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BOREAS FOLLOW-ON DSP-04 1994 ERS-1 LEVEL-4 LANDSCAPE FREEZE/THAW MAPS, VER. 1.0
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Summary

The BOREAS DSP-4 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 correlating with snow melt and soil thaw onset, 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 presented here are gridded maps of landscape freeze/thaw state derived from backscatter change maps. The backscatter change maps were computed from radar backscatter images covering the southern BOREAS sites. The freeze/thaw classifications were determined through application of a change detection threshold based on temporal backscatter change relative to a winter-time frozen reference state. The data are provided as both ASCII text and as binary image (*.gif) format files.

Data Citation

Cite this data set as follows (citation revised on October 30, 2002):

McDonald, K., and J. Nickeson. 2001. BOREAS Follow-On DSP-04 1994 ERS-1 Level-4 Landscape Freeze/Thaw Maps, Ver[sion] 1.0. Data set. Available on-line [http://daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.

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

1.1 Data Set Identification
      BOREAS Follow-On DSP-04 1994 ERS-1 Level-4 Landscape Freeze/Thaw Maps, Ver. 1.0

1.2 Data Set Introduction
      Synthetic Aperture Radar (SAR) data acquired by the European Space Agency's (ESA's) 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 changes in radar backscatter related to varying environmental conditions. The data set presented here consists of maps of estimated landscape freeze/thaw state registered to the BOREAS grid. Data provided were acquired during 1994 over the southern BOREAS sites.

1.3 Objective/Purpose
      The aim of the study was to demonstrate that imaging radar could be utilized to detect the onset of the thaw process during spring transitions. It has previously been demonstrated that imaging radar can be utilized to discern the freeze transition in the fall season (Rignot and Way, 1994). Knowing the dates of onset of freeze/thaw events is useful in determining the length of the growing season, and has obvious implications for hydrologic, meteorological, and ecosystem functional processes (e.g. carbon exchange). These ERS-1 SAR data were compared to in situ air temperature, soil temperature, and xylem flow data (see Section 1.6) collected at the Southern Study Area (SSA) Old Black Spruce (OBS), Old Aspen (OA), and Old Jack Pine (OJP) sites.

1.4 Summary of Parameters
      Gridded landscape freeze/thaw classification as determined through application of a backscatter thresholding scheme applied to Freeze/Thaw Backscatter Change Images (see section 1.6 Related Data Sets). Each grid cell indicates percent of the cell that is frozen, percent thawed, and percent consisting of lakes or open water. Determination of lake and open water regions was performed with a Landsat-based classification map and are provided only over the region covered by the Landsat map (see Section 7, Data Description).

1.5 Discussion
      None given.

1.6 Related Data Sets
BOREAS RSS-17 1994 ERS-1 Level-3 Freeze/Thaw Backscatter Change Images
BOREAS RSS-17 Dielectric Constant Profile Measurements
BOREAS RSS-17 Stem, Soil, and Air Temperature Data
BOREAS RSS-17 Xylem Flux Density Measurements at the SSA-OBS Site
BOREAS TE-18 Landsat TM Maximum Likelihood Classification Image of the SSA

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2. Investigator(s)

2.1 Investigator(s) Name and Title
      Dr. Kyle McDonald, Research Scientist

2.2 Title of Investigation
      Freeze/Thaw Transitions as Observed with ERS-1 Imaging Radar at BOREAS

2.3 Contact Information

Contact 1:
Kyle McDonald
Jet Propulsion Laboratory
Pasadena, CA 91109
(818) 354-9476
kyle.mcdonald@jpl.nasa.gov

Contact 2:
Jaime Nickeson
Science Systems and Applications, Inc.
NASA GSFC
Greenbelt, MD
(301) 286-3373
(301) 286-0239 (fax)
Jaime.Nickeson@gsfc.nasa.gov

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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. ERS-1 SAR data collected over boreal regions have been used to map spring and fall seasonal transitions as related to landscape freeze/thaw state (Way et al., 1997; Rignot and Way, 1994). The technique employed utilizes repeat-pass observations to monitor radar backscatter change relative to a reference state representative of winter-time (frozen) conditions. A temporal series of co-registered SAR images is used to compute backscatter change relative to the frozen reference image. Freeze/thaw state is estimated through application of a threshold on a pixel-by-pixel basis. With the exception of open water areas, this technique has been shown to be relatively independent of landcover type, although over forested terrain, the change in radar backscatter corresponding to freeze/thaw transitions may be slightly lower in magnitude than that recorded on non-forested areas. Where available, a mask is applied to remove regions of open water. The resulting maps are spatially gridded with each grid cell providing percent frozen area, percent thawed area, and where available, percent open water.

