The Crustal Dynamics Data Information System (CDDIS) has supported the International GPS Service (IGS) as a global data center since 1992. The CDDIS activities within the IGS during 2002 are summarized below; this report also includes any changes or enhancements made to the CDDIS during the past year. General CDDIS background and system information can be found in the CDDIS data center summary included in the IGS 1994 Annual Report (Noll, 1995) as well as the subsequent updates (Noll, 1996, Noll, 1997, Noll, 1998, Noll, 1999, and Noll, 2001)

Noll, Carey E.

NASA Technical Reports Server

CDDIS 2002 Global Data Center Report
137
CDDIS 2002 Global Data Center Report
Carey E. Noll
Terrestrial Information Systems Branch
NASA Goddard Space Flight Center, Code 922
Greenbelt, MD  20771
Introduction
The Crustal Dynamics Data Information System (CDDIS) has supported the International GPS
Service (IGS) as a global data center since 1992.  The CDDIS activities within the IGS during
2002 are summarized below; this report also includes any changes or enhancements made to the
CDDIS during the past year.  General CDDIS background and system information can be found
in the CDDIS data center summary included in the IGS 1994 Annual Report (Noll, 1995) as well
as the subsequent updates (Noll, 1996, Noll, 1997, Noll, 1998, Noll, 1999, and Noll, 2001).
System Description
The CDDIS archive of IGS data and products are accessible worldwide through anonymous ftp.
The CDDIS is located at NASA’s Goddard Space Flight Center (GSFC) and is accessible to users
24 hours per day, seven days per week.
Computer Architecture
The CDDIS is operational on a dedicated UNIX server.  All GPS data and product files are
archived in a single filesystem, accessible through anonymous ftp, and are stored in UNIX
compressed format.  At present, nearly 400 Gbytes of on-line disk space is devoted to the
storage of daily GPS tracking data and products.  GPS data since 1995 and IGS products since
1992 are available on-line.
Archive Content
As a global data center for the IGS, the CDDIS is responsible for archiving and providing access
to both GPS data from the global IGS network as well as the products derived from the analyses
of these data.
GPS Tracking Data
The GPS user community has access to the on-line and near-line archive of GPS data available
through the global archives of the IGS.  Operational and regional data centers provide the
interface to the network of GPS receivers for the IGS global data centers.  Nearly forty of these
IGS data centers make data available to the CDDIS from selected receivers on a daily (and in
some cases hourly and/or sub-hourly) basis.  The CDDIS also accesses the archives of the other
two IGS global data centers, Scripps Institution of Oceanography (SIO) in La Jolla California and
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IGS 2001 / 2002 Technical Reports
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the Institut Géographique National (IGN) in Paris France, to retrieve (or receive) data holdings
not routinely transmitted to the CDDIS by an operational or regional data center.  Over 87K
station days from 313 distinct GPS receivers were archived at the CDDIS during the past year; a
complete list of these sites can be found at URL ftp://cddisa.gsfc.nasa.gov/pub/reports/gpsdata/
cddis_summary.2002.  Table 1 below summarizes the types of GPS data archived at the CDDIS.
Daily GPS Data Files
Once the daily RINEX data files arrive at the CDDIS, these data are quality-checked,
summarized, and archived to public disk areas in daily subdirectories; the summary and inventory
information are also loaded into an on-line data base.  These metadata are utilized to generate
various reports on data holdings and data latency.
The CDDIS daily GPS tracking archive consists of observation (in both RINEX and “compact”
RINEX format), navigation, and meteorological data, all in compressed (UNIX compression)
RINEX format.  Furthermore, summaries of the observation files are generated by the UNAVCO
quality-checking program TEQC (Estey 1999) and are used for data inventory and quality
reporting purposes.  During 2002, the CDDIS archived data on a daily basis from an average of
280 stations.  One reason for the large increase in stations is that as of June 11, 2002,
GPS+GLONASS receiver data were archived in what was previously a GPS-only directory
structure in the CDDIS archive.  This new archive structure for the IGS regional and global data
centers was mandated by the Central Bureau under recommendation from the IGLOS-PP.  In
2002, the CDDIS GPS archive of daily GPS data files totaled over 31 Gbytes in volume
(compact RINEX format only); this figure represents data from over 87K observation days.  Of
the 280 or more sites archived each day at the CDDIS, not all are of “global” interest; some, such
as those in Southern California, are regionally oriented.  The CDDIS receives data from these
sites as part of its NASA archiving responsibilities.
