SpaceOps 2008 Conference (Hosted and organized by ESA and EUMETSAT in association with AIAA)

AIAA 2008-3250

Envisat ASAR Interferometry Baseline Results of the Current Envisat Orbit Maintenance Strategy

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D. Kuijper* LogicaCMG c/o ESA-ESOC, Robert-Bosch-Str. 5, D-64293 Darmstadt, Germany, E-mail: [email protected] The biggest and most advanced Earth Observation Satellite in-orbit, developed by the European Space Agency (ESA) and its member states, is Envisat. It was launched on March 1, 2002 by an Ariane V rocket from French Guyana and holds a total of 10 multidisciplinary Earth observation instruments, among which an Advanced Synthetic Aperture Radar (ASAR). The ASAR user community requested the Flight Dynamics division of the European Space Operations Centre (ESOC) to investigate how the orbit control maintenance strategy for Envisat could be changed to optimise ASAR interferometry opportunities overall and in addition support the International Polar Year 2007/2008 initiative. The Polar Regions play a pivotal role in understanding our planet and our impact on it as they are recognized as sensitive barometers of environmental change. One of the main themes of the International Polar Year 2007/2008 is therefore the study of Earth’s changing ice and snow, and its impact on our planet and our lives. Naturally, ESA is supporting this very important initiative by controlling Envisat in such a way ASAR interferometry opportunities are optimised. This paper briefly discusses the orbit control maintenance strategy that has been in place since the launch of Envisat. It also presents the at the start of 2007 adopted orbit control maintenance strategy that aims at improving and increasing the opportunities for Envisat ASAR interferometry, while preserving the fuel on board the spacecraft. The hydrazine on-board Envisat happens to be a precious resource as only approximately 300 kg of it was available at launch, like ERS-2. The difference being that the mass of Envisat is approximately 3.2 times that of ERS-2. The old orbit maintenance strategy effectively resulted in ASAR interferometry baselines of 2000 meters maximum. The new strategy on the other hand performs more regular orbit inclination manoeuvres, which reduces the baselines down to 250 meters over the polar region between certain repeat cycles, making a valuable contribution to the International Polar Year initiative. In addition, the maintenance of the orbit altitude was changed to reduce the baselines of the ASAR interferometry opportunities over the rest of the orbit. The remainder of this paper presents the actual baseline results since the start of 2007 as a consequence of the change in orbit control maintenance to verify its effectiveness. In addition this paper presents an evaluation of the adopted orbit control maintenance strategy in terms of actual fuel consumption, by comparing the actual numbers against the predicted ones that were taken into consideration when selecting the new orbit control maintenance strategy for Envisat.

I. Introduction The biggest and most advanced European Earth observation satellite in-orbit, developed by the European Space Agency (ESA) and its member states to help understand the impact of mankind’s activities on the Earth environment, like the global warming, the hole in the ozone layer, destruction of forests, flooding, and man made natural disasters, is Envisat (see Figure 1). Envisat was launched on an Ariane V rocket from Kourou in French Guyana on March 1, 2002 and was put into a sun-synchronous low Earth orbit with the following orbital characteristics: • semi-major axis = 7159.5 km, • inclination = 98.55 deg, • mean local solar time = 10:00 A.M. (at the Figure 1. Artist impression of Envisat in-orbit descending node) • repeat cycle of 35 days (or 501 orbits) with 14 11/35 orbits/day *

Flight Dynamics Specialist, AIAA Member Grade for first author. 1 American Institute of Aeronautics and Astronautics 092407

