Ice thickness is typically determined using data collected from waveforms with different pulse durations. Generally, all receive channels are used to produce the best result. The two reflections that are of most interest are the ice surface and ice bottom. The difference in the propagation time between the ice surface and ice bottom reflections is then converted into ice thickness using an estimated ice index of refraction of ice (square root of 3.15). The media is assumed to be uniform, i.e. no firn correction is applied.

Data collection modes used for typical operation are described below in the Data Acquisition Methods section.

P-3 Low Altitude

A waveform with a 1-µs duration and lower receiver gain settings is used to measure the round-trip signal time for the surface echo, while a waveform with a 10-µs duration and higher receiver gain settings is used to measure the round-trip signal time for the bed echo. The two different waveforms are used because of the large dynamic range of signal powers that are observed. The 10-µs duration and higher receiver gain settings are more sensitive to the bed echo, but the signal is generally saturated from the ice surface and upper internal layers.

DC-8 Low Altitude

The same concept is used as for the P-3. However, during the first two field seasons (2009 Antarctica DC-8 and 2010 Greenland DC-8), extra antennas inside the cabin were used to detect the ice surface delay time because the Transmit/Receive (TR) switches did not meet their switching time specification. The TR switches have not been fixed in subsequent field seasons, but the TR switch control signals have been set so that the surface echo is generally still detectable, although with diminished power, even for very low altitudes down to 600 feet Above Ground Level (AGL).

P-3 High Altitude and DC-8 High Altitude

The dynamic range between the ice surface and ice bottom echoes is much smaller and a single high-gain and long pulse duration waveform is used to capture both echoes.

2009 Antarctica DC-8 Radar Settings

Bandwidth: 180-210 MHz (DC-8 platform restricted to 189.15-198.65 MHz)
Tx power: 550 W
Waveform: eight channel chirp generation, 14 bit ADC at 111 MHz bandpass sampling
Acquisition: eight channels
Dynamic Range: waveform playlist
Rx Aperture: 1.5 wavelength aperture
Tx Aperture: 1.5 wavelength aperture; fully programmable
Monostatic Rx/Tx
Data rate: 12 MB/sec per channel

2009 Antarctica Twin Otter Radar Settings

Bandwidth: 140-160 MHz
Tx power: 500 W
Waveform: eight channel chirp generation
Acquisition: eight channels
Rx Aperture: 3 wavelength aperture
Tx Aperture: 3 wavelength aperture; fully programmable
Bistatic Rx/Tx
Data rate: 12 MB/sec per channel

2010 Antarctica DC-8 Radar Settings

Dynamic Range: waveform playlist coupled with low gain and high gain channels

2010 Greenland P-3 Radar Settings

Bandwidth: 180-210 MHz (EMI restricted to 10 MHz within 180-210 MHz most segments)
Tx power: 600 W
Waveform: eight channel chirp generation
Acquisition: sixteen channels (multiplexed on to 8 channels), 14 bit ADC at 111 MHz bandpass sampling
Rx Aperture: 2 wavelength, 3.5 wavelength, and 2 wavelength apertures, baseline of 6.4 m between each aperture
Tx Aperture: 3.5 wavelength aperture; fully programmable
Mixed monostatic and bistatic tx/rx
Data rate: 6 MB/sec per channel

2011 Antarctica DC-8 Radar Settings

Dynamic Range: waveform playlist coupled with low gain and high gain channels

2011 Antarctica Twin Otter Radar Settings

Tx power: 1500 W
Rx Aperture: two 3 wavelength apertures with 13.8 m baseline
Tx Aperture: 3 wavelength aperture; fully programmable

2011 Greenland Twin Otter Radar Settings

Bandwidth: 180-210 MHz
Tx power: 500 W
Waveform: eight channel chirp generation
Acquisition: sixteen channels (multiplexed onto 8 channels), 14 bit ADC at 111 MHz bandpass sampling
Rx Aperture: two 3 wavelength apertures with 13.8 m baseline
Tx Aperture: 3 wavelength aperture; fully programmable
Mixed monostatic and bistatic tx/rx
Data rate: 6 MB/sec per channel

