BOREAS RSS-17 1994 ERS-1 Level-3 Freeze/Thaw Backscatter Change Images

Summary

The BOREAS RSS-17 team acquired and analyzed imaging radar data from the ESA's 
ERS-1 over a complete annual cycle at the BOREAS sites in Canada in 1994 to 
detect shifts in radar backscatter related to varying environmental conditions.  
Two independent transitions corresponding to soil thaw and possible canopy thaw 
were revealed by the data.  The results demonstrated that radar provides an 
ability to observe thaw transitions at the beginning of the growing season, 
which in turn helps constrain the length of the growing season.  The data set 
presented here includes change maps derived from radar backscatter images that 
were mosaicked together to cover the southern BOREAS sites.  The image values 
used for calculating the changes are given relative to the reference mosaic 
image. The data are stored in binary image format files.

Due to copyright issues, the 01-March-1994 reference image is not included on 
the CD-ROM and is not publicly available.  See Sections 15 and 16 for 
information about how to possibly acquire the data. 

Note that some of the data files on the BOREAS CD-ROMs have been compressed 
using the Gzip program.  See section 8.2 for details.

Table of Contents

1.  Data Set Overview
2.  Investigator(s)
3.  Theory of Measurements
4.  Equipment
5.  Data Acquisition Methods
6.  Observations
7.  Data Description
8.  Data Organization
9.  Data Manipulations
10. Errors
11. Notes
12. Application of the Data Set
13. Future Modifications and Plans
14. Software
15. Data Access
16. Output Products and Availability
17. References
18. Glossary of Terms
19. List of Acronyms
20. Document Information

1. Data Set Overview

Imaging radar data acquired by the European Space Agency's (ESAs) Earth Remote 
Sensing Satellite (ERS-1) over a complete annual cycle at the BOReal Ecosystem-
Atmosphere Study (BOREAS) test sites in Canada in 1994 were analyzed to detect 
shifts in radar backscatter related to varying environmental conditions.  The 
data set presented here includes change maps in radar backscatter mosaicked 
together over the southern BOREAS sites and the reference mosaic image used for 
calculating those changes.

1.1 Data Set Identification

BOREAS RSS-17 1994 ERS-1 Level-3 Freeze/Thaw Backscatter Change Images

1.2 Data Set Introduction

ERS-1 Synthetic Aperture Radar (SAR) observations of the BOREAS study sites were 
mosaicked together along a north-south transect to study shifts in radar 
backscatter as a result of changing environmental conditions.  The data set 
presented herein shows the measured changes in radar backscatter in reference to 
a scene acquired in winter, when the landscape was in frozen state.

1.3 Objective/Purpose

The aim of the study is to demonstrate that imaging radar can be utilized to 
detect the onset of the thaw process during a spring transition.  It had already 
been demonstrated that imaging radars can be utilized to pick up the freeze 
transition in the fall season (Rignot and Way, 1994).  Knowing the dates of 
onset of freeze/thaw events is required to determine the length of the growing 
season, with obvious implications for carbon exchange.  The ERS-1 remote sensing 
data were compared to insitu air temperature, soil temperature, and xylem flow 
data (see Section 1.6) collected at the Southern Study Area (SSA) Old Black 
Spruce (OBS) site.

1.4 Summary of Parameters

a.  Radar backscatter of reference scene (im13721) expressed in Digital Number 
(DN) values between 0 and 255.  To convert the DN values into radar cross-
section numbers in decibels (dB), use the following formula:

radar cross-section (dB) = 10*log10 (DN*DN*1.2E-5)

b.  Radar backscatter change maps for other scenes in dB relative to the scene 
of reference (Winter scene) expressed in DN values between 0 and 255.  DN = 0 
corresponds to -3dB change or lower.  DN=255 corresponds to +3 dB change.

1.5 Discussion

None given.

