GLOBAL RE-DEPLOYMENT OF THE ENVISAT ASAR TRANSPONDERS FOR

AROUND ORBIT CALIBRATION.

Goetz, Christoph1; Jackson, Harry

2; Rosich, Betlem

3; Tranfaglia, Massimo

4;

Meadows, Peter5; Canela, Miguel

6; Lorza-Pitt, Rafael

6 and Viau, Pierre

6

1RHEA/Rhea Tech Ltd., Westminster, London, W1H 0RG, United Kingdom, c.goetz@rheagroup.com.

2SERCO FM B.V., Ambachtsweg 20a, 2222 AL Katwijk, The Netherlands, hjackon@serco-fm.nl.

3European Space Agency/ESRIN, Via Galileo Galilei, Casella Postale 64, 00044 Frascati, Italy, betlem.rosich@esa.int.

4SERCO S.p.A., Via Sciadonna 24/26, 00044 Frascati, Italy, mtranfaglia@serco.it.

5BAE Sytems plc, 6 Carlton Garden, London, SW1Y 5AD, United Kingdom, peter.meadows@baesystems.com.

6European Space Agency/ESTEC, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands.

ABSTRACT

Following the commissioning phase of the Advanced

Synthetic Aperture Radar (ASAR) instrument on-board

EnviSat decision was taken to re-deploy three of the

existing four ASAR Transponders, specially procured

for this instrument, from locations within The

Netherlands to sites all over the world.

The Re-Deployment of the ASAR Transponders

describes a major activity in order to maintain the high

level of ASAR calibration. Furthermore, for the first

time, it allows ESA to perform around orbit calibration

of the EnviSat ASAR instrument.

We present the initial and final Deployment of all four

EnviSat ASAR Transponders and explain the objectives

and the strategy together with the challenges faced

installing the ASAR Transponder instruments at very

remote locations including Indonesia and the Arctic.

The excellent results of the latest calibration figures

show the success of this campaign.

1. INTRODUCTION

1.1. EnviSat

In March 2002, the European Space Agency (ESA)

launched EnviSat, the largest polar-orbiting Earth

Observation satellite ever built. EnviSat carries ten

sophisticated optical and radar instrument to provide

continuous observation and monitoring of the Earth’s

land, atmosphere, oceans and ice caps [1].

The EnviSat program primary objectives are to provide

continuity of the observations started with the European

Remote Satellites (ERS). EnviSat data collectively

provide a wealth of information in order to study the

system Earth, including insights into factors

contributing to climate change [1].

1.2. Advanced Synthetic Aperture Radar

The Advanced Synthetic Aperture Radar (ASAR)

Instrument, operating at C-band, is the largest single

instrument onboard EnviSat and it ensures continuity of

the image mode SAR and the wave mode AMI data of

the ERS-1/2 satellites. It features enhanced capability in

terms of coverage, range of the incidence angles,

polarisation, and modes of operation. The improvements

allow radar beam elevation steerage and the election of

different swaths, 100 or 400 km wide [1].

1.3. ASAR Transponders

ASAR is one of the most important instruments of the

ENVISAT mission, and it requires frequent external

calibration to maintain the quality of its data products

used by thousands of users worldwide. The ground

based ASAR Transponders (TR), high precision active

radar calibrators, are instrumental in the external

characterisation of the ASAR instrument. They provide

a point target of sufficient accuracy for end-to-end

radiometric calibration of ASAR [2].

2. INITIAL DEPLOYMENT

On year 2000, four ASAR Transponders were deployed

at four different locations in The Netherlands in order to

provide support to the ASAR calibration during the

EnviSat commissioning phase.

Figure 1. Initial Deployment showing the four EnviSat ASAR

Transponders (in yellow) and the four RadarSat Transponders (red). _____________________________________________________

Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

Fig. 1 illustrates the initial deployment of the four

EnviSat ASAR Transponders at the following sites in

The Netherlands:

• TR1 -> Edam

• TR2 -> Zwolle

• TR3 -> Swifterbant

• TR4 -> Aalsmeer

All opportunities on those four sites were used for

calibration until mid 2004.

3. RE-DEPLOYMENT ACTIVITY

As ASAR applications became operational, rules to

limit conflicts between Cal/Val and User requests were

established, significantly reducing the number of passes

available for calibration purposes.

Since mid 2004 the usage of the four ASAR

Transponders located in the Netherlands was limited to

ascending passes (max. 12 acquisitions/cycle) only due

to conflicts with user requests. Initially user request

were of lower priority than calibration requests but after

the end of the ASAR commissioning priorities have

changed in order to satisfy the large growing

community of ASAR users.

