MAPSAR: a small L-band SAR mission for land ... - OBT - Inpe

Acta Astronautica 56 (2005) 35 – 43
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MAPSAR: a small L-band SAR mission for land observation
Reinhard Schröder a,∗ , Jürgen Puls a , Irena Hajnsek a , Fritz Jochim a , Thomas Neff a ,
Janio Kono b , Waldir Renato Paradella b , Mario Marcos Quintino da Silva b ,
Dalton de Morisson Valeriano b , Maycira Pereira Farias Costa b
a German Aerospace Center, DLR, P.O. Box 1116, D-82230 Weßling, Germany
b NationalInstitute for Space Research, INPE, Av. dos Astronautas 1758, Caixa Postal515, 12201-010 São José dos Campos, SP-Brazil
Available online 14 October 2004
Abstract
This paper introduces Multi-Application Purpose SAR (MAPSAR). A new Synthetic Aperture Radar (SAR) mission for
earth observation. MAPSAR is the result of a joint pre-phase A study conducted by INPE and DLR targeting a mission
for assessment, management and monitoring of natural resources. The applicability of the sensor system was investigated
for cartography, forestry, geology, geomorphology, hydrology, agriculture, disaster management, oceanography, urban studies
and security. An L-band SAR, based on INPE’s multi-mission platform (MMP), has been chosen as payload of the satellite.
The key component of the SAR instrument is the SAR antenna, which is designed as an elliptical parabolic reflector antenna.
L-band (high spatial resolution, quad-pol) has been selected for the SAR sensor as optimum frequency accounting for the
majority of Brazilian and German user requirements. At the moment, the pre-phase A has been concluded and the phase A
is planned to start in early 2003.
© 2004 Elsevier Ltd. All rights reserved.
1. Introduction
The ability of a SAR to provide high-resolution imagery
independent of weather or sun light is particularly
important for regions of the planet that present
restrictions to the use of optical sensors due to the presence
of rain, perennial clouds, haze and smoke (e.g.,
the Amazon region) or where solar illumination is
∗ Corresponding author. Tel.: +49 8153 28 2310;
fax: +49 8153 28 1135.
E-mailaddresses: reinhard.schroeder@dlr.de (R. Schröder),
kono@dss.inpe.br (J. Kono).
insufficient (Polar regions). Beside this, imaging
radars also have the ability to penetrate a vegetation
canopy, which is not possible using optical sensors.
In general, the acquisition of information in tropical
regions, particularly in rain forests is complicated due
to the large continental extension, poor accessibility,
and complex nature of the environment. In particular,
Brazil has a vast territory with a tremendous need
for resource management information such as can be
provided by satellite imaging radars. Many obstacles
have been faced for the integration of the Amazon region
to the rest of the country. This integration is necessary
nowadays more than ever with respect to the
0094-5765/$ - see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.actaastro.2004.09.007