1.6 Related Data Sets

BOREAS RSS-17 Dielectric Constant Profile Measurements
BOREAS RSS-17 Stem and Air Temperature Measurements
BOREAS RSS-17 Xylem Flux Density Measurements at the SSA-OBS Site

2. Investigator(s)

2.1 Investigator(s) Name and Title

JoBea Way, JPL
Reiner Zimmerman, JPL
Kyle McDonald, JPL
Eric Rignot, JPL

2.2 Title of Investigation

Freeze/Thaw Transitions as Observed with ERS-1 Imaging Radar at BOREAS

2.3 Contact Information

Contact 1
JoBea Way
Jet Propulsion Laboratory
Pasadena, CA 
(818) 354-8825
way@lor.jpl.nasa.gov

Contact 2
Eric Rignot
Jet Propulsion Laboratory
Pasadena, CA 
(818) 354-1640
eric@adelie.jpl.nasa.gov

Contact 3
Jaime Nickeson
Raytheon STX Corporation
NASA GSFC
Greenbelt, MD
(301) 286-3373
(301) 286-0239 (fax)
Jaime.Nickeson@gsfc.nasa.gov

3. Theory of Measurements

At microwave frequencies, freezing results in a dramatic decrease of the 
dielectric constant of soil and vegetation, which significantly alters their 
radar scattering properties.  Using ERS-1 SAR data collected along a north-south 
Alaskan transect, Rignot and Way (1994) showed that it was possible to detect 
the onset of freezing by mapping areas whose radar backscattering intensity 
dropped by 3 dB or more relative to a reference scene acquired during the summer 
season.  The ERS results were compared to air-temperature data collected at 
weather stations and insitu observations of forest stands.  The technique 
utilizes repeat-pass SAR data that are mosaicked together and coregistered to 
the reference image on a pixel-by-pixel basis.  The spatial resolution of the 
pixel elements is of several tens of meters.  The technique operates independent 
of cloud cover and surface topography (change detection techniques do not 
require topographic information to measure backscatter changes).  In Alaska, the 
technique was also shown to be relatively independent of land cover types, with 
the exception of open water areas, which behave in a completely different 
manner.  Results obtained later over the Bonanza Creek Experimental Forest, AK, 
and at BOREAS, however, indicate that over forested terrain, the change in radar 
backscatter corresponding to freeze/thaw transitions may be slightly lower in 
magnitude than that recorded on nonforested areas.

4. Equipment

4.1 Sensor/Instrument Description

ERS-1 was launched on 17-Jul-1991 by an Ariane 4 launcher from Kourou, French 
Guiana.  Its total mass is 2157.4 kg, 888.2 kg from the payload and 1257.2 kg 
from the platform. The peak power supplied to the payload is 2600 W; payload 
average power is at most 550 W. The voltage of the power supply varies between 
23 V and 37 V, with a maximum onboard energy of 2650 WH.  ERS-1 is a three-axis 
stabilized spacecraft with a design lifetime of 2 to 3 years. 

ESA sponsored the mission. The prime contractor is Dornier (Federal Republic 
Germany). Co-contractors include Fokker (The Netherlands), Laben (Italy), Matra 
(France), MDA (Canada), Marconi (United Kingdom), and Selenia (Italy). 

AMI Image-Mode (SAR) Characteristics: 

   Antenna Size:              10 m x 1 m
   Peak Power:                4.8 kW
   Frequency:                 5.3 GHz (C-Band)
   Bandwidth:                 15.55 +/- 0.1 MHz
   PRF Range:                 1640-1720 Hz in 2-Hz steps
   Polarization:              Linear-Vertical (LV)
   Long Pulse:                37.12 +/- 0.06 microseconds
   Compressed Pulse Length:   64 nanoseconds
   Sampling Window:           296 microseconds (99-km telemetered swath)
   Analog/Digital 
      Complex Sampling:       16.96 million samples/second
   Quantization:              5I, 5Q if range compression on ground
                              (nominal 6I, 6Q if range compression onboard)
   Data Rate:                 < 105 Mbit/s
   Spatial Resolution:        30 m x 30 m
   Radiometric Resolution:    2.5 dB at sigma-naught = -18 dB
   Noise-Equivalent
        Sigma-Naught:         -23 dB
   Incidence Angle:           23 at mid-swath
   Swath Stand-Off:           250 km to side of orbital track
   Swath Width:               100 km

4.1.1 Collection Environment

The ERS-1 satellite orbits Earth in a sun-synchronous, polar, near-circular 
orbit at a mean altitude of 785 km and an inclination of 98.5 degrees. 

