Satellite Altimetry Missions

Transcription

1 Satellite Altimetry Missions SINGAPORE SPACE SYMPOSIUM 30 TH SEPTEMBER 2015 AUTHORS: LUCA SIMONINI/ ERICK LANSARD/ JOSE M GONZALEZ

2 Table of Content General Principles and Applications Technology description Thales Alenia Space solutions 2

3 Radar Altimetry Principle Payload Data Precise Orbit Determination Dual Frequency Altimeter Or Ionospheric Models Microwave radiometer t=2r/c => 1 cm ~ 0.06 ns Microwave radar altimeter 3 3 End User Data Processing Data Processing Altimeter echo retracking Tropospheric correction Ionospheric correction POD

4 Mesoscale in the Oceans(time / space) 4

5 Themes (Operational/Science) Cryosphere Hydrology Ocean Sea Dynamic Topography Oceanic Circulation and Tides Modelling Acoustic communications Bathymetry Ocean Operational Forecasting 5

6 Dynamic Topography Dynamic topography is the sea level driven by thermodynamic processes in the ocean. It includes a "static" part (taking into account features like the main currents, etc.), and a variable part. 6

7 Oceanic Circulation and Tides Oceanic Circulation is the computation of speed and direction of ocean currents through the height gradient from the sea surface (The major ocean currents raise sea surface height) and the reference geoid. 7

8 Modelling Acoustic Communications Acousticwavespeed dependson water pressure and T The soundpropagatesalong«channels» Someareas are «shadowed» 8

9 Altimetry & Bathymetry 1 km 4 km Non-linear transfer function between sea surface bottom topography and sea surface slope Altimetry Bathymetry is limited to km spatial wavelength 1 µrad slope (1 cm/10 km) = 1 mgal anomaly of gravity (10-5 m.s -2 ) 9

10 Ocean Operational Forecasting Sea Surface Temperature(combined) Wind stress (scatterometers) Altimeter sea level anomaly Temperature Currents Tides Value Added Products for fishery, pollution, support to man atsea, defence, 10

11 Hydrology and Land Applications Reception of the return echoes on land is more complex. The backscatter coefficient depends on the state of the observed surface, according to whether it is covered by snow, vegetation, flooded areas, etc. Altimeters provide data on status of Lakes and Rivers for climate and water resources monitoring 11

12 Key drivers for altimetry mission design Random noise in altimeter measurements Speckle and thermal noise Trade-off between along track spatial resolution (averaging of measurements) and geophysical product accuracy Signal to thermal noise ratio (Orbit height) Data Processing of radar echo (retracking) Estimation of range, wave heights, backscattering, antenna pointing Altimeter measurement stability Overall altimeter design Internal Calibration Modes External Calibration (ground proof) Temporal / Spatial sampling Orbit selection (tides, X-track distance vs repeat cycle) Number of satellites (constellation) Type of altimeters : nadir vs swath altimeters 12

13 Altimeter Techniques 13

14 Pulse Altimeter The model is a convolution product of 3 terms The plane surface responding to a Dirac impulse (step) The distribution of the surface roughness (wind waves, tides, gravity perturbations) impulse response of the radar system (thermal noise, significant wave height (SWH), leading edge curvature, satellite or antenna mispointing, Antenna Gain) Ideally calm sea Rough sea 14

15 SAR Altimeter Conventional Pulse Limited Altimeter 1. Non-coherent measurements 2. Real antenna aperture: D phys =1.2 m 3. Antenna Beamwidth and Footprint Θ synth 69º λ/d phys = 1.26º D foot_phys =720km Θ synth 16 km 4. Resolution enhancement based on priori knowledge on the topography (B=320 MHz) =2 /2 1.6km Altimeter with SAR mode 1. Coherent measurement over n pulses 2. Synthetic aperture: D synth >> D phys 3. Synthetic Antenna Beamwidth and footprint Θ synth 51º λ/ D synth = 0.022º D foot_synth =720km Θ synth km 4. Improvement of Signal to Noise Ratio by SAR processing ca. 10 db (B=320 MHZ) 5. Peak shape of the Echo, due the non constant surface ring of the signal Along Track Doppler Modulation Time delay 15

16 Interferometer Altimeter Interferometry is used to better locate the echo reducing the noise Enhance the dynamics at high resolution Interferometric phase: Elevation: cosθ Difference in Elevation:! "! The interferometric phase noise depends on: Signal to thermal Noise Ratio Speckle noise (geometric decorrelation) Interferometric base attitude noise Range ambiguities Number of looks 16

17 A complete off-the-shelf Offer 17

18 TAS Altimeters at a Glance Most altimeters in-orbit since 1990 are developed by Thales Alenia Space 30 years of altimeter development 20 years of successful in-orbit heritage Acknowledged expertise in end to end radar altimetry design Experience at Export 18