36 R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43
increasing Brazilian population and the immense natural
(renewable and non-renewable) resources available
in the region. SAR systems have not only the capability
to image but also to measure bio-/geophysical environmental
parameters of such a “complex environment”
in a systematic and repetitive manner in support of
natural resource assessment and the monitoring processes
that are essential parts of procedures for sustainable
environmental management.
In the range of wavelengths used by imaging
radar, backscatter intensity from terrain surfaces is
strongly controlled by decimetre-scale changes in surface
slopes, or by centimetre-scale roughness of the
surface. Thus, the sensitivity of SAR to the macrotopography
(slopes, tilts, broad undulations in the
terrain), the surface roughness (roughness on the scale
of the SAR wavelength) and to the presence of moisture
or liquid water content of a medium affecting the
complex dielectric constant, makes it an ideal instrument
for thematic mapping and resource management
purposes in the tropical region, and when stereoscopic
capabilities are available, for topographic mapping.
In addition, the large ratio of SAR return from land
versus water is primarily a function of contrast of
dielectric properties and surface roughness, providing
a striking interface which also facilitates coastal
delineation and related mapping application (flooding,
wetland and hydrological studies in the Brazilian
Amazon).
On the other hand, forest biomass is today one of
the most unknown parameters in dynamic ecosystem
change especially with respect to a reliable global
Carbon-flux modelling. In many regions of our planet
even a mere forest classification is missed. Parallel to
the ecological dimension, and under the light of the
Kyoto protocol, this problem has also a political dimension
that increases the responsibility of the scientific
community to provide exact answers. The fact that
biomass information is especially missed in remote
areas (i.e. tropical and boreal forest ecosystems) combined
with the fact that these ecosystems contain the
biggest amount of forest biomass make remote sensing
techniques a challenge. However, optical remote
sensing sensors are in general not capable of measuring
forest biomass or monitoring the dynamics of deforestation
and biomass regeneration. An alternative
methodology is the estimation of forest biomass based
on the estimation of forest height from radar remote
sensing data. One of the key challenges facing imaging
radars is to force evolution from high-resolution
qualitative imaging to accurate high-resolution quantitative
measurement. One very promising way to extend
the observation space is the combination of interferometric
and polarimetric observations. SAR interferometry
is today an established technique for the
estimation of the height location of scatterers through
the phase difference in images acquired from spatially
separated locations at either end of a baseline.
The initiative of the joint study of a small
spaceborne SAR is a consequence of a long-term
Brazilian–German scientific and technical cooperation
that was initiated between INPE and DLR in the
1970s. The decision to perform a pre-phase A study
for MAPSAR was established in 2001 following
several meetings in both agencies ([1,2]). Based on
the specific and complementary experience of both
partners, the sharing of the thematic responsibilities
within the study was agreed. Brazil is responsible
for the platform and integrated satellite analysis and
Germany for the payload and orbit analysis.
2. User requirements
The MAPSAR mission is tailored to optimally support
the potential user groups in both countries taking
into account distinct aspects of specific applications.
Thus, in April 2002, a working group of Brazilian
SAR scientists and technical experts (potential SAR
end-users) met on the campus of INPE (São José dos
Campos) to perform a review of the use of imaging
radar to derive surface characteristics important for
tropical applications. The first workshop of potential
end-users of MAPSAR was structured for the purpose
of (1) performing an assessment of the current understanding
of the applications of SAR in Brazil and (2)
developing end-user requirements and recommendations
aiming at a joint INPE–DLR small spaceborne
SAR program.
The consensus of the Brazilian working group
was that a spaceborne SAR mission will provide a
powerful new tool to acquire data and to derive important
and unique information of vegetated terrain
of the Amazon Region. Due to the enormous scarcity
of up-to-date information, which is fundamental
for planning and strategic decision-making about

R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43 37
environmental assessment, management and monitoring
of natural resources in the Brazilian Amazon, the
proposed small spaceborne SAR initiative should be
strongly oriented to a quasi-operational (“applicationoriented”)
system. This is dedicated to thematic
mapping purposes for topography, vegetation and
deforestation, geology, hydrology, etc. Furthermore,
it was strongly recommended to have a series of
satellites aiming at the operational aspects and the
program continuity. In addition, the MAPSAR concept
should also address aspects of integration with
the Brazilian “Surveillance of the Amazon System”
(SIVAM/SIPAM), due to the complementary nature of
both sources (spaceborne/airborne) of multi-polarized
and fully polarimetric L-band data. Finally, although
recognised in most application fields, the effects of
polarimetry and interferometry on the image information
content are not well understood in some of those
fields.
The Brazilian applications are of high interest for
the German potential user community. Additional applications,
which are of specific importance for the
German user side, as biomass estimation, disaster
monitoring and security complement the Brazilian
disciplines. The common aim is a global biomass
mapping mission covering major forests biomes of
the globe (tropical and boreal regions). This requires
the capability of polarimetric SAR interferometry for
forest height estimation which is directly related to
forest biomass using allometry. The Brazilian and the
German requirements have been merged resulting in
the final user requirements for MAPSAR presented
in Table 1(a) and (b).
Due to several restrictions related to INPE’s multimission
platform (mass, power generation, geometric
envelope and data rate) the following main limitations
were imposed upon the satellite configuration: use of
a single frequency and use of a light weight antenna.
The resulting antenna concept limits the maximum
instantaneous swath width to approximately 55 km.
Table 2 summarizes the final specification.
3. Payload configuration
Based on these user requirements and platform limitations
the design of the L-band SAR sensor is based
on a reflector antenna concept. The main advantage
is the possibility to realize full polarization and high
bandwidth with low technological risk at low costs.
The main drawbacks are the limited instantaneous
swath width and the lack of an electronic beam steering
capability.
3.1. SAR antenna
A key problem for a L-band SAR is the large antenna
size required compared to higher earth observation
frequencies. Fig. 1 shows an approximation for
the minimum antenna size required to access various
incidence angles at different orbit heights taking into
account a sufficient ambiguity suppression of approximately
−20 dB.
It is obvious that a low orbit height reduces the
antenna size requirements significantly. On the other
hand, orbit heights below 500 km can become critical
with respect to maintainability and drag compensation.
To achieve a maximum incidence angle of 45 ◦
an antenna aperture of at least 27 m 2 is necessary at
600 km orbit height. Lower orbit heights would allow
to reduce the antenna aperture, higher orbit heights
would require to increase it. A reduction of the antenna
dimensions as far as possible is mandatory taking
into account the launcher and platform limitations.
Therefore, an orbit height as low as possible is advantageous
for the SAR sensor design. Considering
the requirements of the Brazilian multi mission platform
it was agreed between the partners to concentrate
the investigations on orbits with a height between 600
and 650 km. Therefore, the basis for the SAR sensor
design was chosen to be at a height over ground of
620 km. Variations of ±30 km do not affect the design
significantly.
The final choice of the antenna size is a trade-off between
various parameters. We considered a Cassegrain
configuration with an elliptical parabolic main reflector
with 7.5 m length in azimuth and 5 m width in elevation.
Neither the subreflector mounting nor the horntype
prime radiator need to be folded. The resulting
antenna gain in combination with a low orbit allows
very good sensitivity with a reasonable power budget.
3.2. Access region of the SAR sensor
The antenna configuration and the restrictions imposed
by a sufficient ambiguity suppression in azimuth

