GPS Placement

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The purpose of this study is to evaluate the reception of the GPS signal of a Telematics Electronic Control Unit (ECU) in two different cabin geometries. In the first cabin, the ECU is installed in the center of the dashboard. In the second cabin, the ECU is installed in two different positions: 1-Near the center of the dashboard and 2- On the right side of the dashboard.

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GPS Placement

  1. 1. © 2011 ANSYS, Inc. June 5, 20131GPS PlacementJuliano Mologni, Ph.D.ESSS, Sao Paulo, BrasilOctober 3, 2011
  2. 2. © 2011 ANSYS, Inc. June 5, 20132Purpose of the StudyThe purpose of this study is to evaluate the reception of the GPS signal of a Telematics Electronic Control Unit(ECU) in two different cabin geometries. In the first cabin, the ECU is installed in the center of the dashboard. Inthe second cabin, the ECU is installed in two different positions: 1-Near the center of the dashboard and 2- On theright side of the dashboard.The results are expected to show in details the GPS sensitivity of the ECU at the different positions and cabinsdetailed above. Electromagnetic field plots will be used to better understand the phenomena.
  3. 3. © 2011 ANSYS, Inc. June 5, 20133Telematics GPS ECUIn order to run an accurate electromagnetic analysis a reliable model of the telematics electronic control unit(ECU) is required. The basic 3D model was imported and the GPS antenna was inserted into the model as well asthe ground plane on the PCB.REAL PICTURE
  4. 4. © 2011 ANSYS, Inc. June 5, 20134Telematics GPS ECUThis study investigates only the reception of the GPS signal when the ECU is installed inside the truck cabin. TheECU also comprises a GSM/GPRS/EDGE antenna which most likely operates at 850, 900, 1800 and 1900MHz thatare not considered in this simulation. PCB traces and electrical components are not modeled since the groundplane is the only geometric structure that will affect the performance of the GPS antenna. It is very importantto make sure that the electromagnetic behavior of the 3D ECU model is similar to a real GPS antenna. The tablebelow shows the electrical specification of a typical active GPS antenna used on telemetry modules (GPS antennabrands may vary but electrical specifications are very similar).
  5. 5. © 2011 ANSYS, Inc. June 5, 20135Telematics GPS ECUS-Parameter matrix details the information on the frequency of operation of the GPS patch antenna. Thesimulated S11, also known as the reflection coefficient, shows that the antenna model has an operating bandwidthof 520MHz (range where S11 curve is below -10dB). Hence, the simulated antenna demonstrates a similar S11performance when compared to the electrical specifications of a real antenna.
  6. 6. © 2011 ANSYS, Inc. June 5, 20136Telematics GPS ECUThe antenna impedance calculated in HFSS is also close to 50 ohms at 1.575GHz. This is an important featurebecause the electromagnetic waves used as excitations in the HFSS model must be radiated into free space andnot reflected back to the source.
  7. 7. © 2011 ANSYS, Inc. June 5, 20137Telematics GPS ECUVoltage Standing Wave Ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode(maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, theVSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimumstanding wave value.Accordding to HFSS results the VSWR of the GPS antenna complies with typical electrical specifications (<2).
  8. 8. © 2011 ANSYS, Inc. June 5, 20138Telematics GPS ECUAntenna gain is the gain in the directivity over an antenna radiating in all directions (omni directional or isotropicantenna), For a RHCP antenna the unit of measurement is dBc.The directive gain of a circularly polarized antenna, expressed as the ratio, in decibels, of the antennas directivityto that of an isotropic antenna with the same polarization characteristic. Derived from decibels over isotropic.Plot below shows the antenna RHCP gain which also provides similar characteristics of a typical antenna whenplaced on a 7cm x 7cm ground plane.
  9. 9. © 2011 ANSYS, Inc. June 5, 20139Telematics GPS ECUAntenna gain is the gain in the directivity over an antenna radiating in all directions (omni directional or isotropicantenna), For a RHCP antenna the unit of measurement is dBc.The directive gain of a circularly polarized antenna, expressed as the ratio, in decibels, of the antennas directivityto that of an isotropic antenna with the same polarization characteristic. Derived from decibels over isotropic.Plot below shows the antenna RHCP gain which also provides similar characteristics of a typical antenna whenplaced on a 7cm x 7cm ground plane.
