1. The Geoscience Australia’s Online GPS Processing Service (AUSPOS)
Operation, Limitations
and Best Practices
Gary Johnston and John Dawson
Geoscience Australia Earth Monitoring Group,
Canberra, Australia

2. Outline GPS processing options AUSPos GPS error Sources
Principles
Submitting data
Reading and understanding AUSPos results
GPS error Sources
AUSPos recommended practices
Reference frames and Datums
ITRF, WGS84, GDA94
AHD, AUSGeoid98

3. GPS Processing Options
Baseline from existing geodetic infrastructure
disadvantages
most propriety based software has limited modeling and suits baselines < 100 km
marks can be difficult to find/access
requires two GPS receivers
advantages
not reliant on external data or processing services

4. GPS Processing Options cont.
Baseline from IGS/ARGN
disadvantages
user must find and download IGS/ARGN data
require Internet connection
need to model antennas correctly
IGS/ARGN station may be > 1000km away
advantages
not reliant on processing services
requires one GPS receiver

5. GPS Processing Options cont.
Use AUSPOS
disadvantages
require Internet connection
requires > 6 hours of data
advantages
AUSPOS automatically collects IGS/ARGN data
works anywhere in the world
requires one GPS receiver

6. AUSPOS users submit geodetic quality GPS via web-browser
Internet web application
users submit geodetic quality GPS via web-browser
rapid turn-around precise positions via
cm level coordinates anywhere in the world in an absolute sense
position quality depends on data quantity/quality
local datum within Australia (GDA94)
ITRF coordinates outside of Australia
FREE service

7. AUSPOS Applications :- DGPS reference station positioning
Remote GPS station positioning
Ultra-long GPS baseline positioning
GPS connections to IGS and ARGN stations
High accuracy vertical GPS positioning
In the field high-accuracy processing
GPS network quality control

8. AUSPOS cm level positioning with two receivers
Relative GPS Positioning
cm level positioning with two receivers
requires existing ground geodetic infrastructure ie. coordinated ground marks

9. AUSPOS cm level positioning with one receiver
Absolute GPS Positioning
cm level positioning with one receiver
geodetic infrastructure ‘invisible’ to the user

10. AUSPOS What do you Need? GPS data dual frequency data
RINEX format only
> 6 hrs of continuous GPS data
Internet web browser
i.e. netscape, Internet Explorer
address

11. AUSPOS Overview Input and Output User GPS Data Processing Results
Coordinates ( )

12. AUSPOS Overview What do you Submit? GPS observational data (RINEX)
Your address
the height of your GPS antenna
the type of your GPS antenna
What happens?
Data is uploaded to GA
processed
results ed and available by ftp
**AUSPOS Where do you submit it?

13. AUSPOS Overview Speed and accuracy 6 hour data file
results delivered in ~ 3 minutes
20 mm horizontal, 50 mm vertical
24 hour data file
results delivered in ~15 minutes
<10 mm horizontal and mm vertical

14. AUSPOS Overview Operations
IGS
User GPS
AUSPOS Overview
Operations
‘invisible’ Geodetic infrastructure
International GPS Service (IGS), worldwide network of permanent GPS receivers
‘baseline’ from three closest IGS GPS
station to station distances
typically <1000 km, up to 3500 km
IGS analysis products
precise orbits
precise Earth Orientation Parameters

15. AUSPOS Overview Software MicroCosm
commercial version of the Goddard Space Flight Centre (GSFC) software GEODYN
capable of multiple technique data processing
GA currently uses MicroCosm for GPS, Satellite Laser Ranging (SLR) and DORIS
Processing standards
full implementation of the International Earth Rotation Service (IERS) 1996 computation standards

