ACHIEVING THE ERS-2 ENVISAT INTER-SATELLITE INTERFEROMETRY TANDEM CONSTELLATION.

Transcription

1 ACHIEVING THE ERS-2 ENVISAT INTER-SATELLITE INTERFEROMETRY TANDEM CONSTELLATION M. A. Martín Serrano (1), M. A. García Matatoros (2), M. E. Engdahl (3) (1) VCS-SciSys at ESA/ESOC, Robert-Bosch-Strasse 5, Darmstadt, Germany, (2) ESA/ESOC, Robert-Bosch-Strasse 5, Darmstadt, Germany, (3) ESA/ESRIN, Via Galileo Galilei, Frascati, Italy, ABSTRACT The two ESA Polar-orbiting Earth observation satellites ERS-2 and ENVISAT are equipped with Synthetic Aperture Radar (SAR) sensors. One of the most important observation techniques of SAR systems is the Interferometric SAR (InSAR). This technique has been applied successfully in the retrieval of bio-geophysical parameters from space, for example in the detection of vertical ground movement in the millimeter-meter scale, land-cover classification and estimation of vegetation parameters. Until 2007 very few of the so called ERS-2-ENVISAT inter-satellite interferograms had been processed due to a difference in the carrier frequencies between the ERS-SAR and the ENVISAT- ASAR (Advanced SAR). It is possible to compensate for this difference (for surface scatterers) by controlling the two satellites orbits in a certain formation. This paper presents the analysis of several alternatives and the final operational implementation of the orbit control strategy for ERS-2 in order to achieve a Tandem configuration with ENVISAT that allows performing inter-satellite InSAR observations. Some results obtained from the processing of the acquired targets are also included. The various constraints imposed by the limited ground coverage and the restricted operability of an old spacecraft platform such as ERS-2 (launched in 1995) were a major challenge for the Flight Dynamics team at ESOC and the Mission Planning group at ESRIN. 1. INTRODUCTION One of the most important observation techniques of SAR systems is the Interferometric SAR (InSAR). The SAR and ASAR instruments on board ERS-2 and ENVISAT respectively offer the possibility to perform this kind of data acquisition. The InSAR technique exploits the coherent nature of SAR systems and consists in performing interferometry between two suitable SAR acquisitions over an area. The InSAR technique allows for example, to derive elevation models or to detect small movements of scatterers between the two interfering image acquisitions. The InSAR technique has been applied successfully in the retrieval of bio-geophysical parameters from space, in the detection of vertical ground movement in the millimeter-meter scale (caused for example by ground subsidence, tectonics, landslides or movement of glaciers), land-cover classification or estimation of vegetation parameters. Thanks to the configuration of the reference orbits of ERS-2 and ENVISAT with a common reference ground-track, the two sensors could in principle revisit the same areas on the Earth surface with a time delay of approximately thirty minutes. This system would have a very low temporal decorrelation with the following two main advantages:

2 1) High mapping accuracy 2) Mitigations of atmospheric artifacts Until 2007 very few of the so called ERS-2-ENVISAT inter-satellite interferograms or crossinterferograms had been made. The reason for not having exploited this technique is a difference of 35MHz in the carrier frequencies (CF) between the ERS-SAR and the ENVISAT-ASAR (Advanced SAR). There is a way to compensate for this difference in the CF (for surface scatterers) by keeping the perpendicular baseline between the two satellites close to 2.1 km, being the incidence angle of ENVISAT lower than for ERS-2 1. The perpendicular baseline is the distance from ENVISAT to the SAR line of sight of ERS-2 (see Figure 1). This baseline compensation can be achieved by acquiring a certain flight formation between the two spacecraft. The very favorable solar activity conditions since the end of 2007 enabled an extremely accurate orbit control of both satellites which crystallized into a first successful ERS-2-ENVISAT inter-satellite InSAR campaign at the end of Following the success of the first campaign two more campaigns took place at the end of 2008 and the beginning of DESIGN AND OPERATIONAL IMPLEMENTATION OF THE TANDEM CONSTELLATION In spite of the fact that every campaign was driven by specific requirements and targets, the basic concept of formation flying in order to achieve given baselines is common to the three ERS- 2/ENVISAT interferometric campaigns and will be described in this chapter Ground-track control The nominal orbit control strategy for ERS-2 and ENVISAT is based on Sun-synchronous reference orbits with a repeat pattern of 501 orbits in 35 days and with a mean local solar time of the ascending node (LTAN) of 22:30 and 22:00 respectively. The deviation of the S/C orbit from the reference orbit is measured in terms of perpendicular distance in ground-track and in deviation of the LTAN. The orbit control is achieved by executing two types of manoeuvres in order to keep the distance to the ground-track and the LTAN within a predefined dead-band of 1 km: In-plane (IP) manoeuvres to change the semi-major axis (and eccentricity) which allows to control the ground track deviation from the reference orbit at the Earth Equator. The eccentricity is controlled around a frozen eccentricity vector. Out-of-plane (OOP) manoeuvres to correct the inclination, allowing to control the deviation from the reference ground-track at high latitudes as well as the drift of the ascending node and its local time. Table 1. Reference orbits description ERS-2 ENVISAT Orbit Type Near polar, Sun-synchronous with frozen eccentricity LTDN 10:30 10:00 Cycle length 501 orbits Cycle duration 35 days (3 days sub-cycle) 1 The SAR and ASAR instruments point to the right side of the ground-track

