1. 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

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

3. 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?

4. 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]

5. 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

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

7. 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)

8. NGS Calibration Facility and Methods

9. 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

10. 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

11. Why Robot?
Introduce angle changes for TDSD
Better spatial coverage

12. 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

13. NGS Calibration Resultsmm
NGS Calibration Results

14. 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
1.38
-0.45
69.84
0.92
-0.47

15. Trimble Zephyr Geodetic 2: using IGS05 PCO

16. Trimble Zephyr Geodetic 2: PCV

17. Trimble Zephyr Geodetic 2 differences from IGS05

18. 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

19. Ashtech Geodetic III “Whopper”

20. Ashtech Geodetic III “Whopper”: differences from IGS05

21. Topcon CR.G3

22. Topcon CR

23. Topcon CR

24. Topcon PG-A1

25. Topcon PG-A1

26. Topcon PG-A1

27. 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

28. NGS Calibration Services

29. PC0/PCV Distribution Freely available
Distribution via website:
Data formats for different software:
NGS format (relative and absolute)
ANTEX (absolute)

30. 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 s

31. 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 or

32. End of presentation

34. 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)?

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

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

37. “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

38. 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

39. 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

40. Facility Design

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

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

43. 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

44. 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

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

46. #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

47. 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

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

49. 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)

50. PCV is not time-dependent…
Trimble Zephyr Geodetic (TRM )

51. … but depends on spatial variables

52. Major Contribution from PCV0
PCV0(ENV) =
[ ] mm

53. Leftovers = PCV(elev)