38 R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43
Table 1
MAPSAR user requirements
(a)
Application/MAPSAR parameters Agriculture Cartography Disaster monitoring Forestry
Frequency LLC L ∗ ,C
L C L L
Polarization/polarimetry quad. pol N.E. VV, HH quad. pol.
quad. polN.E. quad. pol quad. pol.
Incidence interval Variable 25–45 ◦ variable (45 ◦∗ ) 20 ◦ –30 ◦ /45 ◦ –60 ◦ 20 ◦ –45 ◦
45 ◦∗ variable 20 ◦ –45 ◦
Spatial resolution 30 m 5 m 30–50/15 m 10 m
3–5 m 3–5 m 3–5 m 10 m
Swath 30 km N.A. 150–350 km 100 km
30 km variable variable variable
Orbit inclination N.A. sun-syn sun-syn sun-syn
sun-syn sun-syn sun-syn sun-syn
Look direction N.A. asc/desc asc/desc N.A.
asc/desc asc/desc asc/desc asc/desc
Revisit 15 days N.A. < 1 day monthly

R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43 39
Table 2
MAPSAR final specification taking into account user requirements
and MMP constraints
Off-nadir angle
Frequency
Polarization
Incidence interval
Spatial resolution
Swath
Orbit inclination
Coverage
Look direction
Revisit
Data access
Additional requirement
L-band
Single, dual and quad
20 ◦ –45 ◦
3–20 m
20–55 km
Sun-synchronous
Global
Ascending/descending
Weekly
Near real time
Stereoscopy and interferometry
620 km
Incidence angle
20˚
48,1˚
Nadir
204 km
395 km
Fig. 3. Access region of the SAR sensor.
Nadir Zone
20 30 40 50 60
30
Orbit height= 500 km
Orbit height= 600 km
Orbit height= 700 km
L-Band (f=1.3 GHz)
5
Ambiguities appr. -20 dB
0
0 10 20 30 40 50 60
Incidence angle / deg
Fig. 1. Minimum antenna size in L-band.
25
20
15
10
Minimum antenna area /m 2
between 20 ◦ and 48.1 ◦ incidence angle covering an
area of 395 km on both sides of the satellite nadir track.
In full polarization mode the PRF must be doubled,
leading to a useful range between 5200 and 6000 Hz.
The corresponding field of view is between 20 ◦ and
39.3 ◦ incidence angle, covering an area of 245 km on
both sides of the satellite nadir track.
Fig. 3 illustrates the whole access region. Since the
sensor has no electronic beam steering the current
swath is selected by rolling the whole satellite in the
necessary position. The same is done to change between
left or right looking configuration. This results
in certain restrictions to image regions within the same
orbit which are close to each other in flight direction
but separated in elevation by more than one instantaneous
swath width.
3.3. MAPSAR radar modes
Offnadir angle/ deg
60
50
40
30
20
transmit interference
nadir interference
PRF-RESTRICTIONS
10
2200 2400 2600 2800 3000 3200
PRF / Hz
Fig. 2. PRF bands with interference.
DLR
Based on the user requirements, MAPSAR offers
three different resolution modes (3, 10, and 20 m) operated
in the conventional stripmap mode. The chirp
bandwidth is designed to operate in the full 85 MHz allowed
by the World Administrative Radio Conference
(WARC). This enables a high-resolution (HR) mode
in the order of 3 m without increasing the complexity
of the front-end and the antenna. For lower resolution
modes the bandwidth is reduced accordingly. A single
polarization mode (SPM), a dual polarization mode
(DPM) and a quad polarization mode (QPM) are foreseen
for all of these resolution modes. Table 3 lists the
sensor and performance parameters for some of these
modes.
As a baseline a RF peak power of 1000 W was
considered. Depending on the priority of the different

