ERS-2 and ENVISAT orbit control Current and future strategy The impact on the interferometric baseline

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1 ERS-2 and ENVISAT orbit control Current and future strategy The impact on the interferometric baseline Berthyl DUESMANN (ESA) & Itziar BARAT (ESA) Contributions by Nuno Miranda, Betlem Rosich, Montse Pinol, Dirk Kuijper

2 The ERS & ENVISAT Orbit, Definition Parameter Value semi-major axis Inclination eccentricity mean local time at ascending node argument of perigee orbital period mean altitude orbits per day repeat cycle km :30 p.m. ( 10:28:15 ERS-2) 10:00 p.m. (EnviSat) min km 14 11/35 35 days (501 orbits) ±0.068 km ± ±5 min ±3

3 Optimising SAR interferometry by Orbit Control The across track component can be controlled by dedicated orbit maintenance. The baseline orbit maintenance strategy, in place since Envisat launch in 2002, effectively results in interferometry baselines of maximum 2000 meters. The average baseline values are around 750 m, providing useful data for most interferometric applications. There is however a significant percentage of data with very large (more than 800 m and up to 2000 m) baselines. A Definition of the perpendicular baseline B i and across track baseline A

4 BASELINE ORBIT CONTROL MAINTENANCE(1) The baseline orbit control maintenance strategy ensures the actual orbit to be within 1 km of the so-called reference orbit. This is achieved by a combination of independent in-plane and out-of-plane manoeuvres. West Limit (-1km) Relative Altitude [m] East Limit (+1km) time In-plane manoeuvre High Density Predicted Density Low Density ~ m Flight Direction Relative Ground Track [km] time The relative altitude and ground track evolution in relation to airdrag and executed in-plane manoeuvres

5 BASELINE ORBIT CONTROL MAINTENANCE(2) Out-of-plane manoeuvre The deviation of the actual ground track from the reference ground track at the north-most point in each orbit

6 SAR Interferometric Baseline Approximation formulas EnviSat cycle n Ba = Acrosstrack Baseline, Orbit α EnviSat cycle m Bg = Ba * (6371 / 7159) = Ba * 0.89 Bn = Ba * cos (α) = Ba * 0.95 = Bg * α = deg Bn = Perpendicular Baseline Approximation Perpendicular Baseline at any (PSO) latitude Bn (PSO) = x ( Bg pole x sin (PSO) + Bg equator x cos (PSO) ) Bg = Groundtrack Baseline SAR Acquisition

7 Current ENVISAT Orbit Strategy Phasing of Out-of-plane Manoeuvres (1) GT deviation Start of cycle Groundtrack deviation at max lat (m) Summer 2007 Summer 2008 Principle: By performing out of plane manoeuvres, after N cycles. The ground track deviation at the pole is expected to be similar for SAR acquisitions N cycles apart. This results in a small (groundtrack) polar baseline /12/ /03/ /06/ /09/ /12/ /03/ /06/ /09/2008 Date = Start of Cycle The expected inclination evolution for the proposed out-of-plane manoeuvre strategy once every 70 days

8 Current ENVISAT Orbit Strategy Phasing of Out-of-plane Manoeuvres (2) Simulations Groundtrack baselines during Summer, at the pole vs vs vs vs vs vs vs vs vs Deviation difference (m) Day in cycle

