ABYSS-Lite Science Requirements and Mission Concept
Walter H.F. Smith(1) , Keith Raney(2) , and David Sandwell(3)
NOAA/NESDIS United States,
1335 East-West Highway #5408,
Silver Spring, MD 20910,
(2) Johns Hopkins Applied Physics Lab, 11100 Johns Hopkins Rd, Laurel MD 20723, United States
(3) Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093-0225, United States
We are delighted to participate in this meeting celebrating 15 years of progress in radar altimetry. For some time we have been suggesting a new altimeter mission concept that would address a mixture of bathymetric, geodetic, and oceanographic needs. We will present our views and also listen to the perspectives and interests of our colleagues.
Altimetry has enjoyed a variety of successes in applications as diverse as sea level rise, variability and change at planetary, basin, and meso- scales, geodesy and bathymetry. Some of these applications require absolute accuracy in the sea surface height measurement, and long term stability of that accuracy, while others require only relative accuracy over defined spatial and temporal scales. In observing mesoscale eddies, for example, only sea surface height changes on spatial scales from tens to a few hundred km and time scales of weeks are needed. For geodetic and bathymetric applications it is the horizontal gradient of the time-averaged sea surface, and not the height itself, that is required.
The Ocean Surface Topography Mission is expected to continue the absolutely accurate sea surface height time series on the Topex and Jason 10-day repeat ground tracks. We would like to see a next-generation, higher-resolution altimeter to complement the OSTM, making mesoscale observations in the 300 km holes left by OSTM coverage while providing new data on the geodetic and bathymetric signals. We believe the ideal solution is one simple low-cost satellite hosting a single-frequency delay-Doppler altimeter (and perhaps a radiometer) in a non-repeating orbit tuned to have oceanographically optimal “near exact repeats”.
Data from the two geodetic missions to date have revolutionized our understanding of the marine gravity field and global bathymetry, confirming the plate tectonic theory, revealing many previously uncharted features, and raising new questions about the details of the seafloor spreading process and the volcanic and meteoritic history of the Earth. However, the resolution of these data is not yet adequate to support advanced inertial navigation systems, to characterize the seafloor roughness spectrum at scales that control ocean mixing, internal wave generation and tidal dissipation, or to determine the heights of seamounts accurately enough for navigation and habitat considerations. A new mission could meet these goals and reveal about 50 000 as yet unknown seamounts.
Requirements for such a mission are simple. Long-term sea-surface height accuracy is not needed; the fundamental measurement is the slope of the ocean surface to an accuracy of ~1 micro-radian (1 mm per km). The main mission requirements are:
Improved range precision. A factor of two or more improvement in altimeter range precision with respect to current altimeters is needed to reduce the noise due to ocean waves.
Improved along-track spatial resolution. The missing seamount and bathymetric data are in the 6-km to 25-km range. The shorter scales can be mapped only if the along-track resolved footprint of the altimeter is ~ 6 km or less. This requirement cannot be met by conventional radar altimeter data, especially in areas of large prevailing significant wave heights such as are typical of the southern oceans.
Fine cross-track spacing and long mission duration. A ground track spacing of 6 km or less is required. A six-year mission would reduce the error by another factor of two.
Moderate inclination. Existing satellite altimeters have relatively high orbital inclinations, thus their resolution of east-west components of ocean slope is poor at low latitudes. The new mission should have an orbital inclination close to 60° or 120° so as to resolve north-south and east-west components almost equally while still covering nearly all the world’s ocean area.
Near-shore tracking. For applications near coastlines, the ability of the instrument to track the ocean surface close to shore, and acquire the surface soon after leaving land, is desirable.