2011 Greenland P-3 Radar Settings

Bandwidth: 180-210 MHz
Tx power: 1050 W
Waveform: Eight channel chirp generation
Acquisition: sixteen channels, 14 bit ADC at 111 MHz bandpass sampling
Dynamic Range: waveform playlist
Rx Aperture: 2 wavelength, 3.5 wavelength, and 2 wavelength apertures, baseline of 6.4 m between each aperture
Tx Aperture: 3.5 wavelength aperture; fully programmable
Mixed monostatic and bistatic tx/rx
Data rate: 32 MB/sec per channel

The layer tracking of ice surface and ice bottom reflections are manually driven processes with basic tools for partial automation. The tools used are determined by the operator picking the data and include:

1. Manual picking and interpolation.
2. Snake tracker which follows the strongest return within a window centered on the last tracked location from range line to range line.
3. Leading edge detector searches for the crossing of a threshold beneath the peak return.
4. Peak detector.

Various processing outputs, for example Minimum Variance Distortionless Response (MVDR), standard, quick look, dynamic range of the image, averaging, and detrending methods are used to better highlight features in the echogram as needed.

For further detail on data processing, including algorithms, see the Data Processing section in the IceBridge MCoRDS L1B Geolocated Radar Echo Strength Profiles documentation.

On April 12, 2011, the 2009 Antarctica MCoRDS L2 data were replaced by V01.1. The original 2009 Antarctica data were processed through a temporary process with quick look data products only. V01.1 data are produced from full Synthetic Aperture Radar (SAR) processing.

On October 17, 2011, the MCoRDS L2 data were replaced by V01.2. The csv_good and CSARP_layer data were removed from the previous version.

The data are not radiometrically calibrated. This means that they are not converted to some absolute standard for reflectivity or backscattering analysis. The MCoRDS science team is working on data processing and hardware modifications to do this.

The primary error sources for ice penetrating radar data are system electronic noise, multiple reflectors also known as multiples, and off-nadir reflections. Each of these error sources can create spurious reflections in the trace data leading to false echo layers in profile data. Multiple reflectors arise when the radar energy reflects off two surfaces more than once (or resonates) in the vertical dimension, and then returns to the receive antenna. Reflections occur in situations when two or more large reflectors are present with large electromagnetic constitutive property changes, such as the ice surface (air/ground), the bottom of the ice, and the aircraft body which is also a strong reflector. The radar receiver only records time since the radar pulse was emitted, so the radar energy that traveled the additional path length appears later in time, apparently deeper in the ice or even below the ice-bedrock interface. Note that multiples of a strong continuous reflector have a similar shape because the propagation time is a multiple of the resonance cavity. The most common multiple is between the air-ice surface and the aircraft. This "surface" multiple shows up at twice the propagation time as the original surface return and all the slopes are doubled.

Off-nadir reflections can result from crevasse surfaces, water, rock outcrops, or metal structures. Antenna beam structure and processing of the data are designed to reduce these off-nadir reflected energy sources.

2009 Antarctica DC-8 Specific Issues

Transmit/Receive Switch: During the first two field seasons (2009 Antarctica DC-8 and 2010 Greenland DC-8), extra antennas inside the cabin were used to detect the ice surface delay time because the TR switches did not meet their switching time specification. The TR switches have not been fixed in subsequent field seasons, but the TR switch control signals have been set so that the surface echo is generally still detectable, although with diminished power, even for very low altitudes down to 600 ft. AGL.

Some of the data collected during this season are from high altitude. The high altitude data are generally lower quality than the low altitude data. This is because:

1. the cross-track antenna resolution is proportional to range creating severe layover problems in mountainous terrain, for example 05 November 2009 high altitude peninsula flight.
2. the sidelobes from the long pulse duration mask out some of the returns that otherwise would have had a high enough signal to noise ratio.
3. the range to target is greater so the spherical spreading power loss is greater leading to a lower signal to noise ratio.

Monostatic elements: All monostatic elements used for transmit and receive have an unknown fast-time gain profile because the transmit/receive switches take about 10 microseconds to fully switch positions. This fast-time gain profile has not been corrected so that using the surface or shallow layer returns for antenna equalization or for radiometric purposes is not recommended. All five of the antennas on the DC-8 are monostatic antennas.

2010 Antarctica DC-8 Specific Issues

Transmit/Receive Switch: This is a similar problem as with the 2009 Antarctica DC-8 season. However, we were able to set the TR switch control signals so that the surface is recoverable with the regular array down to an altitude of 600 ft. AGL but with very much reduced signal strength. The regular array is preferred over the EMI antenna setup because the radiation pattern characteristics are better. At an altitude of 1500 ft., no degradation in signal detectability is observed.

