CDDIS Bulletin - December 1996
Volume 12 No. 2

In this issue:

• Cover: CORS Network (as of 12/96)
• Articles:
  • The CORS GPS Network
  • New CDDIS Web Site
  • Meteorological Data from GPS Sites
  • Review of SLR Data Use
• Data Bits and Bytes:
  • Recall of SLR Data from GPS Satellites
  • EGM96 Web Page Implemented
  • Permanent GPS Network Update
  • CDDIS Bulletin Survey (Reminder)
  • CDDIS Bulletin Survey (Form)

About the cover: The map on the cover of this issue of the CDDIS Bulletin shows the current CORS GPS network. At this time, over 85 receivers from various U.S. and cooperating agencies are participating. The article on page 2 of the bulletin discusses CORS in more detail.

The CORS GPS Network

NOAA's National Geodetic Survey (NGS) manages the Continuously Operating Reference Station (CORS) GPS network. The current (and future) network, as shown on the cover of this issue of the CDDIS bulletin and in the coverage map on the following page (courtesy of NGS), is currently composed of over 85 sites installed by several government agencies and cooperating institutions, including:

• NGS
• U.S. Coast Guard (USCG)
• NOAA Forecast Systems Laboratory (FSL)
• U.S. Army Corps of Engineers (COE)
• International GPS Service for Geodynamics (IGS)
• Federal Aviation Administration (FAA, future)

As stated on the CORS web site (http://www.ngs.noaa.gov/CORS/cors-data.html), a primary objective of CORS is "to monitor a particular site, which is determined to millimeter accuracy and provide local users a tie to the National Spatial Reference System". The network is composed of primarily Ashtech Z-12 and Trimble SSE receivers observing at a thirty second sampling rate. Typically, data are downloaded to a central data facility located at NGS in Silver Spring in real-time for the USCG sites and on an hourly basis for COE sites; other sites, such as IGS sites, are downloaded daily from IGS data centers. At the central data facility, the data are checked for quality, converted to RINEX, and placed in files appropriate for distribution and archiving. Furthermore, daily determination and monitoring of station position are performed as well as precise orbit generation in support of the IGS and other activities. Data are available from the NGS data facility in daily or hourly files; approximately one-month's worth of data are available on-line at one time.

For further information about CORS, visit the web site and/or contact:

William Strange (CORS Administration)
E-mail: bstrange@ngs.noaa.gov
Phone: (301) 713-3222 x135

Paul Spofford (CORS Administration)
E-mail: pauls@ngs.noaa.gov
Phone: (301) 713-3205 x158

Don Haw (CORS Information)
E-mail: don@ngs.noaa.gov
Phone: (301) 713-3208 x167

Jim Drosdak (CORS Information)
E-mail: jimd@ngs.noaa.gov
Phone: (301) 713-3208 x161

Neil Weston (CORS Newsletter)
E-mail: nweston@ngs.noaa.gov
Phone: (301) 713-3169 x146

New CDDIS Web Site

A new CDDIS web site has been designed and implemented using the same URL as before, http://cddis.gsfc.nasa.gov/cddis.html. This new web site uses frames to provide for ease in navigation; a version for browsers that do not support frames is also available. Other new features include descriptions of space geodetic techniques and data sets, updated information about supported data sets, and access to selected on-line data sets (both anonymous ftp and password-protected ftp for authorized users). Unfortunately, the site is rather slow due to internal networking problems and limitations of the VAX server. The CDDIS staff hopes to procure new hardware in the next few months; transfer of the web applications to this new computer will be a top priority. Users should contact Carey Noll if you have any questions about or suggestions for the new web site.

Meteorological Data from GPS Sites

Since April 1995, the CDDIS has been archive meteorological data from several GPS stations collocated at VLBI sites. These sites are Greenbelt (GODE) MD, Fairbanks (FAIR) AK, Kokee Park (KOKB) HI, and Wettzell (WETT) Germany. The GeoForschungsZentrum (GFZ) is now providing meteorological data for several of their GPS sites, including Potsdam (POTS) and Oberpfaffenhofen (OBER) Germany and Kitab (KIT3) Uzbekistan. In addition, the Defence Mapping Agency (DMA) includes meteorological data from their site in Bahrain (BAHR) in their daily data deliveries. All meteorological data are in RINEX format and stored on the CDDIS in subdirectories by day, e.g., GPS#:[GPSDATA.yyddd.yyM], where # is the GPS disk number (1, 2, 5, or 6), yy is the year, ddd is the day of year.

Review of SLR Data Use

Michael Pearlman/SAO

As part of CSTG SLR/LLR Subcommission activities, Dr. Pearlman recently conducted a survey of SLR analysts. The results of the survey were published in the May issue of the CSTG SLR/LLR Subcommission Newsletter. An update to this survey is printed in this issue of the CDDIS Bulletin.

In a recent survey of the analysis groups, the following questions were asked:

• Which satellites are you currently using in your analyses?
• What are the applications for each satellite (station position, motion, gravity field, Earth rotation, altimetry, etc.) ?
• Are you receiving sufficient data for your analyses?
• Are you receiving sufficient geographic coverage?
• Are the data of sufficient accuracy for your applications?
• Are there other satellites that you plan to use in the future? For what applications?
• What else do you need that you are not getting?

