Transcript CryoSat Workshop, 9/2/05 - 1 SIRAL Beam-formation Verification using Transponders Mònica Roca and Mercedes Reche PiLDo Labs, Barcelona PiLDo Labs.

CryoSat Workshop, 9/2/05 - 1
SIRAL Beam-formation Verification
using Transponders
Mònica Roca and Mercedes Reche
PiLDo Labs, Barcelona
PiLDo Labs
ToC
CryoSat Workshop, 9/2/05 - 2
1. Introduction
2. Relevant aspects of the SIRAL on-ground processing and
data
3. The ESA transponder
4. SIRAL calibration using transponder data
5. Conclusions and way forward
PiLDo Labs
CryoSat Workshop, 9/2/05 - 4
2. Relevant aspects of the SIRAL ground processing
and data
2. SIRAL ground processing and data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 5
2. SIRAL ground processing and data
•
SIRAL has 3 operating modes:
•
In LRM, the range compression and incoherent averaging are performed onboard, and the resulting averaged echo forms the level 0 data.
The FBR data in the SAR and SARin modes consists of the individual,
complex (I and Q) echoes. In SARin mode, there are two such echoes, one
for each antenna of the interferometer.
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•
– Low-resolution mode (LRM): Conventional, pulse-limited altimeter, used over ice
sheet interiors and over ocean.
– Synthetic Aperture mode (SAR): used over sea ice. One single antenna.
– SAR-Interferometric mode (SARin): Used over the margins of the ice sheets.
Interferometry between echoes received on each antenna.
In SAR and SARin modes, the radar echoes must first be synthetic
aperture processed (performed on-ground) before incoherent multi-looking
(in SARin mode also phase multi-looking will also be applied). This forms the
Level 1b data.
2. SIRAL ground processing and data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 6
2. SIRAL ground processing and data
Instrument-corrected and
geo-located bursts
FBR
Steering: shifting the orientation
of the beams
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•
FFT over elapsed time
FFT across the burst
Beams steered to specific surface
- Range compression
- 64 Doppler beams with an equal angular
separation
Slant range correction
Computed for each Doppler beam, to align
them with their respective surface samples.
Multi-look of each stack
(+ phase multilooking for SARin)
To reduce speckle.
Echoes from beams of successive bursts,
directed at the same location, are summed.
SAR L1b
SARin L1b
2. SIRAL ground processing and data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 7
3. The ESA Transponder
3. The ESA transponder
PiLDo Labs
CryoSat Workshop, 9/2/05 - 8
3.1. Introduction
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A transponder is seen by a radar as a point target (well known).
•
1 transponder will be available for the CryoSat project (refurbished old
ESA transponder developed for the ERS altimeter calibration).
Located in the ESA Svalbard station.
Initially 2 TRP’s were located 250 meters a part. This strategic location
would have allowed the retrieval of many passes with transponder signal
taken with different along-track and across-track separation.
Finally only 1 transponder. We can still perform the analysis and
interpretation of the data over the transponder, acting as a point target,
for retrieval of the beam formation.
We have identified several specific studies or calibrations that will make
use of the transponder data (after reconstruction of doppler beams).
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3. The ESA transponder
PiLDo Labs
CryoSat Workshop, 9/2/05 - 9
3.2. Transponder location and passes
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Transponder Location: SVALBARD
(selected due to its high latitude)
CryoSat Orbit repeat cycle is 369 days.
Not a sun-synchronous orbit.
SARin mode over the transponder
The passes can be used so long as the
power decay (with respect to the peak)
is not grater than about -7dB, which
implies a separation from the nadir
track of about about 5 kilometres.
Days for CryoSat passes over Svalbard
In 6 months: 13 days
In 16 months: 37 days
Irregular interval,averaging 12 days
3. The ESA transponder
PiLDo Labs
CryoSat Workshop, 9/2/05 - 10
3. The ESA transponder
3.2. Transponder location and passes
PiLDo Labs
3.3. TRP characteristics
CryoSat Workshop, 9/2/05 - 11
Parameter
Type I
Antenna diameter
0.61 m
Radar cross section (s = Gant1 x Gant2 x Gsys x l2/4π)
44.5 – 104.5 dB
Beam width
2.5º
Beam co-linearity
+-0.1 º
Beam pointing
+-0.1º
Antenna gain
35.8 dB
Inter-antenna isolation
105 dB
Amplifier frequency range
13.2 – 14.1 GHz
Amplifier gain
17-77 dB
Amplifier gain variation across band
<+-0.5 dB
Amplifier 1dB suppression point
20 or 23 dBm
Amplifier phase linearity across band
+-7º
Internal gain monitoring precision
+-0.05 dB
Gain stability over temperature
need it !
Internal path-length monitoring precision
+-0.5 mm
Nominal internal path length
9.88 m
Path-length stability over temperature
0.1 mm/º
3. The ESA transponder
Transponder developed by RAL (UK) in 1987
curtsey of ESA
PiLDo Labs
CryoSat Workshop, 9/2/05 - 12
4. SIRAL calibration using Transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 13
4. SIRAL calibration using TRP data