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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 in tandem 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 bit rate 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
      P035PU 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 are 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.

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5. Data Acquisition Methods

The input data were obtained as standard products from the Alaska SAR Facility.

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6. Observations

6.1 Data Notes
      See Way et al., 1997.

6.2 Field Notes
      None given.

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7. Data Description

7.1 Spatial Characteristics
7.1.1 Spatial Coverage
      Data are provided over the region of the BOREAS Southern Study Area (SSA) for which SAR coverage was acquired. The products however, have been projected onto a 60 line by 66 column latitude/longitude 5 by 10 minute grid covering most of the BOREAS region.
      The spatial coverage of freeze/thaw data within that grid is currently being expanded to cover the full BOREAS regional image area, see section 15 for information regarding the availability of subsequent data sets.
      For the Freeze/Thaw maps provided here, the grid corner coordinates are:
Corner       Latitude (degrees)         Longitude (degrees)
-----------------------------------------------------------
NW               57°N                         107°W
NE               57°N                          96°W
SW               52°N                         107°W
SE               52°N                          96°W
The North American Datum of 1983 (NAD83) corner coordinates of the BOREAS region are:
Corner       Latitude     Longitude
------       --------     ---------
Northwest    59.979°N     111.000°W
Northeast    58.844°N      93.502°W
Southwest    51.000°N     111.000°W
Southeast    50.089°N      96.970°W


The NAD83 corner coordinates of the SSA are:

Corner       Latitude     Longitude
------       --------     ---------
Northwest    54.319°N     106.227°W
Northeast    54.223°N     104.236°W
Southwest    53.513°N     106.320°W
Southeast    53.419°N     104.368°W
The northwest corner of the TM image used as a water mask was located at -106.50 degrees longitude and 54.42 degrees latitude. The mask image was 4115 rows by 4809 samples, with north-south resolution of 0.00030 degrees per pixel, and east-west resolution of 0.00045 degrees per pixel.
 

7.1.2 Spatial Coverage Map
     Not available.

7.1.3 Spatial Resolution
      Although these products were derived using 200 meter resolution ERS-1 SAR imagery, this data set has been projected onto a lower resolution 5 minute vertical (latitude) by 10 minute horizontal (longitude) grid.

7.1.4 Projection
      Products are provided in a geographic (latitude/longitude) grid projection.

7.1.5 Grid Description
      The products have been projected onto a latitude/longitude grid extending from 107.0 to 96.0 degrees West longitude, in 66 longitudinal (10 minute) bins, and from 57.0 to 52.0 degrees North latitude, in 60 latitudinal (5 minute) bins. This is consistent with the grid defined and used by the BOREAS Hydro-Meteorological Processes Working Group.


7.2 Temporal Characteristics

7.2.1 Temporal Coverage
      Files were produced using ERS Synthetic Aperture Radar (SAR) imagery from 1994, beginning on day-of-year 45 and extending to day-of-year 347. Data files are provided for each day for which ERS data are available over the region of the SSA. The dates of coverage and the corresponding day-of-year are:
Day of year, 1994            Date
-----------------      ----------------
      045              14-February-1994
      048              17-February-1994
      054              23-February-1994
      057              26-February-1994
      060              01-March-1994
      063              04-March-1994
      066              07-March-1994
      069              10-March-1994
      072              13-March-1994
      075              16-March-1994
      078              19-March-1994
      087              28-March-1994
      099              09-April-1994
      102              12-April-1994
      119              29-April-1994
      122              02-May-1994
      139              19-May-1994
      156              05-June-1994
      159              08-June-1994
      176              25-June-1994
      196              15-July-1994
      213              01-August-1994
      216              04-August-1994
      230              18-August-1994
      233              21-August-1994
      250              07-September-1994
      253              10-September-1994
      287              14-October-1994
      290              17-October-1994
      307              03-November-1994
      324              20-November-1994
      344              10-December-1994
      347              13-December-1994


7.2.2 Temporal Coverage Map
     See section 7.2.1.

7.2.3 Temporal Resolution
      The temporal resolution of ERS-1 SAR data was limited by its orbital geometry and swath width. Not all orbits were acquired by ASF. The highest temporal repeat coverage was acquired during periods that the satellite was in a three day repeat orbit.