High altitude data: See 2009 Antarctica DC-8.

Monostatic elements: See 2009 Antarctica DC-8.

2010 Greenland DC-8 Specific Issues

One of the in-cabin Electromagnetic Interference (EMI) antennas is used to produce the quick look data product. The EMI antennas were not designed to receive the surface return. However, the transmit-receive switch for the primary array was set so that the surface returns for platform altitudes below 1500 feet were greatly attenuated. Because of this, the EMI antennas are used for picking the surface return at low altitude.

2010 Greenland P-3 Specific Issues

Monostatic elements: See 2009 Antarctica DC-8. Only the center seven elements are monostatic on the P-3.

EMI: Due to lack of shielding, a noisy switching power supply, and potentially other unidentified sources the radar was operated with 10 MHz bandwidth (190-200 MHz) for most of the field season. The signal quality is still lower than expected in this band due to broadband noise which is present at all times and periodic burst noise from other pulsed instruments on the P-3 and from random burst noise. The noise present at all times manifests itself as an increase in the noise floor and the burst noise manifests itself as smeared point targets.

2011 Greenland P-3 Specific Issues

The MCoRDS2 data acquisition system fielded for the first time this season has a known issue with radar data synchronization with GPS data. The synchronization time correction that must be added to the radar time stamp is either 0 or -1 second. 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 entire day must include the offset, either -15 or -16 seconds. GPS corrections have been applied to all of the data using a comparison between the accumulation radar and the MCoRDS radar. The accumulation radar from 2011 is known to have only the GPS/UTC problem. The GPS/UTC problem is easily detectable by comparing the data to raster imagery, so the only correction that could be in error is the 0 or -1 second offset and this generally happens when there are no good features in or above the ice to align the accumulation and MCoRDS2 radars. GPS time corrections and frames where no good sync information was available are given in the vector worksheet of the Parameter Spreadsheet.

Two of the missions, SE Glaciers on 11 April 2011 and Helheim/Kanger/Midguard on 19 April 2011, suffered from radar configuration failures and about 40 dB of sensitivity was lost on the high gain channel. Short portions of these data are still good so the datasets are published, but most of the data are not useful.

As described on the CReSIS Sensors Development Radar Page, the Multichannel Coherent Radar Depth Sounder operates over a 180 to 210 MHz frequency range with multiple receivers developed for airborne sounding and imaging of ice sheets. Measurements are made over two frequency ranges: 189.15 to 198.65 MHz, and 180 to 210 MHz. The radar bandwidth is adjustable from 0 to 30 MHz. Multiple receivers permit digital beamsteering for suppressing cross-track surface clutter that can mask weak ice-bed echoes and strip-map SAR images of the ice-bed interface. These radars are flown on twin engine and long-range aircraft including NASA P-3, Twin Otter (TO), and DC-8. GPS time corrections and frames where no good sync information was available are given in the vector worksheet in the Parameter Spreadsheet

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Gogineni, S., D. Tammana, D. Braaten, C. Leuschen, T. Akins, J. Legarsky, P. Kanagaratnam, J. Stiles, C. Allen, and K. Jezek. 2001. Coherent Radar Ice Thickness Measurements Over the Greenland Ice Sheet, Journal of Geophysical Research-Atmospheres 106(D24): 33761-33772.

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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, 2010: 157 – 160.

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• IceBridge MCoRDS L1B Geolocated Radar Echo Strength Profiles
• IceBridge PARIS L2 Ice Thickness
• IceBridge HiCARS 1 L2 Geolocated Ice Thickness
• Greenland 5 km DEM, Ice Thickness, and Bedrock Elevation Grids

• CReSIS Website (https://www.cresis.ku.edu/)
• CReSIS Sensors Development Radar Web Page (https://cms.cresis.ku.edu/research/sensors-development/radar).
• IceBridge Data Web site at NSIDC (http://nsidc.org/data/icebridge/index.html).
• IceBridge Web site at NASA (http://www.nasa.gov/mission_pages/icebridge/index.html).
• ICESat/GLAS Web site at NASA Wallops Flight Facility (http://glas.wff.nasa.gov/).
• ICESat/GLAS Web site at NSIDC (http://nsidc.org/daac/projects/lidar/glas.html).