Seminar at NUS 10th April 2019
Ammar Mian
We consider the problem of SAR change detection through covariance matrix equality testing.
Approach based on mutivariate statistical analysis. Pre-requisites:
Slides are available at: https://ammarmian.github.io/
For questions: ammar.mian@centralesupelec.fr
Considering a time series of SAR images, we want to detect spatial areas where notceable changes have occured.
Illustration on Terrasar X data
We consider a sliding windows approach to consider spatially local data.
On this window, we assume i.i.d observations:
\[ \forall (k,t)\, \mathbf{x}_k^t \sim \mathbb{C}\mathcal{N}(\mathbf{0}_p, \mathbf{\Sigma}_t)\]Idea: Compare the covariances $\mathbf{\Sigma}_t$ over time.
Denote by $\left\{\mathbf{X}_1,\dots,\mathbf{X}_T\right\}$ a collection of $T$ mutually independent samples of i.i.d $p$-dimensional complex vectors: $\mathbf{X}_t = [\mathbf{x}_1^{t},\dots,\mathbf{x}_{N}^{t}]$.
We assume $\forall (k,t),\,\mathbb{E}\{\mathbf{x}_k^{t}\}=\mathbf{0}_p$ and we denote $\mathbf{\Sigma}_t=\tau_t\boldsymbol{\xi}_t$ the shared covariance matrices among the elements of $\mathbf{X}_t$. $\boldsymbol{\xi}_t$ is the shape matrix ($Tr(\boldsymbol{\xi}_t) = p$) and $\tau_t$ is the scale.
We want to choose between the following alternatives:
\[ \left\{ \begin{array}{ll} \mathrm{H}_{0}: & \mathbf{\Sigma}_{1} = \ldots = \mathbf{\Sigma}_{T} = \mathbf{\Sigma}_{0} \, ,\\ \mathrm{H}_{1}: & \exists (t, t'),\, \mathbf{\Sigma}_t \neq \mathbf{\Sigma}_{t'} \end{array}\right. \]
We want to obtain:
a statistic of decision \(T\): \(\begin{aligned} \mathbb{C}^{p\times N}\times \dots \times \mathbb{C}^{p\times N} &\rightarrow \mathbb{R}^+\\ \mathbf{X}_1,\dots,\mathbf{X}_T &\rightarrow T(\mathbf{X}_1,\dots,\mathbf{X}_T) \end{aligned}\)
a threshold \(\lambda\)
So that \(\mathbb{P}\left(T(\mathbf{X}_1,\dots,\mathbf{X}_T)>\lambda/\mathrm{H}_1\right)\) is high while \(\mathbb{P}\left(T(\mathbf{X}_1,\dots,\mathbf{X}_T)>\lambda/\mathrm{H}_0\right)\) is low.
Suppose $\forall t,\, \forall k,\, \mathbf{x}_k^{t} \sim \mathbb{C}\mathcal{N}(\mathbf{0}_p,\mathbf{\Sigma}_t)$ so that $ p_{\mathbf{x}_k^{t};\mathbf{\Sigma}_t}(\mathbf{x}_k^{t};\mathbf{\Sigma}_t) = \dfrac{1}{\pi^p|\mathbf{\Sigma}_t|}\mathrm{exptr}\left\{\mathbf{S}_k^{t}\mathbf{\Sigma}_t^{-1} \right\} $, where $\mathbf{S}_k^{t} = \mathbf{x}_k^{t}{\mathbf{x}_k^{t}}^{\mathrm{H}}$.