  10. 10. © 2011 ANSYS, Inc. June 5, 201310Telematics GPS ECUThe phase animation on the left shows theelectric field distribution on the surface ofthe components (top plot) and on a crosssection of air space (bottom left) consideringan antenna excitation of 2V at 1.575GHz. TheRHCP Gain radiation pattern for the completeECU is shown on the plot below.
  11. 11. © 2011 ANSYS, Inc. June 5, 201311Results of the Study on Cabin #1TRANSIENT ANALYSISThe first electromagnetic analysis is a time domain simulation called transient. In this simulation we areattempting to reproduce an incoming GPS signal from the space. It was detailed before that an electromagneticwave can be received with different angle of arraival (AOA) depending on the positions of the satellites in theGPS constellation and the geographical position of the receiver on earth. In this first simulation an incident waverepresented by a 1V pulse with a center frequency 1.575GHz is inciding normally toward the cabin roof.
  12. 12. © 2011 ANSYS, Inc. June 5, 201312Results of the Study on Cabin #1TRANSIENT ANALYSISPlot below shows the plane wave animation travelling towards the cabin roof. All the reflections of theelectromagnetic wave can be seen in time domain. The electromagnetic field can be quantified for any given timeduring the simulation. This anaysis is a valuable tool to study the influence of the geometry in multi path (whenthe signal reaches the receiver through multi path due to reflections on secondary objects).
  13. 13. © 2011 ANSYS, Inc. June 5, 201313Results of the Study on Cabin #1TRANSIENT ANALYSISIt is possible to compare the electric field received by the ECUwith and without the cabin. Blue curve shows that the signalstrength (magnitude) is higher when the ECU is placed alonein free space. Red curve shows the two peaks with reducedelectric field magnitude. The second peak is delayed by 3ns andis caused by reflections (multipath) on metallic structures and itmight cause errors on the GPS engine.
  14. 14. © 2011 ANSYS, Inc. June 5, 201314Results of the Study on Cabin #1ANTENNA COUPLINGThe second simulations consists on the evaluation of the electromagnetic coupling between the ECU placed in itsoriginal position and a second ECU (a second antenna with same radiation characteristics) positioned 15cm abovethe roof of the cabin. This second position was chosed based on the intuitive assumption that the best position toreceive GPS signals is on the top of the cabin roof. By calculating the coupling between these two antennas we cancreate a comparative study (if there is more than one position for the ECU inside the cabin) estimating how is thebehaviour of the original ECU in the case of receiving signals at 1.575GHz from the ECU positioned on the roof top.
  15. 15. © 2011 ANSYS, Inc. June 5, 201315Results of the Study on Cabin #1ANTENNA COUPLINGPlot shows E-field distribution (logscale) on cross sections and on the surface of the cabin when only the ECU on thetop of the cabin is transmitting. Electric field plots can change due to the excitation of the antennas, nevertheless, thecoupling is calculated on frequency domain and for 1.575GHz it will always be -90.98dB for this arrange.
  16. 16. © 2011 ANSYS, Inc. June 5, 201316Results of the Study on Cabin #1ANTENNA COUPLINGPlot shows E-field distribution (logscale) on cross sections and on the surface of the cabin when only the ECU on thetop of the cabin is transmitting. Electric field plots can change due to the excitation of the antennas, nevertheless, thecoupling is calculated on frequency domain and for 1.575GHz it will always be -90.98dB for this arrange.
  17. 17. © 2011 ANSYS, Inc. June 5, 201317Results of the Study on Cabin #1ANTENNA COUPLINGPlot shows E-field distribution (logscale) on cross sections and on the surface of the cabin when only the ECU on thetop of the cabin is transmitting. Electric field plots can change due to the excitation of the antennas, nevertheless, thecoupling is calculated on frequency domain and for 1.575GHz it will always be -90.98dB for this arrange.E-FIELD ON A CROSS SECTION 1CM ABOVE THE TOP ECUSome of the E-field enters the cabin through the gapbetween the whole cabin roof and the solar roof.
  18. 18. © 2011 ANSYS, Inc. June 5, 201318Results of the Study on Cabin #1RADIATION EFFICIENCYRadiation Efficiency is calculated for the ECU alone in the space and when the cabin #1 is added to the model. Plotbelow shows the electric field distribution when the GPS antenna radiates at 1.575GHz (3V Sinusoidal).