40 R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43
Table 3
MAPSAR radar modes
Parameter HR mode MR mode LR mode
High resolution Medium resolution Low resolution
SPM DPM QPM
Access region near far near far near far
Resolution
Range/m 4,7 3,1 10 10 20 20
Azimuth/m 3,1 3,1 10 10 20 20
Off-nadir angle/ ◦ 20,0 41,8 20,0 41,8 20,0 32,1
Incidence angle/ ◦ 20,3 47,6 20,0 48,1 20,0 36,8
Ground swath width/km 38,3 20,5 45,1 35 43,4 28
PRF/Hz 2650 2718 2660 2680 5320 5320
Pulse bandwidth/MHz 85 85 42,5 21,25 21,25 21,25
Sampling frequency/MHz 96 96 2 × 48 2 × 24 2 × 24 2 × 24
Pulse length/μs 25 25 10 10 10 10
Data rate/Mbit/s 262 300 267 300 247 249
RF peak power/W 1000 1000 1000 1000 1000 1000
Average DC power/W ∼ 490 ∼ 500 ∼ 440 ∼ 440 ∼ 440 ∼ 440
NESZ/dB
Image center −23,6 −19 −28,9 −25,4 −34,6 −33,3
Image edge −17,0 −18,3 −19,9 −23,3 −26,6 −30,9
Number of looks
Range 1 1,33 1,14 1,13 1,04 1,73
Azimuth 1 1 3,3 3,3 6,6 6,6
Radiometric res./dB

R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43 41
Table 4
Radar beam specification
Beam number/ Incidence angle Swath width/ Eff. PRF/
km range/ ◦ km Hz
1 19,93–23,95 45,1 2660
2 23,76–27,32 41,9 3010
3 27,12–30,71 44,5 2660
4 29,69–33,31 46,9 2660
5 32,86–36,52 50,4 2910
6 36,15–39,86 54,8 2660
7 39,50–42,69 50,0 2900
8 42,22–45,24 51,0 2640
9 45,16–46,92 31,6 2870
10 46,28–48,08 33,5 2680
Fig. 6. Average revisit time over Brazil.
Fig. 5. Maximum revisit time over Brazil.
The actual concept of the MAPSAR mission favours
an orbit with a repeat cycle of 37 days and an altitude
of about 620 km. This orbit has an inclination of
about 97.86 ◦ . The sensor field of view for this orbit is
between 20 ◦ and 48.1 ◦ to fulfil the specifications of
the users. Fig. 5 shows the maximum revisit times for
Brazil. The highest value is 132 h, but most regions of
Brazil will be reached in less than 100 h.
In Fig. 6, the average value for the revisit times can
be seen. The maximum value over the northern part of
Brazil is 42.3 h, but most regions have average revisit
times far less than 40 h.
Because of the fact that the latitude of the German
regions ranges between 47 ◦ and 55 ◦ , the revisit times
are quite lower than for some Brazilian regions. In
detail, the maximum values can be seen in Fig. 7, the
average values for Germany are shown in Fig. 8.
Fig. 7. Maximum revisit time over Germany.
Fig. 8. Average revisit time over Germany.

