IceBridge Ku-Band Radar L1B Geolocated Radar Echo Strength Profiles, Version 2

This data set contains elevation and surface measurements over Greenland, the Arctic, and Antarctica, as well as flight path charts and echogram images acquired using the Center for Remote Sensing of Ice Sheets (CReSIS) Ku-Band Radar Altimeter.

Operation IceBridge products may include test flight data that are not useful for research and scientific analysis. Test flights usually occur at the beginning of campaigns. Users should read flight reports for the flights that collected any of the data they intend to use. Check IceBridge campaign Flight Reports for dates and information about test flights.

Table of Contents

  1. Contacts and Acknowledgments
  2. Detailed Data Description
  3. Data Access and Tools
  4. Data Acquisition and Processing
  5. References and Related Publications
  6. Document Information

Citing These Data

We kindly request that you cite the use of this data set in a publication using the following citation. For more information, see our Use and Copyright Web page.

Leuschen, Carl, Prasad Gogineni, Fernando Rodriguez, John Paden, and Chris Allen. 2014. IceBridge Ku-Band Radar L1B Geolocated Radar Echo Strength Profiles, Version 2, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://nsidc.org/data/irkub1b.html.

Summary

Platform:

NASA DC-8
NASA P-3B

Sensor:

CReSIS Ku-Band Radar Altimeter

Spatial Coverage:

Arctic, Greenland, Antarctica

Spatial Resolution:

Varies dependent on along-track, cross-track, and aircraft height characteristics.

Temporal Coverage:

Periodic, ongoing, from 12 October 2012 to the present.

Temporal Resolution:

Seasonal

Parameters:

Elevation
Surface

Data Format:

netCDF (.nc)

Extensible Markup Language (.xml)

Joint Photographics Experts Group (.jpg)

Metadata Access:

View Metadata Record

Version

See the Version History section of this document for previous version information.

Get Data:

netCDF data beginning with the 2012 Antarctica campaign: FTP.

Binary data for 2012 Greenland and earlier campaigns: FTP.

1. Contacts and Acknowledgments

Investigator(s) Name and Title

Carl Leuschen, Prasad Gogineni, Fernando Rodriguez, John Paden, Chris Allen
CReSIS
Nichols Hall
2335 Irving Hill Road
University of Kansas
Lawrence, Kansas 66045

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

Data and data products from CReSIS were generated with support from NSF grant ANT-0424589 and NASA grant NNX10AT68G.

2. Detailed Data Description

Format

The data files are in netCDF format. The echogram and flight path image files are JPEG files.

Each data file is paired with an associated XML file. XML files contain file level metadata and location, platform, and campaign information.

Echogram.jpg files contain depth echograms. The echograms are useful for tracking internal layers and shallow ice thicknesses.

Map.jpg files show campaign flight locations and flight lines.

The y-axis in the JPEG files shows depth relative to a range around the surface. The surface is in the center of the y-axis and the y-axis is set to a fixed range, usually from 0 meters to 60 or 80 meters for the land ice, and 0 meters to 4 meters for sea ice.

Note: Currently IceBridge Ku-Band Radar L1B Geolocated Radar Echo Strength Profiles (IRKUB1B) data for 2009 through the 2012 Greenland campaign are in binary format stored separately as IRKUB1B Version 1. Beginning with the 2012 Antarctica campaign, all data are provided in netCDF format. In the near future, data from all campaigns prior to 2012 Antarctica will be replaced with netCDF data and added to Version 2. For details on the IRKUB1B Version 1 data, see the Version 1 documentation.

File and Directory Structure

Data are available on the FTP site in the ftp://n5eil01u.ecs.nsidc.org/SAN2/ICEBRIDGE/IRKUB1B.002/ directory. Within this directory, the folders are organized by date, for example /2012.10.12/ to /2013.04.12/.

File Naming Convention

Data Files

Data files are named according to the following convention and as described in Table 1.

