PARIS (Pathfinder Airborne Radar Ice Sounder)
PARIS is a NASA-funded airborne active microwave technology demonstration instrument within the IIP (Instrument Incubator Program) designed and developed at JHU/APL (Johns Hopkins University/Applied Physics Laboratory), Laurel, MD. The project of the 150 MHz radar started in July 2005. The primary objective was to demonstrate successful radar sounding of ice sheet layering and bottom topography from a high-altitude platform. The major challenges are: 1) 2)
• Clutter dimensions: Along-track and cross-track clutter suppression
• Weak signal mitigation: Innovative radar design (large dynamic range, very low side-lobes, extrem linearity, generous power).
The radar's architecture is based on two key characteristics: signal modulation at the mean transmitted frequency, and analog-to-digital conversion operating directly on the received signal. These techniques have proven to be essential for radar sounders, since they circumvent the main sources of non-linearity and harmonic generation common to all up- or down-conversion frequency schemes.
Figure 1: High-level architecture of the PARIS radar sounder (image credit: JHU/APL)
The transmit waveform is a linear frequency-modulated chirp at a 150 MHz operating frequency with a trapezoidal envelope (5 MHz bandwidth). Such severe weighting is essential to reduce the ringing commonly associated with the initial on-off transition of weakly-weighted waveforms. The 180-W (peak) pulse has a bandwidth of ~ 6 MHz. The amplifier is class AB to help ensure the high linearity needed to suppress the internal clutter (sidelobes) generated by the chirp waveform.
In the receiver, there is no intermediate frequency, and no analog baseband down-conversion. Instead, the signals are sampled directly following the low-noise amplifier. The sample rate is well below Nyquist, but is chosen so that the resulting spectra shift an alias of the main signal to offset baseband in a clear channel.
The radar ice sounding application demands a dynamic range of at least 90 dB. To help support this requirement, the receiver includes variable attenuators to adjust the voltage range of the signal input to the analog-to-digital converter as well as sensitivity time control (STC) to increase the effective dynamic range of the response as a function of depth of penetration. The overall noise figure of the receiver is less than 5.5 dB with a gain of over 60 dB and a 45 dBm third-order intercept point.
PARIS instrument: The digital components of the radar consists of a FPGA (Field Programmable Gate Array) radar synchronizer, a DDS (Direct Digital Synthesizer), and an under-sampling ADC (Analog-to-Digital Converter). All components of the digital subsection are clocked by a stable 66.6 MHz reference oscillator. The radar data are time-tagged by reference to GPS.
The data processor in the ground system employs the “delay Doppler technique” proposed by Keith Raney and his team at APL in 1994 and verified with the D2P (Delay-Doppler Phase-monopulse Radar) airborne radar altimeter instrument in the spring of 2000 (first campaign). Figure 4 illustrates the the logical flow of this technique as would be required for a radar altimeter. - The ice sounding application requires that the range curvature correction (the “delay shift” operation) and the resolved along-track data co-registration (the “Doppler shift” operation) must account for the retarded EM propagation speed within the ice sheet.
There are three major advantages that follow from this method:
• a) rejection of clutter from Doppler selectivity
• b) finer along-track footprint resolution
• c) more degrees-of-freedom resulting in substantial speckle reduction.
Compatible methods to reduce clutter contributions from off-nadir sources to the side of the surface track are currently under investigation.
Figure 2: Along-track clutter suppression: partially-coherent Doppler (image credit: JHU/APL)
Figure 3: Along-track clutter suppression: Doppler (image credit: JHU/APL)
Figure 4: Logical flow of the delay-Doppler algorithm, which is the optimum processing strategy to reduce self-clutter and improve along-track spatial resolution for a nadir-viewing radar sounder (image credit: JHU/APL)
First field campaign:
In May 2007, the PARIS instrument successfully completed 10 days of test flights and collected over 900 GByte of data over northern Greenland. On these flights aboard the NASA P-3B aircraft, PARIS provided the first field demonstration of radar sounding of both ice sheet layering and basal topography from an airborne platform. The flights proved the feasibility of ice sounding from high altitudes (up to 8.5 km).
Figure 5: Benefits of partially-coherent Doppler processing (image credit: JHU/APL)
Figure 6: PARIS instrument on the NASA-P3 aircraft (image credit: JHU/APL)
Further airborne campaigns with PARIS participation:
• Operation IceBridge Greenland/Arctic Sea Ice 2009: The airborne campaign was flown on the NASA P-3B aircraft from March 31 to May 5, 2009. The following payloads (instruments) were flown: 3)
- ATM (Airborne Topographic Mapper)
- PARIS (Pathfinder Advanced Radar Ice Sounder) of JHU/APL
- Ku-band Snow Radar Altimeter of CreSIS (Center for Remote Sensing of Ice Sheets)
- LVIS (Land, Vegetation, and Ice Sensor)
Note: OIB (Operation IceBridge) is a NASA airborne mission making laser altimetry, radar, and other geophysical measurements to monitorand characterize the Earth's cryosphere. OIB will operate from 2009 until the launch of ICESat-2, scheduled for 2016. The platforms include the NASA DC-8, P-3, and eventually the GlobalHawk UAV. The instruments include laser altimeters, ice penetrating radars, gravimeters, digital mapping cameras, and other new technology. 4)
So far, the PARIS instrument has been flown in IceBridge campains over Greenland in 2009, 2010, 2011, 2012 and 2013.
1) R. Keith Raney, Carl Leuschen, Marshall Jose, “Pathfinder Advanced Radar Ice Sounder: PARIS,” Proceedings of IGARSS 2008 (IEEE International Geoscience & Remote Sensing Symposium), Boston, MA, USA, July 6-11, 2008, WE2.210.2
2) R. Keith Raney, Carl Leuschen, Marshall Jose, “Pathfinder Advanced Radar Ice Sounder: PARIS,” ESTC2008 (Earth Science Technology Conference 2008), June 24-26, 2008, College Park, MD, USA, URL of paper: http://esto.nasa.gov/conferences/estc2008/papers/Raney_Keith_B5P1.pdf, and URL of presentation: http://esto.nasa.gov/conferences/estc2008/presentations/RaneyB5P1.pdf
3) “IceBridge Data - Campaign Data Summaries,” NSIDC (National Snow and Ice Data Center), URL: http://nsidc.org/data/icebridge/campaign_data_summary.html
4) Jeffrey S. Deems, Marilyn Kaminski, Mary J. Brodzik, Ronald L Weaver, “Operation IceBridge: Science Overview and Data Management,” NSIDC, Boulder, CO, URL: http://www.google.de/url?sa=t&rct=j&q=paris%20(pathfinder%20advanced%20radar%20ice%20sounder)
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.