Optimization of future altimeter orbits

1. Optimization of future altimeter orbits

2. Outline • Introduction • Synthesis of main previous results and studies • Detailed presentation of WP4000: Impact of orbit choices on main applications • Mission costs • Precise Orbit Determination • Mesoscale • Climate • Mean Sea Level • Suitability of selected orbits in a Wide Swath configuration • Conclusions

3. Introduction • Objectives: • Propose Post-EPS altimeter orbit candidates for 2020 and onward (Jason-CS, Jason-4) • Nadir altimeter • High resolution • Reference mission • Methodology • Generation of a wide range of orbit candidates (altitude, inclination and period ranges) • Pre-selection of Post-EPS altimeter orbit candidates according several criteria (tides, High frequency signals, climate, altitude …) • Assessment in a multi-satellite context (purely geometrical sampling analysis) • To filter out suboptimalorbitchoices (e.g.: no S3A / S3B redundancy) • To highlightpotentially optimal choices • Evaluation of impact of orbit candidates on different application • Tides and HF • Mesoscale observation • MSL trends • Climate • SWH, hydrology, Ice • POD • Costsevaluation

4. Support from domain experts • Tides: • Aliasing • Observability and separability • R. Ray (NASA/GSFC), F. Lyard (LEGOS), C. Perigaud (JPL), R. Ponte (AER), F. Lefevcre (CLS) • High frequency atmospheric signals: • Crucial issue in terms of errors for altimetry • R. Ponte (AER), C. Perigaud (JPL) • Climate signals (ocean/atmosphere): C. Perigaud (JPL) • Ocean circulation and mesoscale signals: Mercator-Ocean (Y. Drillet, O. Le Galloudec), P.Y. Le Traon (Ifremer) • Mean Sea Level: A. Cazenave (LEGOS) • Dedicated meetings: • At Eumetsat premises (Tides, HF signals) • During the Nice OSTST meeting (tide aliasing) • Internal meetings with experts at CLS

5. Dedicated tools for the study

6. 1- Pre-selection of orbit candidates Global recommendations from analysis of passed and planned altimeter missions Optimisation of orbit geometry Altitude between 800 and 1400 km (Berthias 2008) Air-drag and solar radiation exposure trade-off Repeat cycle between 10 and 35 days Ocean mesoscale signal observation Long term continuity challenge High inclination to get more polar ocean observations No sun-synchronous orbits => > 22000 potential orbits Need to make drastic selection to extract a few suitable orbits for Post-EPS Tide aliasing, Climate, Mesoscale, Costs issues

7. Strong tide aliasing criterion : Good alias is > 2 cpy 14 tidal waves are studied : main+non-linear components Very few orbits meet these criterion Tide aliasing criterion relaxed : We admit some aliasing below 2 cpy S1 alias > 2 cpy (= precession rate; Nerem 2008, Ray 2008) K1 wave aliasing is a priority K1 alias > 2 cpy or not good separation with Sa and Ssa signal (< 2-3 years) Recommendations for climate purposes : Avoid the aliasing band [4-9 cpy] Avoid aliasing at 3 cpy and 6 cpy (60 and 120 days signal) No alias at annual or semi-annual frequencies Mesoscale observation: Scales of ~150 km and ~15 days. 3 days and 4 days sub-cycles preferred Low altitudes preferred to reduce mission costs Orbit selection criteria

8. Post-EPS orbit candidates

9. 2- Assessment in a multi-mission context • Pulse-limitedaltimeters • Operational missions • Near Real Time use (idealcoverage, no onboardanomaly, perfectdeliverywithoutanyadditionaldelay…)  Best case NRT • Samplinganalysis (purelygeometrical, no OSSE) • No universal optimisation : • Depends on contemporaneousaltimeters • Optimal phasingis not possible unless all altimetersfly on a similarorbit • Only a trulyoptimised constellation yields a significantlybettersampling • But itis possible to have a verypoorsampling (muchredundancy) • Depends on the space / time correlationscalesanalysed • Optimisation islimitedwithoutperfectphasing (e.g.: Jason + TP) • Analysislimited to the signals… • Whichcanbe (reasonably) resolved by multi-satellite altimetry • Still as small as possible  7-15 days, 75-150 km

10. Base metrics • Coverage • Observable scales (quickview) • Sampling quality and homogeneity • Structure detection and long-term monitoring • Cross-overs and cross-calibration

11. Coverage • Seaiceignored • Groundtrackdensity (ocean) and rivers / lakesactuallyflown over are neglected (theoreticalcoverage) • Similaranalysis possible on icewithicecoverageclimatology

12. Sampling scales • Eachorbit has 3 important figures in terms of samplingscales • Altitude (number of revs per day, lower altitude isbetter) • Cycle duration (space vs time trade-off) • Sub-cycle duration(scanning pattern and SWH applications) • Ex for the Jason orbit: • 315 km (worst case)at cycle scale, • 280 km if looking fornearest orthogonal point at 66° inclination • 3 x 945 km scans fromthe 3 daysub-cycle

13. Evolution of the cross-track distance as a function of observing duration • J1+TPN best for 7-12 days • A926bis and A923 can resolve smaller scales for 1-3 days time scales • A926bis, A801 and A1104 can resolve smaller scales for longer period >12-13 days (= mesoscale observation) • A1104 is bad for periods below 11 days • A878 is good for 9-10 days periods