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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The spacecraft accommodates a total of 10 multi-disciplinary Earth observation instruments, among which the Advanced Synthetic Aperture Radar (ASAR). In 2006, the ASAR user community requested the Flight Dynamics division of ESOC/ESA to investigate any possible orbit control maintenance strategy for Envisat that would: a) reduce/minimise the average baseline values Orbit n Orbit n+501 anywhere in the orbit and throughout the mission in the future (see Figure 2 for baseline definitions). b) reduce/minimise the baseline values to less than 200-250 m over the Poles for about 3 consecutive cycles during southern Polar winter (i.e. European summer) and repeat this the next 3 years. The investigations concentrated on how to best accommodate these requirements, but not to neglect the A fact that fuel is a precious commodity on-board Envisat that needs to be preserved wherever possible to maximise mission lifetime. The results of these investigations are extensively reported in Ref 1. The next chapter presents the difference between the Figure 2. Definition of the perpendicular baseline B┴ general Envisat orbit control maintenance strategy, in and across–track baseline B (from reference 1) place since its launch, and the at the start of 2007 adopted strategy that tries to accommodate the requirements of the ASAR user community as best as possible while avoiding or limiting as much as possible the impact on the fuel consumption. The subsequent chapter evaluates the new orbit control maintenance strategy in terms of actual fuel consumed and presents some results on actual ASAR interferometry baslines achieved

II. Change in the Envisat Orbit Control Maintenance Strategy This chapter presents the general Envisat orbit control maintenance strategy in place since the launch of Envisat in 2002 and the, at the start of 2007, adopted orbit control maintenance strategy. A. General Envisat orbit control maintenance concept The general Envisat orbit control maintenance strategy is based on the frozen eccentricity reference orbit control concept that was already applied successfully on ERS-1 & -2 (see Ref. 2). In this concept, the orbit of the satellite is controlled such that its ground track is maintained within 1 km of a reference ground track. The reference ground track is based on a reference orbit that has an exact 35 days / 501 orbits repeat cycle and is computed without taking into account the perturbing forces by sun and moon gravitation, air drag, and solar radiation pressure. These perturbing forces do exist however, and therefore an exact repeat orbit cannot be maintained without orbit control. The orbit control strategy therefore aims at compensating the effect of these forces (in particular the first two forces) as far as possible to achieve the ground track control requirement of ± 1 km†. The perturbing forces by sun and moon gravitation cause a secular decrease in inclination showing periodic variations due to the moon and seasonal variations due to the Sun-Earth distance variation. To compensate for the decrease in inclination, out-of-plane Orbit Control Manoeuvres (OCMs) are executed, if possible, centred around the Equator, at the ascending node (an operational requirement to execute the manoeuvre in eclipse). To meet the 1 km ground track requirement, around 3 OCMs per year were performed in the past. The perturbing force by air drag (a non-conservative force) continuously takes away energy from the orbit and thus gradually decreases the orbit semi-major axis and thus the orbital period. The effect of air drag on the ground track is that the satellite shows a westward drift at an altitude above reference, which turns into an eastward drift as soon as the altitude drops below the reference. This drift can only be stopped by a semi-major axis raising manoeuvre, which is controlled by in-plane manoeuvres, i.e. the thrusters’ fire along or against the flight direction in †

This requirement was originally driven by the needs of the altimetry instrument to over fly the same ground track with this accuracy 2 American Institute of Aeronautics and Astronautics 092407

B. Adopted Envisat orbit control maintenance strategy The analysis showed that the first requirement of the ASAR user community can be fulfilled in a fuel economic way by executing one OCM every 70 days (2 repeat cycles), and by increasing the frequency of the in-plane manoeuvres to reduce the (equatorial) across-track differences. It was decided to keep the ground track deviations within 200 meters of the reference at the Equator (ground track deviations at middle latitudes are a combination of the deviation at the Equator and the pole). The analysis also showed that the second requirement of the ASAR user community can be fulfilled by introducing one small OCM during northern summer, this however at the cost of some extra fuel due to the slews associated with each OCM. In summary: 1. In-plane manoeuvres: At the Equator the orbit is allowed to drift 200 meters w.r.t. to the reference, i.e. a 200 meter deadband is maintained. Effectively the number of (smaller) in-plane manoeuvres increases, but this does not affect the overall annual propellant consumption for in-plane manoeuvres. 2. Out-plane manoeuvres In 2007 and 2008, every 2 cycles, an inclination correction is performed at the same relative orbit within the cycle. The only exception is made for OCM #17 (see Table 1 below), which is delayed by one cycle due to the requirement to support the International Polar Year initiative. The OCMs are always performed in the early hours of the day. Note that with this strategy 5 orbit maintenance OCMs are executed each year, with an estimated additional fuel cost of a bit less than 500 grams per year.