4.1.2 Source/Platform

ERS-1 has a sun-synchronous, polar, near-circular orbit at a mean altitude of 
785 km and an inclination of 98.5 degrees.  During the initial 3 months of the 
commissioning phase, the satellite had a 3-day repeat cycle at an altitude of 
785 km (this is known as the reference orbit).  Subsequent satellite height 
adjustments have provided two multidisciplinary phases with a 35-day repeat 
cycle, two ice phases with 3-day repeat cycles, and two geodetic phases with 
168-day with cycles.  The majority of the mission has been performed in the 35-
day repeat cycles.  ERS-1, operating intandem with ERS-2, is expected to remain 
in a 35-day repeat cycle for the rest of its mission.  Since ERS-1 has no 
onboard recorders except for an onboard tape recorder for bitrate data, Active 
Microwave Instrumentation (AMI) data can be obtained only if there is a ground 
station in view of the orbiting satellite. 

4.1.3 Source/Platform Mission Objectives

ERS-1 is an ESA satellite devoted to remote sensing from a polar orbit.  It 
provides global and repetitive observations of the environment using techniques 
that allow imaging to take place irrespective of weather conditions.  ERS-1 has 
a sun-synchronous, polar, near-circular orbit with a mean altitude of 785 km. 

List of Sensors/Instruments:

 1 AMI:

AMI combines the functions of a SAR and a Wind Scatterometer (WNS). The AMI 
measures wind fields and wave spectra over the open ocean and records all-
weather, fine-resolution images over the ocean, polar ice, coastal zones, and 
land. The AMI has an image mode (swath) SAR. SAR mode and Wind/Wave mode are 
mutually exclusive during operation. 

 2  Radar Altimeter (RA):

RA provides measurements of altitude, significant wave heights and surface wind 
speed over the ocean, and various parameters over sea ice and ice sheets. 

 3  Along-Track Scanning Radiometer (ATSR):

ATSR is an experimental four-channel infrared radiometer that provides precise 
and accurate measurements of sea surface temperatures and cloud top 
temperatures. 

 4  Microwave Sounder (MWS):

MWS is a two-channel passive microwave radiometer that provides information on 
the total precipitable water vapor and the total liquid water content of the 
atmosphere. 

 5  Precise Range and Range-rate Equipment (PRARE):

PRARE is an experimental instrument providing high-precision orbit data in 
support of the altimeter mission.  This instrument does not work. 

 6  Laser Retroreflector (LR):

LR permits the use of ground based laser ranging to determine precise orbit and 
calibration information in support of the altimeter mission.

4.1.4 Key Variables

Radar backscatter.

4.1.5 Principles of Operation

In image mode, the SAR obtains strips of high-resolution imagery 100 km in width 
to the right of the satellite track. The 10-m-long antenna, aligned parallel to 
the flight track, directs a narrow radar beam onto Earth's surface over the 
swath. Imagery is built up from the time delay and strength of the return 
signals, which depend primarily on the roughness and dielectric properties of 
the surface and its range from the satellite. 

The SAR's fine resolution in the range direction is achieved by phase coding the 
transmit pulse with a linear chirp and compressing the echo by matched 
filtering.  Range resolution is obtained from the travel time.  Azimuth 
resolution is achieved by recording the phase as well as the amplitude of the 
echoes along the flight path.  The set of echoes over a flight path of about 800 
m is processed (on the ground) as a single entity, giving an azimuth resolution 
equivalent to a real aperture 800 m in length.  This is the 'synthetic aperture' 
of the radar. 

Operation in image mode excludes the other AMI operating modes, and power 
considerations limit operating time to a maximum of 10 minutes per orbit.  
Because the data rate of 100 Mbit/s is far too high to allow onboard storage, 
images are acquired only within the reception zone of a suitably equipped ground 
station.

4.1.6 Sensor/Instrument Measurement Geometry

ERS-1 operates a C-band (5.7-cm wavelength), vertical receive and transmit 
polarization SAR, illuminating the surface at a 23-degree incidence angle from 
nadir.  The swath width is 100 km x 100 km, with 30-m resolution for four looks.  
The data were processed at a 200-m resolution for this regional study.