16. Submitting GPS data to AUSPOS

17. AUSPOS Data Submission

18. Reading and understanding AUSPOS results

19. Reading and understanding AUSPOS results

20. Reading and understanding AUSPOS results

21. Reading and understanding AUSPOS results

22. Reading and understanding AUSPOS results

23. Reading and understanding AUSPOS results

24. GPS Processing GPS Processing Software Modelling and Error Sources
Satellite Orbits
Satellite and Receiver Clock Error
Tropospheric Refraction
Ionospheric Refraction
Antenna Phase Centre Variations
Multipath

25. commonly used observation used by AUSPOS
Geodetic GPS Observations
double difference observation
commonly used observation
used by AUSPOS
GPS Baseline

26. GPS Processing Software
short baselines (< 100 km)
errors sources tend to cancel in the double difference
propriety processing software usually adequate
long baselines (>100 km)
many error sources become significant
good observation modelling is essential
requires sophisticated software systems
E.g. MicroCosm, Bernese, Gispy, Gamit, Epos, page5

27. Geodetic GPS Errors and Modelling Issues
Satellite Orbits
Orbit modelling
Accelerations on the satellite
Solar radiation pressure
other accelerations acting on GPS satellites
**AUSPOS uses these IGS precise orbits

28. Geodetic GPS Errors and Modelling Issues
Satellite Orbits
Orbit error example :-
Precise IGS orbit versus Broadcast orbit
GPS Satellite 27, 1st January 2000
Along track
Cross track
Radial

29. Geodetic GPS Errors and Modelling Issues
Satellite and Receiver Clock Error
can be eliminated by double difference
GPS receiver clock error example :-
Cocos Island Receiver Clock (AOA SNR-12 ACT)

30. Tropospheric Refraction
troposphere is non-dispersive for GPS signals, extending to a height of about 10km
both frequencies impacted identically
total delay due to the troposphere is about 2.3m
relative tropospheric error can impact station heights
1 cm error in troposphere signal delay produces around 3 cm error in height
the troposphere can be modeled using a standard atmosphere but these models have limitations
can overcome model limitations by additional parameter estimation of site specific troposphere parameters
**AUSPOS estimates a tropospheric scale factor every two hours at every site used in the processing

31. Ionospheric Refraction
ionosphere is dispersive for GPS signals
both frequencies impacted differently, the delay is approximately proportional to the frequency -2
GPS signal is delayed in the ionosphere due to the interaction with free electrons
the total delay can vary from 1 to 20m
on short baseline the ionosphere doesn’t need to be accounted for
on long baseline the scale of the baseline can be impacted
for the most part the impact of the ionosphere can be eliminated by combining GPS observations from both frequencies
**AUSPOS uses an ionosphere corrected L1 observation

32. Modelling Earth tides example :-
Earth Tides, Suva Fiji
**AUSPOS makes the IERS recommended tidal corrections

33. GPS Antenna Height and Modeling
GPS heights can now be determined very accurately
accurate antenna heights are important
correct antenna phase centre modeling is important
incorrect identification of antenna make and model can impact the computed coordinate significantly!
Up to 0.1 metre in height regardless of baseline length
**AUSPOS models most commonly used antenna types which can be selected when submitting data

34. GPS Antenna Height
Antenna Reference Point (ARP) - the point from which phase centre offsets are measured
the Bottom of the Ground Plane (BGP) is the usual point for measuring slope heights
Top of Ground Plane (TGP) is also often used
Vertical Height to ARP =
**AUSPOS accepts only vertical height to the ARP

35. GPS Antenna Height ARP to BGP offset Slope Height to BGP
Radius
ARP to BGP offset
Slope Height to BGP
Vertical Height to ARP

36. GPS Antenna effects Antenna Phase Center offset
Consists of two components
First is the mean offset from the Antenna Reference Point
Second is the variable component around this mean
Second component depends on azimuth and elevation of incoming signal
Second component can cause errors in height of up to 0.1m even over very short lines
IGS phase center variation models eliminate the majority of this error
**AUSPOS uses the IGS models

