High Resolution Air- borne & Space-borne Radar with Real Aperture

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1 High Resolution Air- borne & Space-borne Radar with Real Aperture Downward Looking Radar Radar Altimeter 1

2 A. Resti et. al. The Envisat Radar Altimeter System (RA-2), ESA bulletin 98 june

3 1 return Radar Altimeter Scheme, Average Waveform Profile 25 returns 1000 returns Power T R = c/2r Leading Edge Slope ~ wave height ~ σ 0 Trailing Edge Slope ~ instrument pointing Time 3

4 A. Resti et. al. The Envisat Radar Altimeter System (RA-2), ESA bulletin 98 june

5 Altimeter Returns from Ice Returns from sea ice appear more specular than that from ocean (peaked trace). Range measurement variability is of the same order as that from the ocean can therefore be tracked using the altimeter ocean tracking mode. Different for continental ice: echo has unpredictable shape & more importantly can have a larger variability in surface elevation.. 5

6 6

7 Altimeter Operation Modes Altimeter Exploratory Study, ESTEC Contract No:6483/85/NL/BL 7

8 Pacific Ocean: Altimeter Result vs. Sea Floor Topography Altimeter Exploratory Study, ESTEC Contract No:6483/85/NL/BL 8

9 Terrain Type and Condition σ 0 /db Flat swampy areas +23 Ocean: very calm conditions +19 Ocean: average sea state +9 Snow-covered terrain +8 Cropland...+5 Terrain partially covered with short, dry, grass..-2 Wet terrain partially covered with short green grass -5 Terrain covered with tall green weeds -15 Terrain covered with tall dry weeds.-20 Tree-covered terrain.-20 Mountains

10 Sea Surface Topgrapgy & Currents of Planet Earth 1.Atlantic Ridge 2.Fracture Zones 3.Pacific Trenches 4.Volcaic Hawaian Arc 5.Emperor Seamount Chain 6.Gulf Stream 7.Kuroshio Current 8.Agulha Current 9.Brazil Falkland Confluence Courtesy JPL 10Westwind Drift Confluence 10

11 Categories of Altimeter Waveforms Width 10 bins Altimeter Exploratory Study, ESTEC Contract No:6483/85/NL/BL 11

12 Global mean sea surface height derived from radar altimeter scanning the ocean surface The radar beam detects relief variations of about ± 80 m in places. The Ocean relief is stable & is shaped by the geoid - the surface normal to the direction of the Earth's gravity field. 12

13 The Shape of the Earth times surmounted + 65 m North Atlantic -100m Indian Ocean 13

14 Different Satellite Altimeter Specifications 14

15 ENVISAT Altimeter (RA-2), Basic Specifications Orbit range 764km, 825km Operative frequencies GHz, 3.2GHz Pulse length.20microsec Ku chirp bandwidths. 320MHz, 80MHz, 20MHz S chirp bandwidth. 160MHz Ku transmitter peak power. 60W S transmitter peak power.60w Ku-PRF Hz S-PRF HZ I/Q A/D conversion bit number Number of FFT points Max. data rate 91kbit/sec Antenna diameter 1.2m Measured power consumption...126w Mass..112kg A. Resti et. al. The Envisat Radar Altimeter System (RA-2), ESA bulletin 98 june

16 ENVISAT 16

17 ASAR: ATSR: ENVISAT Payload Advanced Synthetic Aperture Radar) (Advanced Along Track Scanning Radiometer) DORIS: Doppler Orbitography and Radiopositioning Integrated by Satellite GOMOS: Global Ozone Monitoring by Occultation of Stars MERIS: MIPAS: MWR: RA-2: MEdium Resolution Imaging Spectrometer Michelson Interferometer for Passive Atmospheric Sounding MicroWave Radiometer Radar Altimeter SCIAMACCI: Scanning Imaging Absorbtion Spectrometer for Atmospheric Cartography 17

18 ENVISAT Radar Altimeter RA-2 DeRamp Mixer Signal Processor Sub Assembly Frequency Generation & Conversions Unit Low Voltage Power Supply Instrument Control Unit 18

19 f Received Frequency Chirp f Deramped Frequency Chirp FFT t f Deramping Function t f Transmitted Frequency Chirp t P Processed Sea Surface Return t t 19

20 Real Altimeter Ocean Waveforms Power ENVISAT Seasat Range Bins 20

21 Multi Peak Waveform Charakterization Conditions p P peak P peak /2 P 2 >0,5P peak P 1 >P peak R ~ T P 1min < 0,7 P 1 P 2min < 0,7 P 2 21