4.1.7 Manufacturer of Sensor/Instrument

ESA sponsored the ERS-1 mission.  The prime contractor is Dornier (Federal 
Republic Germany).  Co-contractors include Fokker (The Netherlands), Laben 
(Italy), Matra (France), MDA (Canada), Marconi (United Kingdom), and Selenia 
(Italy). 

Some of the major participants include:

              Dornier Systems
              P.O. Box 1420
              D-7790 Friedrichshafen 1
              Federal Republic of Germany
              0 75 45 8-0 (tel)

              Marconi Thomsom
              (United Kingdom Branch)
              Anchorage Road
              Portsmouth Hampshire
              PO3 5PU England
              44 705 66 49 66 (tel) 

4.2 Calibration

The ERS data were collected, processed, and fully calibrated at NASA's Alaska 
SAR Facility (ASF) to yield slant-range radar backscatter images.  Earlier 
engineering tests and experiments demonstrated that the data were calibrated 
with an absolute precision of about 2 dB and a relative accuracy of 1/3 dB 
(which is the stability of the ERS-1 system).

4.2.1 Specifications

Calibration of the AMI is undertaken in two steps.  An internal calibration unit 
continuously monitors the out put power and receiver gain of the AMI over short 
intervals, and in SAR modes, the phase characteristics of the transmit signal.  
Antenna patterns and gains were measured on the ground and then, from time to 
time, in orbit.  In the SAR modes, corner reflectors are used. 

4.2.1.1 Tolerance

The parameter derived from the SAR image mode is the normalized radar 
backscattering coefficient, sigma-naught.  ESA engineer Henry Laur has shown 
that the ERS-1 image mode SAR relative accuracy is 0.18 dB (1 sigma). ASF ERS-1 
SAR image data is sufficiently monitored and calibrated to ensure +/- 1.0 dB 
relative accuracy and +/- 2.0 dB absolute accuracy. 

4.2.2 Frequency of Calibration

Each ERS satellite's image mode SAR is checked against external calibration 
targets as often as the orbit and acquisition schedules allow.  The orbit phases 
have repeat times of 3 days, 35 days, and 168 days.  The latter two phases 
provide coverage over the ASF calibration sites more than once per repeat time 
period.  Scheduling conflicts, equipment failures, and other factors reduce the 
number of available calibration passes.  SAR image mode data are checked for 
miscalibration every 2 weeks. 

4.2.3 Other Calibration Information

Image calibration coefficients vary with image type, processor gain setting, 
etc., and are provided in the metadata accompanying each image produced by the 
ASF.  The radiometric calibration has never needed to be adjusted. 

5. Data Acquisition Methods

The data were obtained as standard products from the ASF SAR facility.

6. Observations

6.1 Data Notes

Way, J., Reiner Zimmermann, Eric Rignot, Kyle McDonald, and Ram Oren 1997. 
Winter and spring thaw as observed with imaging radar at BOREAS. Journal of 
Geophysical Research (JGR), BOREAS Special Issue, 102(D24), Dec. 1997, pp. 
29673-29684.

6.2 Field Notes

None given.

7. Data Description

7.1 Spatial Characteristics

7.1.1 Spatial Coverage

The coverage of the imagery is 100 km across by up to 1,000 km in the along- 
track direction.

Two sets of image sizes are included in this data set.  The images acquired 
before the reference date (01-March-1994) are all 1024 samples by 3086 lines in 
size.  These images are shorter in the number of lines because ERS did not 
acquire the entire swath crossing the Prince Albert area.  The shorter images 
coincide with the lower half of the reference and post-reference images.  The 
reference image and the images acquired after that date are all 1024 samples by 
8552 lines in size.  The last line of all the image files are coincident in 
geographic location.  The only coordinates given for these change images is the 
center latitude and longitude of each set.  The center latitude, longitude of 
the shorter (pre-reference) images is 60.2N and 108.6W.  The center latitude, 
longitude of the longer (post-reference) images is 53.3N and 105.4W.  The data 
are in the slant-range radar geometry.