37. Antenna constant phase center offsets examples :-
Trimble: TR GEOD L1/L2 GP (Mod , compact, with groundplane)
TR GEOD L1/L2 W/O GP (Mod , compact, without groundplane)
< L1/L2
/ \
< TGP
< BGP
| |
| |
| |
x < ARP=BPA
< > NOTCHES
< > EDGE
Leica SR299E/SR399E: EXTERNAL WITH GP EXTERNAL WITHOUT GP
/ \
/ \
/ \
/ \ < L1/L2
\ x / < ARP=TOP
| > < |
===| |===
-+ +-
| |
Ashtech: GEODETIC III L1/L2
_____________
/ \
< L1=L2
< TGP
| |
=| |
<
| |
< ARP
< >
Antenna constant phase center offsets examples :-

38. GPS Signal Characteristics
Right Hand Circular Polarised
L1 wavelength 19.05cm
L2 wavelength cm
receipt characteristic depend on signal azimuth and elevation, as well antenna element

39. Phase Center Offset Diagram (ASHTECH)

40. Phase Center Offset Diagram (DMT)

41. Effect of Phase Center Variations

42. Effect of Phase Center Variations

43. Impact of Phase variations, example :-
May 1995 NTF campaign with geodetic quality GPS equipment
differences between solution that has no variable model applied and solutions that have IGS phase models applied
difference shows very little effect in horizontal position.
vertical difference varies from one antenna type to another, and one location to another
Differences are generally the same for like antennae in close proximity to each other

44. Impact of Phase variations, example :-

45. Multipath systematic biases can result up to 20 m in pseudorange
multipath is the result of GPS signals that are reflected from a surface near to the antenna. The GPS antenna receives both the direct and indirect signal
systematic biases can result
up to 20 m in pseudorange
up to several centimeters in the carrier phase measurements
multipath effects greater for low elevation satellites
multipath is difficult to model because it depends on the antenna environment
**AUSPOS tip -- when observing GPS take care to avoid high multipath environments

46. AUSPos recommended practices
Positional Uncertainty (m) 1 (Horiz Vert)
Location 2
Australia
IGS products 3 (minimum standard accepted)
IGS Final
(~14 day delay)
IGS Rapid
(~2 Delay)
IGS Ultra-rapid
(partly predicted)
GPS Receiver 4
Geodetic, dual frequency, carrier phase & code
GPS Antenna 5
IGS/NGS modelled
GPS data format 6
RINEX
GPS data sampling 7
30 sec
Duration of observations 8
Multiple 24 hour sessions
Multiple 6 hour sessions
Multiple 2 hour sessions
Repeatability between sessions (m) 9
Transformation to GDA94 10
Yes
Solution statistics satisfied 11
Antenna type 12
Make, model & serial number
Antenna height 13 mm
Reference stations 14
At least 3 within 1500 km
At least 3
Positional Uncertainty versus GPS data attributes for “absolute” positioning.

47. AUSPos guidelines
Positional Uncertainty is a 95% confidence value, in metres, with respect to the GDA94.
Outside Australia results are ITRF at the epoch of the survey.
Refer to the IGS product guidelines at
Some hand-held receivers may provide phase & code, but the quality of their data cannot be guaranteed for this type of processing

48. AUSPos Guidelines Must apply antenna phase centre variation
Receiver Independent EXchange format (RINEX) is required
Most processes use 30 second data, but will accept any sampling rate less than 30 seconds that can be stripped back to 30 seconds (e.g. 1, 3, 5, 6, 10, 15, 30 sec).
Each session should be entirely within a UT day. Repeat shorter duration sessions should be observed at different times of the UT day to minimise systematic effects from the GPS system and ambient site conditions (e.g. similar satellite constellation).
Multiple sessions are recommended to ensure repeatability and hence confidence in the result. Re setup equipment for each session

49. AUSPos guidelines
Transformation to the GDA94 is required. The time-varying ITRF-GDA94 transformation parameters published by Geoscience Australia are recommended
Must examine coordinate precisions and observation fits to ensure acceptability
The calibration for an antenna can be different, even for the same brand with only slight variations in the model. Exact identification is essential to ensure that the correct calibration is applied (see note 5).
Ensure correct antenna heights used
Check operation of nearest 3 ARGN stations for critical

