On the Stability of Large Antennas as Calibration

Targets

A. I. Zakharov1, P. A. Zherdev

2, M. M. Borisov

2, A. B. Sokolov

2, C. G. M. van’t Klooster

3

1Institute of Radioengineering and Electronics RAS

141120, Fryazino, Vvedensky square, 1 2SDB MPEI

111250, Moscow, Krasnokazarmennaya str, 14 3ESTEC ESA

Keplerlaan 1, P.O.Box 299, 2200 AG Noordwijk, Netherlands

Abstract - Large radar cross section (RCS) and high temporal

stability are the typical requirements expressed nowadays to ex-

ternal calibration targets to be used for spaceborne SAR calibra-

tion. The use of ground-based large parabolic reflector antennas

may be a good alternative to active radar calibrators or corner

reflectors. Outstanding stability of the antennas RCS was dem-

onstrated in a calibration series under AO3-343 in 1999-2000.

Under the extension of the project in 2002 with a goal of prepara-

tion to ENVISAT mission we were able to demonstrate the re-

peatability of the series of observations made before as well as

our ability to control polarisation of backscattered signal and to

show phase stability of the backscatter, what is important for

repeated orbits interferometry.

INTRODUCTION

Today‘s achievements in the area of calibration techniques

and active/passive instruments provide the absolute accuracy

of the measurements better than 1 decibel, what become now

to be a standard requirement. Ground-based large parabolic

reflector antennas may be a good alternative to active radar

calibrators or corner reflectors.

The idea of using large antennas as calibration targets was

realized at Bear Lakes test site (Moscow region, Russia) in

1990th

. Their scattering properties were tested in 1995-2000

in ERS SAR observations of the test site mentioned. More

than 30 SAR calibration sessions were conducted under AO3-

343 project named "Research and development of highly effi-

cient calibration techniques for spaceborne SAR systems on

the base of ground based reflector antennas". In the year 2002

experiment was prolonged in order to continue study of long-

term stability of the antennas scattering properties, ASAR

calibration and intercalibration of ERS and ENVISAT.

EXPERIMENT CONDITIONS

In our study we used ground-based parabolic reflector an-

tennas as a calibration targets at the OKB MEI calibration site.

The calibration scenario was formed by pointing the required

calibration targets to the satellite using ephemeris available via

Internet. The ERS observations of 1999-2000 were made ac-

cording to the requests under AO3-343 project. The peculiar-

ity of the schedule of ERS calibration observations was that in

order to reduce influence of variations of antenna pattern onto

measurements we have planned a series of observations in

repeated tracks geometry. Two interleaved series were used

with track numbers 207 and 479.

All the calibration scenarios at the Bear Lakes test site in

1999-2000 were based on a use of 3 passive calibration targets

- large parabolic antennas of 4.7 m in diameter, located at the

corners of the rectangular triangle with 50 m legs. One of the

targets was an antenna with a conducting disc 0.5 m in diame-

ter located in the focal area. This target was pointed in practi-

cally every SAR session and served as reference target allow-

ing to monitor possible variations of RCS because of onboard

SAR instability and signal propagation media. One of the

most impressing features of such a modernization to be shown

later is an increase of the antenna pattern till 100

in C-band.

Another target was antenna adapted for a use by ERS ground

station, being under tests at Bear Lakes test site in 1998-1999.

It gave us good chance to analyze stability of regular receiving

station antenna as a reference target for calibration studies.

The last target was an antenna usually equipped with diffrac-

tion grid in focal area (see Fig. 1).

Figure 1. Large antenna with diffraction grid in focal area

Because of relatively small distance between the calibration

targets the ERS-2 SLCI images were used in the analysis of

observation results. The integration method of estimation the

antennas RCS was used, when the signal was integrated in the

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area of main lobe. Background noise signal was estimated

outside the impulse response function area and subtracted.

Antenna pattern was taken into account as well as range

spread loss of the signal.

RADIOMETRIC STABILITY

Most of the results on the stability of the measurements

were obtained from measurements of the RCS of antenna with

disk in focal area during 1.5 years of observations in 1999-

2000. From a total number of observations made on tracks

207 and 479 from the period mentioned we excluded few ones

made at heavy meteorological conditions and wrong calibra-

tion targets pointing. The subset of 13 observations results is

presented on the Fig. 2.

According to the RCS numbers obtained we have RCS

mean value 59.03 decibels of square meters (dBm2) with stan-

dard deviation 0.062. From here we made a conclusion, that

the stability of RCS is very high, better then that reported for

the corner reflectors and ESA active radar calibrators. One of

the reasons of high and stable RCS of our antenna was the fact

it had wide ~100 antenna pattern because of insertion of 0.5 m

disc in focal area.

Study of the width of the antenna pattern was made also

during the year 2000. The depointing of the antenna with

conducting disk at the angles 50 and 10

0 from nominal direc-

tion to the satellite led to 1.84 and 39.1 decibel decrease in

RCS. Comparing the measurements with theoretical estima-

tions of the pattern for 4.7 m antenna with 0.5 m disc in focal

area, presented on Fig. 3, we can see, that an increase of the

pattern width has place.

The prolongation of our calibration project in 2002 allowed

us to get new confirmation of long term stability of our cali-

bration target. On the Fig. 4 we presented prolonged series of

measurements for a track 207, including observations of 2002

and fresh results from a beginning of 2003 (subdivided by a

gap on the plot).