Example:
IRKUB1B_20121012_02_034.nc

IRKUB1B_YYYYMMDD_xx_xxx.NNN

Where:

Table 1. Data File Naming Convention
Variable Description
IRKUB1B Short name for IceBridge Ku-Band Radar L1B Geolocated Radar Echo Strength Profiles
YYYY Four-digit year of survey
MM Two-digit month of survey
DD Two-digit day of survey
xx Segment number
xxx Frame number
NNN Indicates file type. For example: netCDF (.nc), or XML (.xml)

JPEG Files

JPEG files are named according to the following convention and as described in Table 2:

Example:
IRKUB1B_20121012_02_034_Echogram.jpg
IRKUB1B_YYYYMMDD_xx_xxx_aaa.jpg

Where:

Table 2. JPEG File Naming Convention
Variable Description
IRKUB1B Short name for IceBridge Ku-Band Radar L1B Geolocated Radar Echo Strength Profiles
YYYY Four-digit year of survey
MM Two-digit month of survey
DD Two-digit day of survey
xx Segment number
xxx Frame number
aaa Image type. Examples: Echogram, or Map
jpg Indicates JPEG file type.

File Size

NetCDF files range from approximately xx MB to xx MB.

XML files are approximately xx MB each.

JPEG files are approximately xx KB to xxx MB

Volume

The entire data set is approximately xxxx TB.

Spatial Coverage

Spatial coverage for the IceBridge ku-band radar campaigns includes the Arctic, Greenland, Antarctica, and surrounding ocean areas. In effect, this represents the two coverages noted below.

Arctic / Greenland:
Southernmost Latitude: 60° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Antarctic:
Southernmost Latitude: 90° S
Northernmost Latitude: 53° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Spatial Resolution

Spatial Resolution varies dependent on along-track, cross-track, and aircraft height characteristics. See the Derivation Techniques and Algorithms section for further detail on resolution and bandwidth.

Projection and Grid Description

Referenced to WGS-84 Ellipsoid.

Temporal Coverage

These data were collected seasonally from 12 October 2012 to the present as part of Operation IceBridge funded campaigns.

Temporal Resolution

IceBridge campaigns are conducted on an annual repeating basis. Arctic and Greenland campaigns are conducted during March, April, and May, and Antarctic campaigns are conducted during October and November.

Parameter or Variable

The Ku-Band Radar Level-1B Geolocated Radar Echo Strength Profiles data set contains elevation and surface measurements.

Parameter Description

The Ku-band Radar netCDF files contain fields as described in Table 3.

Table 3. File Parameter Description
Parameter Description Units
altitude WGS-84 geodetic elevation coordinate of the measurement's phase center. Dimension is time. Meters
amplitude Power detected radar echogram data matrix. The first dimension is fasttime and time is the second dimension. Power is relative to the current range line only. Each range line may contain a different bias and so power comparisons between range lines may not be possible. Relative power (log scale)
Elevation_Correction Represents the number of zeros that were inserted during elevation compensation for each range line to simulate near-level flight. These zeros are not included in the truncation noise statistics. Only available when "amplitude" is truncated. Dimension is time. Range bins
fasttime Fast time. Zero time is the time at which the transmit waveform begins to radiate from the transmit antenna. Microseconds
heading Platform heading attitude (zero is north, positive to east). Dimension is time. Degrees
lat WGS-84 geodetic latitude coordinate of the measurement phase center. Always referenced to North. Dimension is time. Degrees
lon WGS-84 geodetic longitude coordinate of the measurement phase center. Always referenced to East. Dimension is time. Degrees
pitch Platform pitch attitude (zero is level flight, positive is up). Dimension is time. Degrees
roll Platform roll attitude (zero is level flight, positive is right wing tip down). Dimension is time. Degrees
Surface Estimated two way propagation time to the surface from the collection platform. This uses the same frame of reference as the fasttime variable. This information is sometimes used during truncation to determine the range bins that can be truncated. Dimension is time. Seconds
time UTC time of day. This is also known as the slow time dimension. The parameter "units" attribute contains a string of the form "seconds since YYYY-MM-DD 00:00:00" which indicates the day related top this time parameter. This pertains to data sets that wrap over a UTC day boundary which will cause this parameter to be outside the range [0,86400]. Seconds
Truncate_Bins Indices into the original (before truncation) fasttime vector for which the "amplitude" values are available. Only available when "amplitude" is truncated. Dimension is time. n/a
Truncate_Mean Represents a mean of the noise power for the truncated range bins before the surface return. When no range bins were truncated before the surface return the value is NaN. Only available when "amplitude" is truncated. Dimension is time. n/a
Truncate_Median Represents a median of the noise power for the truncated range bins before the surface return. When no range bins were truncated before the surface return the value is NaN. Only available when "amplitude" is truncated. Dimension is time. n/a
Truncate_Std_Dev Represents a standard deviation of the noise power for the truncated range bins before the surface return. When no range bins were truncated before the surface return the value is NaN. Only available when "amplitude is truncated. Dimension is time. n/a