III. Evaluation of the Adopted Orbit Control Maintenance Strategy This chapter presents what has actually been achieved by the change in orbit control maintenance strategy and compares it to what was planned. One issue that was looked at during the orbit control maintenance strategy analysis that is worth mentioning here in view of the evaluation of the adopted strategy, is the optimal duration of an OCM. The out-of-plane OCMs are the biggest fuel consumers on board and it is therefore of interest to find the optimal duration for these OCMs to limit the impact on the fuel consumption. The aspects that affect the optimal duration are on the one hand the constant propellant cost contribution associated with each 11,0 OCM, as Envisat needs to be rotated by 90° twice per out-of-plane manoeuvre, and on the other 10,0 hand the efficiency of an out-of-plane thrust to change the inclination is reduced by the cosine of the latitude, i.e. the efficiency of an OCM 9,0 decreases with its duration. A formula was derived to calculate the 8,0 optimal burn duration that provides the maximum inclination change per propellant mass. It was 7,0 noted that a first approximation of the optimal duration only depends on the mean motion, fuel 6,0 mass required to rotate the spacecraft and the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 mass flow rate. To determine the optimal burn Orbit Control Manoeuvre # duration for future OCMs, it is important to know Figure 3. Mass flow rate values for each orbit maintenance how the mass flow rates have evolved and OCM that has been executed up to February 12, 2008. extrapolate that to the future. Mass flow rate, gr/sec²

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order to have the desired increase or decrease in semi-major axis respectively. This type of manoeuvre is referred to as a Stellar Fine Control Manoeuvre (SFCM) and is performed every 30-50 days, depending on the level of solar activity and thus the rate of orbital decay. The general orbit control maintenance strategy effectively resulted in interferometry baselines of a maximum 2000 meters. Although the collected ASAR data, while this strategy was in place, have an average baseline value of around 750 m and provide useful data for most interferometric applications, a significant percentage of the collected data have very large (more than 800 m and up to 2000 m) baselines. To decrease this percentage, i.e. reduce/ minimise the average baseline values anywhere in the orbit and throughout the mission in the future, the orbit control maintenance strategy in place needed to change. To determine what the best change would be, an analysis of possible orbit control maintenance strategies was conducted (see Ref. 1) that resulted in a new orbit control maintenance strategy being adopted for Envisat at the start 2007.

3 American Institute of Aeronautics and Astronautics 092407

Up to now a total of 22 OCMs have been successfully executed. The first 2 were executed to manoeuvre Envisat into its operational orbit. The other 20 manoeuvres were orbit control maintenance manoeuvres, of which the last 6 followed the adopted orbit control maintenance strategy. Figure 3 presents the mass flow rate for each of these 20 manoeuvres. Looking at the last 6 OCMs it seems that the linear decrease in mass flow rate that was assumed in Ref. 1 for computing the optimal burn duration for future OCMs, is not completely correct, however, the small OCM #17 (see Table 1) spoils the picture by being so small that it hardly reduces the mass flow rate. When this OCM is removed from the picture, the assumption is still correct. The small variations of the mass flow rates are a result of switching tanks once in a while, which is done to balance the pressure between the tanks.

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A. Out-of-plane manoeuvre evaluation The analysis further noted that even a burn where the duration is within about 250 seconds of the optimal burn duration, the inclination change per kilogram of fuel is only 1% less than optimal. From the Tdiff column in Table 1 it can be seen that only OCM #2 and #17 are more than 1% less optimal, where manoeuvre #17 is the small OCM that was introduced last summer to support the International Polar Year initiative. Table 1. Overview of the orbit maintenance OCMs that have been performed by Envisat up to February 12, 2008. Indicated by a yellow and light-blue background are the manoeuvres that followed the general, and the new orbit control maintenance strategy respectively. OCM # Start time burn

Tprev

Tburn

Topt

Tdiff

MFR

(days) (sec) (sec) (sec) (gr/s) 1

10-09-2002 00:35

2

18-12-2002 05:27

3

21-02-2003 04:42

4 5

MT

MTOT

(kg)

(kg)

An

Mincl

∆Vn

∆Vr

∆Vt

(mm/s2) (m/s)

