Real-Time PPP using open CORS Networks andRTCM Standards
Andrea Sturze 1 Leos Mervart 2 Wolfgang Sohne 1
Georg Weber 1 Gerhard Wubbena 3
1Federal Agency for Cartography and Geodesy, Frankfurt, Germany
2Geodetic Institute Czech Technical University, Prague
3Geo++ GmbH Garbsen, Germany
March 27, 2012
Outline
1 IntroductionObservation Space Representation (OSR)State Space Representation (SSR)
2 StandardizationRTCM in generalStatus and plans regarding RTCM-SSR messages
3 RealizationIGS Real-Time InfrastructureBKG example
4 Summary
MCG 2012 (Stuttgart) Real-Time PPP March 2012 2 / 23
Introduction: GNSS Error Components
Precise GNSS positioning requires the knowledge of all error componentswith corresponding accuracy
How this information can be provided in real-time?
MCG 2012 (Stuttgart) Real-Time PPP March 2012 3 / 23
Introduction: Observation Space Representation (I)
Sum of GNSS errors per station, GNSS,frequency and signal
Distance dependent errors are derived from and combined withreference station observations
Technique used with current RTCM standards (VRS, MAC, FKP)RTK networking:
- RTK service uses network of reference stations- RTK rover uses reference station observations and RTK corrections
MCG 2012 (Stuttgart) Real-Time PPP March 2012 4 / 23
Introduction: Observation Space Representation (II)
Disadvantages:
only satellites and signals tracked within the RTK network are usable
no reduction of reference station dependent errors
update rate of the corrections depends on component with highestdynamics (satellite clock, ionosphere)
limited spatial validity of corrections
MCG 2012 (Stuttgart) Real-Time PPP March 2012 5 / 23
Introduction: State Space Representation (I)
Transmission of individual GNSS error components
functional and optional stochastic state description
Precise Point Positioning (PPP):- observations of single GNSS receiver- global or regional real-time network- rover uses state space information (e.g. IGS products)
MCG 2012 (Stuttgart) Real-Time PPP March 2012 6 / 23
Introduction: State Space Representation (II)
Advantages:
high reduction of reference station dependent errors through highredundancy within the network
independent from single reference stations
more realistic physical models for individual errors enables bettermodelling and interpolation
update rate can be optimized for different state parameters
Scalability of accuracy and hence of the derived services
Broadcasting of parameters possible
Disadvantages:
higher standardization effort
higher implementation effort
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Standardization: RTCM
Radio Technical Commission for Maritime Services
Development of international open standards
RTCM SC-104: DGNSS Standards- Working Groups examples regarding
o communication and data transfer: WG Internet Protocol (NTRIP)o modelling: WG RTK network MSG, WG State Spaceo GNSS: WG Galileo, WG GLONASSo multi constellation concepts: WG Version 3
WG State Space
- since 2007- RTCM-SSR messages
MCG 2012 (Stuttgart) Real-Time PPP March 2012 8 / 23
Standardization: RTCM-SSR messages
1 precise orbits, satellite clocks and code biases, quality indicator- Real-Time PPP for dual frequency users- compatible to basic PPP mode using IGS products- RTCM recommended Standard since May 2011, included in RTCM 3
2 vertical Total Electron Content (VTEC) and satellite phase biases- Real-Time PPP for single frequency users
3 Slant Total Electron Content (STEC) and troposphere- enable PPP-RTK
MCG 2012 (Stuttgart) Real-Time PPP March 2012 9 / 23
Standardization: RTCM-SSR - messages Step 1
1057 SSR GPS orbit correction
1058 SSR GPS clock correction
1059 SSR GPS code bias
1060 SSR GPS combined orbit and clock correction
1061 SSR GPS User Range Accuracy (URA),
1062 SSR GPS high rate clock correction
1063 SSR GLONASS orbit correction
1064 SSR GLONASS clock correction
1065 SSR GLONASS code bias
1066 SSR GLONASS combined orbit and clock correction
1067 SSR GLONASS User Range Accuracy (URA)
1068 SSR GLONASS high rate clock correction
MCG 2012 (Stuttgart) Real-Time PPP March 2012 10 / 23
Standardization: RTCM-SSR - Satellite Orbit (I)
Orbit corrections refer to broadcast orbits
reduces bandwidth
XOrbit = XBroadcast − δX
XBroadcast .. satellite position corrected by SSR orbit correction messageXOrbit .. satellite position computed according GNSS ICD
from broadcast parameter set identified by IOD/IODEin SSR correction message
δX .. satellite position correction
MCG 2012 (Stuttgart) Real-Time PPP March 2012 11 / 23
Standardization: RTCM-SSR - Satellite Orbit (II)
Orbit corrections defined radial, along-track and cross-track
ealong =r∣∣r∣∣ ecross =
r × r∣∣r × r∣∣ eradial = ealong × ecross
δX =[eradial ealong ecross
]δO
r = X .. satellite broadcast position vector
r = X .. satellite broadcast velocity vectorei .. direction unit vector, i = {radial , along , cross}δO .. orbit correction vector
Orbit corrections consists of correction and velocity correction term
δO =
δOradial
δOalong
δOcross
+
δOradial
δOalong
δOcross
(t − t0)
δOi , δOi .. orbit correction terms from SSR orbit message,i = {radial , along , cross}