The North American Datum of 1983 (NAD83) corner coordinates of the BOREAS region 
are:
             Latitude     Longitude
             --------     ---------
Northwest    59.979N     111.000W
Northeast    58.844N      93.502W
Southwest    51.000N     111.000W
Southeast    50.089N      96.970W

The NAD83 corner coordinates of the SSA are:

             Latitude     Longitude
             --------     ---------
Northwest    54.319N     106.227W
Northeast    54.223N     104.236W
Southwest    53.513N     106.320W
Southeast    53.419N     104.368W

7.1.2 Spatial Coverage Map

A coverage map is shown in Figure 1 of Way et al., 1997.

7.1.3 Spatial Resolution

The spatial resolution of this product is 200 m.

7.1.4 Projection

The data are in the slant-range radar geometry.

7.1.5 Grid Description

Not applicable.

7.2 Temporal Characteristics

7.2.1 Temporal Coverage

The dates of data acquisition by orbit are: 

Orbit      Date 
-----   ---------
12904   03-Jan-94
13205   24-Jan-94
13377   05-Feb-94
13506   14-Feb-94
13635   23-Feb-94
13678   26-Feb-94
13721   01-Mar-94 (reference)
13764   04-Mar-94
13807   07-Mar-94
13850   10-Mar-94
13893   13-Mar-94
13936   16-Mar-94
13979   19-Mar-94

7.2.2 Temporal Coverage Map

Not available.

7.2.3 Temporal Resolution

The potential temporal resolution of the data is 3 days, but not all orbits were 
acquired by ASF.

7.3 Data Characteristics

7.3.1 Parameter/Variable

The parameters contained in the data are:
   a.  Radar backscatter of reference scene 
   b.  Radar backscatter change maps

7.3.2 Variable Description/Definition

The description of the parameters contained in the data are:

a.  The radar backscatter of the 01-March-1994 reference scene (Orbit 13721) is 
expressed in DN (digital number) values between 0 and 255.  To convert the DN 
values into radar cross-section numbers in decibels, use the formula,

radar cross-section (dB) = 10 log10 (DN*DN*1.2*10-5)

b.  Radar backscatter change maps for other scenes in decibels (dB) relative to 
the scene of reference (winter scene) expressed in DN values between 0 and 255.  

DN =   0 corresponds to more than 3 dB decrease

DN = 255 corresponds to more than 3 dB increase

7.3.3 Unit of Measurement

The images are provided are digital numbers.  Use the equations given above to 
calculate radar backscatter in decibels.

7.3.4 Data Source

European Remote Sensing Satellite, ERS-1 

7.3.5 Data Range

The range of values in each image is 0 to 255.

7.4 Sample Data Record

Not applicable to image data.

8. Data Organization

8.1 Data Granularity

The smallest unit of data available is the entire set of change images.

8.2 Data Format(s)

8.2.1 Uncompressed Format

The entire data set consists of 13 images divided into two sets.  The six images 
acquired before the reference date (01-Mar-1994) contain 1024 pixels in each of 
3086 lines.  These images are shorter in the number of lines because ERS did not 
acquire the entire swath crossing the Prince Albert area.  The shorter images 
coincide with the lower half of the reference and post-reference images.  The 
reference image and the six acquisitions after that date each contain 1024 
pixels in each of the 8552 lines.  The last line of all the image files are 
coincident in geographic location.  Descriptions of the files are:

File    Description      Pixels   Lines
----    -----------      ------   -----
  1     jan-03_mref       1024     3086
  2     jan-24-mref       1024     3086
  3     feb-05_mref       1024     3086
  4     feb-14_mref       1024     3086
  5     feb-23_mref       1024     3086
  6     feb-26_mref       1024     3086
  7     mar-01_refimg*    1024     8552
  8     mar-04_mref       1024     8552
  9     mar-07_mref       1024     8552
 10     mar-10_mref       1024     8552
 11     mar-13_mref       1024     8552
 12     mar-16_mref       1024     8552
 13     mar-19_mref       1024     8552

The description, (date)mref, signifies radar backscatter change between the date 
given and the reference image (01-Mar-1994).  For the orbits associated with the 
image dates, see Section 7.2.1.