In order to maintain the high-level of ASAR calibration

decision was taken to re-deploy three of the four ASAR

Transponders to other locations around the world.

3.1. Objectives

To satisfy the needs of the ASAR user community and

to maintain the high level of ASAR calibration the

ASAR Transponders should be re-deployed worldwide.

An additional requirement was to take advantage of the

re-deployment to support the around orbit calibration by

distributing the targets at different latitudes.

It was therefore decided to leave one Transponder at the

original location in Edam (The Netherlands) and move

the others to sites:

i. Of very high latitude (thus offering a very high

number of opportunities)

ii. Close to the equator / Rain Forest latitude

iii. Close to one of the existing RADARSAT

Transponder (see Fig. 1 – Transponders in red)

3.2. Strategy

In order to meet the above objectives it was decided to

! Deploy TR2 at Resolute Bay, CANADA (i+iii)

! Deploy TR3 at Ottawa, CANADA (iii)

! Deploy TR4 at Balikpapan, INDONESIA (ii)

This would allow having more calibration opportunities

at different types of environment and at different

latitudes.

On top of this all four Transponders should be put into

nominal operation before end of 2006.

3.3. Challenges

One of the biggest challenges the global re-deployment

of the ASAR Transponders had to face were the harsh

environmental conditions at the new selected sites.

All four Transponders were initially designed for The

Netherlands and therefore it had to be assessed whether

the new conditions require modifications to the initial

design.

While in Indonesia extreme hot and very humid

conditions can be found. The arctic (Resolute Bay)

offers extreme cold conditions.

On top of this, the extreme temperature differences

between summer (+35˚C) and winter (-35˚C) in Ottawa

had to be taken into account.

A basic infrastructure had to be put in place at all sites

in order to allow sufficient power supply and remote

operation of the calibration site.

Furthermore the reception and storage of the calibration

site equipment had to be coordinated with the

installation and maintenance teams present at all sites

and importing and transmitting licenses had to be

obtained in due time.

3.4. Design Modifications

Due to the fact that all Transponders were initially

designed to suit the environment of The Netherlands

with temperature ranges from -10˚C to +35˚C additional

external heaters were added to the Transponder 2 and 3

in order to support the mechanical axis of the azimuth-

over-elevation Positioner system during the winter

months.

On top an extra layer of insulation was applied covering

both the Positioner system and the additional heaters in

order to keep the transponder system within the initial

design specifications.

To ensure an automatic control file transfer which

enables remote control of the ASAR Transponder

calibration site the ground segment architecture has

been re-designed and the existing communication

concept based on an analogue modem was changed to a

TCP/IP connection using a satellite internet modem

(except for TR1). The satellite internet modem allows

high data transfer from and to the ASAR Transponder

location and was easier to install than a new analogue

telephone line (which was not existing at any of the

three new sites). The analogue modems inside the

control PCs of all three Transponders was replaced by a

network card in order to allow an Internet connection.

On top of this, a web based interface has been designed

to monitor the Transponder operation, status and

planning (see Chapter 6).

3.5. History of main Activities

The following paragraph sheds light on the main

activities performed during the re-deployment of the

ASAR Transponder in 2006:

• Jan/Feb 2006: Pre-shipment Test TR4

• April 2006: Pre-shipment Test TR2 + TR3

• June/July 2006: Installation TR4 (Balikpapan)

• July 2006: Re-Test TR3

• July 2006: Thermal Insulation TR2

• August 2006: Installation TR2 (Resolute Bay)

• September 2006: Repair TR3 /

• September 2006: Thermal Insulation TR3

• Sep/Oct 2006: Re-Test #2 TR3

• Oct/Nov 2006: Installation TR3 (Ottawa)

During the Pre-shipment test of TR3 misbehaviour of

the automatic gain control of the Transponder was

detected. A more detailed analysis of the problem in

July confirmed that the Phase Locked Dielectric

Resonator Oscillator (PDRO) was broken and did not

lock into the phase anymore. After replacing the

hardware and re-testing of all functionalities TR3 was

successfully installed at Ottawa before November 2006.

Since November 2006 the four ASAR Transponder are

in nominal operation and all Re-Deployment activities

are completed.

4. FINAL TRANSPONDER DEPLOYMENT

Since end 2006 EnviSat ASAR Transponders are

available at Edam (The Netherlands) and at the new

selected locations of Balikpapan (Indonesia), Ottawa

and Resolute Bay (Canada).