(m/s)

(m/s)

(kg)

624

859

235

10.2

0.631

6.984

2.85

1.782 -0.018

0.016

6.853

99

509

861

352

10.3

0.642

5.867

2.86

1.458 -0.021

0.012

5.737

65

627

872

245

9.6

0.623

6.657

2.70

1.692 -0.039

0.003

6.497

20-05-2003 05:11

88

699

884

185

9.5

0.644

7.303

2.68

1.873 -0.019

0.011

7.188

28-10-2003 05:55

161

806

897

91

8.9

0.629

7.804

2.49

2.005 -0.047

0.005

7.607

6

04-02-2004 05:46

99

687

887

200

9.1

0.621

6.881

2.54

1.747 -0.033

0.003

6.744

7

14-04-2004 05:42

70

719

905

186

8.7

0.630

6.882

2.41

1.734 -0.016

0.008

6.786

8

21-09-2004 05:14

160

881

918

37

8.2

0.618

7.835

2.27

2.000 -0.035

0.005

7.681

9

07-01-2005 05:24

108

815

908

93

8.4

0.615

7.460

2.35

1.915 -0.048

0.003

7.265

10

17-03-2005 05:50

69

906

930

24

8.0

0.627

7.854

2.23

2.016 -0.015

0.010

7.758

11

07-09-2005 06:19

174

999

947

52

7.6

0.630

8.202

2.10

2.102 -0.013

0.011

8.106

12

10-01-2006 05:53

125

1020

941

99

7.8

0.632

8.548

2.15

2.198 -0.018

0.006

8.457

13

28-03-2006 05:32

77

1130

958

172

7.3

0.628

8.892

2.02

2.284 -0.037

0.009

8.716

14

13-09-2006 05:21

169

1091

971

120

7.0

0.623

8.226

1.92

2.093 -0.007

0.009

8.164

15

23-01-2007 04:32

132

1123

959

164

7.1

0.614

8.620

1.98

2.227 -0.052

0.005

8.402

16

03-04-2007 04:34

70

931

975

44

6.8

0.619

6.991

1.89

1.763 -0.015

0.009

6.897

17

17-07-2007 04:40

105

135

968

833

6.8

0.607

1.523

1.78

0.241 -0.031 -0.005

1.323

18

27-09-2007 05:15

72

889

986

97

6.6

0.619

6.478

1.80

1.602 -0.014

0.011

6.380

19

04-12-2007 04:42

70

898

991

93

6.3

0.601

6.217

1.74

1.558 -0.044

0.001

6.043

20

12-02-2008 04:42

70

844

986

142

6.7

0.625

6.213

1.83

1.543 -0.015

0.009

6.119

Totals 13.739 156.7962 153.462 Tprev = Days between OCM and previous OCM, Tburn = Actual burn duration, Topt = Optimal burn time, Tdiff = Absolute difference between optimal and actual burn duration, MFR = Mass flow rate, MT = Fuel mass used for rotation, MTOT = Total manoeuvre fuel mass, An = orbit-normal accelerations, ∆Vn = Delta-V cross-track, ∆Vr = Delta-V radial, ∆Vt = Delta-V along-track, Mincl = MTOT minus the fuel mass spend on radial and along-track components of the OCM

Note that Envisat OCMs do not just have a cross track component. Due to small thruster misalignments and attitude control imperfections OCMs also have a radial and along-track component, with the radial component being significantly larger than the along track component. As a consequence, the consumed total fuel masses (MTOT) were not spent completely on changing the orbit inclination, which needs to be considered when evaluating the new orbit control maintenance strategy. The next table presents a comparison between the planned manoeuvres under the new orbit control maintenance strategy (taken from Ref. 1) and the finally implemented and executed manoeuvres. The highlighted planned total fuel masses (MTOT), and the actual fuel masses used for he inclination changes (Mincl), should be compared. 4 American Institute of Aeronautics and Astronautics 092407

Table 2. Characteristics of the planned manoeuvres (left) and the implemented manoeuvres (right) using the new orbit control maintenance strategy. Planned Tburn

MFR

MT (gr)

MTOT (kg)

