The CryoSat Payload

Constantin MavrocordatosCryoSat Payload ManagerESA-ESTEC

• Composition of the CryoSat Payload• SIRAL principle of operation• Instrument measurement corrections• Data product contents

Composition of the CryoSatPayload

SIRAL (SAR/Interferometric Radar Altimetmer• Main payload instrument• Measures the range from the satellite to the surface

DORIS• Shared between platform and payload• Used for precise orbit determination and datation

Star Trackers• Shared between platform and payload• Mainly used in SARIn mode for the determination of the

orientation of the interferometric baseline

Doris measures the radial velocity of the satellite wrt to a network of beacons, by exploiting the Doppler effect.

The precise orbit is computed by CNES and fed into the PDS processor

Doris L1b product consists of Doppler measurements along the orbit, during beacon visibility

Network of Doris beacons

rument plate with SIRAL tronics, antennas and Star-ker heads, before integration

View of the payload, after integration on the satellite

SIRAL principle of operation

SIRAL is a radar altimeter with 3 main measurement modes, for optimum operation over different surfaces:

• LRM mode• Conventional “pulse-width limited” mode• Used over ice interiors (flat surfaces) and over ocean

• SAR mode:• Enhanced along-track resolution, enables measurements over narrow

leads

• Multilooking improves the measurement accuracy• Used over sea-ice

• SARIn mode:• Determination of across-track angle of arrival of echoes, in addition to

SAR: Suitable for sloping surfaces

• Used over ice-margins

Deramp is a “pulse compression” technique, used in all modes.

The transmit pulse is a chirp. The deramp pulse is triggered by the on-board racker, after a delay Ho. The echo is received at a delay τ and mixed with the

deramp chirp.

The resulting signal is a sine wave (assuming a single point target). Its frequency ∆f is proportional to the delay τ.

A spectral analysis (FFT) after deramp resolves the time delay of each ndividual echo. This FFT can take place either on-board (LRM) or on ground SAR/SARIn).

equency

Time

Ho τ

Transmit pulse

Deramp pulse

Received echo

∆f

Properties of the radar echo over flat surface

• In SAR and SARIn modes, several synthetic beams are formed within the antenna beam, by exploiting the Doppler effect inducedby the satellite motion.

• These beams improve the spatial resolution of the altimeter (wrt to conventional pulse-width limited altimeters) in the along-track direction.

• The resulting resolution cells are elongated across-track and narrow along-track.

• All synthetic beams are active simultaneously. Every cell is thus observed several times from different angles.

• In SARIn, the angle of arrival of the echoes (wrt to the baseline vector B) is derived from the phase difference of the echoes received through two separate antennas:θ

∆∆∆∆R

B

∆Φ ∆Φ ∆Φ ∆Φ = 2 ππππB sin θθθθ

λλλλ

• Problem of left/right ambiguities can be solved by exploiting the echo from the closest point of the surface (leadinedge)

Tracking low bandwidth (40 MHz) mode FFT 128 points

Tracking Time = 30 ms

TRACKING CYCLE: 46.7ms

PRF 1970 Hz

ow Resolution ModeTx Rx

92 pu92 pulNoise pulse

PRF Burst 85.7 Hz => 11.7 ms

AR Modet Tx Burst Rx

256 pu256 pu

RF Burst 85.7/4 = 21.4 Hz => 46.7 ms

ARIn Modest Tx Burst Rx 64 pu64 pul

Short range window and large dynamic range of echoes:

=> real time range and gain tracking is needed in all modes

LRM/SAR modes:

Median algorithm for range tracking=> keeps the median point of the echo centred

SARIn mode:

EDP (Earliest Detected Point) algorithm for range tracking:=> keeps the leading edge at a predefined location

Instrument measurement corrections

iodic instrument calibrations performed in-flight, in order to correct for instrumenerfections and drifts. Corrections are computed and applied by L1b Processor:

Delay drift (all modes – CAL1)

Gain drift (all modes – CAL1)

Receiver gain ripple (all modes – CAL2)

AGC gain steps (all modes – CAL1)

Pulse-to-pulse gain and phase variation (SAR/SARIn – CAL1)

Phase-difference (SARIn – CAL1 and CAL4)

Note: CAL1: Calibration of impulse responseCAL2: Acquisition and averaging of noise samplesCAL4: Phase bias of both Rx channels, interleaved with SARIn measurements

CAL1-LRM: range impulse response

CAL2-SARIn – noise

CAL1 – SARIn

CAL1-SARIn

Data Products

Contents:• Geo-located, un-calibrated instrument source packets in SAR and

SARIn modes: • Bursts of 64 echoes in the time domain, consisting of:

• 128 I&Q samples in SAR mode• 2 x 512 I&Q samples in SARIn mode

Possible use:• Independent investigation/troubleshooting of SAR processor• Exploration of alternative SAR processing techniques• Exploration of surface properties, not included in L1b

Not recommended for routine processing because:• Data calibration is a complex process which requires in-depth

knowledge of the instrument• It involves a very large amount of data: about 150 times larger

than L1b• Additional lower level data may be required (e.g. L0 Calibration

data)

L1b product consists of echo waveforms in spectral domain (after range FFT) andafter formation of the synthetic beams and multilooking (SAR and SARIn only). Instrument calibration corrections are also applied.

• LRM: Echo power waveforms (average of 91 echoes of 128 samples)

• SAR: Multilooked power waveform corresponding to each along-track resolution cell. Each echo is composed of 128 samples. The data product also contains stack characterisation parameters

• SARIn: Multilooked power waveform corresponding to each along-track resolution cell. Each echo is composed of 512 samples. The data product also contains phase-difference, phase coherence and stack characterisation parameters

The following simulations show SARIn echoes at different stages of the processin

• Simulation scenario consists of point targets aligned across- trac

• Echo power is shown as observed through each of the 64 synthetic beams

• Maximum power in central beams and minimum range

• Echoes through central beamscover a larger range than fore/aft beams

• Echoes through fore/aft beamscome from greater range

• Inferred angle shows the effecof right/left ambiguity beyondthe leading edge of the echo

• Point targets are seen from different along-track angles

• Point targets remain within thantenna footprint for about 2 sec.

• The range migration of the echoes corresponding to a givenstrip is compensated (for later accumulation): all echoes through the different beams appear at a range correspondingto the range of the nadir beam

• Ocean echoes are now more “peaky” than “Brown” echoes

• Inferred angle shows clearly theeffect of ambiguity beyond the leading edge of the echoes

• All echoes of a given stack areaveraged (multilooking)

• The scenario consists of several point targets aligned along-track. They are now visible as the time spans over 20 seconds (140km along track)

• Ocean echoes have a sharp fall-off in power, as expected

• The inferred angle of the leading edge is close to 0, showing that the closest point is at nadir (horizontal surface)

• The span of the inferred angle increases at greater ranges. This is a consequence of the left/right ambiguity which affects the coherence of the echoes.

Contents:• Estimates of surface elevation (and other surface parameters)• Derived by “re-tracking” each echo from L1b data• The rate is approx 20Hz in all modes

Particular features:• Over sea-ice, data include ice thickness and backscatter properties• Over land ice, the elevation is slope corrected using the

interferometer phase (SARIn) or a DEM of the surface (LRM)• In SARIn, the height estimates are not necessarily below the

satellite track, but can move (left or right), depending on the across-track slope.