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

Outline

• GPS processing options• AUSPos

• Principles• Submitting data• Reading and understanding AUSPos results

• GPS error Sources• AUSPos recommended practices• Reference frames and Datums

• ITRF, WGS84, GDA94• AHD, AUSGeoid98

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

• advantagesnot reliant on external data or processing services

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

• advantagesnot reliant on processing services

requires one GPS receiver

Use AUSPOS

• disadvantagesrequire Internet connection

requires > 6 hours of data

• advantagesAUSPOS automatically collects IGS/ARGN data

works anywhere in the world

requires one GPS receiver

GPS Processing Options cont.

AUSPOS

• Internet web application• users submit geodetic quality GPS via web-browser• rapid turn-around precise positions via email• 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

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

AUSPOS

• Relative GPS Positioning

• cm level positioning with two receivers• requires existing ground geodetic infrastructure ie. coordinated ground

marks

AUSPOS

• Absolute GPS Positioning

• cm level positioning with one receiver• geodetic infrastructure ‘invisible’ to the user

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• Email address

UserGPS Data

Processing ResultsCoordinates (email)

AUSPOS Overview

Input and Output

AUSPOS Overview

What do you Submit?• GPS observational data (RINEX)• Your email address• the height of your GPS antenna• the type of your GPS antenna

What happens?• Data is uploaded to GA• processed• results emailed and available by ftp

**AUSPOS Where do you submit it?

http://www.ga.gov.au/nmd/geodesy/sgc/wwwgps/

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 10-20 mm vertical

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

IGS

IGS

IGS

User GPS

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

http://www.vmsi-microcosm.com

Processing standards• full implementation of the International Earth Rotation Service (IERS) 1996 computation standards• http://www.iers.org/

Submitting GPS data to AUSPOShttp://www.ga.gov.au/bin/gps.pl

AUSPOS Data Submission

Reading and understanding AUSPOS results

Reading and understanding AUSPOS results

Reading and understanding AUSPOS results

Reading and understanding AUSPOS results

Reading and understanding AUSPOS results

Reading and understanding AUSPOS results

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

GPS Baseline

Geodetic GPS Observations

• double difference observation

• commonly used observation• used by AUSPOS

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

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

Geodetic GPS Errors and Modelling IssuesSatellite Orbits

Orbit error example :-• Precise IGS orbit versus Broadcast orbit• GPS Satellite 27, 1st January 2000

Radial

Along track Cross track

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)

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

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

Modelling Earth tides example :-

Earth Tides, Suva Fiji

**AUSPOS makes the IERS recommended tidal corrections

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

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

offsetRadiusSlope 22

GPS Antenna Height

Radius ARP to BGP offset

Slope Height to BGP

Vertical Height to ARP

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

Trimble: TR GEOD L1/L2 GP (Mod. 22020, compact, with groundplane) TR GEOD L1/L2 W/O GP (Mod. 22020, compact, without groundplane)

----+---- <-- 0.0625 L1/L2 / + \++----------------+-----------------+-----------------++ <-- 0.0591 TGP++---------------+-------------------+----------------++ <-- 0.0556 BGP | | | | | | +------x------+ <-- 0.000 ARP=BPA

<-- 0.467 --> NOTCHES<-- 0.483 --> EDGE

Leica SR299E/SR399E: EXTERNAL WITH GP EXTERNAL WITHOUT GP

/ \ / \ / \ / + \ <-- 0.039 L1/L2 +-----------------------------------------------+ +-----------------------------------------------+ \ ---------+ +x+ +---------/ <-- 0.000 ARP=TOP | > < | ===| |=== ===| |=== -+ +- | | | |

Ashtech: GEODETIC III L1/L2

_____________ / \ +--- + ---+ <-- 0.072 L1=L2+------------------------------------------------------+ <-- 0.065 TGP+-------------------+-------------+--------------------+ | | =| | +-------------+ <-- 0.018 | | +--+--+ <-- 0.000 ARP

<-- 0.368 -->

Antenna constant phase center offsets examples :-

GPS Signal Characteristics

• Right Hand Circular Polarised

• L1 wavelength 19.05cm

• L2 wavelength 24.45 cm

• receipt characteristic depend on signal azimuth and elevation, as well antenna element

Phase Center Offset Diagram (ASHTECH)

Phase Center Offset Diagram (DMT)

Effect of Phase Center Variations

Effect of Phase Center Variations

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

Site Name Antenna Name Latitude(m)

Longitude(m)

Height(m)

Hils ASH700718AP -0.0024 -0.0099 0.0419Per2 Geodetic L1/L2 P -0.0027 -0.0087 0.0453Benw 4000ST L1/L2 Geod -0.0060 -0.0075 0.0989Laac 4000ST L1/L2 Geod -0.0060 -0.0078 0.1039Flag Leica Internal -0.0054 0.0048 0.0537948_ Leica Internal -0.0051 0.0048 0.0542Rfnp 4000ST L1/L2 Geod -0.0057 -0.0090 0.0906Stnp 4000ST L1/L2 Geod -0.0063 -0.0093 0.0910Hent TR Geod L1/L2 GP -0.0060 -0.0069 0.0971Port TR Geod L1/L2 GP -0.0069 -0.0084 0.0931Kio_ 4000ST L1/L2 Geod -0.0045 -0.0099 0.0901Mula 4000ST L1/L2 Geod -0.0042 -0.0093 0.0908