7.3 Data Characteristics

7.3.1 Parameter/Variable
     The parameters contained in the data are:
Percent frozen land area
Percent thawed land area
Percent open water area (where discrimination was available using Landsat data)

7.3.2 Variable Description/Definition
     Percent frozen landscape represents the fraction of landscape area, on percent area basis, that contains water in a solid phase.
     Percent thawed landscape represents the fraction of landscape area on percent area basis, that contains water in a liquid phase.
     Percent open water area represents the fraction of landscape area on percent area basis, that is covered by open water. This information is provided only for that area of the dataset covered by the Landsat scene used in classification of open water area.
     The GIF files (*.gif) corresponding to each of the *.dat files are pictorial representations of the data provided in the *.dat files. Each bin is shown in a combination of red, blue, green and black, where:
red = area fraction of the grid cell that is thawed
blue = area fraction of the grid cell that is frozen
green = area fraction of the grid cell that is lake (open water)
black = area fraction of the grid cell that has no data or is missing data.
Some cells (records in the .pct files) have percent frozen, thawed, and lake area that do not sum to 100%. This is caused by missing data, and is represented by black area in the .gif images. Missing data correspond to portions of grid cells for which no ERS data are available during that day. This occurs in the 200-meter resolution ERS-1 backscatter data that are aggregated into the larger 5 by 10 minute resolution grid cells, and accounts also for the large black regions in the GIF images and zeros in the .pct files for areas further outside the ERS swath.

7.3.3 Unit of Measurement
     Variables are provided on a percent by area basis.

7.3.4 Data Source
     European Remote Sensing Satellite, ERS-1.

7.3.5 Data Range
     The range of values of each variable is 0 to 100 percent.


7.4 Sample Data Record
     The following is an example of data extracted from a DAT file. Data shown are from records 1190-1210 of file "94-03-01_ers_ft.dat". The data for each DAT file are arranged with the grid cells beginning in southwest corner and proceeding W->E first. Upon finishing the first latitudinal (x-direction) image line (66 cells or lines/records in .pct file), the data then move up an image line (N latitudinal, y-direction), and begin again at the western edge and again proceed eastward to the end of the second line, and so on. The final entry in each DAT file corresponds to the northeastern-most grid cell. The columns represent:

(column 1) percent frozen landscape
(column 2) percent thawed landscape
(column 3) percent open water area

      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
      0.489040     0.0832409    1.24514
     76.4728      14.8863       2.98114
     70.4806      17.6201      11.8993
     58.2489      23.5921      18.1590
     88.0869      10.7535       1.15960
     94.7569       5.04092      0.202177
     91.9158       8.00836      0.0758165
     89.0510       9.12650      1.82251
     86.3066      13.6555       0.0379082
     93.0833       6.71745      0.199261
     49.8449       4.51666      0.997278
      0.00000      0.00000      0.446151
      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
      0.00000      0.00000      0.00000
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8. Data Organization

8.1 Data Granularity
      The data file names are given by the format "yy-mm-dd_ers_ft.dat" and "yy-mm-dd_ers_ft.gif", where yy is the year (94 = 1994), mm is the month number, and dd is the day of month. Each pair of *.dat and *.gif files provides composite data corresponding to ERS-1 SAR observations obtained on the corresponding year and day-of-year.

8.2 Data Format(s)
      The data set consists of 33 pairs of data files, representing observations from 33 days in the thaw and freeze-up periods of 1994. The DAT files (*.dat) provide ASCII data for each day, each file with 66x60 lines (3960 records/grid cells) containing the three columns below:

  1. percent frozen landscape (column 1)
  2. percent thawed landscape (column 2)
  3. percent open water area (column 3)
Missing data and are not indicated in the ASCII files. The grid begins in the southwest corner of the region covered and proceeds eastward first.
      The GIF files (*.gif) corresponding to each of the *.dat files are graphical representations of the data provided in the *.dat files. Each bin is shown in a combination of red, blue, green and black, where: Return to top of document

9. Data Manipulations

9.1 Formulae
9.1.1 Derivation Techniques and Algorithms
     The technique used to derive the backscatter change images is described in Rignot and Way (1994). The freeze/thaw classifications were determined through application of a change detection threshold based on temporal backscatter change relative to a wintertime frozen reference state.


