GNSS Absolute Antenna Calibration at the National Geodetic Survey Andria Bilich & Gerald Mader Geosciences Research Division National Geodetic Survey With contributions from: Charles Geoghegan, Jarir Saleh, Steven Breidenbach, Dennis Lokken, Kendall Fancher

Outline Motivation NGS methods and observation models Example results NGS Calibration Services

Where do I receive the signal? Antenna element Nonphysical and inconstant point floating in space? Antenna reference point (ARP) Antenna calibration attempts to answer very important question – where do I receive the signal?

What is GNSS Antenna Calibration? PCV (el) PCV (az,el) Create a “map” of antenna characteristics Mean point being positioned (PCO) Spatial variations (PCV) Determination of receiving antenna characteristics required for accurate geodetic positions. What is the location of the point being positioned? How does that point vary in space? How to neutralize antenna contributions (flatten out spatial variation)? PCO [ENU]

What Is The Effect of PCO/PCV? Antenna element introduces elevation- (and azimuth-) dependent advance/delay to the carrier phase observation = PCO + PCV Effect on heights (with respect to elevation cutoff) Alias into troposphere estimate <= 10cm height errors on mixed-antenna baselines Mixed-antenna and longer baselines demand good antenna calibrations Published values are idealized (environment-free) Applies an elevation (and azimuth) 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

Equipment and Environment Effects on PCV Ashtech chokering (700936D_M) with SCIS radome with SNOW radome

NGS Motivations and Goals Serve high precision needs of U.S. surveying and geodesy communities Simultaneous multi-freq, multi-GNSS calibrations 2-D (elevation, azimuth) phase center patterns Free calibration service w/ quick turn-around Calibration values publicly distributed via Internet Compatibility with IGS ANTEX values Although there are other absolute antenna calibration facilities in operation, the National Geodetic Survey felt that a domestic, free calibration service is consistent with our misson. Our overall goal is to serve high precision needs of US surveying/geodesy communities. To do this, we have developed a facility that… (read other bullets)

NGS Calibration Facility and Methods

Calibration Setup Single differences Short baseline (5 m) Simplified multipath environment Common clock (heading receiver) Remaining factors = phase centers (ref, test), differential multipath, hardware bias Experimental setup removes many common phase delay factors via single difference

Time Difference of Single Differences Closely spaced time pairs + robot motion = PCO/PCV at reference antenna removed slowly varying biases (differential MP, hardware bias) minimized When time pairs are closely spaced (< 10 seconds), satellite motion is negligible. Between time 1 and time 2, I move antenna under test by a large pan or tilt angle Results in large change in reception angles for test antenna, but no net change at reference antenna ** Removes phase center effects of reference antenna, and minimizes the slowly varying biases Fixed reference antenna Test antenna

Why Robot? Introduce angle changes for TDSD Better spatial coverage

Modelled Factors PCV (az,el) A priori position Frame rotation(s) between robot and local frame Rotation arm length Phase windup (antenna motion) PCO [ENU] V N E

NGS Calibration Results mm NGS Calibration Results

Trimble Zephyr Geodetic 2 (TRM55971.00) ~/conferences/IONGNSS2010/plotTrmZeph1.m Patterns have been adjusted so that all use same PCO (from IGS). (mm) North East Up IGS 1.07 -0.19 67.17 30212661 1.38 -0.45 69.84 30212716 0.92 -0.47

Trimble Zephyr Geodetic 2: using IGS05 PCO

Trimble Zephyr Geodetic 2: PCV

Trimble Zephyr Geodetic 2 differences from IGS05

Ashtech Geodetic III ‘Whopper’ (ASH700718B) ~/conferences/IONGNSS2010/plotAshWhop2.m (mm) North East Up IGS -1.67 -0.47 69.48 11885 -1.22 0.22 69.13 11869 -1.40 0.23 69.17

Ashtech Geodetic III “Whopper”

Ashtech Geodetic III “Whopper”: differences from IGS05

Topcon CR.G3

Topcon CR

Topcon CR

Topcon PG-A1

Topcon PG-A1

Topcon PG-A1

Next Steps Steered based on satellites in view Better data fill at edges Faster, more automated process Move towards same cutoffs as Geo++ -5 deg in antenna frame +18 deg in local ENV frame