At this time, the CDDIS on-line archive of daily GPS data contains data from January 1995
through the present.  Prior to mid-2002, these data are available in compact RINEX only; later
data are archived in both compact RINEX and uncompacted RINEX formats.  As the disks
supporting this archive fill up, older, uncompact RINEX observation data are deleted.  However,
in recent weeks, the GPS data are typically only available in compact RINEX format due to
severe disk space constraints on host cddisa.gsfc.nasa.gov.
The majority of the data delivered to and archived in the CDDIS during 2002 was available to the
user community within six hours after the observation day.  Nearly sixty percent of the data
from the global sites delivered to the CDDIS were available within three hours of the end of the
observation day.
Hourly GPS Data Files
Since 2000, many IGS operational/regional data centers transmit hourly data files to the global
data centers.  Within minutes of receipt, the files are archived to separate subdirectories
(/gps/nrtdata) by day and hour on the CDDIS.  These data are retained on-line for three days.
After that time, the hourly data files are deleted; the daily file, transmitted through normal
CDDIS 2002 Global Data Center Report
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channels with a typical delay of one to two hours, will have been received and archived already
and thus the hourly data are of little use.  Furthermore, to ensure the most rapid delivery of these
data to the user community, no validation or checks on data quality are performed.  In 2002,
approximately sixty percent of these hourly data files were available to the user community
within twenty minutes of the end of the hour.  Over 160 sites (both GPS and GLONASS+GPS)
transmitted hourly data files to the global data centers in 2002.
The site-specific ephemeris data files for each hour are decompressed and then appended to a
single file that contains the orbit information for all GPS satellites for the day up through that
hour.  This merged ephemeris data file, named hourddd0.yyn.Z (where ddd is the day of year and
yy is the year), is then copied to the daily subdirectory in the hourly filesystem
(/gps/nrtdata/yyddd).  At the end of the day, this file is copied to the corresponding subdirectory
under the daily filesystem (/gps/gpsdata/yyddd/yyn) and renamed to brdcdd0yyn.Z.  Users can
thus download this single daily file instead of all broadcast ephemeris files from the individual
stations.
High-Rate GPS Data Files
In May of 2001, the CDDIS began the archive of high-rate (typically one-second) GPS data in
support of the IGS Pilot Project for Low Earth Orbiting (LEO) Missions.  The data are made
available to the CDDIS from four principal sources, JPL, GFZ, NRCan, and ESA as well as other
operations centers (e.g., ASI, GOPE, UNB, etc.).  The RINEX data are archived in files
containing fifteen minutes of data using the filenaming convention ssssdddhmi.yyt.Z where ssss is
the monument name, ddd is the day of year, h is the hour (a-z), mi is the minute (00, 15, 30, 45),
yy is the year, and t is the file type (d, m, n).  On average during 2002, the CDDIS archived high-
rate data from 52 sites totaling approximately 250 Mbytes per day, and a total of 90 Gbytes for
the year.  
Meteorological Data
The CDDIS currently receives meteorological data from over fifty sites.  The meteorological data
provided are dry temperature, relative humidity, and barometric pressure at thirty minute
sampling intervals.  These data are stored on CDDIS with the daily GPS observation and
navigation data files in parallel subdirectories.
LEO GPS Data
The CDDIS proposed to serve as a data center supporting the IGS Pilot Project for Low Earth
Orbiting (LEO) Missions in 2000.  In 2002, the CDDIS established an archive of space-borne
GPS receiver data from selected missions (e.g., SAC-C and CHAMP); future missions supported
will include Jason-1, GRACE, and ICESat.
IGS Products
The seven IGS data analysis centers (ACs) retrieve the GPS tracking data on a daily basis from
the global data centers to produce daily orbit and clock products as well as weekly Earth rotation
IGS 2001 / 2002 Technical Reports
140
parameters (ERPs) and station position solutions; the seven IGS associate analysis centers
(AACs) also retrieve IGS data and products to produce station position solutions.  The CDDIS
archives the products generated by both types of IGS analysis centers.  These files are delivered
to the CDDIS by the IGS analysis centers to individual user accounts, copied to the central disk
archive, and made available in compressed format on the CDDIS by automated routines that
execute several times per day.  The IGS Analysis Coordinator then accesses the CDDIS (or one
of the other global analysis centers) on a regular basis to retrieve these products and derive the
combined IGS orbits, clock corrections, and Earth rotation parameters as well as to generate
reports on data quality and statistics on product comparisons.  The CDDIS currently provides
on-line access through anonymous ftp or the web to all IGS products generated since the start of
the IGS Test Campaign in June 1992.
