back- plane 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 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? 04/16/03 12.540 Lec 17 21 General results ? free observable (LC) ? Positions can change by 10-cm when phase center models used ? Phase residuals are systematic if wrong antenna type used, but RMS is often less than 10 mm compared to normal noise of ~5 mm ? ? Typical phase variations are quite different at L1 and L2 frequencies and the even larger in the ionospheric Where do we get the “zero phase center antenna”? The IGS has adopted the Dorne-Margolian Choke ring as standard. What are its phase cenetr variations? 04/16/03 12.540 Lec 17 22 Absolute calibration ? Hannover System: 11? Original AOAD/M_T Choke Ring antenna, and antenna on the robot. (Images courtesy of http://www.geopp.com/gnpcvdb/AOA_DM_T/.) 04/16/03 12.540 Lec 17 23 L1 Results 04/16/03 12.540 Lec 17 24 L2 Results 12? The PCV vary for L1 over a range of approximately 20 mm. The PCV vary for L2 over a range of approximately 14 mm. Image courtesy of http://www.geopp.com/gnpcvdb/AOA_DM_T/calibDMT.html. Image courtesy of http://www.geopp.com/gnpcvdb/AOA_DM_T/calibDMT.html. 04/16/03 12.540 Lec 17 25 Absolute calibrations ? The Hanover results are similar to anechoic chamber results although there are problems with this type of measurement: As the antenna is rotated, ground reflections have higher gain. ? Major problem at the moment: 10-cm height changes (14-ppb scale change) in global GPS when absolute models are used. ? Could be at satellite? Where are phase centers on satellites? 04/16/03 12.540 Lec 17 26 Satellite phase centers ? Satellites transmit from an array ? Figure at left gives some idea of size. ? For current GPS precisions, we need phase center to a few centimeters ? See NGS ANTCAL site ? Currently adopted positions of phase centers could be in error by definitely wrong by this amount) over 1 m. (Block IIR satellites are 13? 04/16/03 12.540 Lec 17 27 Use of Signal-to-noise ratio (SNR) Cosine Sine Direct signal Reflected signal As path length changes reflection rotated Received Signal Phasor diagram 04/16/03 12.540 Lec 17 28 SNR analysis o out-of-phase) sign). http://bowie.mit.edu/~tah/snrprog/ ? One method of characterizing multipath at a site is SNR analysis. ? Changing path length difference between direct and reflected signals causes oscillating signal amplitude and phase (90 ? Analysis of signal strength variations can allow prediction of phase errors (but ambiguous in ? Implemenation: 14? 04/16/03 12.540 Lec 17 29 Example HOLC California (LC) Theoretical from SNR Measured Phase residuals 04/16/03 12.540 Lec 17 30 scattering centimeters Summary ? Measurements at mm level require careful evaluation of multipath and near-antenna ? Phase center variations can be many ? Probably largest problem is vegetation near antennas since it changes with time and allows transmission of GPS signals. 15?