Primarily used for calibration of the interferometer baseline.
• Other possible calibrations over the transponder:
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–
–
–
–
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Range bias: the measured range is compared with the expected one
Input to orbit studies: comparison of the range measured by the
radar altimeter with that calculated from an ephemeris
Sigma-0 bias: altimeter measured received power when flying over
the transponder is compared with the theoretical power. The
transponder radar cross-section has to be accurately characterised
Datation: the expected time of minimum range is compared with the
actual time of minimum range seen by the altimeter over the TRP
Mispointing: traditionally by measuring the slope of the echo trailing
edge. New methods using SARin mode echoes over a TRP
SIRAL Doppler beam formation consolidation through the TRP.
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 14
4.1. Range bias
4.1. Range Bias:
• Over TRP:
– using Level 1b data (after multi-looking)
– using stack (after beam steering, 2D FFT, but before slant-range
correction and multi-looking)
• Over a natural target
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 15
4.1. Range bias
Over TRP using Level 1b data
•
Principle: the range is evaluated by retracking the one single multilooked echo (from the Level 1b data) at the transponder position and
compared with the theoretical range at that same point.
•
We need to pay particular attention to the atmospheric corrections
(ionosphere and troposphere).
We do not have in-situ measurements so we should rely on modelling.
Ionospheric correction smaller at high latitudes.
We need the internal TRP delay characterised up to a value that
depends on the range calibration requirement. This is measured by an
TRP internal calibration (about 12 ns).
We can also characterise it through the RA-2 (use RA-2 well calibrated
over the TRP to determine TRP internal delay).
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•
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4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 16
4.1. Range bias
• EnviSat orbits
• CryoSat orbits
• Need to evaluate TRP distance to EnviSat ground track
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 17
4.1. Range bias
Over TRP using a stack of
beams
Principle: the range is
evaluated by modelling
the theoretical function
that describes the range
distance between the
altimeter and the TRP,
accounting for the
satellite trajectory and
velocity [r(t) hyperbola].
This range is compared
with the measured
range.
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 18
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4.1. Range bias
We can use SARin mode, although we only need one antenna received
echo.
In order to retrieve the range measured by the altimeter, the
individual I&Q echoes shall be processed by performing the 2D-FFT
(Doppler beams and range compression), and retracking.
We will compare the above with the computed theoretical range and
retrieve the range bias.
Same considerations about the characterisation of the internal delay of
the transponder apply.
Same considerations about the atmospheric corrections apply.
4. SIRAL calibration using transponder data
PiLDo Labs
4.1. Range bias
CryoSat Workshop, 9/2/05 - 19
Over natural targets: the salar de
Uyuni, Bolivia (PI- H.A. Fricker)
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measurement of the surface height
retrieved by the altimeter is compared
with independent measurement (GPS
survey)
we will use SAR In
ICESat
EnviSat
TOPEX
360
085
we will reproduce the processing
performed for RA-2: retrack the
waveforms with a gaussian retracker
we can cross-calibrate RA-2 and SIRAL
SAR In
we can also use the salar to calibrate
the interferometric baseline
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 20
4.2. Sigma-0 bias
4.2. Sigma-0 bias
•
•
The TRP can be used to calibrate the radar measurements of the
surface backscatter
2 ways of approaching the problem
– using Level 1b data (after multi-looking)
– using stack (after beam steering, 2D FFT, but before slant-range correction
and multi-looking)
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The altimeter measures the power of a target, the transponder, which
has a well known radar cross section (RCS).
Therefore the RCS shall be extremely well characterised, depending on
sigma-0 calibration requirements (aprox. ±0.1 dB).
By applying the radar equation it is possible to compute the theoretical
power the altimeter is supposed to measure.
4. SIRAL calibration using transponder data
PiLDo Labs
4.2. Sigma-0 bias
CryoSat Workshop, 9/2/05 - 21
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Over a TRP, the altimeter is looking at a ground fixed point target and therefore the
altimeter is not measuring at nadir but at a certain angle given by the geometry. At
the same time, the TRP is not seen by the altimeter from zenith.
•
Since the radar cross section of the transponder depends on the transponder antenna
gain, the angle in which the transponder is seen by the altimeter also becomes
important.
 R

