[IGSMAIL-5927]: Status of IGS Real Time Pilot Project

Caissy, Mark Mark.Caissy at NRCan-RNCan.gc.ca
Fri Apr 3 02:12:29 PDT 2009


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IGS Electronic Mail      03 Apr 02:12:32 PDT 2009      Message Number 5927
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Author: Mark Caissy

Status of IGS Real Time Pilot Project

Dear IGS colleagues,

We have compiled this message to update everyone on the activities that
are taking place under the scope of the Real Time Pilot Project. We hope
that this will encourage increased participation to the project as well
as highlighting the good work that is taking place.

Objectives
-------------
The following outline the key objectives of the Pilot Project:
1.	Manage and maintain a global IGS real-time GNSS tracking
network.
2.	Enhance and improve selected IGS products.
3.	Generate real-time products.
4.	Investigate standards and formats for real-time data collection,
data dissemination and delivery of derived products.
5.	Monitor the integrity of IGU predicted orbits and GNSS status.
6.	Distribute real-time observations and derived products to
real-time users.
7.	Support Network DGPS/RTK
8.	Encourage cooperation among real-time activities, particularly
in IGS densification areas.

Data Products
-----------------
There are two systems in operation for delivery of Real Time data
streams to RTPP contributors.

UDPRelay is operated by NRCan and delivers data from approximately 50
stations to authorised users, using standardised data formats and the
UDP protocol. An archiver application (RTIGSA) can decode the streams
and create binary files that can be converted to RINEX using the TEQC
application. An RT-IGS Multicast Receive Software (RTIGSMR) receives the
RTIGS data stream and provides users with a framework to develop
applications. Further information is available at
http://www.rtigs.net/architecture.php.

In the BKG NTRIP infrastructure, the data sources (receivers) transmit
their data to the broadcast servers using the NtripServer application. A
number of NtripCasters serve data to users in different communities
using the HTTP protocol. The IGS Caster www.igs-ip.net serves as the
RTIGS-NTRIP Interface for the IGS Pilot Project. It disseminates data
for nearly 150 stations, mainly in RTCM3 and RTIGS formats. A small
number of stations transmit in various flavours of RTCM2. Ntrip server
and Ntrip client software for various operating systems is available
under GPL. Further information can be found at
http://igs.bkg.bund.de/index_ntrip.htm.

Analysis Products
----------------------
A number of contributing Real Time Analysis Centres provide daily clock
RINEX and SP3 product files to ESOC, which performs the AC coordination
activities. The products are derived from the Real Time infrastructure
of the individual Analysis Centres. ESOC then computes a combination
clock product which is disseminated daily, along with a comparison
summary report. The algorithms used for the combination are the same as
will be used in the near future for providing a Real Time combination
product stream. This will comprise the clock corrections for the GPS
satellites and the IGU-predicted orbits.

The daily combination clock RINEX files and comparison reports are now
uploaded to the CDDIS data server and can be found under
ftp://cddis.nasa.gov/gps/products/rtpp/.

Target combination product performances are:
Satellite Clock Accuracy:		0.3 ns
Station Clock Accuracy:		0.3 ns
Orbit Accuracy:			At the level of the IGS Ultra
predictions
Latency (when available in RT):	10 s

The clock accuracy is measured as the RMS difference between the IGS
rapid or final clock solution and the combination solution, after
removing a common offset per epoch.

Analysis Centres
--------------------
A brief description of the Analysis Centres that are currently
contributing their solutions and their software infrastructure is given
below:

ESOC: The software system at ESOC is called RETINA (System for Real Time
Navigation) and it integrates core infrastructure elements for RT
processing (Job Scheduling and Monitoring, Graphics, History Files
allowing for streaming and retrieving historical data seamlessly,
network monitoring) with Real Time and Batch applications. Orbits are
generated using similar processes to the ultra-rapids at configurable
intervals (currently every 2 hours). A Real Time filter estimates
spacecraft clocks, receiver clocks, tropospheric zenith delays and phase
ambiguities. Automated comparisons are performed between batch and RT
products and with IGS Finals. The ESOC solution stream currently uses
data from the RTIGS server at NRCan. A second solution, ESOC2, is
running in parallel, using data from the NTRIP casters and including
measurements from approximately 40 stations. This makes use of the IGS
Ultra-Rapids for orbit information. Processing frequency is variable and
1-second processing is possible but currently ESOC is using a 5-second
frequency to maximise the online data span. Clock comparisons with IGS
are at the 0.2-0.45 ns level RMS, depending on network availability.