Implemented

An

∆Vn 2

(mm/s ) (m/s)

Topt

Tdiff

Tburn

MFR

MT

(gr/s) (kg)

Mincl (kg)

MTOT (kg)

An

∆Vn 2

(mm/s ) (m/s)

Topt

Tdiff

Mdiff

#

(s)

(gr/s)

(s)

(s)

(s)

(s)

(s)

(gr)

15

1216

7.0

0.628 9.111

1.86

2.252 966

250

1123

7.1

0.614 8.402

8.620

1.98

2.227

959

164

-709

16

937

6.7

0.628 6.907

1.79

1.677 978

41

931

6.8

0.619 6.897

6.991

1.89

1.763

975

44

-10

17

151

6.7

0.628 1.644

1.79

0.272 978

827

135

6.8

0.607 1.323

1.523

1.78

0.241

968

833

-321

18

762

6.4

0.628 5.534

1.71

1.311 991

229

889

6.6

0.619 6.380

6.478

1.80

1.602

986

97

846

19

743

6.2

0.628 5.207

1.64

1.224 1004

261

898

6.3

0.601 6.043

6.217

1.74

1.558

991

93

836

20

1110

5.9

0.628 7.174

1.57

1.743 1018

92

844

6.7

0.625 6.119

6.213

1.83

1.543

986

142 -1055

Totals 3.768 35.577 8.479 1700 3.685 35.164 36.042 8.934 1373 -413 Tburn = Burn duration, Topt = Optimal burn time, Tdiff = Absolute difference between optimal and burn duration, MFR = Mass flow rate, MT = Fuel mass used for rotation, MTOT = Total manoeuvre fuel mass, An = orbit-normal accelerations, ∆Vn = Delta-V cross-track, Mdiff = Difference in fuel mass used for pure inclination change between the implemented and planned manoeuvre.

From this table we can conclude that about 400 grams less fuel was used than was predicted by the analysis. Part of this can be explained by the fact that the burn durations of the manoeuvres were overall closer the optimal values and the fuel used for the rotation of the satellite was less than expected. Also the total inclination change (65.9 mdeg) achieved by these 6 OCMs is higher than was expected (62.5 mdeg) from the analysis. B. Ground track evaluation The previous section showed that the final delta-Vs were slightly different from the planned ones. The reason was an unexpected under performance of OCM #17, which affected some of the following OCMs, e.g. manoeuvre calibration used where it should not have been. Consequently, the ground track deviations at maximum latitude differ from the expected one, as can be seen from the plots below. Figure 4 presents the planned (left) and actual (right) ground track deviation at maximum latitude starting December 19, 2006. Please note that on the right plot after 12 February, 2008 is again planned. GT deviation

Start of cycle

GT deviation Start of cycle 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

1250,0

1250.0

1000,0

1000.0

Ground track deviation at max lat (m)

Ground track deviation at max lat (m)

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OCM

750,0 500,0 250,0 0,0 -250,0 -500,0 -750,0 19-12-2006

17-7-2007

12-2-2008

9-9-2008

750.0 500.0 250.0 0.0 -250.0 -500.0 -750.0 -1000.0 19/12/2006

17/07/2007

Date

12/02/2008

09/09/2008

Date

Figure 4. Planned (left) and actual (up to 12 February, 2008) ground track deviations at maximum latitude, following the new orbit control maintenance strategy. Each jump in the plot is associated with an OCM. The start of each cycle is indicated by cyan dots. The first cycle in each plot is cycle 54. These ground track deviations can be used to compute the baselines at maximum latitude for each day in the cycle and for each possible cycle combination. Figure 5 presents the planned (left) and actual (right) baseline results of cycle combinations of the cycles that cover our northern summer of both 2007 and 2008. As can be seen from both plots, baselines are still within the required 250 metres.