3 Because of the strict constraints on the orbit maintenance of ENVISAT, mainly due to the level of fuel onboard, no changes on its orbit control strategy could be introduced to support the interferometry campaigns. Therefore the ENVISAT orbit was considered as reference and the control of the ERS-2 orbit was modified to provide the desired ERS-2-ENVISAT perpendicular baselines. Currently, the OOP maintenance manoeuvres on ENVISAT are determined in advance because of its own interferometric objectives and the implementation of IP manoeuvres could be scheduled without major alterations thanks to the low solar activity experienced during the campaigns. These two characteristics of the orbit control of ENVISAT facilitated considerably the implementation of the tandem concept. Figure 1. Definition of the perpendicular baseline The required perpendicular baseline of 2.1 km can be translated, in spite of differences in altitude, into a requirement in ground-track deviation between the two spacecraft. In what follows we will refer to the ground-track deviation of ERS-2 with respect to ENVISAT as E2/En gt deviation and to the ground-track deviation of ERS-2 with respect to its reference orbit as E2/E2 gt deviation. Assuming a nominal altitude for both spacecraft of km and a SAR incidence angle for ERS-2 of α = (see Figure 1), it is straightforward to translate the 2.1 km in perpendicular baseline into a value for the E2/En gt deviation on the Earth surface of approximately 2.0 km. This means that keeping a E2/En gt deviation close to 2.0 km at a given latitude leads to a perpendicular baseline for inter-satellite InSAR acquisitions at that latitude of 2.1 km over the Northern Hemisphere, with ENVISAT flying on the right hand side of ERS-2 in the flight direction. Operationally, a dead-band around the ideal value of 2 km has to be defined in order to drive the orbit control of ERS-2. This dead-band is determined by the maximum deviation with respect to the 2.1 km perpendicular baseline that still makes it possible to generate cross interferograms. Even though in principle this deviation could go up to 400 m, when the deviation is greater than 200 m the success in the processing of the data pairs depends strongly on the type of terrain to be monitored. The control dead-band for the E2/En gt deviation was therefore set to 400 m (trying to achieve when possible a control of 200 m) It is also important to notice that because of the asymmetry of the problem it is not possible to keep the required E2/En gt deviation on both arcs of the ERS-2 orbit. Consequently, whenever a latitude is requested to perform InSAR (ERS-2 / ERS-2) or inter-satellite InSAR (ERS-2 / ENVISAT) only