NRCan: A least-squares batch process computes orbits every hour for the
IGS Ultra-Rapid product.  The predicted portions of these orbits are
held fixed in a real-time sequential least-squares filter to estimate
the satellite clocks, receiver clocks, station tropospheric zenith
delays and phase ambiguities every two seconds. Orbit and clock
corrections referenced to the Broadcast ephemerides are disseminated in
a modified-RTCA (MRTCA) format with 4mm precision. Computing
infrastructure includes development, test and production servers.
Correction validation is performed in real-time and hourly. Combined
product accuracy (User Range Error, URE) is at the 10 - 15 cm level. A
real-time ionospheric product is generated over a regional
(North-American) coverage area and provided for single-frequency users.

BKG with TU Prague: The RTNet package is used in the context of the
Pilot Project to estimate clock biases, tropospheric zenith delays and
phase ambiguities every second. It currently uses orbits from the IGS
Ultra-Rapids. RTNet processing includes data from GPS and GLONASS. The
clock corrections are designed to correct the double difference
observable but absolute clock corrections will be provided in the future
for comparison/combination purposes. The accuracy is sub-decimetre over
Europe and is available within 5-7 minutes after filter initialisation.
Two additional software applications have been developed for peripheral
tasks:
(i) BNC, a GPL tool for feeding real-time GNSS engines with streams
coming in SOC, RTCMv2 or RTCMv3 format, converting streams to RINEXv2 or
v3 observation and navigation files and monitoring real-time GNSS
networks
 (ii) BNS, a tool for encoding clock and orbit corrections to RTCMv3 and
streaming to Ntrip broadcasters.

DLR: The RETICLE software estimates Real Time spacecraft clock biases
and drifts, receiver clock biases, tropospheric zenith delays and phase
ambiguities, using a Kalman filter. The software uses the IGS Ultras as
a source of orbit information.

GMV: The software is a GMV's own development, based on software
developed for Galileo for orbit determination and time synchronisation.
It has been tested with Near Real Time GPS and Giove data. It currently
produces products every hour. The SW is under improvement to increase
the frequency of outputs and to eventually include a Real Time data
acquisition and processing capability.

A number of additional centres are in the process of developing their
systems and expect to be able to contribute their solutions in the near
future. They include GFZ, Geo++, IGG, Chalmers, University of Newcastle,
TU Catalonia and KASI.

Formats and Protocols
---------------------------
Formats including RTCM 3.0, BINEX and RTCA are being investigated as
part of the PP activities. The objective is the development of an open
standard that will meet the needs of a future IGS Real-time Service.
This objective is being pursued during the pilot project through the
work of the Real Time Working Group.

In our efforts to develop standards and protocols for Real Time data and
product dissemination, we have found synergies with the RTCM community
(see www.rtcm.org). The IGS has applied for membership to the RTCM and
is actively participating in SC104, the Special Committee dealing with
Differential Global Navigation Satellite Systems (DGNSS). The advantage
of working through the RTCM is that the resulting formats will be
adopted by the receiver manufacturers, allowing seamless integration of
data and products between tracking receivers, data and analysis centres
and users in the field.

Users
-------
A number of proposals from users of the RTPP data and products have been
received. Until the Real Time products become available, users whose
objective is to evaluate the potential accuracies achievable are invited
to start downloading and processing the combination product in the form
of daily clock RINEX files from ftp://cddis.nasa.gov/gps/products/rtpp/.


Loukis Agrotis (RT Analysis Centre Coordinator)
Mark Caissy (RTWG Chair)



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