50. Reference Frames and Datums
The International Terrestrial Reference Frame (ITRF)
ITRF Definition
ITRF History
Relationships to the IGS realisation of the ITRF
The World Geodetic System 1984 (WGS84)
The Geocentric Datum of Australia (GDA)
The relationship between ITRF, WGS84 and GDA
GPS heighting issues

51. Reference Frames and Datums
International Terrestrial Reference Frame (ITRF)
What is the ITRF?
precise station coordinates and velocities
globally consistent
Internationally accepted reference frame
ideal for geodetic applications
History
ITRF92, ITRF93, ITRF94, ITRF96, ITRF97
ITRF2000 (current)
ITRF2000 Primary + Densification

52. Reference Frames and Datum
How is the ITRF Primary Solution Computed?
analysis groups submit their solutions
combined by the International Earth Rotation Service (IERS)
includes data back to 1977
Solutions
3 x Very Long Baseline Interferometry (VLBI)
1 x Lunar Laser Ranging (LLR)
7 x Satellite Laser Ranging (SLR)
6 x Global Positioning System (GPS)
2 x Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS)
2 x multi-technique solutions
Local Ties between techniques

53. Very Long Baseline Interferometry
VLBI
uses observations of quasars
observables from telescopes involved in simultaneous measurements are correlated to produce an experiment
microwave frequency band
~30 stations with global coverage

54. Lunar/Satellite Laser Ranging
LLR and SLR
simple measure of time of flight of a laser pulse
~ 30 SLR stations
global coverage (but biased to northern hemisphere)
~ 60 satellite and lunar targets
canonball geodetic satellites
lageos1/2 ~9000 km altitude

55. Global Positioning System
GPS
dual frequency interferometry
~ 200 permanent IGS stations global coverage (receive)
~27 satellites ~20,000 km altitude (transmit)

56. Doppler Orbitography & Radio Positioning Integrated by Satellite
DORIS
dual frequency doppler
~ 51 stations with global coverage (transmit)
~ 5 satellites (receive)
SPOT2 satellite ~7000 km altitude
developed for precise orbit determination of low orbiting satellites
significant tool for high precision global geodesy

57. Reference Frames and Datum
ITRF2000
GPS, SLR, DORIS, VLBI Network, Pacific

58. ITRF2000 Scale and rate weighted average of the VLBI and SLR solutions
Origin (translations and rates)
weighted average of the SLR solutions
Rotations
ITRF97 at epoch
rates No Net Rotation w.r.t NNR-NUVEL1A
Core Network Stations
continuously observing over 3 years
located away from plate boundaries and deforming zones
velocity accuracy better than 3 mm/yr
velocity residuals > 3 mm/yr < 2 solutions

59. WGS84 United States Department of Defense
origin is the Earth’s centre of mass
WGS84 ellipsoid
semi-major axis
flattening 1/
refined
WGS84 (G730)
ITRF91
29 June 1994
WGS84 (G873)
coordinates re-computed
29 January 1997
Now mapped to align with ITRF

60. The Geocentric Datum of Australia
ITRF92
fixed at epoch
realised by the positions of the ARGN stations within Australia (Australian Fiducial Network)
GRS80 ellipsoid
Transformation parameters exist to convert to GDA94

61. Relationship between ITRF, WGS84 and GDA
ITRF versus GDA
tectonic motion
re-computation of ITRF solutions
ITRF/GDA versus WGS84
WGS84 difficult to realise precisely
practically equivalent
**AUSPOS provides both ITRF and GDA coordinates by using an GA transformation process

62. Relationship between geoid & ellipsoid
AHD
MSL
N is the geoid-ellipsoid separation
In an absolute sense H = h - N where: H = height above the geoid h = height above the ellipsoid N = Geoid-Ellipsoid separation

