NASA Activities in Support of an Advanced Altimeter Mission Tony Freeman

NASA Activities in Support of an Advanced Altimeter

Mission

Tony FreemanOctober 2006

Advanced Altimeter Mission

Current Status• WSOA was descoped from the OSTM mission in 2005• In May 2006, the acting division manager for Earth Science at HQ, Bryant

Cramer, commissioned a study of a new advanced altimeter mission• Study is the 1st (of several) done in anticipation of decadal survey results • Mission objectives were to include land and ocean applications • Study was completed at the end of summer• Since then, three joint meetings involving NASA, NOAA and the Navy have

been held - next meeting in late November• All three agencies need a follow-on altimeter after OSTM, which launches

in 2008 and is expected to last 5 years• All 3 agencies require backwards compatibility with the TOPEX/Poseidon/

Jason-2/OSTM climate data records, i.e. global sea level• An additional driver for NASA is to further our understanding of the Earth

system through advances in measurement capability • The advanced altimeter study resulted in a mission concept that can meet

the requirements of all three agencies at reasonable cost


Science Objectives:• Sea surface height measurements

(continue the Jason/TOPEX series)• Study mesoscale phenomena• Ocean bathymetry (surface slopes)• Land surface water (hts.+ gradients)

Instruments:• Ka-band wide-swath radar altimeter

(1-2 km resolution)• Ku-band nadir altimeter• 3-freq. water vapor radiometer• GPS for POD• Laser retroflector for POD

Orbit:• 993 km altitude, 78 deg inclination• 21-day exact repeat, 10.5 day revisit• Delta II launch

Programmatics:• NASA/NOAA/Navy partnership• Currently no US altimeter planned

after Jason-2• Possible Eumetsat/CNES roles• Launch date 2012 for continuity with

Jason-2• 5-year mission• Mission Cost ~ $500M (FY ‘06)

New technology:• Wide-swath altimeter is a SAR

interferometer• Onboard processing

Mission Concept

Advanced Altimeter Mission1-

Requirements Traceability

Science requirements are derived from three primary mission concepts submitted to the NRC decadal survey RFI:

The WatER mission

The Hydrosphere Mapper mission

The Bathmetry ABYSS mission

Navy and NOAA requirements are derived from:

Navy Altimeter Requirements, Jacobs, G. A. et al, Naval Research Laboratory, NRL/FR/7320--99-9696, Nov. 1999

NPOESS Integrated Operational Requirements Document-II, January 2002

Advanced Altimeter MissionNASA Requirements: Traceability Matrix (1 of 2)

Category Science Objectives Scientific Measurement Requirements Instrument Functional Requirements Mission Functional Requirements Level 2 Data Products

Global monitoring of storage change by measuring changes in water body height and spatial extent with time. Derivation of river discharge from measurements of slope and spatial extent within a hydrodynamic model assimilation. Revisit time ~2 weeks in the tropics and less than 1 week in the Arctic.

Ka-band Interferometer Requirements: Height accuracy 5cm. Slope accuracy 10urad (1cm/km). Spatial resolution finer than a 100 m posting. 120 km total swath.

Orbit /repeat (14 days in the tropics, less than 7 days in the Arctic). Exact repeat is not required, rather samples anywhere within the swath are sufficient for describing water height. Precise orbit determination required.

3 year baseline operation, with a 5 year goal.

3 year reliability, with a 5 year goal.

Minimum 1 year, with a 3 year baseline mission, with a 5 year goal.

Global coverage of sea surface height measurements for ice free oceans with a repeat cycle less than 25 days.

Aliased periods of the eight leading tidal components (M2, S2, K1, K2, P1, Q1, N2, O1) shall be separated from one another with maximum period less than one year.

Ka-band Interferometer : provide above measurement accuracy over a total swath of [120] km.



3 year baseline mission, with a 5 year goal.

Global maps of water surface elevations and area are expected after every period of global coverage (e.g., ~weekly). Individual swaths will be useful for local water managers. Hydrologists need derived products that are global maps of water storage change and discharge. Hydraulic engineers will need maps of h, dh/dt, dh/dx and area.

WatER Science Objectives: Determine surface water storage change and discharge to predict the land surface branch of the global hydrologic cycle; Measure flood hydraulics; assess the role of fresh water storage as a regulator of biogeochemical cycles such as carbon and nutrients. [A]

Hydrosphere Mapper Science Objectives : Measure mesoscale activity, including fronts, eddies, and boundary currents; eddy mean-flow interactions, eddy transports, and the role of eddies in climate; physical-biological interactions and the role of eddies in the carbon cycle; coastal tides and open ocean internal tides; and coastal currents [B]

Monitor global mesoscale activity through the measurement of sea surface height (SSH) with a spatial resolution of 1 km x 1 km with random noise no greater than 1.5 cm.

In the open ocean 30 km away from the coast, the SSH accuracy shall be less than 4.6 cm (1) at wavelengths shorter than 500 km; the total SSH accuracy (all scales included) shall be less than 5.1 cm (2) .