5 American Institute of Aeronautics and Astronautics 092407

58 vs 60

59 vs 60

68 vs 69

68 vs 70

69 vs 70

58 vs 68

59 vs 69

60 vs 70

58 vs 59

200,0

150.0 Baseline values at max latitude (m)

200.0

150,0 100,0 50,0 0,0 -50,0 -100,0 -150,0

58 vs 60

59 vs 60

68 vs 69

68 vs 70

69 vs 70

58 vs 68

59 vs 69

60 vs 70

100.0 50.0 0.0 -50.0 -100.0 -150.0 -200.0

-200,0

-250.0

-250,0

-300.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Baseline values at max latitude (m)

58 vs 59

250,0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Day in cycle

Day in cycle

Figure 6 shows the planned (left) and actual (right) perpendicular baseline results at maximum latitude between the cycles where the inclination of the orbit is higher than the nominal (reference) one, i.e. west of the reference at the northern hemisphere. Note that the baseline values are still less than 250 m for most cycle combinations, but for some the baseline values have increased to about 400 meter. The jumps seen on the right at day 3 of the cycle are caused by the fact that OCM #18 had to be shifted by 2 days due to problems with the satellite when the OCM was actually planned. 55 vs 62 62 vs 74

55 vs 64 64 vs 66

55 vs 66 64 vs 72

55 vs 72 64 vs 74

55 vs 74 66 vs 72

62 vs 64 66 vs 74

62 vs 66 72 vs 74

62 vs 72 55 vs 57

55 vs 62 64 vs 66

250,0 200,0

Baseline values at maximum latitude (m)

Baseline values at maximum latitude (m)

150,0 100,0 50,0 0,0 -50,0 -100,0 -150,0 -200,0 -250,0

55 vs 64 64 vs 72

55 vs 66 64 vs 74

55 vs 72 66 vs 72

55 vs 74 66 vs 74

62 vs 64 72 vs 74

62 vs 66

62 vs 72

62 vs 74

400.0 350.0 300.0 250.0 200.0 150.0 100.0 50.0 0.0 -50.0 -100.0 -150.0 -200.0 -250.0 -300.0 -350.0 -400.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

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Figure 5. Planned (left) and actual (right) perpendicular baseline values at maximum latitude for all cycle combinations of the cycles that cover the northern Summer of both 2007 and 2008.

1 2 3 4 5 6 7 8 9 1011121314151617181920212223242526272829303132333435

Day in cycle

Day in cycle

Figure 6: Expected (left) and actual (right) perpendicular baseline values at maximum latitude for all cycle combinations of the cycles where the inclination of the orbit is higher than the reference. Similar plots can be presented for the cycle combinations of the cycles where the inclination of the orbit is lower than the reference one. C. Baseline evaluation At the time of writing (April 2008) only baseline results for the first 6 cycles flown since the introduction of the new Envisat orbit control maintenance strategy, are available at ESOC. The initial interferometric baseline results for the period April to October 2007 however look very promising: •

500 m - 900 m 0m

•

For the polar regions:

- 250 m

Northern Winter, for baselines with odd cycle difference, “consecutive” Northern Winter, for baselines with even cycle difference, “alternate”

In between 60S – 60N:

0m

- 800 m

0m

- 300 m

Northern Winter, for baselines with odd cycle difference, “consecutive” Northern Winter, for baselines with even cycle difference, “alternate”)

The next 4 graphs illustrate the baseline results that have been obtained for these 6 cycles (courtesy of Betlem Rosich from ESA-ESRIN). 6 American Institute of Aeronautics and Astronautics 092407

cycle diff=1

cycle diff=3

cycle diff=2

cycle diff=4

1400

1000

800

600

400

200

0 26500

27000

27500

28000

28500

29000

29500

First orbit of image pair

Figure 7. Absolute perpendicular baseline values (m) over Greenland (65N-70N & 75N80N) from April to October 2007 for image pairs with 1, 2, 3 , and 4 cycles differences. OCM #16 took place on April 3, 2007 at 4:34, which corresponds to orbit number 26611. Note that each cycle has exactly 501 orbits. So, cycles 58, 59 and 60 covering our northern summer, start at orbit number 27112 and end at orbit number 28615. Taking the images from cycles 58 and 59, we see that baseline values (blue and yellow dots) are in line with the expected baseline values (see Figure 5). cycle diff=1

cycle diff=3

cycle diff=5

cycle diff=2

cycle diff=4

1200

Absolute perpedicular basline vlues, m

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Absolute perpedicular basline vlues, m