Note that the reference backscatter image of 01-Mar-94 is not distributable.  To 
obtain information regarding the acquisition of raw backscatter imagery, see 
Sections 15 and 16.

8.2.2 Compressed CD-ROM Files

On the BOREAS CD-ROMs, image files 1 to 6 and 8 to 13 have been compressed with 
the Gzip (GNU zip) compression program (file_name.gz).  These data have been 
compressed using gzip version 1.2.4 and the high compression (-9) option 
(Copyright (C) 1992-1993 Jean-loup Gailly).  Gzip uses the Lempel-Ziv algorithm 
(Welch, 1994) also used in the zip and PKZIP programs.  The compressed files may 
be uncompressed using gzip (with the -d option) or gunzip.  Gzip is available 
from many websites (for example, the ftp site prep.ai.mit.edu/pub/gnu/gzip-*.*) 
for a variety of operating systems in both executable and source code form.  
Versions of the decompression software for various systems are included on the 
CD-ROMs.

9. Data Manipulations

9.1 Formulae

Radar backscatter changes between -3 dB and +3 dB are linearly stretched between 
0 and 255 (see above).

9.1.1 Derivation Techniques and Algorithms

The technique used is the same as that described by Rignot and Way (1994). 

9.2 Data Processing Sequence

9.2.1 Processing Steps

The data were mosaicked together and automatically registered using image 
offsets determined by Fourier analysis of the radar scenes.  The change in radar 
backscatter was then computed in the logarithm domain and stretched between 0 
and 255.

9.2.2 Processing Changes

None.

9.3 Calculations

9.3.1 Special Corrections/Adjustments

None.

9.3.2 Calculated Variables

The same scale/calibration factor was applied to all images to ensure that the 
changes in radar backscatter were relatively consistent.  The scale factor 
utilized was that of the reference image.  Other images were scaled accordingly.

9.4 Graphs and Plots

Plot 1 shows the variation in absolute radar backscatter of the black spruce 
study site in the SSA versus DDY.

The onset of thawing was detected in between 01-Mar-94 and 05-Mar-94, with a 
relative change in backscatter of about 1.5 dB.  The thawed areas progressively 
extended in size and northward with time.

The early mosaics are shorter in length because ERS data were not acquired along 
the entire transect at that time.  The scenes acquired between 04-Jan-94 and 19-
Mar-94 corresponded to the 1994 Ice Phase of the ERS-1 SAR satellite, which 
provided exact repeat-pass data over the same sites every 3 days.  Later 
acquisition of ERS-1 SAR data over the BOREAS study sites corresponded to a 35-
day repeat cycle that could not be used directly to produce change detection 
maps.  The BOREAS sites could still be imaged about every 2 weeks, but at a 
different location within the SAR swath, therefore limiting the possibilities to 
automatically coregister the data on a pixel-by-pixel basis.  Analysis of 
changes in radar backscatter for those later scenes was therefore conducted over 
small test areas instead of the entire scenes.

Note that lakes, rivers, and other areas of open water behave differently than 
the rest of the landscape, so the change in detection technique is not 
applicable other than these areas, as discussed in Rignot and Way (1994).

10. Errors

10.1 Sources of Error

Given the stability of the ERS instrument, instrument errors are unlikely to 
cause erroneous changes in radar backscatter from the surface.  Environmental 
factors may, however, complicate the interpretation of the data.  To derive the 
freeze/thaw change products from ERS, the influence of snow is neglected because 
Alaskan snow is typically cold and dry and therefore transparent to radar 
signals.  During warm episodes in the fall and/or the onset of spring, however, 
snow will become wet and its radar backscatter will rapidly drop to low values. 
In those circumstances, melting snow and freeze/thaw transitions may be more 
difficult to identify in the absence of ancillary information, for instance 
air/snow temperature.  In the fall, there may be alternating freeze/thaw periods 
before the land freezes in for the whole winter.  During those periods, if snow 
is already present on the ground, it may have a disturbing effect on the radar 
signature from the surface, which may be difficult to interpret in terms of 
freeze/thaw of the soils and vegetation beneath the snow cover.

