High Density Digital Tape (HDDT) Landsat Data Recovery

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Detailed Description

High Density Digital Tapes (HDDT) containing Landsat scenes arrive at EROS from International Cooperators located around the world. Operators use a baking process to recover the imagery from the HDDTs.


Date Taken:

Length: 00:09:29

Location Taken: Sioux Falls, SD, US


For more than 40 years,
Landsat satellites have

acquired data from across
the globe, with the National

Satellite Land Remote
Sensing Data Archive at the

U.S. Geological Survey (USGS) 
Earth Resources Observation

and Science (EROS) Center in
Sioux Falls, SD, holding the

single most geographically
and temporally rich

collection of Landsat data
in the world.

Over those 40+ years,
International Cooperators (ICs)

have played a critical role in

the mission’s success,
acquiring Landsat imagery at

numerous International Ground
Stations (IGS). Until recently,

most of the data held
internationally were unique,

relative to each station's
area of coverage, and were

not duplicated in the
USGS archive. 

In 2010, the Landsat Global
Archive Consolidation (LGAC)

initiative began, with a goal
to consolidate the Landsat data

archives of all international
ground stations, thus making

the data more accessible to
the global Landsat community

and significantly increasing
the frequency of observations

over a given area of interest
to improve scientific uses

such as change detection
and analysis. This initiative

is possible only with the
tremendous support given by

the Landsat International
Cooperators. One critical

component to successfully
fulfilling the LGAC initiative

was the ability to recover
data from old, obsolete media

for which newer media sources
were not available, including

Landsat data contained on
thousands of High Density

Digital Tapes, also known
as HDDTs. Data recovery from

HDDTs is a painstaking process
that is dependent upon

functional aged equipment and
diligent operator support.

In an effort to highlight the
tremendous work performed by

the data recovery technicians
and to shed some light on the

complexities of the data
recovery process, this

presentation will summarize
the HDDT Landsat data

recovery process.
First, boxes and crates

containing HDDTs arrive at
EROS from ICs located around

the world. HDDTs are not
small or light, with each

tape weighing approximately

a standard-sized reel is about

contains one satellite downlink.

HDDTs labelled as Landsat
Thematic Mapper (TM) are sorted

out and processed first.
HDDTs labelled as Multispectral

Scanner (MSS) or tapes that
show signs of mold are set

aside initially. MSS tapes
are processed at a later time

by changing a setting in the
Ingest software.  Moldy tapes,

on the other hand, need to be
evaluated, run through cleaners,

and then can usually proceed
through the data

recovery process.
Once the sorting has been

completed, operators retrieve

about 20 of the TM HDDTs and
cart them to the Digital

Camera Lab, which consists of
a staging area for the HDDTs,

four ovens for baking, BOW
cleaners for cleaning the

tapes, drives used for
exercising the tapes, ingest

drives used to read in the
data, and a computer station

that displays the data being
captured. Some of the

equipment used in the data
recovery process was graciously

donated by the ICs and has
been vital to the success

of the project.
There are four primary steps

in the HDDT data recovery
process: baking, cleaning,

exercising, and reading.
HDDTs from different countries

require different variations
of the data recovery process,

or recipe, and in some cases,
multiple recipes for the same

International Ground Station
have been necessary.

Identifying the proper recipe
can be challenging and

requires patience and
persistence from the operators.

The first step in the HDDT
data recovery process is baking.

Baking is necessary because
of two different but related

scenarios. First, the tape
is sticking to the rollers

and guides on the drive,
which causes damaged or

broken tape, and second,
there is a marginal or bad

quality read of data
from the tape.

Each of the four ovens
in the Digital Camera Lab

have the same specifications.
The ovens are 120 volts,

consist of an enclosed unit

with a door and contain a
low level heat source and

small fan for air circulation.
The ovens are big enough to

handle 10 HDDTs at a time and
have adjustable settings for

temperature and automatic
shutoff. The optimal number

of tapes that can be loaded
at once varies by International

Ground Station based on trial
and error. Once the HDDTs are

loaded in the oven, the door
is closed and the oven is

turned on. The HDDTs bake
in the oven for 24 hours at

after which the operators

either let them cool in the
oven or set them on a wire

rack to cool. It is important
that the tapes are read soon

after the baking process as
the tape tends to quickly

revert back to the condition
it was in before the bake.

The second step in the HDDT
data recovery process is

cleaning. Cleaning is necessary
to remove dirt from the tape

and is performed by using a
BOW cleaner. The BOW cleaner

contains a cloth ribbon that
runs against the tape to help

remove dirt off of the tape.
Operators load the tape onto

the BOW cleaner and run it
through twice, with back-to-back

cleanings. The ribbon turns
really slowly and lasts about

two weeks before needing to
be replaced. If a tape has

not been baked, the ribbon
will contain much more black

dirt or soot on it when

The third step in the HDDT
data recovery process is

exercising. To begin with,
operators have to first clean

the drive. They use alcohol,
Q-tips, and wipes to clean

the tape path. Exercising
the HDDT packs the tape

better on the reel, which
then leads to a better read.

The optimal forward and
rewind speeds can vary by

International Ground Station.
The fourth and final step in

the HDDT data recovery process
is reading. Similar to the

exercising step, the tape
path is cleaned before

reading a new tape. Next,
an HDDT is loaded on the

Ingest drive and the drive
is set to run at the speed

at which the data was recorded,
also known as the downlink

frequency. The HDDT cannot
be read at a faster

or slower speed.
As the tape is being read,

there are different things

operators need to watch for,
including light activity.

When the Status and Control
display shows only a couple

flickers of lights, it is
an indication of a good read.

If the Status and Control
display shows an abundance

of flickering lights or if
solid lights appear, it is

an indication of a bad read.
The read head contains 28

grooves and each groove on
the head corresponds to a

channel. If a light is
flashing for one of the

channels, it is that
section of the tape that

the corresponding groove
is reading that may have

dirt or soot on it. 
At the same time, operator

must watch the computer
monitors that are

displaying the data
capture. As the data is

read off the tape, the
image scrolls on the left

monitor along with a data
range. The right monitor

displays statistics relating
to the image quality.

If the data range rises
and the data quality drops,

that is an indication of a
bad read. When the read is

not going well, the operator
will stop the tape, rewind

it 100 to 200 feet to when
the read was going well,

clean the tape path, and
then retry reading the tape.

Rewinding the tape and
re-cleaning the equipment can

make a huge difference in
the read quality of the data.

This can be seen very clearly
in the before-and-after image

It is rare that tapes from

some International Ground
Stations can be read all the

way through with no stops,
and some tapes need to be

exercised multiple times to
try to repack the tape more

smoothly. Particularly problematic
tapes are set aside and returned

to at a later time. These tapes
will need to go through the entire

data recovery process again when
they are reattempted.

As tapes are completed, they are
placed in a finished pile and

later sent to the warehouse
for disposal.

Recovering data from HDDTs is
a challenging process.

Operators have to make frequent
adjustments to recipes,

equipment must be diligently
cared for, and multiple read

attempts are frequently required.
While the process is labor

intensive and requires
tremendous patience and

persistence, success results
in the addition of new Landsat

data to the USGS EROS archive,
in some cases unlocking new

regions of the world to the
user community, and in all cases

increasing the frequency
of observations globally.