3. Data Access and Tools

Get Data

netCDF data beginning with the 2012 Antarctica campaign: FTP.

Binary data for 2012 Greenland and earlier campaigns: FTP.

Software and Tools

See the NetCDF Resources at NSIDC page for tools to work with netCDF files.

CReSIS netCDF files are compatible with HDF5 libraries, and can be read by HDF readers such as HDFView. If the netCDF file reader you are using does not read the data, see http://www.unidata.ucar.edu/software/netcdf/ and http://nsidc.org/data/netcdf/tools.html for information on updating the reader.

CReSIS MATLAB readers are available for loading, plotting, and elevation compensation for CReSIS Level-1B radar products. These tools are provided by the Principal Investigator as-is as a service to the user community in the hope that they will be useful. Please note that support for these tools is limited. Bug reports, comments, and suggestions for improvement are welcome; please send to nsidc@nsidc.org.

JPEG files may be opened using any image viewing program that recognizes the JPEG file format.

XML files can be read with browsers such as Firefox and Internet Explorer.

4. Data Acquisition and Processing

Data Acquisition Methods

The Ku-band radar uses a Frequency Modulated Continuous Wave (FMCW) architecture (Carrara 1995). This is done to reduce the required sampling frequency of the Analog to Digital Converter (ADC) and is possible when the range gate is limited. Currently, the range gate is limited to low altitude flights to achieve the full bandwidth. In the FMCW radars, a long chirp signal of approximately 250 μs is generated which sweeps linearly in frequency from the start frequency to the stop frequency. This signal is transmitted and also fed to a mixer in the receiver to be used to demodulate the received signal. Signals outside the range gate are suppressed by the Intermediate Frequency (IF) filter and aliased by the system.

The dominant scattered signal is the specular or coherent reflection from the air-snow surface and shallow layers beneath this surface. A bistatic antenna configuration is used to provide isolation between the transmit and receive paths which is important because the FMCW system receives while transmitting and too little isolation means that the direct path from the transmitter to the receiver will be too strong and saturate the receiver. The antennas are mounted so that the main beam is pointed in the nadir direction to capture the specular surface and layer reflections.

The Pulse Repetition Frequency (PRF), or along-track sampling rate, does not necessarily capture the full Doppler bandwidth for point scatterers without aliasing. However, as the target energy is mostly coherent, it occupies only a small portion of the Doppler spectrum so the undersampling in along-track is not generally a problem. Since the coherent portion of the surface and layer scattering is the primary signal of interest, presumming is used to lower the data rate, which effectively low-pass-filters and decimates the Doppler spectrum.

The narrow beam width of the antennas have a fixed pointing direction, which means that when the aircraft rolls beyond approximately 10 degrees, the specular reflection falls outside the main lobe of the antennas and therefore the signal strength is reduced.

Derivation Techniques and Algorithms

Echograms posted include altitude correction, but the data files do not. Correction can be applied by shifting a record from bottom to top by the altitude correction value. Altitude variations within a data file are removed by subtracting the minimum altitude from all values. The result is variation in meters from the minimum. These values are then converted to whole pixel values given the radar parameters: sampling frequency = 58.32 MHz, pulse length, FFT length, and bandwidth. Note: sampling frequency after the 2009 Greenland campaign is 62.5 MHz.