We can see here remarkably good repeatability of the meas-

urements for the antenna with conducting disk except for the

measurements in winter 2002-2003, where the numbers a 1

decibel lower because of cover of the ice/snow on the antenna

surface.

Figure 2. Plot of RCS measurements for calibration antenna with conducting

disk in focal area.

Figure 3. Modeling results of the antenna pattern with a disc in focal area (solid line) and measurements (stars).

Figure 4. Plot of extended series of RCS measurements (track 207 only) for

calibration antenna with conducting disk in focal area.

As for the antenna of regular receiving station as reference

target, we can say, that narrow antenna pattern requires high

pointing accuracy. Our 10 estimations of the RCS of ERS

receiving station at Bear Lakes, usually being pointed to the

ERS-2 satellite also in 1999-2000, presented on the Fig. 5,

show mean value 55.9 dBm2 and standard deviation 0.45. We

can see monotonous trends in the measurements, what may be

explained by antenna pointing errors. Also average RCS of

receiving station is ~3 decibels lower.

Figure 5. Plot of RCS measurements for antenna of ERS receiving station at

the Bear Lakes calibration site.

INTERFEROMETRIC STABILITY

One of the important tasks to be solved during implementa-

tion of the prolongation phase of the project was study of sta-

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bility of antenna phase center location in the case of possible

pointing errors. It is well known that the presence of such

strong and stable scatterers with known elevations is very im-

portant for interferometric observations of the topography,

especially in the repeated orbits observations scheme.

From the series of observations of our calibration site avail-

able we have selected two interferometric pairs and measured

the relative heights of the antennas. Antenna with conducting

disk and antenna with diffraction grid constituted the calibra-

tion scenario here. We have to underline that because of small

distance between the antennas (no more than 50 meters) the

influence of the troposphere should be considered to be negli-

gible. In the Table I we presented the details of observations

and results obtained.

From the results presented in the Table above we can con-

clude that the calibration antennas may be considered as a

targets with stable location of the backscattered signal phase

center.

POLARIZATION STABILITY

Another topic investigated in the series of calibration meas-

urements of the year 1999 was scattering properties of the

antenna with diffraction grid in a focal area. An insertion of

the scatterer into the focal area modifies scattering matrix of

the antenna. In the case of a disc it will be proportional to that

of sphere. Antenna with diffraction grid acts as a dipole, so

the cross-section at co-polarized mode xx will be:

,4cosmax

θσσ =xx

where θ – angle between the grid orientation and signal po-

larization plane. Prior to the experiments the polarization properties of the

diffraction grid with dielectric layer were tested in laboratory.

The attenuation of the signal with linear polarization travelling

from receiver to transmitter via the grid was measured at dif-

ferent orientation of the grid. The ~20 decibels variations of

the signal attenuation as a function of the orientation angle θwere obtained in the laboratory conditions.

In order to compensate possible instability of ERS-2 SAR

and propagating media, the scattering properties of the antenna

with a grid are presented in the Table II related to the target

with disk. Comparing the measured values of RCS decrease

with theoretical model in a form of cos4(θ), we can see good agreement of the values with theory except for the orthogonal

case, when–26 decibels decrease takes place. We have shown here also, that the cross-section of the an-

tenna could be controlled by the orientation of the diffraction

grid with respect to ERS-2 polarization plane in the range

from 0 till –28 decibels. The results are in good agreement

with theoretical predictions and laboratory measurements.

TABLE I CONDITIONS AND RESULTS OF INTERFEROMETRIC EXPERIMENT

Observation dates 990905-991010 990905-000227

Interferometric baseline (perpen-dicular component), m

61 140

Height in meters per 2 phase 162 64

Relative heights of antennas, m 2 3

Relative phase center locations in

fractions of wavelengths

0.013 0.05

TABLE II

RELATIVE RCS LEVEL OF THE TARGET WITH DIFFRACTION GRID, DECIBEL

Date Relative RCS 10lg cos4θ θ990817 0.4 0 00

991010 -5.7 -6 450

991114 -1.4 -1.38 22.50

991130 -16.7 -16.7 67.50

991219 -5.7 -6 -450

050900 -28.42 -∞ 900

291000 -26.83 -∞ 900

An interesting feature discovered here is that for the orienta-

tion angles ±450 we got practically the same values of the calibration target RCS, what characterizes also good linearity

of ERS VV polarization and polarisation stability of the an-

tenna with diffraction grid. This stage of our study confirms high stability of the para-

bolic antennas and the possibility to control antenna RCS for a

single polarization SAR system via control of the polarization

plane of the backscattered signal.

CONCLUSION

The passive calibration targets on the base of parabolic an-

tennas with 4.7 m mirror were shown to have large ~59 dBm2

and stable ~0.06 dBm2 radar cross section. An insertion of a

conducting disc significantly increases the width of the an-tenna pattern and reduces the requirements to the accuracy of

the antenna pointing. An insertion of the diffraction grid in

the antenna focal area allows to control the RCS level for a

single polarization SAR system and to obtain new polarization

properties of the entire scatterer, which may be of high interest

for the polarimetric calibration of the prospective SAR sys-tems. The stability of the phase center location makes the

antennas of Bear Lakes calibration site to be suitable for the

interferometric calibration also.

ACKNOWLEDGMENTS

Authors would like to express thanks to ESA for ERS SAR

data provided under AO3-343 project "Research and devel-

opment of highly efficient calibration techniques for space-

borne SAR systems on the base of ground based reflector an-

tennas" in 1999-2000 and for the prolongation of the project in 2002.

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