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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
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