10.2 Quality Assessment

10.2.1 Data Validation by Source

ERS-1 data are calibrated within 1/3 dB (Rignot et al., 1994; Rignot and Way, 
1994).  One data point (25-Jun-94) was difficult to interpret because it 
exhibited a higher than expected radar signature, but nothing was found wrong 
with the processing of the data.

10.2.2 Confidence Level/Accuracy Judgment

The data quality is good over low-land vegetation and forest.  The change 
detection approach does not work on open water, so the signatures of lakes and 
rivers will appear as erroneous in the map. 

10.2.3 Measurement Error for Parameters

Ground truth data are not available to validate the mapping and estimate the 
measurement error except at one study site.  At that one site, the procedure was 
found to detect the freeze/thaw transition correctly. 

10.2.4 Additional Quality Assessments

[Include figure 7 here]

10.2.5 Data Verification by Data Center

BOREAS Information System (BORIS) staff has viewed the imagery to verify image 
sizes and format and to verify the coincidence of the last line in the two sets 
of image sizes.

11. Notes

11.1 Limitations of the Data

None given.

11.2 Known Problems with the Data

ERS-1 data are calibrated within 1/3 dB (Rignot et al., 1994; Rignot and Way, 
1994).  One data point (25-Jun-94) was difficult to interpret because it 
exhibited a higher than expected radar signature, but nothing was found wrong 
with the processing of the data.

11.3 Usage Guidance

Before uncompressing the Gzip files on CD-ROM, be sure that you have enough disk 
space to hold the uncompressed data files.  Then use the appropriate 
decompression program provided on the CD-ROM for your specific system.

11.4 Other Relevant Information

ESA has a policy that ERS-1 data should be distributed only to ESA-approved 
investigators; hence, the reference ERS-1 mosaic should not be utilized per se 
without first requesting an authorization from ESA.  ERS change maps, which 
correspond to a higher level product, can be utilized with no restriction.

To obtain ERS data, interested users need to contact ESA and in particular the 
ESA/ESRIN Facility at Frascatti.  U.S. investigators interested in data 
available at theASF should contact Greta Reynolds at the Alaska SAR Facility, 
University of Alaska, Fairbanks in Fairbanks, AK.  The investigator will have to 
submit a short research project summary explaining the need for the data and how 
they will e utilized, and ASF will provide ERS data free of charge.

12. Application of the Data Set

These data can be used to determine the onset of thawing at different latitudes 
along the BOREAS study sites.

13. Future Modifications and Plans

This type of analysis may be pursued using SAR data collected by the Radarsat 
Canadian SAR at C-band horizontal polarization, or by the Ku-band NSCATT 
scatterometer launched in 1996.

14. Software

14.1 Software Description

Software has been developed at the Jet Propulsion Laboratory (JPL) to generate 
ERS-1 mosaics.

Gzip (GNU zip) uses the Lempel-Ziv algorithm (Welch, 1994) used in the zip and 
PKZIP commands.

14.2 Software Access

Programs to generate ERS-1 mosaics were written in IDL and are available upon 
request.

15.   Data Access

15.1  Contact for Data Center/Data Access Information

These BOREAS data are available from the Earth Observing System Data and 
Information System (EOS-DIS) Oak Ridge National Laboratory (ORNL) Distributed 
Active Archive Center (DAAC). The BOREAS contact at ORNL is:

ORNL DAAC User Services
Oak Ridge National Laboratory
Oak Ridge, TN 
(865) 241-3952
ornldaac@ornl.gov
ornl@eos.nasa.gov

15.2  Procedures for Obtaining Data

BOREAS data may be obtained through the ORNL DAAC World Wide Web site at 
http://www-eosdis.ornl.gov/ or users may place requests for data by telephone, electronic mail, or fax.

15.3  Output Products and Availability

Requested data can be provided electronically on the ORNL DAAC's anonymous FTP 
site or on various media including, CD-ROMs, 8-MM tapes, or diskettes.

The complete set of BOREAS data CD-ROMs, entitled "Collected Data of the Boreal 
Ecosystem-Atmosphere Study", edited by Newcomer, J., et al., NASA, 1999, are 
also available.

