1. IGARSS 2003 Toulouse, France 1 Abyss-Lite: Improved Bathymetry from a Dedicated Small Satellite Delay-Doppler Radar Altimeter R. K. Raney 1, W. H. F. Smith 2, D. T. Sandwell 3, J. R. Jensen 1, D. L. Porter 1, and E. Reynolds 1 IGARSS 2003 Toulouse, France 1 Johns Hopkins University Applied Physics Laboratory 2 NOAA, Laboratory for Satellite Altimetry 3 Scripps Institution of Oceanography

2. IGARSS 2003 Toulouse, France 2 Underlying Physics Sea surface slopes at length scales of 12 km to 200 km Slopes required to one microradian ( 6 mm in 6 km) ( 1 microrad corresponds to ~ 1 mgal gravity anomaly )

3. IGARSS 2003 Toulouse, France 3 (Watts and Talwani, 1975) Gravity anomalies and depth are closely correlated over the ~12 km to 200 km band Isostatic compensation - e.g., the Hawaiian massif depresses the Earth’s mantle

4. IGARSS 2003 Toulouse, France 4 Short Wavelength Coverage Abyss-Lite will improve slope resolution by an order of magnitude, and spatial resolution by a factor of 3 (avg) Better ! Spatial resolution (kilometers) Better ! As derived for the ABYSS Proposal to NASA (Smith, Raney, et al, 2002) “Altimetric Bathymetry from Surface Slopes”

5. IGARSS 2003 Toulouse, France 5 Bathymetry Profiles (km)

6. IGARSS 2003 Toulouse, France 6 Slope error is due mainly to waves Slope RMS variability from Geosat ERM (1 frequency, no radiometer). Seasonally averaged wave heights, courtesy P. D. Cotton, UK Higher precision requires an altimeter less prone to random noise induced by ocean surface waves.

7. IGARSS 2003 Toulouse, France 7 Area-Averaged Errors in N-S vs E-W Slopes Courtesy, D. Sandwell, SIO Inclination 150120 Better coverage More balanced errors

8. IGARSS 2003 Toulouse, France 8 Better Bathymetry from Space A better orbit (inclination; repeat period ) A better altimeter Why? Need to satisfy…. > finer precision > suppress “noise” from waves > better near-shore performance How? > signal processing techniques: Doppler and Migration (APL invention; US Patent) REQUIRES:

9. IGARSS 2003 Toulouse, France 9 Pulse length Annuli of equal areas Pulse limited footprint 1 2 3 Pulse- Doppler limited footprint 1 0 Altimeters Compared Two-dimensional section of the angular scattering function at each and every subsatellite point Processing: remove extra delay due to wavefront curvature, which converts all data along-track to height measurements Doppler knowledge supports closer approach to shore ConventionalDelay/Doppler Doppler modulation Advantage: along-track incidence and Doppler equivalence (modulo PRF) Multi-looks at each location Delay Doppler Radar Altimeter Delay-Doppler Radar Altimeter

10. IGARSS 2003 Toulouse, France 10 NASA-Funded proof-of-concept Aircraft Altimeter (D2P) Field campaigns 2000, 2001, 2002, 2003 Johns Hopkins University http://fermi.jhuapl.edu/d2p

11. IGARSS 2003 Toulouse, France 11 Abyss-Lite Requirements To provide sea surface slope data to 1 micro-radian over scales as small as 6 km (half-wavelength) by a radar altimeter on a dedicated small spacecraft Altitude (average, circular)~800 Nominal inclination (degree)50 – 63 (or 113 – 120 ) Height precision* < 1 cm @ 3 m SWH Along-track footprint*< 3 km (all SWH) Range pulse width (m)< 0.5 Mission duration (y)5 Resources requiredMinimize power, mass * By implication, requires a delay-Doppler radar altimeter

12. IGARSS 2003 Toulouse, France 12 Abyss-Lite Height Precision > Acceptable precision AND less sensitivity to SWH ! * Derived from white Gaussian noise process over the Abyss-Lite band => one-sigma 1  rad slope error ~1-cm height precision * Section F.3.2 SWH PDF, Summer (after Lefevre and Cotton, 2001)

13. IGARSS 2003 Toulouse, France 13 The Baseline Design ** Target cost * : less than $60 M *Excluding reserves and launch vehicle # Additional cost of retrograde orbit TBD ** Design study at JHU/APL funded by NOAA Radar mass (kg)~ 28 Spacecraft mass (kg)148 Antenna diameter (m)1.0 Science data rate (kb/s)25 (average) Radar power (W)< 75 (fixed solar arrays) D/L data rate (Mb/s)4 (two days of data, 10 min) NavigationStar-trackers & GPS Attitude controlPitch wheel and torque rods LaunchPegasus (60 degrees # )

14. IGARSS 2003 Toulouse, France 14 Abyss-Lite coverage Dense track network (< 5 km average spacing) Better altimeter precision, and better orbit inclination One-time, relatively low cost satellite mission Abyss-Lite will have good E-W control, low noise, and very dense track spacing. Current altimeter data have poor E-W control, high noise (ERS/GM), and uneven track spacing (Geosat/GM). Abyss-Lite

15. IGARSS 2003 Toulouse, France 15 Physical limit on spatial resolution at the sea surface (upward continuation) Resolution km (full wavelength) Lower resolution limit now (Geosat, ERS-1) Lower resolution limit as proposed (Abyss-Lite) Abyssal Mapping from Space Bathymetry derived from space-based altimetry is as good as gravity measurements at the sea’s surface

16. IGARSS 2003 Toulouse, France 16  Exact-repeat orbits and a Topex-class altimeter are not necessarily required for mesoscale determination Contrary to generally accepted doctrine On Meso-scale Field Determination  Fact: Relatively fine spatial coverage is the primary requirement Satisfied by many altimeters in exact-repeat orbits, and also by one altimeter in a non-repeat orbit  Fact: Height precision over meso-scales is required Satisfied by a “precise” single-frequency altimeter without WVR  Implication: Abyss-Lite would meet geodetic and meso-scale oceanographic mission requirements

17. IGARSS 2003 Toulouse, France 17 Analysis and graphics by Remko Sharroo, NOAA

18. IGARSS 2003 Toulouse, France 18 Conclusions > Delay-Doppler: New generation of radar altimetry Offers the precision needed for state-of-the-art bathymetry > Bathymetric mapping from space by altimetry Requires a precision radar altimeter, single frequency, no water vapor radiometer, moderate-inclination non-repeat orbit Abyss-Lite > Meso-scale requirements met by the same mission The Abyss-Lite altimeter, in the context of background absolute SSH data, will also satisfy operational requirements