D2P (Delay-Doppler Phase-Monopulse Radar) - New Technology for Altimeters

D2P (Delay-Doppler Phase-monopulse Radar) - New Technology for Altimeters

Overview

D2P is a JHU/APL (Johns Hopkins University/Applied Physics Laboratory) designed, built, and operated airborne radar instrument sponsored by NASA's Instrument Incubator Program (start in 1998). The goal of this project is to demonstrate the use of two enhancements to satellite radar altimetry and to reduce the risk to a future flight program that would employ an enhanced altimeter. ^{1) 2) 3) 4) 5)}

The underlying concepts behind the D2P radar altimeter are those employed by the TOPEX/Poseidon and GFO (Geosat Follow-On) satellite altimeters. The altimeters flown on these spacecraft are capable of measuring the distance between the satellite and the mean ocean surface with a precision of 2-3 cm. The D2P radar differs from those instruments in two ways:

• Pulse-to-pulse coherence and full Doppler processing to allow for measurement of the along-track position of the range measurement

• Use of two antennas and two receiver channels that allow for measurement of the across-track angle of the range measurement (bistatic configuration).

The pulse-to-pulse coherence allows for a group of pulses to be Doppler processed so that the backscattered energy is separated into contributions that each represent a different along-track position of the scattering surface. This allows for an improvement in the resolution of the altimeter measurement. This is useful in ocean applications near coastlines, in the observation of smaller ocean features, and in solid surface altimetry such as ground ice applications. The second antenna and receiver channel allow for an angle measurement to accompany the range measurement.

The two enhancements provide accurate altimetry over the ice sheets of Greenland and Antarctica and contribute valuable data to an analysis of the ice mass balance as part of global climate change studies. The coherent processing constrains each range measurement to a known position in the along-track direction. The phase-monopulse angle measurement determines the across-track position of the measurement area. The range measurement therefore corresponds to a known position on the ice surface. A conventional radar altimeter must infer the position of the measurement and this inference represents a significant source of error.

The first two test flights with the D2P instrument were conducted in the spring of 2000 (March 30 and April 3) on a Navy-P3 aircraft operated by NRL (Naval Research Laboratory). Further demonstrations of the D2P radar over the ice sheets of southern Greenland were performed in June 2000 (conducted from Goose, Bay, Canada). Several auxiliary measurements with various (standard GPS, DGPS, nadir-looking video, IMU) instruments were performed on all flights with the objective to verify basic concepts and performance and to support the functions of data processing and analysis.

Radar System

The D2P system is composed of two logical portions: the flight system, and the ground system. The flight system has also two logical sections:

• A set of RF and digital components that create, amplify and transmit the radar pulse, and receivers to capture the returning radar echoes

• A set of computers that control the operation of the RF/digital components as well as digitizing and recording the received data.

The airborne system has an interactive screen for operator control, and a processor that provides real-time range/Doppler processing and display of data samples so that proper system performance can be monitored. Data are recorded on high-capacity disks, and returned to the ground for processing and analysis.

All delay/Doppler and phase-monopulse processing was performed in the laboratory, rather than in the aircraft. The delay/Doppler principle involves only a Fourier transform over a group (burst) of samples from pulse to pulse, followed by a range curvature correction (integral shift and phase-multiply), and a range Fourier transform. In practice, more steps are required. These additional steps are needed to compensate for changes in aircraft altitude or mean terrain slope, which necessitate radar system timing changes from burst to burst and within a burst. Processing also must compensate for changes in platform vertical velocity, and organize the data in all Doppler bins so that their averaged waveforms are spatially aligned from burst to burst.

Background

In 1994, Keith Raney and his team at APL began developing a concept with a new type of radar altimeter that incorporates features derived from SAR technology. The result is the DDA (Delay Doppler radar Altimeter) technology.

