Retrieving High Precision River Stage and Slope from Space

E. Rodriguez, D. Moller

Jet Propulsion Laboratory

California Institute of Technology

Surface Water Measurement Requirements(Alsdorf & Lettenmaier, Science, 2003)

• 5-10 cm height accuracy (need height change for storage change, not absolute height)– River discharge, wetland/lake storage change

• Map rivers > 100m width– Would like to go to smaller rivers

• River slope accuracy: 10 rad (1cm/1km)– River discharge

• Revisit time:– Ideal: 3 days in the Arctic, 7 days in the tropics– Acceptable: 7 days in the arctic, 21 days in the tropics

• Imager with resolution better than 100 m– River width, wetland/lake extent– Should distinguish vegetated/non-vegetated

• Global coverage, sampling all major contributors to surface water, is not affected by clouds– Wetlands, rivers, lakes in tropics, Arctic thaw

Surface Water Interferometer Concept

• Ka-band SAR interferometric system with 2 swaths, 10km-60km on each side of the nadir track

• Produces heights and co-registered all-weather imagery

• 200 MHz bandwidth (0.75 cm range resolution) for highr resolution imaging

• Uses near-nadir returns for SAR altimetry to fill in nadir swath

• No data compression onboard

16 Day Repeat Coverage

120 km Swath

Pulse Limited Swath

Global Lake Coverage Histogram

Global River Coverage Histogram

Visits/Cycle for Major Rivers

Visits/Cycle for Major Lakes

Interferometric Measurement Concept• Conventional altimetry measures a single range and assumes the return is from the nadir point

• For swath coverage, additional information about the incidence angle is required to geolocate

• Interferometry is basically triangulation

• Baseline B forms base (mechanically stable)

• One side, the range, is determined by the system timing accuracy

•The difference between two sides (r) is obtained from the phase difference () between the two radar channels.

= 2r = 2 sin

h = H - r cos

Simulated Interferometer Return

The interferometer return signal contains both radar brightness (for water boundary delineation), range, and phase (color) for height estimation

Image geolocation accuracy given by timing accuracy, not platform attitude, unlike optical imager

Ran

ge

Along-track Position

Interferometer Height and Slope Precision

Height and slope estimates are made by using radar image to isolate water body and fitting a best fit linear height change over the swath.

Precision depends on water brightness and the length and width of the imaged water body

Attenuation Effects at Ka-Band

Rain rate mm/hr 35 GHz Attenuation

(5-km path)

3 6 dB

5 11.5 dB

10 24 dB

20 50 dB

30 73 dB

100 215 dB

WatER will only be able to collect valid data at rain rates smaller than 3-5 mm/hr (depending on surface water brightness)

Walsh, et al., Rain and Cloud Effects on a Satellite Dual-Frequency Radar Altimeter System Operating at 13.5 and 35 GHz, IEEE Trans. GRS, 22, 1984

Rain Probability

Petersen WA, Nesbitt SW, Blakeslee RJ, et al.TRMM observations of intraseasonal variability in convective regimes over the Amazon  JOURNAL OF CLIMATE 15 (11): 1278-1294 JUN 1 2002

The total WatER data loss, if data were uncorrelated with time of day, will be less than 10% in the tropics. Similar numbers also hold at other latitudes.

Radar Layover and its Effect on Interferometry

€

δ =arg 1+PLandPWater

γ LandγWater

exp i ΦLand − ΦWater( )[ ] ⎡

⎣ ⎢

⎤

⎦ ⎥

Brightness Ratio (land darker than water)

Correlation Ratio (land less correlated than water)

Volumetric Layover (trees)

Surface Layover

Points on dashed line arrive at the same time

Amazon Tree Layover Simulation-1

Amazon vegetation/water mask courtesy of L. Hess, UCSB

Amazon Tree Layover Simulation-2

• Assumed tree height: 20 m

• Fraction of land which is tree covered: 100%

What is the Global Topographic Layover Probability?

Ohio River Valley Topographic Layover Example-1

Cross-track range

Alo

ng T

rack

Ohio River Valley Topographic Layover Example-2

Referencing River/Wetland Height to LandSource: S. Kheim, JPL

Range delay variability from ground measurements

Roll/systematic phase and tropospheric range errors prevent absolute centimetric referencing relative to the center of the Earth

Solution: The main effect of these errors is to introduce long wavelength tilts and biases. Use existing DEM’s (or DEM’s derived during the mission)for least squares adjustments so that height will be referenced to DEM (not Earth center)

Disadvantage: DEM’s may have systematic errors. However, these errors will be the same for all passes and will cancel when making time series of change (=> storage change)

Tropospheric delays have correlation distances > 50 km. Order of magnitude slope biases: 5cm/50km ~ 1cm/10km. Amazon slope: 1cm/1km larger by one order of magnitude.

Backups

Data Loss vs Time of Day

• In non-tropical regions, the correlation between time of day and rain events is weak– Rainfall dominated by fronts– Data loss expected to be < 10%

• Tropical rainfall is governed by convective instabilities which arise due to daytime heating– Thunderstorms tend to happen in the late afternoon or

evening– Rainfall pattern is patchy– Correlation distance between rain events < 50 km to 100 km– 6 am is near the rainfall probability minimum, while 6 pm is

close to the rainfall probability maximum– See results below for more details

Spatial Distribution of Rain Cells-1

Local Time of Maximum Precipitation

Monthly Weather Review, 129, 2001

Rainfall vs Time of Day

Journal of Climate, 16, 2003

Amazon/LBASoutheast US

Monthly Weather Review, 108, 1980