Flat Surface Range Resolution

For a flat surface the range resolution is expressed by Equation 1:

Equation 1 (Equation 1)

Where:

Table 4. Flat Surface Range Resolution
Variable Description
kt kt = 1.5 due to the application of a Hanning time-domain window to reduce the range sidelobes of the chirped transmit waveform.
c Speed of light in a vacuum
B Bandwidth, nominally 3500 MHz (13 to 16.5 GHz range)
n Index of refraction for the medium

Bandwidth

The bandwidth for a particular segment can be determined by reading the param_radar structure in the echogram data file or by looking at the parameter values f0, f1, and fmult and doing the calculation in Equation 2:

Equation 2 (Equation 2)

Where:

Table 5. Bandwidth
Variable Description
B Bandwidth
param_radar.f1 Stop frequency of chirp out of Direct Digital Synthesis (DDS) and into Phase-Locked Loop (PLL)
param_radar.f0 Start frequency of chirp out of DDS and into PLL
param_radar.fmult PLL frequency multiplication factor

The range resolutions for several indices of refraction are shown in Table 6.

Table 6. Range Resolutions
Index of Refraction Range Resolution (cm) Medium
1 6.4 Air
sqrt(1.53) 5.2 Snow
sqrt(3.15) 3.6 Solid Ice

Index of Refraction

The index of refraction can be approximated by the calculation in Equation 3:

Equation 3 (Equation 3)

Where:

Table 7. Index of Refraction
Variable Description
ρsnow Density of the snow in grams per cm3

A dielectric of 1.53 is used which corresponds to a snow density of 0.3 g per cm3 (Warren 1999).

Along Track Resolution

In the along-track dimension, the raw data, before any hardware or software coherent averages, have a resolution derived in the same manner as the cross-track resolution. However, a basic form of focusing is applied called unfocussed SAR processing, also known as stacking or coherent averaging. If all effects are accounted for, the data may be coherently averaged to a SAR aperture length using Equation 4.

Equation 4 (Equation 4)

Where:

Table 8. SAR Aperture Length
Variable Description
H Height above ground
λc Wavelength

For H = 500 m and a center frequency of 14.75 GHz, the data may be averaged to a length of 2.25 m. The resolution turns out to be approximately equal to this with the exact definition given below. However, these data are only coherently averaged 16 times which includes both hardware and software averaging, and decimated by this same amount. At a platform speed of 140 m/s this is an aperture length, L, of 1.12 m. The sample spacing is likewise 1.12 m. Therefore the actual resolution is less fine, given by Equation 5.

Equation 5 (Equation 5)

Where:

Table 9. Along Track Resolution
Variable Description
H Height above ground
λc Wavelength
L SAR aperture length

For H = 500 m, the along-track resolution is 4.54 m.

A 1 range-bin by 5 along-track-range-line boxcar filter is applied to the power detected data and then decimated in the along-track by 5 so the data product has an along-track sample spacing of 5.6 m.

Fresnel Zone and Cross Track Resolution

For a smooth or quasi-specular target, for example internal layers, the primary response is from the first Fresnel zone. Therefore, the directivity of specular targets effectively creates the appearance of a cross-track resolution equal to this first Fresnel zone. The first Fresnel zone is a circle with diameter given by Equation 6.

Equation 6 (Equation 6)

Where:

Table 10. First Fresnel Zone Diameter
Variable Description
H Height above the air/ice interface
T Depth in ice of the target
λc Wavelength at the center frequency

Table 17 gives the cross-track resolution for this case.

Table 11. Cross-track Resolution Case
Center Frequency (MHz) Cross-track Resolution
H = 500 m
T = 0 m
14750 4.5

For a rough surface with no appreciable layover, the cross-track resolution will be constrained by the pulse-limited footprint, approximated in Equation 7.

Equation 7 (Equation 7)

Where:

Table 12. Pulse-Limited Footprint
Variable Description
H Height above the air/ice interface
T Depth in ice of the target
c Speed of light in a vacuum
kt kt = 1.5 due to the application of a hanning time-domain window to reduce the range sidelobes of the chirped transmit waveform
B Bandwidth in radians

Table 19 gives the cross-track resolution with windowing.

