GPS ANTENNA CALIBRATION AT THE NATIONAL GEODETIC SURVEY Dr. Gerald L. Mader Geosciences Research Division National Geodetic Survey, NOS/NOAA Silver Spring, MD

What is GPS Antenna Calibration? Determination of GPS antenna characteristics that are required for accurate geodetic positions. –What is the location of the point being positioned? –What does the antenna contribute to the observed phase measurements?

090 -π-π +π+π Ideal Point Antenna Connected to Phase Meter Source moves over spherical surface elv R = n.f λ 0.f λ

090 -π-π +π+π Typical Real Antenna Antenna Reference Point (ARP) Phase Center elv R = n.f λ 0.f λ 0°0° 90°

090 -π-π +π+π Typical Real Antenna ARP Phase Center 2 elv R = n.f λ 0.f λ

Our Hypothetical Experiment Shows That: A phase center may be estimated An offset and phase center variation (PCV) must be applied to yield correct distance to source Different phase centers, when corrected for offset and PCV give identical results There is no phase center for which PCV is zero – but can be minimized

Antenna Phase Center Variations (PCV) All real GPS antennas have a phase variation pattern These phase variations depend primarily on elevation Phase variations also depend on the physical components of the antenna and its environment Combine with satellite phase and are included in the observed phase

What’s The Effect of PCV? Applies an elevation dependent phase variation to the observed phase data –Makes height sensitive to elevation cutoff –Looks like troposphere variation - leads to incorrect tropo scale factors and heights –Especially pronounced for mixed-antenna baselines - 10cm height errors are not unusual –Mixed-antennas and longer baselines demand good antenna calibrations

How Can We Do Our Experiment In Practice? Could use Anechoic Chamber –Expensive –Scheduling Use GPS constellation, but … –Orbits not spherical on antenna –Satellite/receiver clocks mask antenna effects –Propagation variation masks antenna effects

Use Differential GPS Short baseline Standard reference antenna Relative calibrations Orbits errors cancel out Propagation cancels out Satellite clocks cancel out Receiver clocks solved for What’s left over is antenna contribution

5 m Reference AntennaTest Antenna GPS RCVR Clock Calibration Technique Δ = c 0 + ( b 1 θ + b 2 θ 2 + b 3 θ 3 + b 4 θ 4 ) - ( a 1 θ + a 2 θ 2 + a 3 θ 3 + a 4 θ 4 ) Δ = c 0 + ( b 1 - a 1 )θ + ( b 2 - a 2 ) θ 2 + ( b 3 - a 3 ) θ 3 + ( b 4 - a 4 ) θ 4 )

5 m 1.8 m North (Reference) Pier South (Test) Pier NGS Antenna Calibration Range, Corbin VA

5 m Reference AntennaTest Antenna GPS RCVR Clock Calibration Technique ANTCAL

Δ = b 0 + b 1 θ + b 2 θ 2 + b 3 θ 3 + b 4 θ 4

Sample from ANT_INFO.003 ASH A1 SNOW Geodetic III 'Whopper' (USCG) +radome NGS ( 7) 97/10/ AOAD/M_T Dorne Margolin T, chokerings (TurboRogue)NGS ( 0) 97/10/

Earth curvature causes different antennas to see the same satellite at different elevations. Absolute PCV of reference antennas cannot be assumed to be flat.

Reference AntennaTest Antenna Absolute Calibration θtθt θrθr Δ = a 1 ( θ t - θ r ) + a 2 ( θ t 2 - θ r 2 ) + a 3 ( θ t 3 - θ r 3 ) + a 4 ( θ t 4 - θ r 4 )

Reference AntennaTest Antenna Absolute Calibration θtθt θrθr Δ = c 0 + ( b 1 θ t + b 2 θ t 2 + b 3 θ t 3 + b 4 θ t 4 ) - ( a 1 θ r + a 2 θ r 2 + a 3 θ r 3 + a 4 θ r 4 )

Automated Absolute Field Calibration of GPS Antennas in Real-Time. Wübbena, G., Schmitz, M., Menge, F., Böder, V., Seeber, G., (2000). Proceedings of the 13 th International Technical Meeting of the Satellite Division of the Institute of Navigation, Salt Lake City, Utah. September , 2000.

Calibrating the L1 and L2 Phase Centers of a Block IIA Antenna

BLK II A Reference Surface L m L m LC m The locations of the L1/L2/LC phase centers above the reference surface for the BLK IIA Previously assumed L1, L2 LC phase centers

1.2 G ANTEX VERSION / SYST A PCV TYPE / REFANT END OF HEADER START OF ANTENNA LEIAT504GG TYPE / SERIAL NO ROBOT Geo++ GmbH METH / BY / # / DATE 5.0 DAZI ZEN1 / ZEN2 / DZEN 2 # OF FREQUENCIES # COMMENT # (C) Geo++ GmbH COMMENT … # Switzerland COMMENT # COMMENT created by ant2atx 25-SEP-06 15:13:56 COMMENT G01 START OF FREQUENCY NORTH / EAST / UP NOAZI … … ANTEX Format

Absolute Calibration Status Hardware is deployed at Corbin in temporary status Data collection software complete Tentative pan/tilt scenario complete Processing software is being written Expect to begin calibrations March ‘09

ISSUES IGS currently maintaining ANTEX file –Only IGS antennas –Only absolute calibrations Should avoid multiple versions of ANTEX –But multiple versions inevitable –Many more antennas than IGS uses NGS will probably continue relative ant_info file and establish companion ANTEX file

Why Do Antenna Calibrations? GPS determines position of antenna phase center Phase center must be related to physical point Phase center locations are not fixed Phase center locations depend on direction to satellite Phase center variation is different for each antenna type

NGS Antenna Calibration Permanent antenna testing facility at Corbin, VA Dedicated equipment and personnel Produce reliable antenna phase offsets/patterns for ‘all’ GPS antennas Evaluate reliability and repeatability of antenna measurements Establish public distribution of ‘standard’ antenna calibrations