63. Brief history of the Australian Height Datum (AHD)
Mainland basic network (5 May 71)
30 tide gauges MSL epoch
Predominantly 3rd order observations
Tasmanian basic network (17 Oct 83)
2 tide gauges MSL epoch 1972
3rd order observations
Supplementary networks
On-going network upgrades
Latest geoid AUSGeoid98

64. Australian Height Datum Basic Network
AHD tide gauge
NTF ABSLMA tide gauge

65. AUSGeoid98 AUSGeoid98 computed using :-
EGM96 global geopotential model
GRS80 ellipsoid
1996 Australian Gravity data base from AGSO
GEODATA 9” DEM
Satellite altimeter-derived free-air gravity anomalies offshore

67. AUSGeoid98 :-
Validation data set consisted of 906 points with AHD and GPS heights
Standard deviation of 36cm resulted
relative accuracy at 3rd order or better
validation points usually used older GPS and AHD spurs

68. AHD tide gauge GPS survey :-
ICSM agencies 1999/2000
Geodetic GPS receivers
5 day continuous observations
AHD tide gauge BM or suitable nearby BM
Included some NTF ABSLMA sites
GPS data set compiled by GA
Junction Point Survey
2000 – present
> 200 AHD junctions points
ITRF2000 ellipsoidal heights observed

69. GPS Points used for Height Modernisation

70. AHD(constrained to Geoid - AHD(new)

71. Adjustment comparison AHD(new) - AHD71
AHD71 does not reproduce.
Updates and corrections to AHD since 1971.
s.d.= 0.061m
Min resid = m
Max resid =0.364m

72. Avoiding the errors :-
The use of Ausgeoid98 in a relative sense will produce 3rd order AHD heights
Use AUSPOS on a known AHD benchmark to determine the geometric N value then use the difference in N from the WINTER software to transfer the AHD height

73. Geodetic GPS infrastructure
Permanent GPS Networks
International GPS Service (IGS)
IGS Data and Products
IGS GPS data
IGS precise orbits and Earth orientation parameters
Accessing IGS Data and Products
The Australian Regional GPS Network (ARGN)
ARGN Data and Products
Accessing ARGN Data and Products

74. Permanent GPS Networks
International GPS Service (IGS) Permanent GPS
200+ Dual frequency GPS
operated national agencies, research organisations, universities,…
Permanently tracking
Dorne Margolin Type antenna
+ domes in some cases
30 second interval RINEX
freely available
IGS related information

75. International GPS Service (IGS) Permanent GPS

76. International GPS Service (IGS) Permanent GPS
IGS Products
precise orbits
precise Earth Orientation (Earth Pole Position)
precise coordinate of IGS stations
Accessing IGS products :-
ftp://igscb.jpl.nasa.gov/igscb/product/XXXX
where XXXX is the GPS week
GPS week 0 starts Sunday 6 January 1980
GPS week 1124 starts Sunday 22 July 2001

77. Accessing ARGN RINEX data
ftp ftp.ga.gov.au/gpsdata/yyddd
anonymous login
RINEX data
UNIX compressed
where yy is the year and ddd is the day of year
Accessing IGS RINEX data
ftp cddisa.gsfc.nasa.gov/gps/gpsdata/yyddd/yyo

78. Australian Regional GPS Network (ARGN)
15 Dual frequency GPS operated by GA
Permanently tracking
Dorne Margolin Type antenna
+ domes in some cases
30 second interval RINEX
freely available
all data contributed to the IGS
major applications
primary geodetic infrastructure
geosciences
atmospheric science
GPS precise orbit determination

79. Australian Regional GPS Network (ARGN)

80. Geodetic GPS Data Geodetic GPS Observations GPS Data formats
Propriety data formats
The RINEX data format
Propriety data formats translation
Translation options
TEQC
TEQC translation
TEQC Quality checking