Table 13. Cross-track Resolution with Windowing
Bandwidth (MHz) Cross-track Resolution
H = 500 m
T = 0 m
3500 16.0

For a rough surface where layover occurs, the cross-track resolution is set by the beamwidth, β , of the antenna array. The antenna beamwidth-limited resolution is expressed by Equation 8:

Equation 8 (Equation 8)

Where:

Table 14. Antenna Beamwidth-limited Resolution
Variable Description
H Height above ground level
T Depth in ice of the target
βy Beamwidth in radians

Footprint

The antenna installed in the bomb bay of the NASA P-3 aircraft, the wing roots of the DC-8, and the nadir port of the Twin Otter is a Pasternack Enterprises 9854-20 standard gain horn antenna. The E-plane of the antenna is aligned in the along-track. The approximate beamwidths are 19 degrees in along-track and 19 degrees in cross-track. The footprint is a function of range as shown in Equation 9.

Equation 9 (Equation 9)

Where:

Table 15. Footprint
Variable Description
β Beamwidth in radians
H Height above ground level

For H = 500 m, the footprint is 167 m in along-track and 167 m in cross-track.

Trajectory and Attitude Data

The trajectory data used for this data release was from a basic GPS receiver. Lever arm and attitude compensation has not been applied to the data.

Processing Steps

The following processing steps are performed by the data provider.

  1. Set digital errors to zero. Error sequences are four samples in length and occur once every few thousand range lines.
  2. Synchronization of GPS data with the radar data using the UTC time stored in the radar data files.
  3. Conversion from quantization to voltage at the ADC input.
  4. Removal of DC-bias by subtracting the mean.
  5. For 2013 Greenland P3 and later, a tracking and truncation function has been implemented in the hardware which reduces the recorded data volume. Each range line is tracked and truncated separately and a step is added here to undo the tracking and truncation step so that the data can be placed in a matrix with constant time bins. This requires zero padding and time shifting of the data to get each range line to line up.
  6. The quick look output is generated using presumming or unfocused SAR processing for a total of 16 coherent averages which includes hardware and software averages. If the PRF is 2000 Hz, the new effective PRF is 125 Hz.
  7. A fast-time FFT is applied with a Hanning window to convert the raw data into the range domain, analogous to pulse compression. The data are flipped around based on the Nyquist zone.
  8. A high pass filter is applied in the along-track to remove coherent noise.
  9. A 1 range-bin by 5 along-track-range-line boxcar filter is applied to the power detected data and then decimated by 5 in along-track.
  10. The quick-look output is used to find the ice surface location, fully automated.
  11. 11. The output is elevation compensated to the nearest radar range bin and then truncated in fast time to reduce the data volume.

The purpose of the elevation compensation, when applied, is to remove the large platform elevation changes to make truncation more effective. The process is not designed to perform precision elevation compensation and is probably not sufficient for scientific analysis. The following steps are performed:

  1. Let:
    1. Elevation_Orig be the 1 by N elevation vector before elevation compensation
    2. Data_Orig be the M_orig by N data matrix before elevation compensation
    3. Time_Orig be the M_orig by 1 fast-time time axis before elevation compensation
    4. Elevation be the 1 by N vector from the data product file
    5. Dat be the matrix from the data product file
    6. Time be the M by 1 fast-time time axis from the data product file
    7. maxElev = max(Elevation_Original)
  2. dRange = maxElev - Elevation_Original
  3. dt = Time_Orig(2) - Time_Orig (1)
      1. Sample spacing in fast-time (i.e. one range bin)
  4. dBins = round(dRange / (c/2) / dt)
    1. This is a 1 by N vector of the number of range bins for each range line used to shift "Data_Orig". In other words, this is the elevation compensation for each range line written in terms of range bins.
  5. M = M_orig + max(dBins)
  6. The original data matrix is zero padded to M and then each range line is shifted by the corresponding entry in dBins.
    1. Because of the round function for creating dBins, the elevation compensation is only done with range bin accuracy.
    2. The new Data matrix is similar to what would have been collected if the aircraft had flown at a constant elevation of maxElev.
  7. The elevation matrix is modified according to the elevation compensation so that: Elevation_Orig = Elevation - dBins*dt*c/2. Once again, because of the round function, the Elevation vector will be nearly constant, but not quite: the quantization noise caused by the round function remains.
  8. The Time_Orig vector is extended in length by the maximum bin shift to create the new Time vector.