16. Output Products and Availability

16.1 Tape Products

ESA has a policy that ERS-1 data may be distributed only to ESA-approved 
investigators; hence, the reference ERS-1 mosaic may not be utilized without 
first requesting an authorization from ESA.  The ERS change maps, which 
correspond to a higher level product, can be utilized with no restriction.

The RSS-17 freeze/thaw image data can be made available on 8-mm, Digital Archive 
Tape (DAT).

16.2 Film Products

None.

16.3 Other Products

None.

17. References

17.1 Platform/Sensor/Instrument/Data Processing Documentation

Excerpts were taken from the following to document this data set:

http://www.asf.alaska.edu/source_documents/ers1_source.html

http://www.asf.alaska.edu/sensor_documents/ami_sensor.html

Welch, T.A. 1984, A Technique for High Performance Data Compression, IEEE 
Computer, Vol. 17, No. 6, pp. 8 - 19.

17.2 Journal Articles and Study Reports

Rignot, E., and J. Way. 1994. Monitoring freeze/thaw cycles along north-south 
Alaskan transects using ERS-1 SAR. Rem. Sens. Environ. 49:131-137.

Rignot, E. et al. 1994. Monitoring of environmental conditions in taiga forests 
using ERS-1 SAR. Rem. Sens. Environ. 49:145-154.

Sellers, P. and  F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment 
Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). 

Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. 
Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. 
Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and 
early results from the 1994 field year. Bulletin of the American Meteorological 
Society. 76(9):1549-1577. 

Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere 
Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). 

Sellers, P.and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment 
Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). 

Sellers, P., F. Hall, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere 
Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). 

Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, 
K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, 
D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, 
D.E. Wickland, and F.E. Guertin. (1997). "BOREAS in 1997: Experiment Overview, 
Scientific Results and Future Directions", Journal of Geophysical Research 
(JGR), BOREAS Special Issue, 102(D24), Dec. 1997, pp. 28731-28770.  

Way, J., Reiner Zimmermann, Eric Rignot, Kyle McDonald, and Ram Oren 1997. 
Winter and spring thaw as observed with imaging radar at BOREAS. Journal of 
Geophysical Research (JGR), BOREAS Special Issue, 102(D24), Dec. 1997, pp. 
29673-29684.

17.3 Archive/DBMS Usage Documentation

None.

18. Glossary of Terms

None.

19. List of Acronyms

AMI      - Active Microwave Instrumentation
ASF      - Alaska SAR Facility
ATSR     - Along-Track Scanning Radiometer
BOREAS   - BOReal Ecosystem-Atmosphere Study
BORIS    - BOREAS Information System
DAAC     - Distributed Active Archive Center
DN       - Digital Number
DOY      - Day of Year
EOS      - Earth Observing System
EOSDIS   - Earth Observing System Data and Information System
ERS      - European Remote Sensing Satellite
ESA      - European Space Agency
GSFC     - Goddard Space Flight Center
JPL      - Jet Propulsion Laboratory
LR       - Laser Retroreflector
MDA      - McDonnell Detweiler Associates
MWS      - Microwave Sounder
NASA     - National Aeronautics and Space Administration
NSA      - Northern Study Area
OBS      - Old Black Spruce
ORNL     - Oak Ridge National Laboratory
PANP     - Prince Albert National Park
PRARE    - Precise Range and Range Rate Experiment
RA       - Radar Altimeter
RSS      - Remote Sensing Science
SAR      - Synthetic Aperture Radar
SSA      - Southern Study Area
URL      - Uniform Resource Locator
UTC      - Coordinated Universal Time
WNS      - Wind Scatterometer

20. Document Information

20.1 Document Revision Date(s)

     Written:         30-Oct-1996
     Last Updated:    14-Sep-1998

20.2 Document Review Date(s)

     BORIS Review:    27-Aug-1998
     Science Review:  

20.3 Document ID

20.4 Citation

Users of this data set should acknowledge Eric Rignot and JoBea Way at NASA JPL 
for providing the ERS change maps and the ESA for providing the ERS data.

20.5 Document Curator

20.6 Document URL

Keywords:
--------
Radar
ERS-1
SIR-C
Backscatter

RSS17_FreezeThaw_Maps.doc
09/14/98