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SVY207: Lecture 17 Third party products and GPS

processing procedures• Motivation

– Ordnance Survey has established a National Network of over 30 Continuously Operating GPS Reference Stations (CORS)

– Data from these stations is served, free of charge, across the internet and can be used by anyone

– GPS is now a standard technique in topographic and engineering surveying

• Aim– To understand what GPS products are available, their format,

and how/when to use them

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Available GPS Products

• What kind of products are available?

– Data from CORS GPS Stations

» Static

» Real Time

– Data types

» RINEX

» sp3

– GPS satellite orbit products

» Final

» Rapid

» Predicted/Real Time

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What is a CORS?

• The principle components of a CORS – Geodetic dual frequency receiver

– Choke ring antenna (Optional ray-dome)

– Communications to central data collection unit

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Typical CORS site monuments

Drilled Gournd Marker

Wall Mount

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Who uses CORS and for what?

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What are the advantages of CORS?

– Provide 3-dimensional data 24hrs/365 days per year

– The are permanent

» Users do not need to reconnaissance control points

» Users do not need to set up instruments at control points

• Saves time and money!

» Data is often available for FREE• Saves money!

– CORS positional coordinates are more precise than those of other control points

» Positions and velocities are available together with metadata (more later)

» Positions are continuously monitored and will be updated if station moves

– CORS provide a direct tie to National Reference System

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What are the disadvantages of CORS?

– Distances to sites is currently excessive

» UK Ordnance Survey active network of CORS provides one station per 100km on average

– Receiver hardware at CORS site will may differ from users hardware

» Antenna phase centre offsets

» Is data recorded in the same way?• Fritz Brunner (2001), found that mixing of the Trimble 4000 SST

with the Trimble 4000 SSE and Trimble 4000 SSI geodetic receivers over short observation times (less than a few hours) can cause vertical height errors of up to five centimeters.

• These errors appear to be caused by differences in how each receiver internally processes the incoming carrier phase signals.

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CORS sites in the UK

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CORS sites in Europe

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CORS sites USA

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Metadata for CORS sites

• Metadata should include– Coordinates (positions and velocities)

– Data availability profiles

» Charts showing time for which data has been collected

– Data sheets

» Information about receiver type, antenna type, station construction, raydome, log files

– Site photos

– Time series of positional coordinates

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A typical Time Series for a CORS

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Surveying Applications using CORS

• Precise Positioning– Full processing of code and phase

» Double differencing ++

– Tie in surveys to Ordnance Survey Reference System

– Establish GPS base station for local survey

» Rapid Static

» Static

» RTK

– Blunder detection - resolving conflicting results

– Base station for monitoring

– NEW data provision for one man RTK

» Requires high CORS data rate + communication of data to users in real-time

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Surveying Applications using CORS

• Sub-meter Applications– Process code only

» GIS asset surveys

» Environmental inventories• Tree locations

• Species locations

» Precision Agriculture (after the event)

» NEW - Real time via communications link to user

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Who provides CORS data?

– http://igscb.jpl.nasa.gov/

– http://www.epncb.oma.be/

– http://www.ngs.noaa.gov/CORS/

– http://swepos.lmv.lm.se/english/index.htm

– http://igscb.jpl.nasa.gov/

– http://www.ordsvy.gov.uk/

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Summary of CORS

– To position points at the cm level over 20km use precise orbit products (see later) and phase data

» Ensure that you record >4hrs data

» Use ion-free solutions for long baselines

» Ensure that either estimate or model Troposphere

» Remember final orbit products have a significant time delay (see later)

» Ensure that all antenna information is correct - otherwise large height errors will result

» Sub-meter positioning is possible with shorter data spans

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Summary of CORS

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RINEX data format • Receiver INdependent EXchange Format

– Motivation for development» All manufacturers have developed their own ‘BINARY’ file

formats for data storage specific to their processing software

• Problems occur when processing data from another manufactures receiver

» TWO potential solutions

• Front-end software to interpret and convert the raw data for each receiver type

• Unique data exchange format + software to convert the raw data into this exchange format

» RINEX developed by the Astronomical Institute of the University of Berne to allow easy exchange of the GPS data

» Principle driver was the large European GPS campaign EUREF 89 - involved more than 60 GPS receivers of 4 different manufacturers.

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RINEX data format cont’d

• Receiver INdependent EXchange Format– Includes formats for observation, navigation and

meteorological data

» Observation and Navigation are the most important– Each file type consists of a header section and a data section

– Header section contains global information for the entire file and is placed at the beginning of the file.

» Contains header labels in columns 61-80 for each line contained in the header section

» These labels are mandatory and must appear exactly as per format description

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RINEX data format cont’d• RINEX VERSION / TYPE

• PGM / RUN BY / DATE

• COMMENT (Opt)

• MARKER NAME

• MARKER NUMBER (Opt)

• OBSERVER / AGENCY

• REC # / TYPE / VERS

• ANT # / TYPE

• APPROX POSITION XYZ

• ANTENNA: DELTA H/E/N

• WAVELENGTH FACT L1/2

• # TYPES OF OBSERV

• INTERVAL (Opt)

• TIME OF FIRST OBS

• TIME OF LAST OBS (Opt)

• LEAP SECONDS (Opt)

• # OF SATELLITES (Opt)

• PRN / # OF OBS (Opt)