14. Sampling quality & homogeneity • Approach • Use actual correlation scales of mesoscale observed (without OI) • For each time step, compute a map of the best correlation between observation (along-track point from each satellite) and unknown (grid point) • Analysis limited to the lowest inclination boundary • Classification • Quality indicator (blind to good) built from correlation • Arbitrary thresholds but results are not much sensitive to this choice

15. Page 15 Instantaneous correlation • Instantaneous correlation between available NRT measurements and a random day • Scanning patterns and optimisation are visible • Allow building the qualityindicator • The maps’ space/time variations give info about the space/time samplinghomogeneity of the constellations. S3+A1104 S3+J1 S3+A878 J1+EN

16. 2 Sats  20 % of the Atlantic is not correctly observed (Class 1 & 2) • 4 Sats  less than 8% • S3+A878 slightly better, S3+A801 equivalent to J1+EN

17. Page 17 • Validation of the method = equivalence to LD2002 • Mapping error given to each class (based on covariance thresholds) • Compute sum of class error weighted by percentage of points in class • Roughly equivalent to results fromLD2002 •  Crude simulation but the optimisation potential for multi-satellite maps is accessible

18. Structure detection and long-term monitoring • Application need : • to detect new features (or a change in shape/intensity) as fast as possible • to monitor them as long as possible (tracking) • Scalesneeded for SSH : ideally75 to 150km (or as small as possible for conventional altimetry) • Approach : purely geometrical • To be detected, a feature must have an observation within 2.dx/3 of its central location • To lose tracking, a feature must have no observation within 2dx/3 and 2dt/3 • Simulations are run over a few weeks • For each snapshot : if simulated altimeter data are within the scan bubble, the feature (grid point) is detected/tracked • Statistical approach (results averaged over the entire Atlantic = probability for a random sample if the distribution is reasonably uniform) • Stationary assumptions acceptable (no strobo-scopy possible with scanning patterns) 2 weeks

19. Detection capability • From 2 to 4 satellites : miss probability of 16 to 4% after 5 days • Still 38% miss chance after 3 days for only 2 satellites (optimised or not) • 3 or 4 satellites needed to detectfastenough for NRT applications

20. DetectionCapability and Monitoring Probability to miss a new 150km wide feature (left) and to lose the tracking (right) on a 150km wide feature for 2 satellite constellations with post-EPS candidates and references scenarios • J1+TPN has the lower probability to miss a new feature after 5 days • S3+A926 and S3+A923 show similar performances for mesoscale structure detection and monitoring: both have better detection performances than J1+EN and S3+J1 for scanning times between 3-7 days (both have 4-days sub-cycle), and have similar monitoring performances to S3+J1. • S3+A923 is very good for 14 days signals due to its short repeat cycle (9 days). • S3+A1104 shows bad performances both for detection and for monitoring of mesoscale. • To detect new or modified features, all configurations cannot compete with true optimisation.

21. Crossovers • Crossover angle (or track angle to equator plane) = impact on geostrophic velocity observations • 45° track (90° crossover) = isotropic velocity obs • Lower cross angles create better observations on U and worse on V • Latitudes of 40-50°: J1, GFO, A878, A926 • 45-60° (ACC) : A801 • A878, A926 : crossovers at most latitudes and a reasonably uniform distribution • A801 has many but aggregated crossovers  problematic correlation of local (solar) time • A1104 : few and not uniform

22. 3 orbits selected • A878_i66_c10 (~878 km, ~66°, ~10 days) or A878 • basically equivalent to a Jason orbit, but in a 850-900 km range (139 revolutions per cycle vs 127  10% denser sampling than Jason) • sub-cycle duration is different : 1 day vs 3 days • scanning over 3000 km (with a resolution of 300km) without any interlacing • A926_i67_c13 or A926 • it has a GFO-like repeat cycle and a T/P-like inclination • Sub-cycle duration is 4 days • interlaced scans of 3000km at a resolution of 750km, every 4 days • A801_i71_c22 or A801 • a longer repeat cycle of 22 days and a 7 day sub-cycle • 3 interlaced eastward propagating scans performed within a given cycle • spatial resolution of these sub-cycle scans is roughly equal to 430km

23. Preliminary conclusions • A comprehensive methodology has been followed: • Dedicated tools have been developed (orbit and ground track generation, assessment with different metrics) • A method has been put in place, taking into account different criteria • Estimation methods depending on the applications, from the simplest to the more sophisticated • Expert support and collaboration for each domain of interest • A robust protocol: • Validated against other methods • Could be easily re-used: • With any other different hypotheses (mission requirements) • In other satellite configurations (single satellite, constellation...) • Could even be improved (assimilation experiments...) • Good starting point for future extensive definition study

24. Preliminary conclusions • Lower altitudes (than 1330 km) are possible for the future high precision altimetry missions • 3 orbits have been selected and assessed (though many other orbits could be envisioned) • Comparable results with Jason, or even better • Results depend on the application specificity (mesoscale, climate, Mean Sea Level) • A926_I67_C13 seems a good trade-off, A878_I66_C10 better for climate studies • Compatibility with the Sentinel-3 mission • These orbits should be compliant to POD requirements, but requiring optimised configuration (platform and payload) • Mission lifetime should be improved while cost would remain constant (worst case) • A synthesis report will be produced presenting different trade-offs