9.2 Data Processing Sequence

9.2.1 Processing Steps
      Frozen and thawed states, and lake-covered landscape, were quantified on a pixel-by-pixel basis using a temporal series of georeferenced, co-registered ERS SAR imagery. The temporal sequence of SAR images were co-registered to a regional ERS SAR mosaic representative of wintertime frozen conditions.
      A Landsat TM classification was co-registered to the mosaic and used to identify the location of lakes (open water). The lake mask only partially covered the study region. The northwest corner of the TM image is located -106.50 degrees longitude and 54.42 degrees latitude. The lake mask was 4115 rows by 4809 samples, with north-south resolution of 0.00030 degrees per pixel, and east-west resolution of 0.00045 degrees per pixel. Outside of the TM mask region, the effect of lakes was ignored in production of the freeze/thaw products, with open water regions being treated the same as the surrounding landscape.
      Landscape freeze/thaw state was estimated on a pixel-by-pixel basis using 200 meter resolution ERS SAR images, with the lake regions masked out. A 1 dB threshold was used in estimating the landscape freeze/thaw state such that:
  1. a pixel is assumed thawed if ERS SAR backscatter increased by 1 dB or more over the wintertime frozen condition.
  2. a pixel is assumed frozen otherwise.
The resulting pixel-by-pixel estimates (with lake regions masked out) were aggregated to the 66x60 grid such that:
  • percent frozen area = 100*(number of frozen non-lake pixels in grid cell)/(total pixels in grid cell)
  • percent thawed area = 100*(number of thawed non-lake pixels in grid cell)/(total pixels in grid cell)
  • percent lake area = 100*(number of lake pixels in grid cell)/(total pixels in grid cell)
Some cells (records in the .pct files) have percent frozen, thawed, and lake area that do not sum to 100%. These are due to missing data (black area in the .gif images) in the input 200-meter resolution ERS-1 backscatter data that were aggregated to the 5 by 10 minute resolution grid. When data were redundant for a bin (due to repeat coverage within a single day), the ERS SAR pass with the smallest number of missing values was used.

9.2.2 Processing Changes
      None.


9.3 Calculations

9.3.1 Special Corrections/Adjustments
      None.

9.3.2 Calculated Variables
      None.


9.4 Graphs and Plots
      None.

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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. When cold and dry, the snowpack found in the BOREAS region is nearly transparent to C-band radar signals. During warm episodes prior to the onset of final spring thaw or after the fall freeze-up, snow may become wet, leading to a marked change in its radar backscatter. In these circumstances and in the absence of ancillary information, melting snow may be more difficult to distinguish from freeze/thaw transitions in other landscape components.
      Another source of error results from noise introduced by freezing and thawing of open water and bogs. Backscatter response to freeze/thaw transitions for open water and boggy regions can vary dramatically from that of forested landscape. The error introduced by the open water effect is significantly reduced over the region where the Landsat water mask was applied prior to the classification of landscape freeze/thaw state. Generally, heterogeneous landscapes will lead to increased error in the freeze/thaw classification results. The scale of heterogeneity that influences the classification scheme are higher or on the order of the spatial resolution of the baseline SAR imagery (200 meters).
      Rain events may also significantly alter the landscape backscatter, influencing the accuracy of the freeze/thaw classification if the SAR imagery were obtained during or shortly after periods of rainfall.

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). Freeze/thaw classification results have been compared with in situ vegetation and air temperature station data at the SSA-OBS, SSA-OJP and SSA-OA sites (See Section 1.6, Related Data Sets.)

10.2.2 Confidence Level/Accuracy Judgment
      Based on comparison with in situ vegetation and air temperature station data, we estimate the freeze/thaw estimates to have maximum error of about 10-12% over the validation sites, depending upon the density and distribution of lakes and bogs. This accuracy degrades outside the bounds of the open water mask. However, trends in landscape freeze/thaw transition during spring thaw and autumn freeze-up are still accurately defined.

10.2.3 Measurement Error for Parameters
      None.

10.2.4 Additional Quality Assessments
      None.

10.2.5 Data Verification by Data Center
      None.

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11. Notes

11.1 Limitations of the Data
      None given.

11.2 Known Problems with the Data
      None.

11.3 Usage Guidance
      None given.

11.4 Other Relevant Information
      Freeze/thaw classifications and ERS change maps, which correspond to higher level derived product, can be utilized with no restriction.
      ERS-1 data can only be distributed to ESA-approved investigators. To obtain ERS data, interested users need to contact ESA and in particular the ESA/ESRIN Facility in Frascatti, Italy. U.S. investigators interested in data available at the ASF should contact the Alaska SAR Facility, University of Alaska, Fairbanks in Fairbanks, AK.

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12. Application of the Data Set

These data can be used to determine the onset of freeze/thaw transitions, and as temporal series maps of landscape freeze/thaw state at the BOREAS southern study sites.