4 the ascending or the descending arc of the orbit will give the required baseline to allow the acquisition ERS-2 orbit control limitations The change of the orbit control of ERS-2 had to account for several limitations of an old platform. After a gyro failure in January 2001 the Attitude and Orbit Control System cannot provide accurate pointing when performing OOP manoeuvres. As a result of this attitude degradation, large parasitic in-plane components are observed after the execution of OOP manoeuvres. A dedicated procedure must be followed when executing OOP manoeuvres on ERS-2, based on the implementation of large pre-emptive IP manoeuvres to compensate for the expected in-plane contributions of the OOP manoeuvre. This procedure implies some operational risk; therefore the number of OOP manoeuvres executed on ERS-2 to support the inter-satellite InSAR campaigns had to be as reduced as possible. As a direct consequence no OOP manoeuvres to reduce the inclination of ERS-2 were desired. In addition to the operational risk, the opportunities to execute OOP manoeuvres on ERS-2 are constrained by the dazzling of the Earth digital sensor, resulting roughly in one opportunity per month always around New Moon. Finally, the requirements on the orbit control of ERS-2 that assure the success of the ERS-2/ERS-2 InSAR acquisitions (between two observations acquired at different repetition cycles) had to be maintained to the maximum extent possible. The manoeuvres implemented to support the intersatellite InSAR had to be optimized to guarantee a minimum impact on the ERS-2 nominal mission and enabling at the same time, a safe and fast recovery of the nominal mission The role of the Sun The two main perturbations originated by the Sun played an important role in the definition of the Tandem constellation: As a perturbing mass, the Sun causes a drift in the inclination of the orbital plane that depends on the distance of the satellite to the Sun, which varies along the year due to the eccentricity of the Earth orbit around the Sun. This seasonal dependence of the drift rate was used to achieve the return to the nominal ERS-2 mission at different stages during the inter-satellite campaigns, avoiding implementation of OOP to reduce the inclination of ERS-2. The perturbation due to the Sun affects differently the two spacecrafts in the Tandem because they have different LTAN. Consequently it is not possible to establish a constant E2/En gt deviation at maximum latitude during a whole campaign. As a source of radiation, changes in the activity of the Sun have a direct influence in the air drag force experienced by LEO satellites. The prediction of LEO trajectories is typically affected by a relatively high inaccuracy due to the uncertainty in the estimation of the atmospheric density, which is directly related to the solar activity. Bad estimations of the atmospheric density imply bad predictions of the air drag force and ultimately a degradation in the orbit predictions. In general, the prediction of the solar activity is not reliable, especially in the long term. The historic low solar activity registered at the time the campaigns took place without major unexpected storms, was a major advantage enabling a quite accurate control of the orbital periods, and therefore an unusually reliable control of the ground-track deviation at low latitudes.

5 3. FIRST CAMPAIGN. SEPTEMBER 2007 FEBRUARY 2008 The observation targets for the first inter-satellite InSAR campaign were all located in the Northern Hemisphere, aiming to provide observations in the frame of the Polar Year The duration of this campaign was originally four 35-days cycles. The targets for the ERS-2 / ENVISAT InSAR were located at high latitudes between 59 and 74 deg N, whereas the more relevant targets for the ERS-2 nominal mission interferometry were located at mid-latitudes (around 40 deg N) and close to the Equator. The orbit control proposed to acquire to the maximum extend possible these targets consisted of: 1. OOP manoeuvres in order to achieve a E2/En gt deviation of 2 km at the North Pole of the orbit (latitude close to 81.5 deg N) 2. IP manoeuvres with double objective: a. Control of the E2/E2 gt deviation. i. During the first half of the campaign (until 2007/12/05) the IP control aimed to enable the ERS-2/ERS-2 interferometry with respect to acquisitions previous to the inter-satellite campaign at least in areas close to the equator. ii. During the second half, the IP control aimed to perform ERS-2/ERS-2 interferometry using acquisitions made after the beginning of the campaign at latitudes close to 40 deg N. Therefore the target was set to repeat to the maximum extend possible, the ground track evolution at 40 deg N with respect to the previous 35-days cycle. b. Control of the E2/En gt deviation. The IP control aimed to keep the E2/En gt deviation close to 2 km at high latitudes in the descending part of the ERS-2 orbit during the first half of the campaign ERS-2 / ENVISAT targets A E2/En gt deviation at maximum latitude (or north pole of the orbit) of 2 km is equivalent to a shift of deg in inclination of the orbital plane of ERS-2 with respect to the ENVISAT orbital plane. A sequence of OOP manoeuvres for ERS-2 was prepared aiming to keep an inclination shift between the two satellites orbits close to deg. Since ERS-2 was chasing ENVISAT, the execution of any nominal maintenance OOP on ENVISAT immediately implied the execution of an OOP manoeuvre on ERS-2 at the next available manoeuvering window. The inclination changes on ENVISAT during the campaign were fixed and known in advanced (see Table 2). Table 2. ENVISAT OOP manoeuvres scheduled during the first campaign ENVISAT inclination manoeuvres Epoch Size 2007/09/ m/s 2007/12/ m/s 2008/02/ m/s (End of the campaign) At least two OOP manoeuvres were needed on ERS-2 in order to achieve the inclination shift during the whole campaign; the first one to acquire the inclination shift and with it the desired deviation in ground-track, and a second one around the 2007/12/04 to keep the inclination shift after the ENVISAT maintenance OOP. In the end three OOP manoeuvres had to be performed on ERS-2, due to the large size of the first one. The final sequence of OOP implemented on ERS-2 is shown in Table 3.