  19. 19. © 2011 ANSYS, Inc. June 5, 201319Results of the Study on Cabin #1RADIATION EFFICIENCYRadiation Efficiency is calculated for the ECU alone in the space and when the cabin #1 is added to the model. Plotbelow shows the electric field distribution when the GPS antenna radiates at 1.575GHz (3V Sinusoidal).
  20. 20. © 2011 ANSYS, Inc. June 5, 201320Results of the Study on Cabin #1RADIATION EFFICIENCYRadiation Efficiency of the ECU (GPS antenna) in cabin #1 is 65.72%. The efficiency of only the ECU in free space is not100% due to the fre space losses and the GPS antenna and the parts of the ECU module (e.g. FR-4 substrate).
  21. 21. © 2011 ANSYS, Inc. June 5, 201321Results of the Study on Cabin #1RADIATION EFFICIENCYAll the solids interacts with the electromagnetic waves and every metallic detail (conductive solid) is considered. Theplot below shows the electric field at the brackets near the ECU. The reflections of the wave can be observed.
  22. 22. © 2011 ANSYS, Inc. June 5, 201322Results of the Study on Cabin #1RADIATION EFFICIENCYBesides the radiation efficiency, we can calculate the RHCP Gain of the antenna at any distance from the ECU. Plotbelow shows the near radiated RHCP E-Field at a sphere positioned at different distances from the GPS antenna.
  23. 23. © 2011 ANSYS, Inc. June 5, 201323Results of the Study on Cabin #1RADIATION EFFICIENCYThe RHCP gain is calculated for the far field for GPS reception. Plot below showsthe near ERHCP patterns for a sphere with different radius and the plots on thetop right shows the Far Field RHCP Gain pattern of the ECU placed in cabin #1 incomparison with the ECU alone. By integrating the gain in the range of 0<Ɵ<360and -90<φ<90 we can estimate an average RHCP gain . The graphic on the bottomright shows the polar plot of the Far Field RHCP gain with the structure. It ispossible to observe that the gain pattern is stronger towards the front of thecabin. No gain is found on the bottom because the floor was modeled as concret.
  24. 24. © 2011 ANSYS, Inc. June 5, 201324Results of the Study on Cabin #2POSITION OF THE ECUsThis simulation consists on the evaluation of the electromagnetic coupling between the ECUs placed insidethe cabin (Position 1 and Position 2) and a second ECU (a second antenna with same radiation characteristics)positioned 15cm above the roof of the cabin. This second position was chosed based on the intuitiveassumption that the best position to receive GPS signals is on the top of the cabin roof. By calculating thecoupling between the antennas inside the cabin and the Tx antenna positioned over the roof we can createa comparative study estimating how is the behaviour of the original ECU in the case of receiving signals at1.575GHz from the ECU positioned on the roof.
  25. 25. © 2011 ANSYS, Inc. June 5, 201325Results of the Study on Cabin #2ANTENNA COUPLINGThis simulation consists on the evaluation of the electromagnetic coupling between the ECUs placed insidethe cabin (Position 1 and Position 2) and a second ECU (a second antenna with same radiation characteristics)positioned 15cm above the roof of the cabin. This second position was chosed based on the intuitiveassumption that the best position to receive GPS signals is on the top of the cabin roof. By calculating thecoupling between the antennas inside the cabin and the Tx antenna positioned over the roof we can createa comparative study estimating how is the behaviour of the original ECU in the case of receiving signals at1.575GHz from the ECU positioned on the roof.
  26. 26. © 2011 ANSYS, Inc. June 5, 201326Results of the Study on Cabin #2ANTENNA COUPLINGThis simulation consists on the evaluation of the electromagnetic coupling between the ECUs placed insidethe cabin (Position 1 and Position 2) and a second ECU (a second antenna with same radiation characteristics)positioned 15cm above the roof of the cabin. This second position was chosed based on the intuitiveassumption that the best position to receive GPS signals is on the top of the cabin roof. By calculating thecoupling between the antennas inside the cabin and the Tx antenna positioned over the roof we can createa comparative study estimating how is the behaviour of the original ECU in the case of receiving signals at1.575GHz from the ECU positioned on the roof.
