ers-2 and envisat orbit control current and future...

ESRIN, 26 Nov 2007 09:45-10:10 ERS-2 and ENVISAT orbit control: current and future strategy the impact on the interferometric baselines

Berthyl Duesmann (ESA), Itziar Barat (ESA)

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



The ERS & ENVISAT Orbit, Definition

35 days (501 orbits) repeat cycle

14 11/35orbits per day

799.790 kmmean altitude

100.6 minorbital period

± 3°90.0°argument of perigee

± 5 min

10:30 p.m. ( 10:28:15 ERS-2)10:00 p.m. (EnviSat)

mean local time at ascending node

0.0050.001165eccentricity

± 0.009°98.549387°Inclination

± 0.068 km7159.4927 kmsemi-major axis

ValueParameter



Optimising SAR interferometry by Orbit ControlThe 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.

Definition of the perpendicular baseline Bi and across–track baseline A

A



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.

Relative Ground Track [km]

The relative altitude and ground track evolution in relation to airdrag and executed in-plane manoeuvres

timeIn-plane manoeuvre

time

East Limit (+1km)West Limit (-1km)Relative Altitude [m]

Flight Direction

High Density

Predicted Density

Low Density

~ 20 - 30 m



BASELINE ORBIT CONTROL MAINTENANCE(2)

The deviation of the actual ground track from the reference ground track at the north-most point in each orbit

Out

-of-p

lane

m

anoe

uvre



SAR Interferometric BaselineApproximation formulas

EnviSatcycle n

EnviSatcycle m

SAR Acquisition

α = 20.355

deg

Bn = Perpendicular

Baseline

Ba = AcrosstrackBaseline, Orbit

Bg = GroundtrackBaseline

αBg = Ba * (6371 / 7159) = Ba * 0.89

Bn = Ba * cos (α) = Ba * 0.95 = Bg * 1.067

Approximation Perpendicular Baseline at any (PSO) latitude

Bn (PSO) = 1.067 x ( Bgpole x sin (PSO) + Bgequator x cos (PSO) )



Current ENVISAT Orbit StrategyPhasing of Out-of-plane Manoeuvres (1)

-1000.0

-750.0

-500.0

-250.0

0.0

250.0

500.0

750.0

1000.0

1250.0

18/12/2006 18/03/2007 18/06/2007 18/09/2007 18/12/2007 18/03/2008 18/06/2008 18/09/2008

Date

Gro

undt

rack

dev

iatio

n at

max

lat (

m)

GT deviation Start of cycle

The expected inclination evolution for the proposed out-of-plane manoeuvre strategy – once every 70 days

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.

= Start of Cycle

Summer 2007 Summer 2008



Current ENVISAT Orbit StrategyPhasing of Out-of-plane Manoeuvres (2)

SimulationsGroundtrack baselines during Summer, at the pole.

-4 0 0

-3 5 0

-3 0 0

-2 5 0

-2 0 0

-1 5 0

-1 0 0

-5 0

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2 3 3 3 4 3 5

D a y in c y c le

Dev

iatio

n di

ffere

nce

(m)

5 8 v s 5 9 5 8 v s 6 0 5 9 v s 6 0 6 8 v s 6 9 6 8 v s 7 0 6 9 v s 7 0 5 8 v s 6 8 5 9 v s 6 9 6 0 v s 7 0



Current ENVISAT Orbit Strategy (summary)The new orbit maintenance has been introduced in January 2007, to be executed during 2007 and 2008.

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 UTC2008: 12 Feb, 22 Apr, 1 Jul, 9 Sep, 18 Nov, always close to 5 UTCThe 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:max @ 32.0 deg < 472 m (during Northern Summer)max @ 61.9 deg < 850 m (during Northern Winter, for baselines with odd cycle difference)max @ 32.0 deg < 472 m (during Northern Winter, for baselines with even cycle difference)

@ 0.0 deg < 400 m all baselines



Current Envisat Orbit StrategyThe Results

Antarctica, 65S:75S, Apr-Oct 2007

0

200

400

600

800

1000

1200

1400

26500 27000 27500 28000 28500 29000 29500

first orbit

abso

lute

per

p B

s di

ff [m

]

c=1c=3c=5c=2c=4

For the polar regions400 m (during Northern Summer)500 m -1000 m (during Northern Winter,

for baselines with odd cycle difference)0 m - 400 m (during Northern Winter,

for baselines with even cycle difference)

Oscillation due to Moon

europe,40N:50N, Apr-Oct 2007

0

200

400

600

800

1000

1200

26500 27000 27500 28000 28500 29000 29500

first orbit

abso

lute

per

p Bs

diff

[m]

c=1c=3c=5c=2c=4

For Tropical and Mid latitudes:max @ 32.0 deg < 472 m (during Northern Summer)max @ 61.9 deg < 850 m (during Northern Winter, for baselines

with odd cycle difference)max @ 32.0 deg < 472 m (during Northern Winter, for baselines

with even cycle difference)

For the polar regions400 m (during Northern Summer)500 m -1000 m (during Northern Winter,

for baselines with odd cycle difference)0 m - 400 m (during Northern Winter,

for baselines with even cycle difference)

Oscillation due to Moon

Equator, 10S:10N, Apr-Oct 2007

0

50

100

150

200

250

300

350

26500 27000 27500 28000 28500 29000 29500

c-1c=3c=2c=4

For Tropical latitudes:@ 0.0 deg < 400 m all baselines



Current ERS-2 Orbit Strategy

• Higher Inclination +0.018 deg (further away from pole) for ERS-2, since 10 September 2007.