Figure 2. Location of the four ASAR Transponder after the Re-

Deployment activity.

Fig.2 shows the locations of the four ASAR

Transponder after the Re-Deployment activity.

A summary of the details of the new ASAR calibration

sites is given in Tab. 1. Please note that southern

latitudes and western longitudes are indicated by a

minus sign.

Table 1. WGS-84 coordinates of the four ASAR Transponders after the

Re-Deployment activity of 2006.

Location Latitude Longitude Altitude

1 Edam 52.524550 5.049330 +42.460

2 Resolute 74.734440 -94.996660 +68.030

3 Ottawa 45.294400 -75.757950 +96.290

4 Balikpapan -1.056510 116.980190 +163.680

5. RESULTS

The following chapter presents the location of the new

sites selected for the three ASAR Transponders to be re-

deployed. The Transponder in Edam has not been

moved and remains at the same location as before.

For each site results of the commissioning are added.

5.1. Installation and Commissioning

5.1.1. TR1 - Resolute

The ASAR Transponder located at Resolute is the

highest in latitude. Located in the middle of the arctic

(at almost 75˚ North) it provides up to 51 possible

acquisitions per cycle to be used for calibration (Fig. 3).

Figure 3. ASAR Transponder location at Resolute Bay (~75˚ North)

Figure 4. ASAR Transponder set-up at Resolute Bay, Canada.

The figure above (Fig. 4) shows the protection of the

ASAR Transponder against any sized wild life (e.g.

polar bears). No fences have been used due to the fact

that they would block and build up snow during

snowstorms. The container hosts all communication and

control equipment. Internet connection is achieved via

satellite modem connection.

The ASAR Transponder at Resolute Bay is fulfilling

two of the above-mentioned objectives. Not only is the

transponder placed at very high latitude, it is also

located close to one of the existing RadarSat

Transponders.

Figure 5. EnviSat ASAR and RadarSat Transponder located at

Resolute clearly visible in radar image.

Fig. 5 shows the location of the EnviSat ASAR

Transponder and the RadarSat Transponder based in

Resolute. Both Transponders are clearly visible in the

ASAR image (left). Compared to the RadarSat

Transponder the RCS of the EnviSat ASAR

Transponder is the brighter one.

Example results as obtained during the commissioning

of the new ASAR Transponder sites at Resolute Bay are

given in Fig. 6. On the left you can see the bright RCS

of the EnviSat ASAR Transponder used for the

calibration. On the top right the ASAR signal as

recorded by the ASAR Transponder is shown while on

the bottom an azimuth over range plot retrieved from

the calibration product shows the intensity of the

returned radar signal.

Figure 6. Commissioning results of the ASAR Transponder located at

Resolute Bay, Canada.

5.1.2. TR3 - Ottawa

The following figure (Fig. 7) displays the set-up of the

ASAR Transponder at Ottawa.

Figure 7. ASAR Transponder set-up at Ottawa, Canada.

The Transponder #3 is located in the Greenbelt

surrounding Ottawa. It also satisfies the objective of

being close to one of the existing RadarSat

Transponders (see Fig. 7 left). The Transponders has

been raised some centimetres over ground in order to

protect the Transponder against snow.

The satellite dish mounted onto the side of the white

shed (clearly visible in the right picture) is used for the

satellite Internet connection.

The left of the next figure (Fig.8) shows the ASAR

image used during commissioning while the right shows

the ASAR signal as recorded by the ASAR

Transponder.

Figure 8. Commissioning results of the ASAR Transponder located at

Ottawa, Canada.

5.1.3. TR4 - Balikpapan

The ASAR Transponder #4 was the first Transponder to

be re-deployed in 2006.

This Transponder is operational since July 2006.

Not only has it been the first EnviSat ASAR

Transponder outside Europe, it is also ESA’s first

EnviSat ASAR Transponder on the southern

hemisphere.

Located at about 1˚South, the Transponder perfectly

fulfils the requirement of being close to the equator.

Figure 9. ASAR Transponder location at Balikpapan (~1˚ South).

The Transponder itself is located on top of a tower in

the middle of the Kalimantan rain forest (see Fig. 10).

Figure 10. ASAR Transponder set-up at Balikpapan, Indonesia.

Due to the fact that the Transponder is located in the

middle of the forest, the surrounding noise is very low.

The top right of Fig. 11 shows a perfect return signal of

an ASAR Transponder. The ASAR image used to

obtain the measurements is given on the left.