• END OF HEADER

RINEX Observation file header labels

e.g. NEWC067a.02o

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RINEX data format cont’d

2 OBSERVATION DATA RINEX VERSION / TYPENational GPS Network Ordnance Survey Mar 8 01:15:51 2002 PGM / RUN BY / DATEActive Station at Dept of Geomatics, Newcastle University COMMENTNEWC MARKER NAME13227S001 MARKER NUMBERNational GPS Network Ordnance Survey OBSERVER / AGENCYUX50172 ASHTECH UZ-12 UX50172 REC # / TYPE / VERSCR16375 ASH700936E SNOW ANT # / TYPEThe following coordinates are NOT APPROXIMATE COMMENTApprox coords replaced by official precise ETRS89 values COMMENT 3667109.2251 -103493.4404 5200153.1949 APPROX POSITION XYZ 0.0000 0.0000 0.0000 ANTENNA: DELTA H/E/N 1 1 WAVELENGTH FACT L1/2 7 L1 L2 C1 P1 P2 D1 D2 # / TYPES OF OBSERV 2002 3 8 0 0 0.000000 TIME OF FIRST OBS 2002 3 8 23 59 45.000000 TIME OF LAST OBS END OF HEADER02 3 8 0 0 0.0000000 0 7 24 4 27 13 8 10 2 -0.000000000 -8115988.180 7 -6291624.25845 22578446.143 22578444.9364 22578451.6024 -2761.132 -2151.532 -420903.227 4 -294899.41543 25080187.396 25080186.3754 25080194.8814 -3779.171 -2944.818 -19461449.079 8 -15136270.99947 21126819.230 21126818.2444 21126822.8474 -380.931 -296.834 -14271234.631 7 -11081530.53445 22748169.818 22748169.4584 22748173.8364 -3013.443 -2348.110 -22897791.319 8 -17834431.79347 20795033.815 20795031.8934 20795037.3214 2114.395 1647.588 -20179780.249 8 -15689412.42447 20944580.077 20944578.6934 20944584.1374 1643.149 1280.362 -11525677.445 6 -8965475.84645 23855205.815 23855204.4674 23855210.7514 -142.950 -111.382

An example of RINEX Observation data

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RINEX data format cont’d

• RINEX filename convention:– SSSSDDDT.YYo

» Four site SSSS

» On day-of-year DDD

» Observations session T

» Year YY

» o = Observation• Other options - n = Navigation, g = GLONAS, m =

Meteorological data

» E.g., NEWC067a.02o is observation data for CORS site Newcastle, day 067, session a, year 2002.

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RINEX data format cont’d

• RINEX Navigation data» RINEX VERSION / TYPE

» PGM / RUN BY / DATE

» COMMENT (opt)

» ION ALPHA (opt)

» ION BETA (opt)

» DELTA-UTC: A0,A1,T,W (opt)

» LEAP SECONDS (opt)

» END OF HEADER

RINEX Navigation file header labels

e.g. NEWC067a.02n

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RINEX data format cont’d

2 N: GPS NAV DATA RINEX VERSION / TYPEXXRINEXN V2.0 AIUB 12-SEP-90 15:22 PGM / RUN BY / DATEEXAMPLE OF VERSION 2 FORMAT COMMENT .1676D-07 .2235D-07 -.1192D-06 -.1192D-06 ON ALPHA .1208D+06 .1310D+06 -.1310D+06 -.1966D+06 ION BETA .133179128170D-06 .107469588780D-12 552960 39 DELTA-UTC: A0,A1,T,W 6 LEAP SECONDS END OF HEADER 6 90 8 2 17 51 44.0 -.839701388031D-03 -.165982783074D-10 .000000000000D+00 .910000000000D+02 .934062500000D+02 .116040547840D-08 .162092304801D+00 .484101474285D-05 .626740418375D-02 .652112066746D-05 .515365489006D+04 .409904000000D+06 -.242143869400D-07 .329237003460D+00 -.596046447754D-07 .111541663136D+01 .326593750000D+03 .206958726335D+01 -.638312302555D-08 .307155651409D-09 .000000000000D+00 .551000000000D+03 .000000000000D+00 .000000000000D+00 .000000000000D+00 .000000000000D+00 .910000000000D+02 .406800000000D+06

An example of RINEX Navigation data

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Orbit Products

• Types of obit product– Variety of orbit products are available from a variety of

providers

» Precise orbit (Final)

» Rapid orbit

» Ultra Rapid orbit

– These orbits tend to be more accurate

– Typically not available for real-time applications

– BUT don’t forget about Broadcast orbit

» Braodcast orbit - transmitted from GPS satellite in real-time

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Orbit Products cont’dIGS Orbit Products

Orbit type Accuracy Latency Updates SampleInterval

Broadcast ~260 cm/~7 ns True real time - - -.-Predicted(Ultra-Rapid)

~25 cm/~5 ns Pseudo realtime

twice daily 15 min/

Rapid 5 cm/0.2 ns 17 hours daily 15 min/5 minFinal <5 cm/0.1 ns ~13 days weekly 15 min/5 min

Orbit Product Providershttp://igscb.jpl.nasa.gov/components/compindex.htmlhttp://www.ngs.noaa.gov/http://sopac.ucsd.edu

NOTE: Predicted, Rapid and Final Orbit Products often come in .sp3 format

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Orbit Products cont’d

• Which orbit type should I use?Rule of Thumb:

Where d is baseline length in km and the errors are in metres.Errors at different baselines lengths:

errororbit20000

error Baselined

<<1mm<<1mm<<1mmPrecise

<1mm<<1mm<<1mmRapid

2.5mm<1mm<1mmUltra-rapid

20-100mm5-25mm1-5mmNavigation

200km50km10km