understanding and the ability to predict change. The 
Center is also participating in NASA's Earth Observing 
System (EOS) Program and plans to process and 
archive land-related data from sensors aboard the EOS 
polar-orbiting platforms. The Center was selected to 
serve as one of the principal EOS data archive and 
distribution facilities because of its demonstrated 
capabilities to archive, process, and distribute large 
volumes of data and related products from remote 
sensing satellite programs. 
In its role as the National Satellite Land Remote 
Sensing Data Archive, the Center is continually 
investigating and exploiting new technologies to ensure 
the long-term availability of these data. For example, 
the Center is converting its current holdings of the U.S. 
Landsat archive to new, more stable archive media. 
This media conversion effort is providing the opportunity 
to assess the overall quality of the Landsat archive data 
and metadata and to collect digital browse images for 
user viewing through state-of-the-art information 
systems, such as the USGS Global Land Information 
System (GLIS). In addition, the Center is experimenting 
with image data subsetting and compression techniques 
to improve the overall effectiveness of the delivery of 
large-scale satellite images over conventional wide-area 
networks. Following are brief overviews of these 
examples of technology exploitation by the Center. 
2. LANDSAT ARCHIVE MEDIA CONVERSION 
Landsat data constitute the largest data set in the 
National Satellite Land Remote Sensing Data Archive. 
This data set includes more than 634,000 scenes of 
Landsat Multispectral Scanner (MSS) data, representing 
approximately 26 terabytes (TB) of data, that were 
acquired between 1972 and 1989 and are stored on 
34,920 high density tapes (HDT). The archive also 
contains more than 214,000 Landsat Thematic Mapper 
(TM) scenes, representing approximately 41 TB of data, 
that have been acquired since 1982 and are stored on 
approximately 26,000 HDT. 
To ensure the long-term availability of these data, the 
Center is converting these data to new, more stable 
archive media. In 1991, the Center funded the 
development of a specially configured system called the 
Thematic Mapper and Multispectral Scanner Archive 
Conversion System (TMACS) to transcribe the Landsat 
archive data to Digital Cassette Recording System 
Incremental (DCRSI) cassette tapes. The DCRSI tape 
drive implementation was selected because of its 
proven reliability, 48 gigabyte (GB) storage capacity, 
incremental buffering capability, and 107 megabyte 
(MB) per second transfer rate. 
In addition to transcribing media, TMACS also supports 
data quality assessment and metadata generation and 
update operations. After the data have been 
transcribed to the DCRSI cassette, the tape is played 
back and a subset of the data is extracted. An orbital 
model is used to delineate individual scenes by 
calculating scene center latitude and longitude and the 
beginning and ending scan lines for each scene. 
Reduced resolution browse images are produced and 
visually assessed for cloud cover and image quality. 
The image data quality information is used to update 
archive metadata. The browse images are compressed 
and subsequently transferred to optical disks for access 
by users of the Global Land Information System. 
Converting the HDT data to DCRSI cassettes improves 
the reliability of the data and saves archive space. 
When the conversion project is completed, more than 
39,000 HDT's will have been transcribed to 
approximately 2,000 digital cassette tapes. The smaller 
physical dimensions of the DCRSI medium will allow the 
Center to achieve a 40-to-1 reduction in required 
archive storage space. 
The Center has begun a program to monitor data 
integrity during the lifetime of the DCRSI tapes by using 
test patterns written at the beginning, middle, and end 
of each tape during data transcription. The DCRSI 
recorder's internal bit-error-rate (BER) test generator is 
used to write and then read the patterns and to 
compute BER statistics during verification processing. 
By establishing a baseline BER for each tape and 
maintaining a history of BER values through subsequent 
operations, the Center will establish quantitative quality 
criteria to help identify media degradation as early as 
possible. In addition, weekly system tests are 
performed on several prerecorded test tapes, allowing 
operations staff to verify, by comparing BER values, 
that the hardware is functioning at an acceptable level. 
Conversion of the MSS data was begun in December 
1992. Concurrent conversion of the TM data was 
begun in November 1993. At the current production 
rate, the MSS archive conversion will be completed in 
June 1994 and the TM data conversion should be 
completed by the end of 1996. 
3. ARCHIVE DATA RECOVERY 
A major problem encountered during the Landsat 
archive media conversion project was the recovery of 
data from tapes affected by hydrolysis. Hydrolysis is 
the process in which tape binders absorb atmospheric 
moisture that causes the binders to change chemically, 
creating a sticky residue buildup on the tape winder and 
cleaner tissues and rollers, as well as on the tape paths 
and heads of the tape recorders. This buildup can 
cause head wear, damage to the tape transport, 
physical damage to the tape, and the potential 
contamination of other tapes. Hydrolysis is exacerbated 
by humid conditions (while tapes are in storage or 
during transportation), but it also occurs at low humidity 
over extended periods of time, possibly as part of the 
natural aging process of older magnetic tapes. Most of 
the affected tapes at the Center were manufactured 
between 1978 and 1981 by two manufacturers. The 
hydrolysis process cannot be stopped; however, it can 
be temporarily reversed long enough to retrieve the data 
and copy it to new media. 
Since April 1993, the Center has successfully recovered 
more than 15,000 Landsat scenes from nearly 700 
affected tapes by applying low levels of heat (130? F) 
to the tapes for 24 hours to temporarily reverse the 
degrading condition. This technique, called baking, 
allows the tape to be played without physical damage to 
the tape or the drive. Local tests show that data can be 
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