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13. Future Modifications and Plans

Development of similar data products over broader areas of the BOREAS region are in progress and at the time of the completion of this document are nearing release. Application of more robust freeze/thaw discrimination schemes are also underway. Similar analyses have been performed utilizing Ku-band scatterometer data from the NASA Scatterometer (NSCAT) and analyses are also underway utilizing SeaWinds data (Frolking et al. 1999; Kimball et al., 2001a,b; Running et al., 1999). Users interested in access to these products should contact Kyle McDonald at JPL (see contact information, Section 2.3).

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14. Software

14.1 Software Description
      Software has been developed in IDL at the Jet Propulsion Laboratory (JPL) to generate the gridded data sets.

14.2 Software Access
      None given.

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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
(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://daac.ornl.gov [Internet Link] or users may place requests for data by telephone or electronic mail. 15.1.

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.

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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. The ERS-1 SAR-based freeze/thaw products, which correspond to higher level derived products, can be utilized with no restriction.

16.2 Film Products
      None.

16.3 Other Products
      The BOREAS RSS-17 freeze/thaw image data are available on the original BOREAS CD-ROM series.

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17. References

17.1 Platform/Sensor/Instrument/Data Processing Documentation
      Excerpts were taken from the following to document this data set:  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
Frolking, S., K. C. McDonald, J. Kimball, J. B. Way, R. Zimmermann, and S. W. Running, 1999. Using the space-borne NASA scatterometer (NSCAT) to determine the frozen and thawed seasons of a boreal landscape, Journal of Geophysical Research, Vol. 104, No. D22, pp. 27,895-27,907, November 27, 1999.

Kimball, J., K. C. McDonald, S. Frolking, A. R. Keyser, and S. W. Running, 2001a. "Radar Remote Sensing of the Spring Thaw Transition Across a Boreal Landscape," Remote Sensing of Environment, BOREAS special issue, (submitted.)

Kimball, J., K. C. McDonald, A. R. Keyser, S. Frolking, and S. W. Running, 2001b. Application of the NASA Scatterometer (NSCAT) for Classifying the Daily Frozen and Non-Frozen Landscape of Alaska, Remote Sensing of Environment, 75:113-126

Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. 2000. Collected Data of The Boreal Ecosystem-Atmosphere Study. NASA. CD-ROM.

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.

Running, S., J. B. Way, K. C. McDonald, J. Kimball and S. Frolking, 1999. Radar remote sensing proposed for monitoring freeze-thaw transitions in boreal regions, American Geophysical Union EOS Newsletter, Vol. 80 (19), pp. 220-221, May 11, 1999.

Sellers, P. and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 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. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94).

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

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.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 102(D24): 28,731- 28,770.

Way, J.B., R. Zimmermann, E. Rignot, K. McDonald, and R. Oren. 1997. Winter and spring thaw as observed with imaging radar at BOREAS. Journal of Geophysical Research 102(D24): 29,673-29,684.
 

17.3 Archive/DBMS Usage Documentation
      None.

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18. Glossary of Terms

None.

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19. List of Acronyms

AMI      - Active Microwave Instrumentation
ASCII    - American Standard Code for Information Interchange
ASF      - Alaska SAR Facility
ATSR     - Along-Track Scanning Radiometer
BOREAS   - BOReal Ecosystem-Atmosphere Study
BORIS    - BOREAS Information System
CD-ROM   - Compact Disk-Read-Only Memory
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
GIS      - Geographic Information System
GSFC     - Goddard Space Flight Center
HTML     - HyperText Markup Language
JPL      - Jet Propulsion Laboratory
LR       - Laser Retroreflector
MDA      - McDonnell Detweiler Associates
MWS      - Microwave Sounder
NAD83    - North American Datum of 1983
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
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20. Document Information

20.1 Document Revision Date(s)

Written: 12-Feb-2001
Last Updated: 20-Feb-2001 (citation revised on 30-Oct-2002)

20.2 Document Review Date(s)

BORIS Review: 13-Feb-2001
Science Review:

20.3 Document ID

dsp04_ers1maps

20.4 Citation

Cite this data set as follows (citation revised on October 30, 2002):

McDonald, K., and J. Nickeson. 2001. BOREAS Follow-On DSP-04 1994 ERS-1 Level-4 Landscape Freeze/Thaw Maps, Ver[sion] 1.0. Data set. Available on-line [http://daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.

20.5 Document Curator:

webmaster@daac.ornl.gov

20.6 Document URL:

http://daac.ornl.gov/BOREAS/FollowOn/guides/dsp04_gridded_ft_maps_doc.html

Keywords:
ERS-1
SIR-C
Backscatter

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