A delay-Doppler altimeter [Raney, R. K., The delay Doppler radar altimeter, IEEE Transactions on Geoscience and Remote Sensing 36 (5), 1578-1588, 1998] meets the requirements for lower noise level, robustness of noise in the presence of large surface waves, fine-scale resolution, and better near-shore tracking. Abyss-Lite, comprised of a single-frequency Ku-band radar, on-board processor, and essential subsystems, is a relatively simple, low-cost, small-satellite design. This instrument and signal processing has proven heritage. Under NASA Instrument Incubator funding, the Johns Hopkins University Applied Physics Laboratory developed and proven through airborne trials an airborne prototype that emulates the innovative features central to the delay-Doppler concept. Thanks to signal processing techniques adapted from the field of synthetic aperture radar, the resulting delay-Doppler radar altimeter has significantly better measurement precision than is possible with any conventional radar altimeter [Jensen, J. R. and Raney, R. K., Delay Doppler radar altimeter: Better measurement precision, in Proceedings IEEE Geoscience and Remote Sensing Symposium IGARSS'98ed, IEEE, Seattle, WA, 1998, pp. 2011-2013]. Furthermore, its canonic post-processing footprint is ~250 meters along-track; several of these can be accumulated to generate ~5 km spatial resolution, a dimension that does not expand with increasing wave height. The precision and spatial resolution of this instrument are ideally suited to meet the demands of high resolution gravimetry and bathymetry.
The altimeter in principle is similar to current conventional oceanographic instruments, and virtually identical to the SAR-mode of the SIRAL altimeter on CryoSat [Raney, R. K. and Jensen, J. R., An Airborne CryoSat Prototype: The D2P Radar Altimeter, in Proceedings of the International Geoscience and Remote Sensing Symposium IGARSS02, IEEE, Toronto, 2002, pp. 1765-1767]. However, unlike CroySat, the Abyss-Lite altimeter payload includes a real-time processor, which has been true for all ocean-viewing radar altimeter satellites since Seasat. Consequently, the data storage and down-link rates are very small. (The inherent data rate from the instrument is less than 30 kHz.) Thus only one ground station is required to support the Abyss-Lite mission, with a factor of two reserve. Further, on-board processing sorts reflections by Doppler (along-track angle of the arrival), which is the basis for “smart” range-gate tracking to assure reliable near-shore operation.
This is a very low-cost mission, in comparison to other altimeters. The ROM cost of the spacecraft, two-string (redundant) altimeter, and water-vapor-radiometer (WVR) is $75M, based on a Phase A/B start in FY 2006, and a launch in CY 2009. The spacecraft will fit the mass and size constraints of a Pegasus launch vehicle reaching the desired orbit. Thus, the ten-year cost, including implementation, launch, on-orbit operations, one ground station with embedded ground support, and science, is much less than $200M. Cost estimates are based on the ABYSS ESSP proposal for the ISS instrument (peer-reviewed by NASA) and a NOAA-funded internal study at JHUAPL [Raney, R. K., Smith, W. H. F., and Sandwell, D. T., Abyss-Lite: A high-resolution gravimetric and bathymetric mission (AIAA-2004-6006), in Proceedings, Space-2004, AIAA, San Diego, CA, 2004] that considered spacecraft trade-offs including those in the GSFC Rapid Spacecraft Development Office catalog.
We believe that this mission could provide mesoscale observations to fill the gaps left by OSTM, despite the fact that our scenario does not use a conventional exact-repeat orbit. For the geodetic application it is enough that the ground track pattern eventually becomes dense (order 6 km spacing between tracks) after sufficient time (order 15 months). However, the sequence in which those tracks are acquired is not important, so that the orbit may be designed to be rich in near-repeats, meaning that the orbit yields space and time sampling intervals that are small compared to the space and time scales of correlation of the mesoscale ocean signal. Since the mean sea surface is already well-known at mesoscale wavelengths, thanks to the existing geodetic missions, one may refer height profiles to the mean sea surface, rather than a history of exact repeats, to obtain the mesoscale anomaly signal. Two independent studies [Scharroo and Smith, unpublished; Monaldo and Porter, Oceans2005 conference] have already demonstrated this idea by recovering the eddy field from ERS-1 geodetic phases E and F and comparing that to the result obtained from Topex in the same time interval. Monaldo and Porter find that the rms difference between the non-repeat eddy height field and the Topex eddy height field is about 9 cm.
We regret the loss of CryoSat. We had hoped that CryoSat would give us the space demonstration of many of these ideas, allowing us to recover mesoscale signals from its non-repeat orbit, and offering a test of the delay-Doppler paradigm via its SAR mode.