Impact of Phase variations, example :-

Multipath

• 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

AUSPos recommended practicesPositional Uncertainty (m) 1 (Horiz

Vert) 0.025 0.05 0.05 0.1 0.1 0.2

Location 2 Australia Australia 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

Geodetic, dual frequency, carrier phase & code

Geodetic, dual frequency, carrier phase & code

GPS Antenna 5 IGS/NGS modelled IGS/NGS modelled IGS/NGS modelled

GPS data format 6 RINEX RINEX RINEX

GPS data sampling 7 30 sec 30 sec 30 sec

Duration of observations 8 Multiple 24 hour sessions

Multiple 6 hour sessions Multiple 2 hour sessions

Repeatability between sessions (m) 9 0.025 0.05 0.05 0.1 0.1 0.2

Transformation to GDA94 10 Yes Yes Yes

Solution statistics satisfied 11 Yes Yes Yes

Antenna type 12 Make, model & serial number

Make, model & serial number

Make, model & serial number

Antenna height 13 mm mm mm

Reference stations 14 At least 3 within 1500 km

At least 3 At least 3

Positional Uncertainty versus GPS data attributes for “absolute” positioning.

AUSPos guidelines

1. Positional Uncertainty is a 95% confidence value, in metres, with respect to the GDA94.

2. Outside Australia results are ITRF at the epoch of the survey.

3. Refer to the IGS product guidelines at http://igscb.jpl.nasa.gov/components/prods.html

4. Some hand-held receivers may provide phase & code, but the quality of their data cannot be guaranteed for this type of processing

AUSPos Guidelines5. Must apply antenna phase centre variation6. Receiver Independent EXchange format (RINEX) is

required7. 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).

8. 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).

9. Multiple sessions are recommended to ensure repeatability and hence confidence in the result. Re setup equipment for each session

AUSPos guidelines

10.Transformation to the GDA94 is required. The time-varying ITRF-GDA94 transformation parameters published by Geoscience Australia are recommended

11.Must examine coordinate precisions and observation fits to ensure acceptability

12.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).

13.Ensure correct antenna heights used 14.Check operation of nearest 3 ARGN stations for

critical

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

Reference Frames and DatumsInternational 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

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

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

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

Global Positioning System

• GPS

• dual frequency interferometry

• ~ 200 permanent IGS stations global coverage (receive)

• ~27 satellites ~20,000 km altitude (transmit)

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

Reference Frames and Datum

ITRF2000

• GPS, SLR, DORIS, VLBI Network, Pacific

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 1997.0 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

WGS84

• United States Department of Defense• origin is the Earth’s centre of mass• WGS84 ellipsoid

• semi-major axis 6378137• flattening 1/298.257223563

• refined• WGS84 (G730)

• ITRF91• 29 June 1994

• WGS84 (G873)• coordinates re-computed• 29 January 1997

•Now mapped to align with ITRF

The Geocentric Datum of Australia

• ITRF92

• fixed at epoch 1994.0

• realised by the positions of the ARGN stations within Australia (Australian Fiducial Network)

• GRS80 ellipsoid

• Transformation parameters exist to convert ITRF2000@epoch to GDA94

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

Relationship between geoid & ellipsoid

N is the geoid-ellipsoid separation

In an absolute sense H = h - N where:H = height above the geoid h = height above the ellipsoidN = Geoid-Ellipsoid separation

AHDMSL

Brief history of the Australian Height Datum (AHD)

• Mainland basic network (5 May 71)• 30 tide gauges MSL epoch 1966-1968• 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

AHD tide gaugeNTF ABSLMA tide gauge

Australian Height Datum Basic Network

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

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

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

GPS Points used for Height Modernisation

AHD(constrained to Geoid - AHD(new)

Adjustment comparison AHD(new) - AHD71• AHD71 does not

reproduce.• Updates and

corrections to AHD since 1971.

s.d.= 0.061mMin resid = -0.212mMax resid =0.364m

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

AUSPOS Web Pages

• GA Geodesy home

http://www.ga.gov.au/nmd/geodesy/

• Step by step user guide

http://www.ga.gov.au/nmd/geodesy/sgc/wwwgps/wwwstep.htm

AUSPOS Web Pages

• Frequently asked questionshttp://www.ga.gov.au/nmd/geodesy/sgc/wwwgps/wwwfaq.htm

Trouble shootinghttp://www.ga.gov.au/nmd/geodesy/sgc/wwwgps/faq6.htm

Contact• Geoff Luton

Telephone• 02 6249 9050

Email• geoff.luton@ga.gov.au • geodesy@ga.gov.au

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

Internet Resources

Geoscience Australia home page www.ga.gov.au

IGS home page igscb.jpl.nasa.gov

NGS Antenna Calibrations http://www.ngs.noaa.gov/ANTCAL/

CDDIS Data Information System http://cddisa.gsfc.nasa.gov/