Regional Network Associate Analysis Centers (RNAACs) routinely generate station position
solutions for regional networks in Software INdependent EXchange (SINEX) format.  The three
Global Network AACs (GNAACs) perform a comparison of these files and submit the resulting
SINEX files to the CDDIS.  The GNAACs also access the SINEX files from the IGS ACs and
RNAACs and produced comparison and combined, polyhedron station position solutions.  The
CDDIS provides “short-SINEX” files, designated with an .ssc extension, for all AC and AAC
SINEX files.  These files contain the site information from the SINEX file but no matrices.  All
RNAAC solution files are also stored in the weekly IGS product subdirectories.  The official IGS
combined weekly SINEX solutions and cumulative combined SINEX solutions generated by the
IGS Reference Frame Coordinator are also available in the weekly IGS product subdirectories.
Both the rapid (designated IGR) and the predicted orbit, clock and ERP (designated IGP)
combined products generated by the IGS Analysis Coordinator continued to be available through
2002.  Furthermore, a new product, the IGS ultra-rapid combination (designated IGU) were made
available twice daily. The IGS global data centers, including the CDDIS, download the rapid,
predicted, and ultra-rapid products from the Analysis Coordinator and made them available in a
timely fashion to ensure their usefulness to the user community.
The CDDIS also continued to archive combined troposphere estimates in directories by GPS
week (i.e., /gps/products/wwww/trop, where wwww is the GPS week number).  Global ionosphere
maps of total electron content (TEC) from the IONEX AACs were also archived in
subdirectories by day of year (i.e., /gps/products/ionex/yyyy where yyyy is the four-digit year).
The CDDIS archived products generated by the individual analysis centers contributing to the
IGS LEO Pilot Project (LEO-PP).  Thirteen AACs have thus far submitted products for review
by the LEO-PP analysis coordinator; these files are archived in subdirectories by AAC within
filesystem /gps/products/leopp.
Supporting Information
Daily status files of GPS data holdings, reflecting timeliness of the data delivered as well as
statistics on number of data points, cycle slips, and multipath continue to be generated by the
CDDIS.  By accessing these files, the user community can receive a quick look at a day’s data
availability and quality by viewing a single file.  The daily status files are available through the
CDDIS 2002 Global Data Center Report
141
web at URL ftp://cddisa.gsfc.nasa.gov/pub/reports/gpsstatus/.  The daily status files are also
archived in the daily GPS data directories.
Ancillary information to aid in the use of GPS data and products are also accessible through the
CDDIS.  Weekly and yearly summaries of IGS tracking data (both daily and high-rate) archived
at the CDDIS are generated on a routine basis and distributed to the IGS user community through
IGS Report mailings.  These summaries are accessible through the web at URL
ftp://cddisa.gsfc.nasa.gov/pub/reports/gpsdata       and        ftp://cddisa.gsfc.nasa.gov/pub/reports/
igshrdata.  The CDDIS also maintains an archive of and indices to IGS Mail, Report, Network,
and other IGS-related messages.
GLONASS Data and Products
During 2002, the CDDIS continued as a global data center for GLONASS data and products in
support of the IGLOS-PP Call for Participation issued in 2000.  The CDDIS archived
GLONASS data from over fifty sites.  As of June 2002, data from GPS+GLONASS receivers are
archived within the GPS directory structure to improve data retrieval for the user community;
data from GLONASS-only receivers continue to be archived in the /igex/data filesystem.
GLONASS products from three analysis centers (BKG, ESA, and MCC) as well as the Analysis
Coordinator (at the Technical University of Vienna) were also made available to the public.
GLONASS products are accessible via anonymous ftp in the filesystem /igex/products.
System Usage
Figures 1 through 3 summarize the monthly usage of the CDDIS for the retrieval of GPS and
GLONASS data and products for 2002.  Figure 1 illustrates the amount of GPS and GLONASS
data retrieved by the user community during 2002, categorized by satellite (GPS or GLONASS)
and type (daily, hourly, high-rate).  Nearly forty million files were transferred in 2002, with an
average of over three million files per month.  Furthermore, nearly 170K GPS product files were
retrieved each month from the CDDIS; less than 1,000 GLONASS product files were retrieved
each month.  Figures 2 and 3 illustrate the profile of users accessing the CDDIS IGS archive
during 2002.  Most accesses were through network gateways, which did not yield sufficient
information about the user.  Figure 3 displays the usage information by geographic region; the
majority of CDDIS users are from hosts in North America.
