The next-generation U.S weather satellite, the National Polar-orbiting Operational Environmental Satellite System (NPOESS), carries the Conical-scanning Microwave Imaging/Sounder (CMIS) instrument. One of the major deliverable products (Environmental Data Records, or EDRs) from this instrument's measurements is the ocean EDR suite that includes ocean surface (skin) temperature, and wind speed and direction over the ocean. An algorithm has already been chosen by which these EDRs are derived from a suite of radiometric measurements of brightness temperature [ref. 1-2]. Each “measurement” is characterized by a centerline radiometer wavelength and one of the 4 Stokes polarization components (1st, 2nd, 3rd or 4th Stokes), with the 3rd and 4th Stokes polarizations determined by 2 physical polarimetric brightness temperature measurements each. Measurements at 4 wavelengths are used to infer wind speed and direction (not all polarization components are measured so the number of physical measurements, n, is smaller than the fully-populated measurement array size of 24 measurements). The same n measurements plus two additional measurements at a 5th wavelength are used to infer skin temperature. The existing algorithm (in its slower-but-better form) performs retrieval in the following sequence:
This invention addresses the following inherent weaknesses in the existing algorithm:
These noise-associated retrieval errors can become a dominant source of error in the theoretical Tbi values.
This invention delays the evaluation of the skin temperature and atmospheric properties so that they are evaluated together with the wind speed at each candidate wind direction. The atmospheric properties are evaluated from a direct model, with arguments that include (in the simplest such model) Ts and the atmospheric columnar water vapor content V. The invention uses initial estimates of Ts, V and uw along with 4 evaluations of the model equations to numerically evaluate ∂Tbi/∂Ts ∂Tbi/∂V and ∂Tbi/∂uw for each of the measurement channels. The first three terms in a Taylor's series of Tbi(Ts,V,uw) are then used to generate an expression for Tbi in the neighborhood of the initial estimates. A figure-of-merit is defined, with a minimum value determining the most likely values of skin temperature and wind speed; this FOM consisting of the difference between measured brightness temperature and Tbi from the Taylor's series, squared and summed over the measurement channels. The expression for this FOM is then minimized wrt Ts, V and uw to yield three algebraic equations linear in Ts, V and uw. This classic least-squares-optimization yields updated estimates of skin temperature, atmospheric water vapor and wind speed. Optionally, a final evaluation of the model equations using the updated Ts, V and uw values yields a more accurate evaluation of the Tbi values and a better estimate of the FOM. After performing this process at all of the candidate wind directions, there has been generated an array of FOM, Ts, V and uw values vs wind direction. The final Ts, V, uw and wind direction best-guess-values correspond to the minimum FOM value.
For each measured brightness temperature Tbmi the corresponding theoretical brightness temperature in the neighborhood of estimated values Ts0, V0 and uw0 is represented by the truncated Taylor's series
Tbi≈f(Ts0,V0,uw0,φ)+∂Tbi/∂Ts(Ts−Ts0)+∂Tbi/V(V−V0)+∂Tbi/∂uw(uw−uw0)+
The partial derivatives are evaluated numerically from evaluations of the model equations using perturbed arguments, f(Ts0+ΔTs,V0,uw0,φ), f(Ts0,V0+ΔV,uw0,φ) and f(Ts0,V0,uw0+Δuw,φ). There are a large number (n) of these equations and three unknowns, Ts, V and uw. If only three of the equations were used to equate measurement to model, Ts, V and uw could be determined exactly. The remaining n-3 equations are redundant, but all n of the equations can be used by asking for a “best fit” instead of an exact solution; i.e. a classical least-squares-fit of Tbi to Tbmi. The difference between measurement and theory is squared and summed over the n measurements to yield the FOM,
FOM=Σ[Tbi−Tbmi]2
This is minimized wrt Ts, wrt V and wrt uw in turn:
These are a set of three linear algebraic equations of the form
a Ts+b V+c uw=d
that can be solved directly for those values Ts, V and uw that minimize the FOM. Because the model function fi depends on the wind direction, the optimized values Ts, V and uw will vary slightly with wind direction. The candidate wind direction bin that results in the smallest minimized FOM is most likely to contain the true wind direction and the associated true values of Ts, V and uw.
The prior art (see references 1-2) referenced by this invention was funded by the U.S. government and there are no known associated patents. The present invention is an improved version of an earlier patent by the same inventor (see reference 3). This invention is the sole property of the inventor, receiving no support from any outside sources.