22 Multi Peak Waveform Charakterization p P peak P peak /2 P 2 >0,5P peak P 1 >P peak R ~ T P 1min < 0,7 P 1 P 2min < 0,7 P 2 22

23 p Centre of Gravity Parameters Heuristic Estimates of Waveform Shape & Location 1 Width (W) Leading Edge Amplitude (A) R n ~ T R 1 R m ΔR Position Area = WxA = R R Centre of Gravity n 0 P( R )dr Seasat ΔR = 46,8 cm 23

24 Heuristic Altimeter Measurement Parameters Return Wave Form: P(R n ) with n= (Seasat) = Power in the n th Rangebin Horizontal Position of Gravity Centre: Position m ΔRP( R n= 1 = m n= 1 P( R n n ) ) Amplitude = 2xVertical Gravity Centre Part: Amplitude m n= 1 = 2 m 0,5P n= 1 2 P( R ( R n ) n ) Width = Area of Return Waveform/Amplitude: Width = m n= 1 P( R n ) Amplitude 24

25 25 25 Institut für Hochfrequenztechnk und Radar Heuristic Centre of Gravity Parameters In order to reduce the Effect of Low Bin Values R 2 (n) is used instead of R(n) = = = m 1 n n 2 m 1 n n 4 ) R ( P ) R ( P Amplitude 2 m 1 n n m 1 n n 2 ) R P4( ) R ( P Width = = = Leading Edge Position = Position - 0,5 Width = = = m 1 n n 2 m 1 n n 2 ) R ( P ) R ( RP Position Δ

26 Example of Waveform Classification Seasat Result, 60 Range Bins Altimeter Exploratory Study, ESTEC Contract No:6483/85/NL/BL 26

27 Radar Altimeter Waveform Classification Altimeter Exploratory Study, ESTEC Contract No:6483/85/NL/BL 27

28 Envisat Altimeter absolut Calibration A Wave Rider GPS Buoy determines the in Situ Water Level Both altimeter & buoy observe the instantaneous water level. The buoy moves up and down with the waves as an independent water level measurement for altimeter to compare with. 28

29 Mean sea level (MSL) monitored by Envisat, TOPEX/Poseidon & Jason-1. 29

30 Radar Altimeter Terminology In-Lock: Altimeter is in Tracking Mode & Telemetry is o.k. Width (W) Leading Edge Position (LEP): LEP = Position - 0,5 Width Retracking: Process of locating the LEP of the Return within the Range Window. Locate at the First Range Bin whose Power exceeds 50% of Peak Centre of Gravity Position (GOP): Parameter: horizontal & vertical Position (P H, P V ), Amplitude (A), W, LEP. A, W, LEP define a rectangular Box with same Area & COG as the Waveform Sea wave Height (SWH ) = 4 σ s for plane, homogenous, rough, Gaussian Surface Not Simple Waveform: If in-lock but Multi Peaked and/or W > 38 and/or 15< LEP < 45 and/or Amplitude >800 Ocean Like Waveform: 20 LEP 32 & (COG + ½ W) 57 30

31 'Wet' radar altimetry Signal returns enable the detection of river and lake levels, as seen here for South America 31

32 Radar altimetry-derived slope map of the Greenland ice sheet 32

33 Institut für Hochfrequenztechnk und Radar Radarsysteme Süddeutsche Zeitung,

34 Radar Altimetry-derived Slope Map of Antarctica 34

35 Altimeter-based chart of Gulf Stream variability The inset shows modelled stratification of the temperature field red is warmest, purple is coldest SEMPI66Y3EE_index_1.html 35

36 Altimeter Map of Planet Venus 36

37 The Planet Venus 37

38 Altimeter Literatur N.N.: RA2-MWR Product Handbook, Jérôme Benveniste: The ENVISAT RADAR ALTIMETRY MISSION DATA PRODUCTS, European Space Agency (ESA/ESRIN), Via Galileo Galilei, I-00044, Frascati (RM), Italy, A. Resti, J. Benveniste*, M. Roca & G. Levrini, J. Johannessen:, The Envisat Radar Altimeter System (RA-2), ESA-Bulletin 98 June 1999 N.N. University College, London:An Exploratory AStudy of Inland Water and Land Altimetry using Seasat Data, European Space Agency, Contract Report, ESTEC Contract No:6483/85/NL/BI, Feb N.N.:Radar Altimetry Tutorial - 3 Altimetry N.N.:Radar Altimetry Tutorial - 5 Products 38