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Orbit Selection Issues for Wide-Swath Altimeter

Orbit Selection Issues for Wide-Swath Altimeter. Don P. Chambers Center for Space Research The University of Texas at Austin Wide-Swath Ocean Sciences and Hydrology Meeting 31 October 2006. Overview. Consider orbits that are not sun-synchronous

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Orbit Selection Issues for Wide-Swath Altimeter

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  1. Orbit Selection Issues forWide-Swath Altimeter Don P. Chambers Center for Space Research The University of Texas at Austin Wide-Swath Ocean Sciences and Hydrology Meeting 31 October 2006

  2. Overview • Consider orbits that are not sun-synchronous • Examine orbits at higher inclination and lower altitude than T/P-Jason • Use tidal aliasing frequencies to evaluate orbits • 8 largest constituents and S1 • Examine min. frequency and frequency separation • Discuss how these characteristics change in general vs. inclination, altitude, groundtrack repeat period 2

  3. Calculating Tide Alias Period • Alias period depends on the satellite’s exact repeat period and the period of the tide • TOPEX/Poseidon exact repeat period is 9.915625 days (“10-days”), or 237.975 hours • M2 period is 12.420601 hours • Although the tidal periods are known, the exact repeat period is a non-linear function of semi-major axis and inclination • Hundreds of orbits exist within a reasonable range of inclination and semi-major axis, each with very different aliasing frequencies 3

  4. Minimum Aliasing Frequency • Want to examine a few parameters related to alias frequencies to eliminate certain orbits from consideration • One parameter is minimum of all aliasing frequencies • Greater than 2 cycles-per year (period less than half-year) • This ensures multiple cycles of the any alias period within a few years so the tide can be estimated from the data 4

  5. Orbits & Tides • Repeat orbits can be found at any inclination that meet this criteria for some tides 5

  6. Orbits & Tides (cont) • Other tides have a problem as satellite inclination goes above 80° • Especially a problem for S1, S2 as orbit approaches sun-synchronous inclination where alias is very low-frequency (approaching a bias) 6

  7. General Observations • Only orbits with an inclination < 66° alias all major constituents to frequencies > 2 cpy • Retrograde orbits have poor aliasing of solar tides • If we do not require that the K1 alias > 2 cpy • Orbits that meet the requirement for all other constituents exist up to 80° inclination • This covers more of the Arctic Ocean, all of the Antarctic boundary, and most rivers in Siberia 7

  8. Frequency Separation • Another important factor to consider is the separation between alias frequencies and between the alias and annual/semi-annual frequency (Df) • Determines time needed to separate two tides in altimeter observations for an estimation • Time to separate ~ 1/Df • T/P did not have optimal separation for some constituents • K1 aliased to within 0.11 cpy of semi-annual • 9-years to separate 8

  9. There are orbits near 80° inclination that have better aliasing properties than T/P 9

  10. Average Frequency Separation • Find minimum separation between each constituent and all others (incl. annual, semiannual): fi • First requirement is the smallest be larger than some minimum value • For T/P, this was 0.167 cpy (~ 6-year separation time) • From these, calculate average • Higher numbers mean separation of all constituents in shorter time f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 10

  11. Average f = 0.4 cpy implies mean separation time of 2.5 years, 0.7 cpy = 1.42 years • 800-1300 km altitude 11

  12. Inclination = 78° • Altitude ~ 845 km Exact Repeat Period (days) 12

  13. Conclusions • There are quite a few orbits at high inclinations that have excellent tidal aliasing properties • Inclinations as high as 78° • Altitudes ~ 800 to 900 km • Repeat periods cluster around 10- to 11-days and 20 to 22-days • Properties are even better than T/P-Jason if we ignore K1 in the minimum frequency calculation • Suggest that these orbits be studied in more detail for the Wide-Swath Altimeter 13

  14. Conclusions (cont) • Current “compromise” orbit being used in studies is based on a report I wrote 5 years ago • At that point, I was ignoring all orbits significantly lower than 1000 km because of orbit determination concerns • Also, was not considering S1 tide • We need to be careful not to hardcode this “compromise” orbit into any mission documents (as was done with NPOESS) until more studies are done

  15. Extras

  16. 800-1000 km altitude 16

  17. Calculating Tide Alias Period • Alias period () depends only on the satellite’s exact repeat period (Prepeat) and the period of the tide constituent (PTide) • Prepeat is the precise repeat period, not the nearest integer repeat period • e.g., TOPEX/Poseidon exact repeat period is 9.915625 days (“10-days”), or 856710 seconds 17

  18. Orbit Period & Repeat Period • Although the tidal constituent periods are known, the exact repeat period (Prepeat) is a non-linear function of semi-major axis (a) and inclination (i)

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