Version History

Version 2 of the IRKUB1B data. Beginning with the 2012 Antarctica campaign, data are provided in netCDF format. In the near future, data from all campaigns prior to Fall 2012 will be replaced with netCDF data. For details on the Version 1 data, see the V1 documentation.

Error Sources

The CReSIS ku-band, accumulation, and snow data acquisition systems have a known issue with radar data synchronization with GPS time. When the radar system is initially turned on, the radar system acquires Universal Time Coordinated (UTC) time from the GPS National Marine Electronics Association (NMEA) string. If this is done too soon after the GPS receiver has been turned on, the NMEA string sometimes returns GPS time rather than UTC time. GPS time is 15 seconds ahead of UTC time during this field season. The corrections for the whole day must include the offset -15 second correction. GPS corrections have been applied to all of the data using a comparison between the accumulation, snow, and kuband radars which have independent GPS receivers. A comparison to geographic features and between ocean surface radar return and GPS elevation is also made to ensure GPS synchronization. GPS time corrections are given in the vector worksheet of the parameter spreadsheet.

Sensor or Instrument Description

As described on the CReSIS Sensors Development Radar Web site, the ku-band radar operates over the frequency range from 13 to 17 GHz. The primary purpose of this radar is high precision surface elevation measurements over polar ice sheets. The data collected with this radar can be analyzed in conjunction with laser-altimeter data to determine thickness of snow over sea ice. The radar has been flown on the NASA DC-8 and P-3 aircrafts, and the National Science Foundation-provided Twin Otter aircraft.

5. References and Related Publications

Carrara, W. G., R. S. Goodman, and R. M. Majewski. 1995. Spotlight Synthetic Aperture Radar: Signal Processing Algorithms, Artech House, Norwood, MA, pp. 26-31.

Patel, A. E., P. S. Gogineni, C. Leuschen, F. Rodriguez-Morales, and B. Panzer. 2010. An Ultra Wide-band Radar Altimeter for Ice Sheet Surface Elevation and Snow Cover Over Sea Ice Measurement, Abstract C41A-0518 presented at 2010 Fall Meeting, AGU, San Francisco, California, 13-17 December, 2010.

Rodriguez-Morales, F., P. Gogineni, C. Leuschen, C. T. Allen, C. Lewis, A. Patel, L. Shi, W. Blake, B. Panzer, K. Byers, R. Crowe, L. Smith, and C. Gifford. 2010. Development of a Multi-Frequency Airborne Radar Instrumentation Package for Ice Sheet Mapping and Imaging, Proc. 2010 IEEE Int. Microwave Symp., Anaheim, CA, May 2010, 157-160.

Warren, S., I. Rigor, and N. Untersteiner. 1999. Snow Depth on Arctic Sea Ice, Journal of Climate, 12: 1814-1829.

Related Data Collections

Related Web Sites

6. Document Information

Acronyms and Abbreviations

The acronyms used in this document are listed in Table 16.

Pulse Repetition Frequency
Table 16. Acronyms and Abbreviations
Acronym Description
ADC Analog to Digital Converter
CIRES Cooperative Institute for Research in Environmental Science
CReSIS Center for Remote Sensing of Ice Sheets
DC-8 Douglas DC-8 aircraft
DDS Direct Digital Synthesis
FFT Fast Fourier Transform
FMCW Frequency Modulated Continuous Wave
FTP File Transfer Protocol
GPS Global Positioning System
IF Intermediate Frequency
JPEG Joint Photographic Experts Group
KML Keyhole Markup Language
Level-1B Processing Level-1B
MATLAB MATrix LABoratory numerical computing file
NASA National Aeronautics and Space Administration
NMEA National Marine Electronics Association
NSF National Science Foundation
NSIDC National Snow and Ice Data Center
P-3 Lockheed P-3B Orion aircraft
PLL Phase-Locked Loop
PRF Pulse Repetition Frequency
SAR Synthetic Aperture Radar
URL Uniform Resource Locator
UTC Universal Time Code
WFF Wallops Flight Facility
WGS-84 World Geodetic System 1984

Document Creation Date

12 September 2014

Document Revision Date

Document URL

http://nsidc.org/data/docs/daac/icebridge/irkub1b/index.html