t, t0 .. time, reference time obtained from SSR orbit messageMCG 2012 (Stuttgart) Real-Time PPP March 2012 12 / 23
Standardization: RTCM-SSR - Satellite Clock (I)
Clock corrections refer to broadcast clocks
clock correction terms: C0,C1,C2 polynomial coefficientsreduces bandwidth
tSatellite = tBroadcast −δC
c0
tBroadcast .. satellite time computed according to GNSS ICDfrom broadcast clock parameters, identified by IOD/IODEof corresponding SSR orbit correction message
tSatellite .. satellite time corrected by SSR clock correction messageδC .. clock correction obtained from SSR clock correction message
δC = C0 + C1(t − t0) + C2(t − t0)2
Ci .. polynomial coefficients from SSR clock correction message,i = {0, 1, 2}
t .. timet0 .. reference time obtained from SSR clock correction message
MCG 2012 (Stuttgart) Real-Time PPP March 2012 13 / 23
Standardization: RTCM-SSR - Satellite Clock (II)
High rate clock
additional and optional message type
both, polynomial and high rate clock describes the complete clockstate
enables higher resolution and update rates
Satellite code bias
absolute correction term
for every signal and tracking mode
enables higher resolution update rates
MCG 2012 (Stuttgart) Real-Time PPP March 2012 14 / 23
Standardization - RTCM-SSR: Consistency
Basic concept:
SSR messages support different update rates, accuracy requirementsadditional SSR message types adds additional resolution and thereforeposition accuracy
SSR update interval and GNSS epoch time:
define changes of parameters and ensure consistency of dataprocessingMCG 2012 (Stuttgart) Real-Time PPP March 2012 15 / 23
Realization: RTCM-SSR - IGS Real-Time Infrastructure
IGS (International GNSS Service)
voluntary federation of more than 200 worldwide agencies
pool of resources and permanent GPS and GLONASS station data
generate precise GPS and GLONASS products
IGS real-time infrastructure
IGS Real-Time Pilot Project (IGS RT PP) from 2007-2011
- development (RTCM member) and investigation of standards andformats for real-time data and product dissemination
- maintaining a global GNSS real-time tracking network- generation of real-time products
10 participating Analysis Centres (ACs)
AC Coordinator: ESOC (European Space Operations Centre)
Ambiguity fixing working group
- studying techniques and formats for PPP ambiguity fixing
Formats and processes are now ready to allow an initial operationalservice
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Realization: RTCM-SSR - IGS Real-Time Infrastructure
IGS real-time tracking network
station operators provide more than 100 observation data streams
using NTRIP technique
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Realization: RTCM-SSR - IGS Real-Time Infrastructure
Real-Time PPP performance (North, East, Up displacements)
24h time series of PPP applying RTCM-SSR of different AC’sPPP monitor at http://igs.bkg.bund.de/ntrip/ppp
2D RMS of 4-5 cm after convergenceMCG 2012 (Stuttgart) Real-Time PPP March 2012 18 / 23
Realization: RTCM-SSR - BKG
Support of IGS real-time pilot project and infrastructure
hosting several NTRIP broadcaster for data and productdissemination
- http://igs-ip.net/home (observations)- http://products.igs-ip.net/home (products)
RTCM member: format (SSR) and protocol (NTRIP) development
RT-IGS AC
- providing several RTCM-SSR product data streams- generation of clock combination product based on Kalman Filter
approach
development of RTCM, NTRIP and PPP supporting software
websites regarding NTRIP and PPP monitoring
- http://igs.bkg.bund.de/ntrip- http://igs.bkg.bund.de/ntrip/ppp
MCG 2012 (Stuttgart) Real-Time PPP March 2012 19 / 23
Realization: RTCM-SSR - BKG NTRIP Client
BNC Contributors
Concept and coding: CTU, Leos MervartTest, validation, documentation: BKG, Georg Weber and colleaguesRTCM stream encoding and decoding: Alberding GmbH, Dirk Stocker
BNC purposes
multi stream NTRIP clientfeed real-time GNSS enginessynchronization of decoded observations via IP portRTCM steam decoding and encodingsaves RINEX observation and navigation fileshandles and saves RTCM-SSR message content..Precise Point Positioning (PPP)combination of clock correction
Open Source
source code at http://software.rtcm-ntrip.orgintention: push development of open standards for PPP
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Realization: RTCM-SSR - BKG NTRIP Client
Real-Time Precise Point Positioning with BNC
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Summary and Outlook
SSR status
RTCM-SSR products have been available since 2008
interested users who may want to test these products are able to startwith
- BKG NTRIP client available fromhttp://igs.bkg.bund.de/ntrip/download
- RTCM-SSR data available from http://products.igs-ip.net/home
SSR can replace OSR techniques for all types of GNSS processing
SSR future steps
further standardization effort required
next steps are more complex
next steps add accuracy and therefore applications
Final goal
Open Standard for PPP up to PPP-RTK
MCG 2012 (Stuttgart) Real-Time PPP March 2012 22 / 23
Thank you for your attention
http://products.igs-ip.net/home
http://igs.bkg.bund.de/ntrip
http://igs.bkg.bund.de/ntrip/ppp
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