  27. 27. © 2011 ANSYS, Inc. June 5, 201327Results of the Study on Cabin #2ANTENNA COUPLINGPlot shows E-field distribution (logscale) on cross sections and on the surface of the cabin when only the ECUon the top of the cabin is transmitting. Electric field plots can change due to the excitation of the antennas. Theback part of the model was modeled as a metallic conductor and the floor was modeled as a concrete ground.
  28. 28. © 2011 ANSYS, Inc. June 5, 201328Results of the Study on Cabin #2RADIATION EFFICIENCYRadiation Efficiency is calculated for two positions of the ECU inside cabin #2. Plot below shows the electricfield distribution when the GPS antenna radiates at 1.575GHz (3V Sinusoidal).POSITION 1 POSITION 2
  29. 29. © 2011 ANSYS, Inc. June 5, 201329Results of the Study on Cabin #2RADIATION EFFICIENCYRadiation Efficiency is calculated for two positions of the ECU inside cabin #2. Plot below shows the electricfield distribution when the GPS antenna radiates at 1.575GHz (3V Sinusoidal).POSITION 1 POSITION 2
  30. 30. © 2011 ANSYS, Inc. June 5, 201330Results of the Study on Cabin #2RADIATION EFFICIENCYRadiation Efficiency is calculated for two positions of the ECU inside cabin #2. Plot below shows the electricfield distribution when the GPS antenna radiates at 1.575GHz (3V Sinusoidal).POSITION 1 POSITION 2
  31. 31. © 2011 ANSYS, Inc. June 5, 201331Results of the Study on Cabin #2RADIATION EFFICIENCYRadiation Efficiency is calculated for two positions of the ECU inside cabin #2. Plot below shows the electricfield distribution when the GPS antenna radiates at 1.575GHz (3V Sinusoidal).POSITION 1 POSITION 2
  32. 32. © 2011 ANSYS, Inc. June 5, 201332Results of the Study on Cabin #2RHCP GAIN PATTERNThe RHCP gain of the antennas can be integrated on all angles so we can have an average of the RHCP gainpattern. Distortions on the far field pattern when compared to the ECU in free space are displayed on the plotsbelowPOSITION 1 POSITION 2
  33. 33. © 2011 ANSYS, Inc. June 5, 201333Results of the Study on Cabin #2RHCP GAIN PATTERNThe RHCP gain of the antennas can be integrated on all angles so we can have an average of the RHCP gainpattern. Distortions on the far field pattern when compared to the ECU in free space are displayed on the plotsbelow
  34. 34. © 2011 ANSYS, Inc. June 5, 201334Summary of ResultsFrom the parameters presented below, the Normalized Radiation Efficiency is usually used to quantify the GPSsensitivity. It shows that when the ECU is placed in the specified position inside cabin #1, the efficiency of theGPS antenna decreases to 71.76% (or reduces 28.24% in comparison with the ECU alone). For cabin #2, position1 shows a better performance compared to position #2.Only ECU Cabin #1Cabin #2Position 1Cabin #2Position 2Peak Directivity 19.1815 7.40758 10.5449 8.06356Peak Gain [dBi] 17.5678 4.86819 7.52303 5.19307Radiated Power [W] 0.00589929 0.00390389 0.00423796 0.00382564Radiation Efficiency 0.915871 0.6572 0.713427 0.644017Normalized Radiation Efficiency 100.00% 71.76% 77.9% 70.32%RHCP Gain Average [dBc] 1 -491.48 -1152.91 -909.30 -1175.622 Antenna Coupling [dB] -69.09 -90.98 -93.31 -96.25Transient Average2 0.7921 0.6476 1.7708 0.79721 RHCP gain average was calculated by integrating the RHCP gain in a far field surface of 0<φ<360 -90<θ<902 Transient average is a time integration of the electric field received by the GPS antenna of the ECU. Cabin #2 shows higher values due to the high number of reflectionsin the case of position 1, which retains the electromagnetic field, and the proximity of the ECU to the metallic sheets in the case of position #2.NOTE: The studies were carried out considering only the geometry and the material properties of the cabin. In cabin #2,the ECU when placed in position 1 is very close to the electrical center, which works with fuses and relays at very highcurrents. It is also close to the radio which works with very high speed digital signals. The proximity of the ECU to thesedevices can decrease the GPS performance due to electromagnetic interference (EMI). Further studies can be madeincluding additional ECUs to study EMI between them.

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