9 Current ENVISAT Orbit Strategy (summary) The new orbit maintenance has been introduced in January 2007, to be executed during 2007 and in-plane manoeuvres: At the equator the orbit is allowed to drift 200 meters (i.e. a 200 meter deadband). Effectively increasing the number of (smaller) in-plane manoeuvres, without affecting the overall propellant consumption. The increased manoeuvre frequency allows the manoeuvres to be scheduled as a single manoeuvre. Out-òf-plane manoeuvres: Every 2 cycles, an inclination correction is performed at the same relative orbit within the cycle. These manoeuvres are now scheduled for 2007 & 2008: 2007: 23 Jan, 3 Apr, 17 Jul, 25 Sep, 4 Dec, always close to 5 UTC 2008: 12 Feb, 22 Apr, 1 Jul, 9 Sep, 18 Nov, always close to 5 UTC The propellant consumption is just 2% more, compared to the most propellant effective manoeuvre scenario. The expected baselines for this optimal orbit control maintenance are as follows For the polar regions 250 m (during Northern Summer) 500 m -750 m (during Northern Winter, for baselines with odd cycle difference) 0 m -250 m (during Northern Winter, for baselines with even cycle difference) For Tropical and Mid latitudes: 32.0 deg < 472 m (during Northern Summer) 61.9 deg < 850 m (during Northern Winter, for baselines with odd cycle difference) 32.0 deg < 472 m (during Northern Winter, for baselines with even cycle 0.0 deg < 400 m all baselines

10 Current Envisat Orbit Strategy The Results absolute perp Bs diff [m ] Antarctica, 65S:75S, Apr-Oct 2007 For the polar regions 400 m (during Northern Summer) 500 m m (during Northern Winter, for baselines with odd cycle difference) 0 m m (during Northern Winter, for baselines with even cycle difference) Oscillation due to Moon first orbit c=1 c=3 c=5 c=2 c= Equator, 10S:10N, Apr-Oct 2007 For Tropical 0.0 deg < 400 m all baselines c-1 c=3 c=2 c=4 europe,40n:50n, Apr-Oct 2007 absolute perp Bs diff [m] For Tropical and Mid latitudes: 32.0 deg < 472 m (during Northern Summer) 61.9 deg < 850 m (during Northern Winter, for baselines with odd cycle difference) 32.0 deg < 472 m (during Northern Winter, for baselines with even cycle difference) c=1 c=3 c=5 c=2 c=4 For the polar regions 400 m (during Northern Summer) 500 m m (during Northern Winter, for baselines with odd cycle difference) 0 m m (during Northern Winter, for baselines with even cycle difference) Oscillation due to Moon first orbit

11 Current ERS-2 Orbit Strategy Higher Inclination deg (further away from pole) for ERS-2, since 10 September The objective is to achieve ASAR perpendicular baselines of 2000 meters, over the Northern Polar regions. Due to the SAR frequency difference between ERS-2 and EnviSat, a perpendicular baseline between 1600 and 2400 meters is required to obtain the necessary correlation for interferometry. The position of Envisat needs to be closer to the Groundtrack imaged area. difference at Maximum Latitude The inclination difference between ERS-2 and ENVISAT is kept constant. Results: ERS - ENVISAT InSAR Campaign - performances 1 quarter of the orbit suits for ERS/Envisat cross-insar: Bperp > 1600m Latitude North ERS Envisat Perpendicular Baseline ascending descending min Max avg. stdev avg stdev Polar regions are accessible on both ascending/descending passes Mid-latitude region only accessible on descending passes Baseline control very efficient First results already obtained: DEM : Netherlands and Siberia Rapid glacier sliding : Greenland

12 Current ERS-2 Orbit Strategy, fine tuning Approximation formula s (page 6) are applicable BGPole Current Proposal Sec since ANX BGEq Ascending True Latitude (rad) Baseline at Min Latitude Baseline at Mid Latitude Delta (m) Sec since ANX Descending True Latitude (rad) Baseline at Min Latitude Baseline at Mid Latitude Delta (m) ERS-2 Ba = Acrosstrack Baseline, Orbit α α = deg Bn = Perpendicular Baseline EnviSat Bn = 2.10 km Ba = Bn / cos (α) = 2.24 km Bg = Ba * (6371 / 7159) = 1.99 km 2.0 km ERS-2 Inclination: deg Approximation Perpendicular Baseline at any (PSO) latitude Bn (PSO) = x ( Bg pole x sin (PSO) + Bg equator x cos (PSO) ) Bg = Groundtrack Baseline SAR Acquisition Secondary goals: ERS-2 & Envisat Interferometry seems possible for Descending Passes for Mid Northern Latitudes (Etna etc) ERS-2 (old) & ERS-2 (new) interferometry seems possible for Ascending Passes for Subtropical Northern latitudes (California etc) Final orbit adjustment to be studied and agreed.