• 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 imaged area.Groundtrack difference at Maximum Latitude

• The inclination difference between ERS-2 and ENVISAT is kept constant.

• Results:ERS - ENVISAT InSAR Campaign -

performances

153.178580.318151.06-

219.878100

156.657938.035144.128163.0452010

149.1751265.64133.14540.1553020

141.1011555.39119.17900.6564030

129.7411798.45102.6551233.655040

115.2431986.3984.87181528.716050

98.48592114.0868.42661777.227060

80.91232177.5256.90671971.328070

65.16182174.5255.65332105.169080

stdevavgstdevavg.

descendingascending

Maxmin

ERS – EnvisatPerpendicular Baseline

Latitude North

• 1 quarter of the orbit suits for ERS/Envisat cross-InSAR:– Bperp > 1600m

• 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



Current ERS-2 Orbit Strategy, fine tuning

• Approximation formula’s (page 6) are applicable

• 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 Sub-

tropical Northern latitudes (California etc)• Final orbit adjustment to be studied and agreed.

ERS-2 EnviSat

SAR Acquisition

α = 20.355 degBn = Perpendicular

Baseline

Ba = AcrosstrackBaseline, Orbit

Bg = GroundtrackBaseline

α

Bn = 2.10 km

Ba = Bn / cos (α) = 2.24 km

Bg = Ba * (6371 / 7159) = 1.99 km ≈ 2.0 km

ERS-2 Inclination: + 0.018 deg

Approximation Perpendicular Baseline at any (PSO) latitude

Bn (PSO) = 1.067 x ( Bgpole x sin (PSO) + Bgequator x cos (PSO) )

BGPole 2015.000 Current Proposal

BGEq -360.000 -360.000 -1100

Ascending DescendingSec

since ANX

True Latitude

(rad)

Baseline at Min

Latitude

Baseline at Mid

LatitudeDelta (m)

Sec since ANX

True Latitude

(rad)

Baseline at Min

Latitude

Baseline at Mid

LatitudeDelta (m)

2150.005 2150.005

80 90 1361 1.416735 2065.596 2107.801 2.641 1636 1.702997 2181.878 2165.941 -8.579

70 80 1178 1.226241 1893.893 1979.745 8.425 1826 1.900778 2158.473 2170.175 -7.345

60 70 1004 1.045116 1666.965 1780.429 3.209 2001 2.082945 2062.384 2110.429 -3.651

50 60 833 0.867113 1390.764 1528.864 0.154 2172 2.260947 1902.527 1982.456 -3.934

40 50 663 0.690151 1072.595 1231.679 -1.971 2342 2.437909 1683.997 1793.262 -5.188

30 40 494 0.514230 723.069 897.832 -2.824 2511 2.613830 1414.601 1549.299 -6.091

20 30 325 0.338309 351.224 537.147 -3.008 2680 2.789751 1101.539 1258.070 -7.570

10 20 157 0.163429 -29.190 161.017 -2.028 2849 2.965672 754.474 928.007 -10.028

0 10 6016 6.262366 -428.795 -228.992 -9.114 3017 3.140552 386.358 570.416 -9.902



Envisat Orbit Extension• The current orbit strategy for Envisat can be maintained until end of 2010. 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.SSeelleecctteedd OOrrbbiitt CCoonnttrrooll SSttrraatteeggyy

35/501, nominal control

new repeat cycleinclination drift

35 / 501 Altitude New altitude.

Altitude

22:00

21:50

22:10

MLST

98,5° inclination

new sun sync. inclination

Inclination

II III IVmilestones



Envisat Orbit ExtensionSELECTED ORBIT CONTROL

Control phasesThe 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).



Envisat Orbit ExtensionSELECTED ORBIT SCENARIO

– The forced altitude change at the end of phase E3 equals -17.4 km.– Sun-synchronous characteristics at the new altitude are:

• Semi-major axis: 7142047 m• Eccentricity: 0,001158• Inclination: 98.476°

– The repeat cycle in phase E4, i.e. after altitude change, will be: 30 days / 431 orbits.

– End of phase E3 (date II) should be in October 2010.

– Predicted fuel quantity available:• before the altitude change:

56 kg ± 15 kg• after the altitude change:

23 kg ± 15 kg

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

01/0

9/20

10

01/1

2/20

10

01/0

3/20

11

01/0

6/20

11

01/0

9/20

11

01/1

2/20

11

01/0

3/20

12

01/0

6/20

12

01/0

9/20

12

01/1

2/20

12

01/0

3/20

13

01/0

6/20

13

01/0

9/20

13

01/1

2/20

13

01/0

3/20

14

01/0

6/20

14

01/0

9/20

14

01/1

2/20

14

01/0

3/20

15

01/0

6/20

15

01/0

9/20

15

TLSTMLSTEquation of TimeMax TLST 22:00Min TLST 22:00Max TLST 22:05Min TLST 21:55Max TLST 22:10Min TLST 21:50



Envisat Orbit ExtensionInterferometry still possible?

Negative: Inclination drifts 47 mdeg / year (= 5 km towards pole / year)Positive: Repeat Cycle / Cycle Length concept maintained

Groundtrack at equator controlledInclination drift over summer close to zeroApproximation 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 mdegERS-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



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 Envisatover 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

ers-2 and envisat orbit control current and future...

Documents