Publications
The CDDIS staff attended several conferences during 2002 and presented papers on or conducted
demos of their activities within the IGS, including:
• Noll, Carey E. “2001 IGS Data Center Reports”, 2001 IGS Annual Report (submitted in
2002)
• Noll, Carey E.  “CDDIS 2001 Global Data Center Report”, 2001 IGS Technical Report
(submitted in 2002)
• Noll, Carey E., “The CDDIS Data Center – NASA’s Space Geodesy Data Archive”, EOS
Transactions American Geophysical Union, May 2002.
IGS 2001 / 2002 Technical Reports
142
• Noll, Carey E.  “Current Status of IGS Data Centers” IGS Network, Data, and Analysis
Center Workshop, Ottawa, Canada, April 2002, in press.
• Noll, Carey E. and Maurice Dube.  “The IGS Global Data Center at the CDDIS – an
Update” IGS Network, Data, and Analysis Center Workshop, Ottawa, Canada, April
2002, in press.
Electronic versions of these and other publications can be accessed through the CDDIS on-line
documentation page on the web at URL http://cddisa.gsfc.nasa.gov/reports.html.
Future Plans
The AlphaServer 4000 computer supporting the CDDIS has been operational for over five years.
In 2003, a Linux-based system (and backup server) will be procured.  Over four terabytes of
RAID storage and a dedicated tape backup system will also be purchased for this new computer
facility.  Migration to the Linux operating system will begin in late 2003.
Contact Information
To obtain more information about the CDDIS IGS archive of data and products, contact:
Ms. Carey E. Noll Phone: (301) 614-6542
Manager, CDDIS Fax: (301) 614-5970
Code 920 E-mail: Carey.E.Noll@nasa.gov
NASA GSFC WWW: http://cddisa.gsfc.nasa.gov/cddis_welcome.html
Greenbelt, MD 20771 http://cddis.nasa.gov
Acknowledgments
The author would once again like to thank the Raytheon Information Technology and Scientific
Services (RITSS) contractor staff, Dr. Maurice Dube, Ms. Ruth Kennard, and Ms. Laurie
Batchelor.  The recognition and success of the CDDIS in many international programs is
significantly helped by their continued dedicated, consistent, professional, and timely support of
its daily operations.
References
Estey, L.  “TEQC Summary” in IGS Network Systems Workshop Proceedings.  March 1999.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 1994 Annual Report.  September 1995.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 1995 Annual Report.  September 1996.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 1996 Annual Report.  November 1997.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 1997 Technical Report.  October 1998.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 1998 Technical Report.  November 1999.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 1999 Technical Report.  November, 2000.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 2000 Technical Report.  In press.
Noll, C. E.  “CDDIS Global Data Center Report” in IGS 2001 Technical Report.  In press.
CDDIS 2002 Global Data Center Report
143
Table 1:  GPS Data Summary
Data Type
Sample
Rate
Avg. No.
Sites/Day
Avg.
Volume/Day
Total
Volume/Year
Data Format
Available
On-line
Daily GPS 30 sec. 260 (1) 90 Mb 31 Gb
RINEX and
compact
RINEX (2)
Since 1995
(3)
Hourly
GPS
30 sec. 160 (1) 70 Mb 350 Mb
Compact
RINEX
Last 5 days
High-rate
GPS
1 sec. 50 250 Mb 90 Gb
Compact
RINEX
Since May
2001
LEO GPS 10 sec. 2 (4) 2 Mb 750 Gb
Compact
RINEX
Since 2002
Notes: (1) Includes data from GPS+GLONASS sites
(2) Amount of non-compact RINEX data available on-line dependent upon available disk space
(3) Some older data migrated to temporary disk areas until more disk space available; data since
1992 archived in CDDIS
(4) Indicates number of LEO satellites
0
500,000
1,000,000
1,500,000
2,000,000
2,500,000
3,000,000
3,500,000
4,000,000
4,500,000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month (2002)
N
um
be
r 
of
 F
ile
s
GLONASS Data
GPS High Rate
GPS Hourly
GPS Daily
Figure 1:  Number of GPS data files transferred from the CDDIS in 2002
IGS 2001 / 2002 Technical Reports
144
Commercial
22%
Education
23%
Network
40%
Non-Profit
1%
Unknown
0%
Government
14%
Figure 2:  Distribution of IGS users of the CDDIS in 2002
Asia
9%
Europe
29%
Pacific
0%
Africa
1%
North America
57%
Australia
4%South America
0%
Figure 3:  Geographic distribution of IGS users of the CDDIS in 2002


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