NGS Calibration Services

PC0/PCV Distribution Freely available Distribution via website: http://www.ngs.noaa.gov/ANTCAL Data formats for different software: NGS format (relative and absolute) ANTEX (absolute)

NGS Calibration Services Formal policy document Calibration process and stages Eligibility for calibration Rights and responsibilities Request calibration via web form Tracking system with automated customer notification emails

Conclusions and Outlook Good agreement with IGS type means Pending approval from International GNSS Service (IGS) Soon to be “open for business” For more information http://www.ngs.noaa.gov/ANTCAL Email andria.bilich@noaa.gov or NGS.AbsAntCal@noaa.gov

End of presentation

What is GNSS Antenna Calibration? OLD VERSION Determination of receiving antenna characteristics required for accurate geodetic positions. What is the location of the point being positioned? How does that point vary in space? How to neutralize antenna contributions (flatten out spatial variation)?

Ideal Point Antenna R = λ φ Source moves over spherical surface Connected to Phase Meter +π φ -π elv 90

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

“Experiment” Results Measured phase = function of angle of reception Phase center may be estimated Phase center variation (PCV) must be applied to yield correct distance to source

Antenna Phase Center Variations (PCV) All GPS antennas have frequency-variant PCV Every antenna is different PCV depends primarily on elevation, some azimuth L1 Lots of unintended consequences to antenna design which really come out with advanced processing strategies L2 mm

Goals of NGS Calibration Facility Fast, free service Simultaneous multi-frequency calibration 2-D (elev, azim) patterns Calibration values publicly distributed via Internet Serve high precision needs of surveying and geodesy communities Produce calibrations to International GNSS Service (IGS) standards

Facility Design

NGS Antenna Calibration Facility Corbin, VA Adjacent to relative calibration piers Potential of remote operation via antenna robot, web cam, controller PC on Internet

Forming the Observations How to take raw carrier phase measurements and remove everything except phase center variations (PCV)

Single Difference Already removed geometric range (orbits) and biases Station A, satellite 1 Already removed geometric range (orbits) and biases Factors in phase meas: Clocks (SV and RX) Atmosphere (iono, tropo) Multipath, noise RX hardware Antenna (PCV) Single difference removes satellite clock contribution Station B, satellite 1

SD with Heading Receiver Dual freq tracking of 2 antennas (Septentrio PolaRx2eH) Common oscillator for all tracking loops = both antennas share clock Remaining hardware bias Small magnitude Slowly time varying differential hardware bias = Signals from 2 antennas travel through different pieces of hardware in RX front end, as well as additional cable lengths between 2 ants, different antenna elements, etc

Additional Modeled Factors Projection of RPTU onto LOS Phase windup Rotation Tilt Must know orientation of and measurements for PTU

#4 - precise equipment measurements PCV (az,el) Arm length between point-of-rotation and antenna ARP (RPTU) Monument location RMON Relative frame orientation (between PTU and local ENV frame) Why care? Robot position and orientation affect angles of reception Why measure instead of estimate? Same factors can look like PCV! PCV0 V RPTU N E RMON

TDSD observable Removed via single difference Atmospheric delays Satellite and receiver clocks Removed via time difference PCV at test antenna Hardware bias Multipath (?) Accounted for known values Robot orientation Tilt arm length Monument position

Motion Strategy Tilt iterations over full range of pans Time pairs = tilted vs untilted Closely spaced in time Large angular contrast

Estimation strategy Pre-Edit Phase Geometric range Angles in local frame Satellite XYZ/velocity calcs (for windup) Single Difference Phase Cycle slip editing Phase windup PTU tilt arm Time Difference Form time pairs Angles in antenna frame Form / Solve Normal Equations PCV0 (east, north, up components) PCV (elevation and azimuth angle)

PCV is not time-dependent… Trimble Zephyr Geodetic (TRM41249.00)

… but depends on spatial variables

Major Contribution from PCV0 PCV0(ENV) = [-0.1 1.2 55.8] mm

Leftovers = PCV(elev)