1200

1000

800

600

400

200

0 26500

27000

27500

28000

28500

29000

29500

first orbit of image pair

Figure 8. Absolute perpendicular baseline values over Europe (40N-50N) from April to October 2007 for image pairs with 1, 2, 3 ,4, and 5 cycles differences. The results over Europe for consecutive cycles look better than over the Polar regions as these are a combination of the deviations at maximum latitude and Equator, where a deadband is maintained of 200 meter. 7 American Institute of Aeronautics and Astronautics 092407

cycle diff-1

cycle diff=3

cycle diff=2

cycle diff=4

300

250

200

150

100

50

0 26500

27000

27500

28000

28500

29000

29500

First orbit of image pair

Figure 9. Absolute perpendicular baseline values (m) over Equator (10S-10N) from April to October 2007 for image pairs with 1, 2, 3, and 4 cycles differences. It is obvious that a deadband of 200 meters is maintained at the Equator. cycle diff=1

cycle diff=3

cycle diff=5

cycle diff=2

cycle diff=4

28500

29000

1400

Absolute perpendicular basline value, m

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Absolute perpendicular basline value, m

350

1200 1000 800 600 400 200 0 26500

27000

27500

28000

29500

First orbit of image pair

Figure 10. Absolute perpendicular baseline values over Antarctica (10S-10N) from April to October 2007 for image pairs with 1, 2, 3 ,4, and 5 cycles differences. It should be clear from Figure 4 that certain combinations of cycles give better baseline results than others, as is confirmed again by the Antarctica and Greenland baseline results.

IV. Conclusions The International Polar Year initiative triggered some investigations on how the orbit control maintenance strategy of Envisat could be changed to support this initiative and on top to improve overall ASAR interferometry opportunities. These investigations resulted in the adoption of one of the proposed orbit control maintenance 8 American Institute of Aeronautics and Astronautics 092407

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strategies at the start of 2007. After 6 out-of-plane manoeuvres the strategy has been evaluated in terms of planned versus implemented fuel consumption and in terms of baseline results. In terms of fuel consumption it can be concluded that so far 400 grams of fuel have been spent less than was actually predicted. Basically due to the fact that the finally implemented manoeuvres had a more optimal burn duration, and needed less fuel for the rotation of the Envisat each OCM. In addition, the total inclination change achieved by these 6 out-of-plane manoeuvres was 3.4 millidegrees higher than expected. In terms of baseline results it is expected that results will be slightly worse than expected due to the fact that some manoeuvres did not quite achieve the expected changes in inclination due to under performances of certain manoeuvres and using invalid manoeuvre calibration values when determining the number of thrusters pulses required to achieve the requested delta-V. Nevertheless, the baseline results of the 6 cycles presented here look to be very promising. The small out-of-plane manoeuvre that was executed during our northern Summer, did provide excellent baselines between consecutive Summer cycles (all below the 250 meter level) to support the International Polar Year initiative well.

References 1

Kuijper, D., Garcia Matatoros, M.A., “Analysis of Envisat orbit maintenance strategies to improve/increase Envisat ASAR interferometry opportunities,” Proceeedings of the 20th International Symposium on Space Flight Dynamics [CD-ROM], CP2007-214158, NASA, Annapolis, MD USA, 24-28 September 2007. 2 Rosengren, M., “Orbit Control of ERS-1, ERS-2 and ENVISAT to support SAR Interferometry,” Proceedings of the ERSEnvisat Symposium 2000, SP-414, ESA, Gothenburg, Sweden,16-20 October 2000. 3 Rudolph, A., Kuijper, D., Ventimiglia, L., Garcia Matatoros, M.A., Bargellini, P. “ENVISAT Orbit Control – Philosophy, Experience and Challenges,” Proceedings of the 2004 Envisat & ERS Symposium, SP-572, ESA-ESTEC, ESA, Salzburg, Austria, 6-10 September 2004. 4 Milligan, D., “ENVISAT Platform Operations: Providing a Platform for Science,” Proceedings of the 2007 Envisat Symposium, SP-636, ESA, Montreux, Switzerland, 23-27 April 2007.

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