Some general comments received from the respondents regarding areas that need attention were:

• Data quality:
  • Large disparity in data quality among stations
  • Often, data of poor quality are worse than no data at all
  • Too many systems with poor data quality
• Data Volume:
  • Tracking coverage from some stations is very sporadic and sparse
  • Some stations do not provide a minimum level of contribution
  • Tracking gaps of more than ten days are a problem
  • Weeks with less than 100 LAGEOS passes are sparse
  • Much more data required on ETALON-1 and -2, GPS-35/36, and GLONASS
  • Significant amount of bad ETALON data
  • GFZ-1 data is insufficient
  • STELLA and STARLETTE often marginal (but still useful)
• Geographic Coverage:
  • Better geographic coverage needed in the Southern hemisphere, Russia, and China
• Temporal Coverage:
  • Tracking gaps over weekends are a problem
  • LAGEOS-1 and -2 data is adequate for three-day resolution, but inadequate for one-day resolution
  • More data on STELLA, STARLETTE, and AJISAI required for one-day resolution of higher order perturbations
• Local Surveys:
  • More reliable local surveys and more information about the surveys and local eccentricities required
• Operational Follow-up:
  • Lack of communications with some stations regarding problems
• Satellites:
  • More satellite targets of high quality needed

So far, sixteen groups have responded to the survey. Information from the survey is tabulated in Table 1 on the following pages.

Table 1. SLR Analysis Survey Results

DATA BITS AND BYTES

This section is designed to give brief notices of special events, new data sets, or problems encountered in recent months. The CDDIS staff believes the user community should be informed of these problems in order to ensure that the best results possible are produced from data issued by the archive.

Recall of SLR Data from GPS Satellites

Van Husson of AlliedSignal Technical Services (ATSC) reports that a number of SLR normal point pass segments from the NASA SLR network for the GPS satellites potentially contain no useful data (i.e., all noise) and are being recalled from the CDDIS to avoid the future use of these known problem data. The data in all of these recalled passes, listed in Table 2 below, are currently under evaluation for the possibility of useful data. If any data are found in these pass segments, normal points will be reissued at a later date. Furthermore, as of December 06, 1996, new procedures have been implemented at ATSC to prevent SLR data with known problems from being distributed to the user community. The ATSC and CDDIS staff regret any inconvenience these data problems may have caused the user community.

Table 2. GPS-35 and -36 SLR Data Recalled

Note: * The SLR data files for GPS-35 are stored in the on-line CDDIS data archive in the directory SLR_DATA:[SLRQL.GPS35.1996]; the GPS-36 SLR data files are stored in the directory SLR_DATA:[SLRQL.GPS36.1996].

In addition, since they contain no useful data, the following daily GPS-36 files have been deleted from the CDDIS on-line data archive (directory SLR_DATA:[SLRQL.GPS36.1996]).

NEW_QL961012.GPS36
NEW_QL961030.GPS36
NEW_QL961116.GPS36
NEW_QL961118.GPS36
NEW_QL961123.GPS36

EGM96 Web Page Implemented

Frank Lemoine (NASA GSFC) has recently implemented a set of web pages to describe the NASA and National Imagery and Mapping Agency (NIMA) joint geopotential model, EGM96. EGM96 is a spherical harmonic model of the Earth's gravitational potential complete to degree and order 360. The web pages summarize the development of the model and include information presented at the International Symposium on Gravity, Geoid, and Marine Geodesy in Tokyo, Japan, September 30 through October 04, 1996. The URL for these pages is http://cddis.gsfc.nasa.gov/926/egm96/egm96.html . The model coefficients and other products are also available via anonymous FTP on cddis.gsfc.nasa.gov, in the directory ANON_DIR:[EGM96].

Permanent GPS Network Update

Since the October 1996 issue of the CDDIS Bulletin, the CDDIS has archived data from the following new GPS tracking sites:

Table 3. New GPS Tracking Sites

CDDIS Bulletin Survey (Reminder)

A survey on the utility and distribution methods was included in the October issue of the CDDIS Bulletin. Many readers responded to the survey and the CDDIS staff thanks these people for their prompt replies. We have included the survey once again in this issue in the hopes that others will be inclined to respond. Once again, our thanks in advance to all who take the time to respond.

CDDIS Bulletin Survey

This survey has been formulated to increase the utility of the CDDIS Bulletin and to assess the need for continuation of hardcopy distribution. We would appreciate forthright answers to these questions and have included a return envelope for your convenience. Alternatively, the survey can be faxed to the number provided or e-mailed to noll@cddis.gsfc.nasa.gov.

Name:
Agency:
Address:

__ I regularly read the CDDIS Bulletin
__ I sometimes read the CDDIS Bulletin
__ I never read the CDDIS Bulletin; please remove my name for distribution

Suggestions for improving the bulletin:

Suggestions for future articles:

__ I would prefer to read the electronic version of the bulletin on the WWW
__ I would prefer to ftp a postscript version of the bulletin if available
__ I prefer to receive a hardcopy version of the bulletin through the mail

Please return this survey to:
Carey E. Noll
NASA GSFC
Code 922
Greenbelt, MD 20771
Fax: 301-286-0213