# Data compression and storage formats

In order to deal efficiently with data from the AIRSAR and SIR-C polarimetric radars, scientists at the Jet Propulsion Lab (JPL) sought a way of storing and distributing the data that was simple, compact and contained all the essential information for data interpretation and classification. Rather than storing the four complex elements of the scattering matrix, possibly using 32 bytes per pixel, they chose the Stokes

**Did you Know?**

Did you know that the Stokes and covariance matrices contain phase information, even though they are power representations? This is because the cross terms such as the expected value of (E_{hh} E_{vv}*) are complex numbers, and the angle of the complex number depends upon the phase angle between the HH and VV channels.

(Kennaugh) matrix for the AIRSAR data, and compressed each sample (or group of averaged samples) into a 10-byte word. Other radar systems have used the covariance matrix for data compression , page 292.

In the JPL scheme, the total power of each sample is computed and stored in 2 bytes, one for the mantissa and one for the exponent. The remaining 8 unique elements of the Stokes matrix are normalized by the top left element, M11, and stored in 1 byte each. The four smallest of these, related to the cross-products of the co-pol and cross-pol channels, are square rooted as well. The original elements of the Stokes matrix are easily recovered from the stored values .

With higher storage capacities becoming available, it is possible to store the full scattering (Sinclair) matrix for each sample, without averaging to reduce the data volume. More sophisticated methods of compressing radar image data have been developed for single channel data, e.g. based upon the DCT or wavelets, but these methods have not been fully tested on polarimetric data yet.

Whiz quiz

**Question 1:** What is meant by a "scattering mechanism"? The answer is...

**Question 2:** How is a "scattering mechanism" defined? The answer is...

**Question 3:** Why is the covariance matrix considered to be a "power" representation? The answer is...

Whiz quiz - answer

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