Within 30 km from the coast, the SSH accuracy at wavelengths shorter than 500 km shall be less than 4.9 cm (3) and the total SSH accuracy shall be less than 5.3 cm (4) .

Wind speed measurement shall have an accuracy of 2 m/s.

Orbit repeat periods shall be between 10-25 days.

Non-sun-synchronous orbit is required.

Exact repeat ground tracks with cross-track separation less than +/- 1 km.

The attitude of the Ka-band interferometer shall be maintained steady (no yaw-steering)

Fully calibrated and validated science data are required within 30 days. Quick-look data with less quality are required within 1 day for coastal and other operational applications.

Ka-band Interferometer: 1.5 cm precision with 2 km x 2 km resolution.Nadir Radiometer:Resolve tropospheric correction with 1.2 cm accuracy. GPS:Determine orbit with radial accuracy of 2 cm.

Fully calibrated, validated science data : Geolocated, calibrated interferometer strip data of sea surface height on a per orbit basis on a 2 km grid near coastlines. Wind speed strip data on a similar grid. Both products with less than 30 day latency.

Quick look data : Geolocated, partially calibrated interferometer strip data of sea surface height on a per orbit basis on a 2 km grid near coastlines. Wind speed strip data on a similar grid. Both products with less than 1 day latency.

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NASA Requirements: Traceability Matrix (2 of 2)

Sea Surface Topography

Measure ocean surface topography covering at least -66 to +66 degrees in latitude, with 3.4 cm accuracy at 6.2 km along-track resolution, with measurement drift less than 1mm/year.

Measurement products to continue the TOPEX/Poseidon, Jason, and OSTM/Jason-2 time series, although different orbit inclinations and ground tracks are acceptable.

Nadir Altimeter (Ku and C band): 2.6 cm height accuracy (1.6 cm Ku noise, 2 cm sea-state bias, 0.5 cm ionospheric error) with along-track resolution of 6-7 km.

Exact repeat ground tracks with cross-track separation less than +/- 1 km



3 year baseline mission, with a 5 year goal.

Measurement of East-West & North-South sea surface slope to 1 micro-radian over scales of 6-8 km.

Ka-band Interferometer: Height precision (uncorrelated random error) at 1km postings of 1.5 cm. Systematic error contribution smaller than 1 micro-radian over scales of 6-8 km (residual slopes after calibration)

The mission orbit design should include all orbit shifts required to get full ocean coverage over the mission lifetime.

Global coverage between +/- 66 degrees latitude with no spatial gaps.

A mission lifetime of at least 2 years is required to average ocean mesoscale signal contamination.

Geolocated, calibrated strip height data on a per orbit basis on a 6.2 km grid. Wind speed strip data on a similar grid.

Products to have less than a 30 day latency.

Data to be cross-calibrated versus OSTM nadir altimeter at associated crossover points.

Global gravity & bathymetry maps at 6-8 km resolution.

Averaging of mesoscales signals requires at least 1 year data collection for each point. Full coverage requires instrument lifetime of at least two years.

Measurement of sea surface slope with 100% coverage between +/- 66 degrees latitude.

Nadir Altimeter Science Objectives : Basin-scale ocean circulation, heat transport, El Nino/La Nina, sea level rise.

Bathymetry Science Objectives : Improve understanding of marine gravity field & global bathymetry at (1) scales sufficient to resolve roughness that drives ocean mixing, internal wave generation, and tidal generation; and (2) to determine the heights of seamounts accurately enough for navigation and habitat considerations [C].

Category Science Objectives Scientific Measurement Requirements Instrument Functional Requirements Mission Functional Requirements Level 2 Data Products

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References:1. WatER: The Water Elevation Recovery Satellite Mission, Response to the National Research Council Decadal Survey Request for Information, Doug

Alsdorf1, Ernesto Rodriguez2, Dennis Lettenmaier3, and Jay Famiglietti4 1Ohio State Univ., 2NASA JPL, 3Univ. of Washington, and 4Univ. of California, Irvine

2. The WatER Mission, www.geology.ohio-state.edu/water3. The Global Hydrosphere Mapper, Response to the National Research Council Decadal Survey Request, L-L. Fu, E. Rodriguez, Jet Propulsion

Laboratory, California Institute of Technology4. ABYSS-Lite: A radar altimeter for bathymetry, geodesy and mesoscale oceanography, Smith et. al. 20055. Altimetric BathmetrY from Surface Slopes, A Proposal in response to NASA AO01-OES-01, July 20, 2001, Principal Investigator: Dr. Walter H. F. Smith


Navy Requirements• Sea Surface height, atmospheric correction parameters, and orbit solutions are required within

48hours. Wave heights are required within 3 hours.• The required instrument white noise level must be below 3 cm rms.• The required total range error must be under 5 cm (peak error) after all atmospheric corrections

are applied.• Required real time orbit solutions must contain under 1 m error at 1 cycle per satellite orbit

revolution (cpr) and less than 2 cm integrated errors at higher frequencies.• An exact repeat orbit must be required, and the satellite must be held to within a 1 km swath of a

predefined ground track.• The required repeat period must not be less than 20 days*• A minimum of one instrument is required. With only one instrument, this data must be used in

conjunction with systems such as MODAS and NRL Layered Ocean Model (NLOM).• Two altimeter are recommended on the NPOESS Polar-Orbiting Operational Environmental

Satellite System (NPOESS) for error reduction and redundancy.• Major tidal constituents should not be aliased to frequencies that contain significant ocean

variability. These frequencies include the annual, semi-annual, and mean.