 sin180  t 
R  h

 t   sin1 
CryoSat
rt 

rd
or
T
G A ,t   G T ,  t  Gelec
 l4

P  P
2
TRP

r
t
4. SIRAL calibration using transponder data
2
 4 
4
 r t   L
4
PiLDo Labs
[W]
CryoSat Workshop, 9/2/05 - 22
4.2. Sigma-0 bias
• Power received by
the altimeter over
the TRP
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 23
4.2. Sigma-0 bias
Processing (conceptually similar to range bias):
•
•
•
•
•
•
We can use SARin mode, although we only need one antenna received
echo.
If using the stack beams: in order to retrieve the power measured by
the altimeter, the individual I&Q echoes shall be processed by
performing: the 2D-FFT (Doppler beams and range compression), and a
retracking (gaussian fitting and integration).
If using the Level 1b: the multi-looked echo shall be retracked.
We will compare the above with the computed theoretical power and
retrieve the power bias.
We need the TRP RCS characterised up to a value that depends on the
sigma-0 calibration requirement (about ± 0.1 dB).
Otherwise we will characterise it through the RA-2 (use RA-2 well
calibrated over the TRP to determine TRP internal delay).
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 24
4.3. Mispointing
4.3. Mispointing
•
Historically, due to the nature of the past altimeters, mispointing have
only been evaluated irrespectively of the direction (pitch and roll).
•
We have now the opportunity of measuring mispointing as a function of
the angle of arrival.
Considerations:
– along-track mispointing can be misinterpreted as a datation error;
– across-track mispointing can be misinterpreted as interferometric
baseline error.
We will therefore estimate the mispointing with the traditional way:
slope of the trailing edge of 10 minutes averaged waveform over ocean
(LRM).
•
•
4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 25
4.4. Datation
4.4. Datation:
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TRP’s can be used to
measure the datation
bias.
The expected time
of minimum range
(black) is compared
with the actual time
of minimum range
seen by the altimeter
over the TRP.
We can use a stack
of echoes or single
multi-looked echo.
4. SIRAL calibration using transponder data
PiLDo Labs
4.5. Angle of arrival
CryoSat Workshop, 9/2/05 - 26
4.5. Angle of arrival
•
The across track angle of an incoming ray, Fmeas, can be inferred by
determining the phase difference between the 2 received signals (one
from each antenna).
l  phase
Fmeas t   sin 

 2B 
1
where B is the distance between the 2 antenna phase centres.
•
The theoretically computed angle of arrival, Ftheo, can be compared with
the angle of arrival retrieved from the data.

h t 
where r(t) is a hyperbola and
satellite
height.
Ftheoht is the
cos 

r
t
   
1
•
If using Level 1b data we compare a single angle measured against
theoretical angle. If using the stack we compare the 2 equations.

4. SIRAL calibration using transponder data
PiLDo Labs
4.5. Angle of arrival
CryoSat Workshop, 9/2/05 - 27
r1
r0
h0
h
-1
r
h1
d1
-1
d0
l  phase
Fmeas t   sin1

 2B 
h t 
Ftheot   cos 

r
t




1
TRP
d-1

4. SIRAL calibration using transponder data
PiLDo Labs
CryoSat Workshop, 9/2/05 - 28
5. Conclusions and way forward
PiLDo Labs
CryoSat Workshop, 9/2/05 - 29
5. Conclusions and way forward
• Transponders have demonstrated through the years the suitability
of certain type of calibrations.
• Doppler beam formation processing can be verified over the TRP.
• We will use the transponder deployed in Svalvard for SIRAL
calibration of angle of arrival and:
• range bias,
• sigma-0 bias,
• datation.
• A transponder RCS characterisation is recommended for Sigma-0
bias determination.
• We will compare the retrieved biases using the stack beams
(before multi-looked) and using the single multi-looked echo.
5. Conclusions and way forward
PiLDo Labs
CryoSat Workshop, 9/2/05 - 30
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Labs
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