6 Table 3. ERS-2 OOP manoeuvres executed during the first campaign ERS-2 inclination manoeuvres Epoch Size 2007/09/ m/s 2007/09/ m/s 2007/12/ m/s The first OOP manoeuvre for ERS-2 was split up into two manoeuvres taking into account: The execution of the manoeuvres had to made use of the complete window around the new moon (2007/09/11) and had to take into account the necessary margin to implement preemptive IP manoeuvres before each OOP. The size of the first manoeuvre was chosen in a way that in case of cancellation of the second one, no violation of the ERS-2 nominal orbit control occurred. The time interval between the execution of the two OOP manoeuvres was long enough to carry out a calibration of the first OOP manoeuvre and its parasitic IP component, prepare the pre-emptive IP manoeuvre for the second OOP manoeuvre and adjust the size to correct for a potential underperformance of the first OOP. The beginning of the first ERS-2 / ENVISAT inter-satellite InSAR campaign was the inclination manoeuvre performed on ENVISAT on the 2007/09/25, when the E2/En gt deviation at high latitudes reached the required value of 2.0 km. Figure 2 shows the E2/En gt deviation at maximum latitude. As it has been mentioned before, this deviation does not remain constant because the perturbation of the inclination due to the Sun is different for each spacecraft. An over-performance of the OOP manoeuvre on ERS-2 in December led to a not completely optimal evolution of the E2/En gt deviation from that point onwards. Figure 2. Ground-track deviation at the maximum latitude of ERS-2 orbit wrt the ENVISAT ground-track At high latitudes in the Northern hemisphere and during the first half of the campaign, the main priority was the inter-satellite InSAR. A simplified expression of the ground-track deviation, GTdev, as a function of the latitude ϕ is given in Eq.1:

7 GTdev ( ϕ) = GTdev( Equator)cos( ϕ) + GTdev(maxlatitude)sin( ϕ) (Eq. 1) Assuming a E2/En gt deviation at maximum latitude of [-1.9, -2.1] km one can conclude that a control of the node crossings at the descending part of the orbit in a dead-band of ±0.2 km centred at -0.6 km leads to a E2/En gt deviation at latitudes close to 60 deg N descending evolving close to the required 2 km. As it has been remarked before, obtaining a 2 km E2/En gt deviation at the required high latitudes was only possible at either the ascending or the descending part of the orbit. The evolution of the E2/En gt deviation is shown in Figure ERS-2 / ERS-2 targets The interferometric acquisition targets for the nominal ERS-2 mission were a lower priority during the first half of the inter-satellite campaign. Nevertheless, during this period the equator crossings at both ascending and descending arcs were controlled inside the nominal 1 km dead-band around the reference orbit (Figure 4) Figure 3. E2/En Ground-track deviation at deg latitude, descending arc. During the second half of the campaign the ERS-2/ERS-2 the targets at mid-latitudes were of larger interest. Due to the offset in inclination with respect to its nominal reference orbit, it was not possible to use acquisitions made before the beginning of the campaign together with acquisitions made within the campaign. Therefore the IP control aimed to reproduce as close as possible (within 400 m at least) the ground track evolution at 40 deg N that had been visited during the previous 35- days repetition cycle (Figure 5). A procedure was developed in order to achieve this IP control consisting on: 1. Shifting the last 35 days of the operational orbit to the future. 2. Generating a reference ground-track based on the previous 35 days shifted orbit. 3. Optimizing the corresponding manoeuvres sequence to keep the E2/E2 gt deviation within a dead-band of 400 m.

