Large Impedance Ground Plane Reference Station Antenna: Basics of Design and Field Test Results

Transcription

1 Large Impedance Ground Plane Reference Station Antenna: Basics of Design and Field Test Results Prof. Dr. Dmitry Tatarnikov Antenna Design Chief, Topcon Positioning Systems, Moscow Munich, April 2014

2 Specular reflections θ Antenna θ Ground surface

3 Fresnel zones θ θ 2 m 25 m (for 10 degrees elevation)

4 Antenna pattern Ideal flat up to horizon with sharp cut- off then antenna pattern, db 0 Current F - 12 /F +12 = - 5-7dB regardless to antenna type F anti zenith horizon zenith (nadir) ( F -12 F nadir

5 Antenna Down-up ratio θ θ DU ( θ ) = F( θ ) F( θ ) Choke Ring and Rover antennas θ, degrees DU, db typical rover Choke Ring -40

6 Carrier phase multipath error (specular reflections) refl mult DU ( θ)sin( Δψ ) ψ = arctan 1 ( )cos( refl + DU θ Δψ ) refl Δ ψ = 2khsinθ + π degrees Θ,degrees typical rover Choke Ring

7 Cut-off type of antenna pattern Ideal flat up to horizon with sharp cut- off then antenna pattern, db 0 Current F - 12 /F +12 = - 5-7dB regardless to antenna type F anti zenith horizon zenith (nadir) ( F -12 F nadir Desired F - 12 /F +12 = dB

8 GNSS antenna with cut-off pattern Antenna size in the wavelength scale is to be substantial Vertical array antenna, ~2.5meters height, Lopez, A.R., GPS Landing System Reference Antenna, IEEE Antennas and Propaga0on Magazine, Vol. 52, No. 1, pp , What could be achieved with a regular ground plane?

9 Diffraction over the ground plane edges - to provide the fastest decay possible versus distance from the source; - to increase the ground plane size

10 2-dimensional model x Flat metal ground plane: A ~ 1 x 1 Impedance surface: A ~ 3 2 x

11 Impedance surfaces Choke grooves >0.25 wavelength ~ 60mm Impedance surface

12 Impedance surfaces Mushrooms >0.1 wavelength ~ 20mm narrowband

13 Impedance surfaces Straight pins >0.25 wavelength ~ 60mm Most broadband; easy to make PN- A5 antenna

14 Omnidirectional source atop an impedance ground plane θ x Current common Choke Ring: - 18dB ~- 5-8 db extra would make performance inacceptable L L,wavelengths inf θ

15 Raised directional source Antenna element with dB down/up h L z θ x

16 Antenna gain towards low suppression for low F +12 DU 12 F +12 DU 12 L=7λ db L=30λ db h, wavelengths h, wavelengths

17 -antenna element is to be raised by 7-8 cm (at GNSS frequencies) against the impedance surface -antenna element is to have dB gain in nadir versus zenith most of commercially available GNSS antennas

18 Scaled model for 5.7 GHz operation Patch antenna with flat metal ground plane Radiation pattern, measured 13.5λ (71cm) 20dB

19 GNSS receiving antenna Commercially available antenna for high precision 13.5λ (3 m)

20

21 SNR, db*hz Novel antenna with Spherical antenna element (MGA8) JPL- spec Choke Ring Novel antenna with common Choke RIng as antenna element

22 Time, hours +/- 5mm CR4 (JPL spec Choke Ring) Vertical coordinate, real time Time, hours +/- 2mm Novel antenna, Spherical antenna element Time, hours Time, hours +/- 2mm Novel antenna, Choke Ring antenna element +/- 1mm Zero baseline 16 hours Sampling rate: 5 sec.

23 +/- 5mm CR4 (JPL spec Choke Ring) Time, hours Time, hours Time, hours +/- 2mm +/- 2mm +/- 1mm Novel antenna, Spherical antenna element Novel antenna, Choke Ring as antenna element Zero baseline Time, hours 2 hours

24 0.5m

25 Height offset 0 cm Height offset 50cm Time, hours Time, hours Time, hours Time, hours Zero baseline mm Time, hours Sampling rate: 1 sec.

26 Height offset 0 cm Dual Difference Residuals, cycles L1 GPS G8- G9 Height offset 50cm Time, min Time, min Time, min Zero Baseline Time, min Time, min

27 Height offset 0 cm Dual Difference Residuals, cycles L2 GPS G8- G9 Height offset 50cm Time, min Time, min Time, min Zero Baseline Time, min Time, min

28 Melbourne-Wubenna linear combination (courtesy Dr. Andrey Veitsel, TTC Moscow) ONSALA IGS BigAnt, Concordia, Italy

29 Absolute Antenna Phase Center (PCV) for BigAnt- 1 & D/M_T Choke Ring (Courtesy Dr. Gerald Mader, NGS USA) L1 PCO L2 N E U N E U AOAD/M_T BigAnt The absolute calibradon of the choke ring antenna is used to convert the reladve calibradon of BigAnt into an absolute calibradon. The PCVs & PCOs for the standard choke ring and BigAnt (standard choke ring + high impedance ground plane) are shown here.

30 Antenna Summary ; G.Mader NGS USA Reference Antenna 45 m Test Antenna Get L1 & L2 Phase Center Offsets (PCO) of test antenna with respect to reference From single differences solve for clock & Phase Center Variadon (PCV) Use absolute calibradon for reference to convert test antenna to absolute Since: geometry removed no troposphere no ionosphere no reladve clock Residuals show only remaining effect PCV Problems: limited sky coverage poor coverage at zenith no azimuth soludon muldpath

31 Conclusion -GNSS receiving antenna with large impedance ground plane has been built; -antenna has a cut-off pattern in elevation plane with down/up ratio of 20dB at 12 degrees elevation and 45dB at nadir; -multipath error to positioning in real time falls below system noise level and is estimated to be +/-2mm in vertical coordinate -certain improvement in data quality compared to traditional Choke Ring antenna is achieved