Figure 11. Commissioning results of the ASAR Transponder located at

Balikpapan (Indonesia).

5.2. Re-Calibration campaign

Immediately after the end of the ASAR Transponder

Re-Deployment a re-calibration campaign was

undertaken in the beginning of 2007 and all previous

RadarSat Transponder and ASAR Transponder

measurements have been re-calibrated.

The following two figures Fig. 12 and Fig. 13 show the

differences between the relative Radar Cross Section

(RCS) as derived from the ASAR Transponder

measurements. The figures clearly show the increased

number of measurements from mid 2006 on which

allowed to perform a re-calibration.

Figure 12. Relative RCS of all ASAR Transponders before the re-

calibration.

Figure 13. Relative RCS of all ASAR Transponders after the re-

calibration (note the changed y-scale).

Fig. 14 draws a distinction between the different

Transponder locations used for the re-calibration

campaign. While a different colour code is used to

indicate the new ASAR Transponder sites all

measurements taken in The Netherlands (including the

ones before the re-deployment) are indicated with the

same colour (green). Measurements taken in the period

between end 2004 and mid 2006 are of the Edam

Transponder only.

Figure 14. ASAR Transponder measurements sorted by location.

The table below gives the radiometric accuracy (mean)

and stability (standard deviation) of the relative RCS for

the four sites.

Table 2. Radiometric Accuracy and Stability of the four ASAR

Transponder sites.

Location Radiometric

Accuracy

(dB)

Radiometric

Stability

(dB)

Measure

ments

1 Edam 0.04 0.32 252

2 Resolute -0.05 0.22 30

3 Ottawa -0.15 0.67 7

4 Balikpapan -0.62 0.29 41

It shows that the radiometric accuracy and stability for

TR1 and TR2 are both very good. The radiometric

accuracy of TR4 is lower than expected. While for TR3

the stability is quite large but only a few measurements

have been made so far.

Even if the results for Balikpapan are lower than

expected the results demonstrate no around orbit

differences between the site in The Netherlands and the

one in Resolute.

6. ONLINE STATUS

During the Re-Deployment activity a web based

interface to monitor the Transponder operation, status

and planning has been developed.

The following sites allow online monitoring of the all

Transponder activity.

Status:

http://uranus.esrin.esa.int/ASAR/operation_status.shtml

Planning:

http://uranus.esrin.esa.int/ASAR/planned_operations.sht

ml

Those sites allow scientists to verify the actual

calibration success. They also provide information to

on-site maintenance teams who are supposed to witness

at least once a month the Transponder operation and

check the health of all on-site equipment.

7. CONCLUSION

The quality and calibration of images from the EnviSat

Advanced Synthetic Aperture Radar (ASAR) is very

important for the thousands of users of the data.

This paper describes the effort undertaken by ESA to

maintain the high-level of ASAR calibration without

interfering with any user requests by re-deploying the

ASAR Transponders on a global scale. It presents the

basis for the new EnviSat ASAR Transponder site

selection, the activities performed to re-deploy the

Transponders worldwide and the resulting benefits.

With the successful deployment of one ASAR

Transponder in Balikpapan, Indonesia (July 2006) ESA,

for the first time, is operating one ASAR Transponder

on the southern hemisphere.

Together with the other two Transponders successfully

deployed in Canada (Ottawa and Resolute Bay) ESA is

now in the position of operating three ASAR

Transponders outside Europe.

With the one ASAR Transponder left in The

Netherlands currently four ASAR Transponders are

operational and available for calibration purposes.

The valuable experience gained during the re-

deployment activity puts ESA in a very good position

for future SAR systems in general and the enhanced

ASAR calibration in particular.

ACKNOWLEDGEMENTS

The authors wish to thank the complete Post-Launch

support team at ESTEC, the ASAR expert teams at

ESRIN, at SERCO and at BAE Systems, the ESA

Transport Office and Site Service teams (ESTEC) and

Directorate of External Relations (HQ) and of course

the ASAR Transponder expert, Harry Jackson, for their

continuing support and commitment to the Re-

Deployment of the EnviSat ASAR Transponder.

REFERENCES

[1] EnviSat Mission Overview, ESA Homepage,

available on line:

http://www.esa.int/esaEO/SEMWYN2VQUD_index_0_

m.html

[2] Jackson, H., Sinclair, I. & Tam, S., 1999,

“ENVISAT ASAR Precision Transponders”, CEOS

SAR Workshop 2-29 October 1999, ESA-SP450EOP-

SMO/1151/MD