Conventional radar altimeters are considered “pulse-limited” instruments and, as opposed to their “beam-limited” laser altimeter counterparts, have footprints whose location and size are determined by the surface slope and system resolution, respectively. Simply put, a conventional radar altimeter “sees” the closest reflective surface. Over land ice, the closest surface may not be directly below the altimeter, which imposes an error on the altimeter's height measurement. The SIRAL (SAR Interferometer Radar Altimeter) instrument of CryoSat-1 of ESA (launch failure in Oct. 2005), and of CryoSat-2, (launch in 2009) as well as D2P share the system design features that overcome this fundamental limitation. ^{6) 7)}

As shown in Figure 1a, when the measurement location is assumed to be at nadir over sloped terrain, the radar altimeter height estimate will be incorrect in both value and geophysical location. This error is significant even over ice sheets whose mean slopes are as small as 1º or less. To mitigate slope-induced measurement errors in the across-track direction, the D2P (and SIRAL) design uses two receive channels with their respective antennas separated in the across-track direction by a baseline b, as shown in Figure 1b.

Legend:

• (a) The measurement occurs at a location normal to the surface. When the surface is sloped and the measurement is assumed at nadir, the height estimate will be incorrect.

• (b) By using two receive channels, with the antennas separated by a baseline, the angle can be determined from the range (phase) difference between the two channels.

When the two receive channels are combined interferometrically, the phase difference is invertible to estimate the across-track surface slope. In the along-track direction, slope-induced errors are mitigated by collecting data at a sufficiently high pulse repetition frequency to synthesize a set of beam-limited measurements using the delay-Doppler method, as shown in Figure 2.

The delay-Doppler method has been shown to reduce range errors caused by along-track surface slopes, improve along-track resolution, and provide additional processing gain that in turn reduces the measurement error (and lowers transmit power requirements). Since the synthesized measurements are beam limited, the surface slope can be estimated from the peak Doppler bin. Together, these across- and along-track improvements increase surface height measurement accuracy and minimize geophysical location errors that sloping ice sheets induce on measurements from conventional pulse-limited radar altimeters.

Legend:

• (a) The measurement area of a conventional radar altimeter is pulse limited.

• (b) Through Doppler processing, a set of synthesized narrow antenna beams is formed to provide beam-limited measurements, which correct the errors in height estimates over a sloped surface.

The D2P radar system is in effect an airborne prototype for the SIRAL altimeter aboard the CryoSat spacecraft. Both instruments operate at Ku-band, have similar bandwidths, include two receive channels, and produce coherent data that may later be processed by the delay-Doppler algorithm. Table 2 lists the significant parameters and compares the D2P and SIRAL systems.

Campaigns With D2P Instrument Participation

Recognizing the similarities of the D2P and SIRAL instruments, ESA co-sponsored the D2P altimeter to participate in several prelaunch CryoSat calibration and validation field activities.

• LaRA (Laser and Radar Altimeter) campaign in the Arctic region of Greenland and Svalbard: A D2P radar campaign took place in May 2002 under joint NASA/ESA sponsorship to support calibration and validation activities, and science investigations in advance of the CryoSat and ICESat missions. The D2P radar altimeter was flown aboard the NASA-P3 aircraft along with the ATM (Airborne Topographic Mapper) laser altimeters to collect simultaneous laser and radar altimeter (hence, the LaRA campaign) measurements over land and sea ice. ⁸⁾

The main objective of that experiment was to provide insight into the differences between the estimated height values extracted from coincidental laser (optical) and radar (RF) measurements over various ice and snow surface conditions. The respective height measurements of the two instruments were cross-calibrated to a few centimeters using runway overflights at the NASA Wallops Flight Facility.

The field geometry is illustrated in Figure 3. The laser and radar collected both coincidental and simultaneous measurements. Depending on the aircraft altitude, the D2P footprint ranged from 5 to 20 m in the along-track and 30 to 100 m in the across track, while the conical scanning laser swath sufficiently covered the entire D2P footprint with many laser shots of an approximately 1 m radius.

• CryoVEx (CryoSat Validation Experiment) campaign: As a follow-on to the LaRA campaign, the D2P system was flown again in 2003 under joint NASA/ESA sponsorship as part of the CryoVEx field campaign. As in 2002, simultaneous laser and radar altimeter measurements were collected in the Arctic.