8 Figure 4. E2/E2 gt deviation during the 1st half of the campaign Figure 5. ERS-2 gt deviation with respect to previous 35-days cycles during the second half of the campaign 3.3. Nominal ERS-2 mission recovery The execution of the OOP manoeuvre on ENVISAT on the 2007/02/12 was the end of the first inter-satellite campaign. The control of the ERS-2 mission continued a strategy aiming to provide

9 good interferometry baselines at mid-latitudes with respect to the previous 35-days cycles until the end of the fourth 35-days cycle (2008/03/20). At that time, the natural inclination drift of the orbital plane of ERS-2 had led to a decrease in the inclination offset down to a nominal control value, which translates into the E2/E2 gt deviation at maximum latitude being within a dead-band of 1 km. An IP manoeuvre was then optimized in order to start a nominal control cycle at the Earth Equator, resuming this way the nominal control of the ERS-2 mission. Figure 6. Recovery of the ERS-2 nominal mission 4. SECOND CAMPAIGN. NOVEMBER 2008 JANUARY 2009 The observation targets for the second inter-satellite campaign were located at latitudes between 50 and 70 deg on the Northern Hemisphere. The targets for the ERS-2 / ERS-2 interferometry were given lower priority. The orbit control proposed for ERS-2 in order to acquire these targets consisted of: 1. OOP manoeuvres in order to achieve a E2/En gt deviation close to 2 km at the north pole of the orbit (latitude close to 81.5 deg N). 2. IP manoeuvres in order to achieve a E2/En gt deviation close to 2 km at the requested range of latitudes, [50,70] deg N ERS-2 / ENVISAT targets The inclination changes on ENVISAT during the second campaign were fixed and known in advanced (see Table 4)

10 Table 4. Envisat inclination manoeuvres during the second campaign ENVISAT inclination manoeuvres Epoch Size 2008/11/ m/s 2009/01/ m/s In principle a E2/Ev gt deviation at maximum latitude of 2 km was not required for this campaign since the latitudes that had to be observed were not reaching the highest latitude. However, in order to reduce the impact on the ERS-2 nominal node crossing control, the target of the OOP control was to achieve an inclination shift equivalent to a 2 km E2/En gt deviation at maximum latitude. Due to the large size of such an OOP manoeuvre it was split up into two manoeuvres at two consecutive windows of the digital sensors. The size of each OOP was chosen in a way that in case of cancellation of the inter-satellite campaign after the execution of the first OOP, no inclination correction would have been necessary in order to resume the nominal ERS-2 mission. Table 5. ERS-2 inclination manoeuvres to support the second campaign ERS-2 inclination manoeuvres Epoch Size 2008/10/ m/s 2008/11/ m/s Following the approximation given by Eq.1, a control of the E2/En gt deviation at the descending node around a value of -0.5 km in a dead-band of ±0.2 km led to a E2/En gt deviation close enough to 2 km at the requested latitudes on the Northern Hemisphere (see Figure 7) Figure 7. ERS-2 / Envisat ground-track deviation at several latitudes 4.2. ERS-2 / ERS-2 targets The ERS-2 / ERS-2 acquisitions were not a constraint to be taken into account during the second campaign. Nevertheless, the strategy selected aimed to keep the E2/E2 gt deviation at the node crossings inside the nominal 1km dead-band.

11 4.3. Nominal ERS-2 mission recovery During the course of the second campaign the possibility of carrying out a third campaign was analyzed and finally approved. Consequently no recovery of the ERS-2 nominal mission was performed at the end of the second campaign. Otherwise the inclination would have drifted naturally back to the nominal control dead-band by mid-march. Until this recovery, the node crossings would have been controlled to perform acquisitions at mid-latitudes in the Northern hemisphere. 5. THIRD CAMPAIGN. JANUARY 2009 APRIL 2009 During the execution of the second campaign the interest to extend the campaign, possibly covering targets at mid-latitudes materialized. The ENVISAT OOP on the 2009/01/27 was the end of the second campaign and a continuation required the implementation of another OOP manoeuvre on ERS-2. The duration of a third campaign was given by the next OOP scheduled for ENVISAT on the 2009/04/ ERS-2 / ENVSAT targets Figure 8. E2/En gt deviation during the third campaign The optimization of the inclination change for ERS-2 in order to start the third inter-satellite campaign was a trade-off between the acquisition of the requested targets and the recovery of the nominal ERS-2 mission within a reasonable time frame. An OOP manoeuvre for ERS-2 to achieve an E2/En gt deviation at maximum latitude of 2 km would have implied a return of the E2/E2 gt deviation to its nominal dead-band in 2009/08, whereas the end of the inter-satellite campaign was given by the ENVISAT OOP on the 2009/04/07. Consequently between April and August ERS-2 would have followed neither its nominal mission nor the inter-satellite constellation (unless an OOP had been performed on ERS-2 to reduce the inclination, which was to be avoided as mentioned in section 2).