81. AUSPOS requires L1 and L2 carrier phase observation
some pseudorange observations are also needed
GPS Data Formats
GPS providers each support their own data format
these formats are generally binary
often format details are difficult to obtain or considered commercial in confidence
RINEX Data Format
"Receiver Independent Exchange Format" or RINEX has been developed by the Astronomical Institute of the University of Berne for the easy exchange of the GPS data
International standard ASCII format
ftp://igscb.jpl.nasa.gov/igscb/data/format/rinex210.txt
**AUSPOS only accepts RINEX data

82. RINEX Data Format 2.00 OBSERVATION DATA G (GPS) RINEX VERSION / TYPE
teqc 2000Jul :28:34UTCPGM / RUN BY / DATE
Solaris 2.7|Ultra 2|cc SC5.0|=+-|*Sparc COMMENT
RGRINEXO V2.5.5 UX AUSLIG JAN- 0 12: COMMENT
Australian Regional GPS Network (ARGN) COMMENT
ALICE SPRINGS AU COMMENT
HARDWARE CALIBRATION (S) COMMENT
CLOCK OFFSET (S) COMMENT
BIT 2 OF LLI (+4) FLAGS DATA COLLECTED UNDER "AS" CONDITION COMMENT
ALIC MARKER NAME
50137M MARKER NUMBER
auslig auslig OBSERVER / AGENCY
C126U AOA ICS-4000Z ACT / REC # / TYPE / VERS
AOAD/M_T DOME ANT # / TYPE
APPROX POSITION XYZ
ANTENNA: DELTA H/E/N
WAVELENGTH FACT L1/2
4 L1 L2 P1 P # / TYPES OF OBSERV
INTERVAL
teqc windowed: 2001 Jan 1 00:00: COMMENT
teqc windowed: delta = sec COMMENT
GPS TIME OF FIRST OBS
END OF HEADER
G14G17G15G26G28G23G30G 6G22G21
G14G17G15G26G28G23G30G 6G22G21

83. RINEX Data Format
L1, L2: Phase measurements on L1 and L2
C1 : Pseudorange using C/A-Code on L1
P1, P2: Pseudorange using P-Code on L1,L2
D1, D2: Doppler frequency on L1 and L2
T1, T2: Transit Integrated Doppler on
150 (T1) and 400 MHz (T2)
S1, S2: Raw signal strengths or SNR
values as given by the receiver
for the L1,L2 phase observations

84. Propriety data formats translation
RINEX translators freely available
TEQC (UNAVCO )
BERN
JPL
ASHTECH
Source

85. TEQC Translation example :-
fbar.bin contains the the time-sequential, real-time binary data records for GPS week 866, 11 Aug Aug 1996 from a Trimble SSE receiver. Then, execute
teqc -tr so -week 866 fbar.bin > fbar o
Quality checking example :-
teqc +qc BASE100a.01o
results available in BASE100a.01S
multipath
cycle slips
ionosphere

86. AUSPOS Modeling IERS96 conventions orbit error
state-of-the-art processing system (MicroCosm)
IERS96 conventions
orbit error
uses IGS precise orbits
receiver and satellite clock error
eliminated by double difference
troposphere refraction
 scale factor estimated
ionospheric refraction
 dual frequency observations
Antenna phase centre variations
 uses IGS phase centre models
Reference frame
holds IGS station coordinates fixed at their IGS values (ITRF)

87. AUSPOS Web Pages Step by step user guide GA Geodesy home
Step by step user guide

88. AUSPOS Web Pages Frequently asked questions

89. Trouble shooting Contact Geoff Luton Telephone 02 6249 9050 Email
Contact
Geoff Luton
Telephone
Firewall
some users have firewall problems at their end so use the FTP option if the UPLOAD doesn’t work
if you don’t have an ftp server you can use the GA server :-
Contact Geoff Luton

90. Internet Resources Geoscience Australia home page  www.ga.gov.au
IGS home page
 igscb.jpl.nasa.gov
NGS Antenna Calibrations 
CDDIS Data Information System 