c.Oberpfaffenhofen, Germany


Titles of Investigations:

I.  Multi-Frequency, Multi-Polarization External Calibration
of the SIR-C/X-SAR Radars

II.  X-SAR/SIR-C Radiometric Calibration Experiment

III.  Information Extraction from Shuttle Radar Images for
Forest Applications

Principal Investigators:

I.  Dr. Anthony Freeman
  Jet Propulsion Laboratory

II.  Dr. Franz Heel
  DLR

III.  Dr. Rudolf Winter
  DLR

Site Description:

 The SIR-C/X-SAR supersite at Oberpfaffenhofen, Germany,
near Munich, is centered at the Oberpfaffenhofen DLR area
(48.09 latitude, 11.29 longitude).  DLR has good communication
facilities, numerous processors, laboratories, and the technical/technological
infrastructure to support sophisticated calibration experiments
in the area.  Therefore, a couple of years ago, we selected
this area as a calibration test site.  Additionally, due
to site characteristics such as agricultural fields, grassland,
forests, cities, and lakes, scientists from other institutions
will use this test site for land use experiments.  Land use
studies are predominantly focused on agricultural, forest,
and hydrological investigations.
To meet mission conditions for SIR-C/X-SAR, ERS-1, JERS-1,
and PRIRODA, the test site area will be extended up to about
10,000 km2.
 In August 1989, the first DC-8/E-SAR airborne SAR campaign
took place at the Oberpfaffenhofen test site.  For calibration
purposes, forty-two trihedral and four dihedral corner reflectors,
one C-band ARC and one C-band receiver prototype were deployed
within the SAR swaths.  Both SAR systems were operated at
three different incidence angles (30, 45, 55), requiring
realignment of all the calibration devices after each flight
path.  Additionally, ground truth measurements were conducted
during the SAR campaign.  Our calibration analyses covered
all the main measurement objectives including absolute, polarimetric,
cross-track, and cross-sensor calibration.  Another team
is correcting for geometrical distortions to obtain geocoded
SAR images.   
 Calibration activities continued during the second DC-8/E-SAR
campaign (July 1991) at this site.  In addition, a first
attempt was made at determining the cross-track inflight
antenna pattern.  For this purpose, eighteen C-band receivers
and nine C-band ARCs with receiving paths were located across
track within the -10 dB points covering a distance of about
35 km.  A GPS receiver was used to survey the ground receivers
and to synchronize their internal clocks.  The procedure
applied to the receiver data was successful in obtaining
the desired cross-track inflight antenna pattern.  Antenna
squint angle, as well as misalignment between A and V patterns
and pulse shapes, could also be determined by this method.
 During this campaign, ground truth measurements included
documenting plant geometry, moisture content, biomass, and
forest stand determination.
 For spaceborne SAR missions like SIR-C/X-SAR, X-band and
L-band receivers and ARCs will be deployed.  This equipment
 includes twenty receivers and five ARCs for each frequency
band.

Objectives:

I.  a) Assess the accuracy at which the SIR-C/X-SAR standard
data products can be calibrated;

  b) Study the cross-calibration between three independent
multi-polarization systems:  SIR-C, the NASA/JPL DC-8 SAR,
and the University of Michigan ground-based polarimetric
scatterometer;

  c) Evaluate the calibration "stability" of the SIR-C/X-SAR;


  d) Develop a cost-effective calibration plan which includes
the development of inexpensive polarimetric active calibrators.

II.  a) Determine error sources and their relative contributions
to the absolute calibration accuracy of the overall system.

III.  a) Extract all possible information from shuttle-borne
radar images for areas with forest stands.

Field Measurements:

I.  a) Calibrate all the calibrators at standard ranges prior
to deployment.

  b) Use polarimetric active radar calibrators to estimate
end-to-end polarization distortion of the SIR-C system. 
Use the estimated distortion parameters to extract a "best
estimate" of the polarization scattering matrix of other
targets on the ground.

  c) Deploy inexpensive trihedral corner reflectors to characterize
co-polarized channel imbalance in magnitude and phase over
an area wider than that covered by the active calibrators
in the primary calibration area.

  d) Use multi-polarization ground receivers to record the
system transmit azimuth pattern at several elevation cuts
and to estimate the transmit polarization distortion.

  e) Use calibrated multi-polarization ground scatterometers
to provide in situ data over extended targets and compare
them with SIR-C/X-SAR data of the same targets.

  f) Evolve a cost-effective calibration strategy by comparing
the "calibrated'' results obtained utilizing different calibration
philosophies.

II.  a) A series of five different experiments covering internal
and external procedures are planned.  Oberpfaffenhofen and
its surrounding area will serve as a master test site.  The
five experiments are: 1)  Internal calibration; 2) Measurement
of operational SAR antenna pattern; 3) Absolute calibration
of SAR image data; 4) Mutual radiative coupling of clutter
surrounded calibration targets; and 5) Attenuation of microwaves
caused by woodlands.

III.  a) Classification results will be based mainly on SAR
information, however, non-satellite data (air photos, soil
maps, forest maps, and ground truth) will be used as reference
data and to verify the results.

Crew Observations:

1) Crew Journal: Document weather conditions, forest and
agricultural areas, and any burning activities at the site.

2) Cameras: Hercules and Linhoff cameras will be used to
obtain stereo color photos of the site.

Coverage Requirements:

The minimum coverage requirements for the Oberpfaffenhofen,
Germany test site are four (4) south-looking passes, preferably
48.

Anticipated Results:

I.  a) Full polarimetric end-to-end characterization of the
SIR-C/X-SAR system in the primary calibration target area
within the limitations of instrument accuracy;

  b) A better understanding of polarimetric calibration of
space-borne microwave synthetic aperture radar.

II.  a) Quantitative analysis of SAR image data.

III.  a) Differentiation of forested and non-forested areas
with an assessment of the accuracy of separation.

  b) Information on the tree stand geometry, age classes
of trees, and seasonal changes.