12 On the other hand, a direct consequence of not targeting a sufficient inclination shift between the two satellites was the stronger dependency on the control at the node crossings in order to achieve a 2 km E2/En gt deviation at the requested latitudes. The final compromise was to execute an OOP on ERS-2 that lead to a return of the E2/E2 gt deviation at maximum latitude to its nominal 1 km dead-band at the end of the third inter-satellite campaign (see Figure 9). This OOP translated into an E2/En gt deviation at maximum latitude of 1.7 km evolving to 1.9 km at the end of the 70 days campaign. The major interest was set on mid-latitudes on the Northern Hemisphere. This was the driving factor to determine the IP control of ERS-2 during the third campaign. According to Eq.1, a control of the E2/Env gt deviation at the descending Equator crossings around values varying from 1.3 to 1.1 km during the 70 days campaign translates into a E2/En gt deviation at 40 deg N (descending arc) close to 2.0 km. The objective of the IP control of ERS-2 consisted then on keeping the E2/En gt deviation at 0 deg descending around an adjusted target between [1.1, 1.3] km, accounting at the same time for the effects of the ENVISAT IP executed during the campaign (see Figure 8 above): A first IP aimed to start a westwards drift at the descending node crossings. A second IP was executed when the desired value of the E2/Env gt deviation close to 1.3 km. The objective of this IP was to reverse the drift and start a short cycle for the E2/En gt deviation at 0 deg descending around 1.3 km. A third IP aimed to control the E2/en gt deviation at 0 deg descending around a value of 1.1 km (at that time the E2/en gt deviation at maximum latitude was close to 1.8 km accounting for the impact of an ENVISAT IP executed some days before). The last IP lead to a final control of the E2/en gt deviation at 0 deg descending around -1.0 km until the end of the campaign, when the deviation at maximum latitude was close to 1.9 km. This control of the descending node crossings resulted on values of the E2/En gt deviation close to 2 km at a range of latitudes between 30 and 60 deg N (40 deg N was taken as reference latitude). Figure 9. Evolution of the E2/E2 gt deviation at max. latitude

13 5.2. ERS-2 nominal mission recovery The end of the third inter-satellite InSAR campaign was the OOP executed on ENVISAT on the 2009/04/07. By that time, the deviation of the ERS-2 inclination with respect to its reference was within the nominal dead-band (see Figure 9) as well as the ground-track deviation at the Equator. The nominal ERS-2 mission could be then resumed by starting a ground-track control cycle on the 2009/04/ DATA ACQUIRED DURING THE CAMPAIGNS A large amount of ERS-ENVISAT Tandem InSAR data was acquired during the three campaigns. Table 6 lists the number of segments acquired during the campaigns (a segment refers to a SAR data-take and in this case a Tandem data-take). Table 6. Number of acquired segments during the three campaigns Number of segments acquired Campaign Number of acquired segments 1st nd rd 3827 Total Figure 10 illustrates the location of the acquired segments for the 3 rd Tandem campaign as an example. Figure 10. Location of the segments acquired during the 3 rd Tandem campaign

14 7. RESEARCH ON THE ERS-ENVISAT TANDEM INSAR DATASETS Research into the Tandem InSAR datasets has already begun; the research topics include highresolution (horizontal & vertical) DEM generation over relatively flat areas, motion of land-ice, seaice studies, retrieval of vegetation parameters and studies of deserts. Figure 11 shows an example of a high-resolution (sub-metric in the vertical direction) DEM generated from ERS-ENVISAT Tandem generated at Wadden-Sea, East Frisia. Figure 11. A DEM with sub-metric vertical resolution at Wadden-Sea, East Frisia. Generated from ERS- ENVISAT Tandem InSAR data by Gamma Remote Sensing AG. 8. CONCLUSION The objectives of the three ERS-2 / Envisat inter-satellite interferometry campaigns were achieved, providing a large set of acquired segments (more than 12000) to the scientific community. The approach taken and described in the present paper was successful exploiting the capabilities of an old spacecraft platform such as ERS-2 (launched in 1995 and with several hardware failures) in combination with Envisat, thus finding a new set of applications for ERS-2 data. BACK TO SESSION DETAILED CONTENTS BACK TO HIGHER LEVEL CONTENTS