13 Envisat Orbit Extension The current orbit strategy for Envisat can be maintained until end of By that time the available of fuel for orbit maintenance manoeuvres is depleted. The extension has its price: the orbit flown by ERS-1, ERS-2 and ENVISAT since the early nineties must be abandoned. Selected Orbit Control Strategy Inclination 98,5 inclination new sun sync. inclination 22:10 MLST 22:00 21:50 milestones Altitude 35 / 501 Altitude II III IV New altitude. 35/501, nominal control new repeat cycle inclination drift

14 Envisat Orbit Extension SELECTED ORBIT CONTROL Control phases The following phases will be implemented chronologically: Phase E3 up to about end 2010 (milestone II): The current mission plan is continued, i.e. nominal operations are pursued with no change in the orbit maintenance strategy based on inclination and altitude control. Phase E3/E4 transition (milestone II): The satellite altitude is lowered to the one of the disposal orbit. Phase E4 from milestone II till mid-2014 (milestone IV): The satellite is in free inclination drift, and operation continues in altitude control only until fuel is exhausted. This point will mark the end of the EnviSat mission (milestone IV).

15 Envisat Orbit Extension SELECTED ORBIT SCENARIO The forced altitude change at the end of phase E3 equals km. Sun-synchronous characteristics at the new altitude are: Semi-major axis: m Eccentricity: 0, Inclination: The repeat cycle in phase E4, i.e. after altitude change, will be: 30 days / 431 orbits End of phase E3 (date II) 1500 should be in October Predicted fuel quantity available: before the altitude change: kg ± 15 kg after the altitude change: kg ± 15 kg /09/ /12/ /03/ /06/ /09/ /12/ /03/ /06/ /09/ /12/ /03/ /06/ /09/ /12/ /03/ /06/ /09/ /12/ /03/ /06/ /09/2015 TLST MLST Equation of Time Max TLST 22:00 Min TLST 22:00 Max TLST 22:05 Min TLST 21:55 Max TLST 22:10 Min TLST 21:

16 Negative: Positive: Envisat Orbit Extension Interferometry still possible? Inclination drifts 47 mdeg / year (= 5 km towards pole / year) Repeat Cycle / Cycle Length concept maintained Groundtrack at equator controlled Inclination drift over summer close to zero Approximation formula applicable for Orbit Extension phase. Invitation to think about interferometry for this extension orbit. Small baselines still available from cycle to cycle (max 500 m) During summer, between several cycles. Study dataset is available ERS-2 and Envisat! Envisat database: image pairs with inclination difference of up to 18 mdeg ERS-2 database: image pairs with inclination difference of up to 50 mdeg (New ERS-2 orbit versus ERS-2 orbit, (No Out-of-Plane) in 2001

17 Conclusions During 2007 & 2008, Orbits control of Envisat optimised, to favour short and predictable interferometry baselines Autumn 2007, Inclination of ERS-2 orbit changed, to obtain interferometric baselines between ERS-2 and Envisat over the Northern Polar Region During 2011 and beyond, Envisat Orbit and orbit control changed to allow lifetime extension. Drifting inclination causes large impact on Interferometry. Current SAR database contains image pairs with large inclination difference (comparable to 1 year drift) Invitation to study interferometry for extension phase

Impact of the Envisat Mission Extension on SAR data

Impact of the Envisat Mission Extension on SAR data Impact of Envisat Mission Extension on SAR data - 1.0 Prepared by Nuno Miranda, Berthyl Duesmann, Monserrat Pinol, Davide Giudici, Davide D Aria Reference