In addition to data collection, the CryoVEx mission was an experimental “test run” to exercise procedures planned for future calibration and validation activities to be conducted by ESA in support of CryoSat. CryoVEx was a major multi-agency enterprise. In situ snow and ice measurements as well as helicopter-based electromagnetic induction ice thickness soundings were collected by researchers aboard the Alfred Wegner Institute 'Polarstern' icebreaker research vessel from Germany. ⁹⁾

• AASI (Antarctic AMSR-E Sea Ice) calibration and validation field campaign. The AMSR-E (Advanced Microwave Scanning Radiometer-EOS) instrument aboard the Aqua satellite of NASA retrieved sea ice concentration data using passive radiometric temperature measurements over a variety of microwave frequency bands.

In late summer 2003, the D2P radar altimeter was deployed again on the NASA-P3 aircraft during the AASI campaign. The role of the D2P altimeter during this campaign was to provide additional estimates of sea ice concentration, freeboard, and possible snow cover from the precision height measurements. - Unfortunately, the 2003 campaign was canceled after aircraft difficulties occurred during the first data flight.

However, the AASI campaign was completed in 2004 on a Naval Research Laboratory NRL-P3 aircraft. Figure 5 illustrates the flight tracks during both the 2003 and 2004 AASI campaigns. ¹⁰⁾

In conclusion it can be stated that the airborne JHU/APL D2P radar altimeter, when flown in company with a co-located laser altimeter over a variety of ice sheets, has collected data that illustrate the frequent occurrence of differentials between surface heights measured by the two types of instruments. These differences are due largely to the penetration of the radar pulse through the snow cover to the ice surface, in contrast to the laser which responds primarily to the top of the snow.

References

1) R. K. Raney, J. R. Jensen, “An Airborne CryoSat Prototype: The D2P Radar Altimeter,” Proceedings of IGARSS 2002, Toronto, Canada, June 24-28, 2002

2) http://fermi.jhuapl.edu/d2p/

3) R. K. Raney, “The delay Doppler radar altimeter,” IEEE Transactions on Geoscience and Remote Sensing, Vol. 36 No 5, 1988, pp.1578-1588

4) . R. Jensen, ”Design and performance analysis of a phase monopulse radar Altimeter for Continental Ice Sheet Mapping,” IEEE Geoscience and Remote Sensing Symposium IGARSS'95, Florence, Italy: IEEE, 1995, pp. 865-867

5) “Delay-Doppler (or SAR) Altimetry,” URL: http://www.altimetry.info/html/alti/principle/alti_doppler_en.html

6) C. J. Leuschen, R. K. Raney, “Initial Results of Data Collected by the APL D2P Radar Altimeter Over Land and Sea Ice,” Johns Hopkins APL Technical Digest, Vol. 26, No. 2, 2005, pp. 114-122, URL: http://www.jhuapl.edu/techdigest/TD/td2602/Leuschen.pdf

7) R. K. Raney, “CryoSat Data as Delay-Doppler Proof of Concept,” CryoSat 2005 Workshop, Frascati, Italy, March 9, 2005, URL: http://earth.esa.int/workshops/cryosat2005/participants/68/pres_raney.pdf

8) R. K. Raney, C. J. Leuschen, R. D. Chapman, J. R. Jensen, B. L. Gotwols, “LaRA-2002: results of the airborne laser and radar altimeter campaign over Greenland, Svalbard, and Arctic sea ice,” Proceedings of IGARSS 2003, Vol. 7, pp. 4392 - 4394, Toulouse, France, July 21-25, 2003

9) “CRYOVEX - first dedicated CryoSat validation campaign gets underway,” ESA, Dec. 19, 2002, URL: http://www.esa.int/esaMI/Cryosat/ESAGFY7708D_0.html

10) Donald J. Cavalieri, Thorsten Markus, “EOS Aqua AMSR-E Arctic Sea-Ice Validation Program: Arctic2006 Aircraft Campaign Flight Report,” NASA/TM-2006-214142, Oct. 2006, URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070008103_2007006855.pdf

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 (eoportal@symbios.space).