Source: Navy Altimeter Requirements, Jacobs, G. A. et al, Naval Research Laboratory, NRL/FR/7320--99-9696, Nov. 1999.

* "The NPOESS IORD-II contains approved/validated altimeter requirements that adequately define the Navy's needs; however, recent studies show that Navy would consider an Exact Repeat Period threshold of slightly greater than the currently cited 20 days." Cmdr. Mark Gunzelman, personal communication, Aug 2006


NPOESS Requirements

Source: NPOESS Integrated Operational Requirements Document-II, January 2002


NPOESS Requirements

Source: NPOESS Integrated Operational Requirements Document-II, January 2002


5- Mission Concept- Orbit Parameters

• 78 degrees inclination.

• Exact repeating groundtrack after ~21 days (286 orbits at 993km).

• This leaves ~17x100km diamond-shaped areas (see graphic) not covered (~1% of area on earth in +-30deg lat; ~0.5% +-78deg Lat.)

• Orbit will be shifted in longitude after 1.5 years to fill gaps for bathymetry science coverage.

• 993 km altitude was chosen to provide an even distribution of coverage during cycle completion. Range of altitude explored: 800 to 1000km.

Ref: Francois Rogez (JPL) this study may 2003


5- Mission Concept: Orbit Parameters: Coverage map after 21 days

Maps near the equator showing the 120km swath and the ~17 km gaps.A 1 degree lat/lon grid is shown with dotted red lines.

Ref: Francois Rogez (JPL) this study

•After 21 days, 286 swaths are uniformly distributed in longitude, leaving gaps of 17 km between swaths at the equator.

•Adding up the coverage from ascending and descending passes leaves diamond shaped gaps with a cumulative area measuring about 800k m^2 at the equator.

•The gaps disappear above 30 deg latitude.

•All coverage Gaps are filled when the orbit is shifted at 1.5 yr Intervals over mission life


10- Key Instrument Performance Parameters: Ka-Band Interferometer Performance Summary

Ref: Louise Veilleux Advanced Altimeter Mission Study, (JPL) April 2006

4/18/06


Ref: Louise Veilleux Advanced Altimeter Mission Study, (JPL) April 2006

4/18/06

10- Key Instrument Performance Parameters: Ku & C-Band Altimeter & Radiometer Summary


Science Measurement Requirements

Measurement OSTM Adv. Alt.OceanographyMeasure ocean surface topography between +/- 66 deg lat., with 3.4 cm accuracy, 6.2 km resolution, drift < 1 mm/yr. Yes YesContinue TOPEX/Poseidon, Jason and OSTM time series Yes Yes3 year mission lifetime, with 5-year goal Yes YesAvoid tidal aliasing Yes YesMonitor global mesoscale activity with 1x1 km spatial resolution YesWithin 30 km of coast, measure SSH with 5 cm accuracy YesGlobal coverage of SSH for ice-free oceans at intervals < 25 days Yes

BathymetryMeasurement of E-W and N-S sea surface slopes to 1 micro-radian over 6-8 km Yes, in 3 yearsMeasurement of sea surface slope with 100% coverage between +/- 66 deg lat. Yes, in 3 years

CryosphereCentimeter accuracy topographic measurements Yes Yes

Adequate spatial sampling

Yes, except for the northernmost part of

Greenland and the Antarctic interior

Surface Water Hydrology

Global monitoring of water storage changesOnly largest rivers

and lakes YesRiver discharge rates YesLocal River Slope YesAdequate spatial sampling YesAdequate temporal sampling YesFloodplain topography For low relief topography

EcologyCarbon fluxes at air-water boundaries Yes

• Continues the TOPEX/Jason/OSTM record while adding new capabilities


Summary

• Study Conclusions:– Trade study examined many options– An advanced altimeter mission can meet the diverse

requirements of NASA, the Navy and NOAA– Estimated cost to each agency is less than a stand-

alone conventional altimeter mission– International partnering arrangement with

CNES/Eumetsat in the mode of Jason-1 is feasible– For a late 2011 launch and overlap with OSTM (Jason-

2), need a Phase A start in 2007

==> Decision point shortly after release of decadal survey

Documents

NASA Activities in Support of an Advanced Altimeter Mission Tony Freeman