42 R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43
Fig. 9. MAPSAR satellite configuration.
Fig. 11. MMP pictorial view (blow-up).
Table 5
MAPSAR payload nominal mass budget
Sub-system
Mass (kg)
4. MAPSAR satellite
Fig. 10. MMP block diagram.
Mechanical structure 30.0
Thermal control 4.0
Radar sensor & data acquisition 193.6
Data storage & transmitter 42.5
Total mass 270.1
Specified mass 280.0
Nominal margin 9.9
4.1. Satellite concept
The MAPSAR satellite utilizes a modular concept
(Fig. 9), consisting of a payload module and the Brazilian
Multi-Mission Platform (MMP).
The MMP concept provides for a capability to support
a variety of low earth orbit missions using the
same basic three-axis stabilized platform with different
payload modules. The MMP block diagram and
pictorial view are shown in Figs. 10 and 11. The MMP
is already being developed by INPE and its first flight
model shall be ready by the end of 2005.
Thermal Control
1%
Structure
11%
MAPSAR
Nominal Mass Budget
Radar Sensor
69%
Margin
4%
Data Storage
15%
4.2. Mass budget
Table 5 presents a summary of the nominal mass
budget for the MAPSAR payload.
The total mass for the MAPSAR payload is
270.1 kg. Compared to the specified value (280.0 kg),
this leads to a margin of 9.9 kg. This shows that, in
Fig. 12. MAPSAR payload mass breakdown.
terms of mass, the proposed configuration is feasible.
Fig. 12 shows the payload mass breakdown in a pie
diagram.

R. Schröder et al. / Acta Astronautica 56 (2005) 35 – 43 43
The ground segment for MAPSAR, as a bilateral
project, has to fulfil the requirements and account for
the existing infrastructure of each individual partner.
In principle, the ground segment is, as usually in all
large Earth observation projects, comprised of two essential
elements, the FOS (flight operations segment)
and PDS (payload data segment). Being a multilateral
program, a clear definition of the responsibilities and
the data policy have to be developed during the (future)
engineering phase, defining the operational responsibilities,
duties and rights of the individual partner. The
following guidelines are in principle assumed:
• The scientific community will be served by national
institutions, a possible commercial user community
will be served by data distributors. Both have to be
served by the PDS.
• Each partner will serve its user community in
its own region (e.g. Brazil—Latin America;
Germany—Europe). The data distribution over
other regions has to be decided during the next
program phases.
• The FOS will be arranged in a primary and a secondary
satellite control center, with equal functions
managing the spacecraft according to a jointly defined
timely structure. Both control centres use a
joint network of TT & C ground stations for the
LEOP and operational phase.
Fig. 13. MAPSAR satellite and launcher fairings.
5. Launcher
In terms of dimensions, the MAPSAR payload
layout is compatible with the majority of MMP
considered standard launcher family. Fig. 13 shows
the MAPSAR satellite inside the launcher fairing
dynamic envelopes. Some candidate launchers are
excluded due to the envelope constraints imposed by
the stowed reflector antenna envelope.
6. Ground segment
7. Conclusions
An innovative small satellite mission with an L-band
SAR was described. The study was prepared by INPE
and DLR, taking into account Brazilian and German
user requirements. The preliminary feasibility was
demonstrated for a reflector antenna concept. A high
degree of innovation is presented in the design considering
the small SAR payload, the reflector antenna
and the remarkable sensor performance. The applications
will take advantage of high spatial resolution
L-band SAR with enhanced capabilities (polarimetry,
stereoscopy, interferometry), particularly suitable for
the Amazon region and Boreal forest operations.
The deployment of the reflector antenna and the
solid state power amplifier are main technological
challenges for this project, but nevertheless, after
careful analysis, the SAR sensor is considered feasible
and its development is estimated in 5 years.
MAPSAR launch is envisaged for 2008.
References
[1] INPE/DLR, MAPSAR Pre-Phase A: Final Report, National
Institute for Space Research (INPE) and Deutsches Zentrum
für Luft und Raumfahrt (DLR), December 2002, 142pp.
[2] www.obt.inpe.br/mapsar/mapsarindex1.htm.