Many statistic exists but the options can be reduced to$^1$ :
\begin{equation} \hat{\Lambda}_\mathrm{G} = \frac{\left|{\hat{\mathbf{\Sigma}}_0^{\mathrm{SCM}}}\right|^{TN}}{\displaystyle\prod_{t=1}^{T} \left|{\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}}}\right|^N} \underset{\mathrm{H}_0}{\overset{\mathrm{H}_1}{\gtrless}} \lambda,\, \mathrm{where:} \label{eq : GLRT Gaussian} \end{equation} \begin{equation} \begin{aligned} \forall t, \hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}} = \frac{1}{N}\displaystyle\sum_{k=1}^N \mathbf{S}_k^{t} \text{ and }\hat{\mathbf{\Sigma}}_0^{\mathrm{SCM}} = \frac{1}{T} \displaystyle\sum_{t=1}^T \hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}}. \end{aligned} \end{equation}
$^1$ D. Ciuonzo, V. Carotenuto and A. De Maio, "On Multiple Covariance Equality Testing with Application to SAR Change Detection," in IEEE Transactions on Signal Processing, vol. 65, no. 19, pp. 5078-5091, 1 Oct.1, 2017.
\begin{equation} \hat{\Lambda}_{\mathrm{t}_1} = N\displaystyle \sum_{t=1}^T \mathrm{Tr} \left[ \left(\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}}\left(\hat{\mathbf{\Sigma}}_0^{\mathrm{SCM}}\right)^{-1} -\mathbf{I}_p\right)^2 \right] \underset{\mathrm{H}_0}{\overset{\mathrm{H}_1}{\gtrless}} \lambda. \end{equation}
\begin{equation} \begin{aligned} \hat{\Lambda}_{\mathrm{Wald}} = & N \displaystyle\sum_{t=2}^T \mathrm{Tr}\left[\left(\mathbf{I}_p - \hat{\mathbf{\Sigma}}_1^{\mathrm{SCM}} (\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}})^{-1} \right)^2\right] \\ & - q\left(N \displaystyle\sum_{t=1}^T (\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}})^{-T} \otimes (\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}})^{-1} , \mathrm{vec}\left({\displaystyle\sum_{t=2}^T \boldsymbol{\Upsilon}_t}\right)\right) \underset{\mathrm{H}_0}{\overset{\mathrm{H}_1}{\gtrless}} \lambda, \end{aligned} \end{equation} \begin{equation*} \begin{aligned} \mathrm{where}\, \boldsymbol{\Upsilon}_t & = N \left( (\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}})^{-1} - (\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}})^{-1} \hat{\mathbf{\Sigma}}_1^{\mathrm{SCM}} (\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}})^{-1} \right).\\ q(\mathbf{x},\mathbf{\Sigma}) & = \mathbf{x}^{\mathrm{H}}{\mathbf{\Sigma}}^{-1} \mathbf{x} \end{aligned} \end{equation*}
CFARness:
The GLRT, $t_1$ and Wald statistic have the CFAR property with regards to the covariance parameter.
Proof:The statistics are invariant for the group of transformation $ \mathcal{G} = \left\{\mathbf{G}\, \mathbf{x}_k^{t}| ,\, \mathbf{G} \in \mathbb{S}_p^{\mathbb{H}} \right\}. $
Distribution under $\mathrm{H}_0$ (F-Approximation$^1$):
Under null hypothesis, we have: $ 2(1-c)\ln(\hat{\Lambda}_{\mathrm{B}}) \sim \chi^2\left((T-1)p(p+1)\right)$, where $c=\dfrac{T^2-1}{T(N-1)}\times\dfrac{2p^2+3p-1}{6(T-1)(p+1)}$ and $\hat{\Lambda}_{\mathrm{B}}$ is a modified version of $\hat{\Lambda}_{\mathrm{G}}$
There are similar results for others statistics.
$^1$ Box, G. E. P. “A General Distribution Theory for a Class of Likelihood Criteria.” Biometrika, vol. 36, no. 3/4, 1949, pp. 317–346.
Under $\mathrm{H}_0$ we have $^1$:
\begin{equation*} \begin{aligned} P\left\{ 2\rho\log(\hat{\Lambda}_\mathrm{G}) \leq z \right\} & \approx P\left\{\chi^2(f^2) \leq z \right\} + \omega_2 \left[ P\left\{\chi^2(f^2 + 4) \leq z \right\} - P\left\{\chi^2(f^2) \leq z \right\} \right] \\ f&=\,(T-1)p^{2}, \, \rho =\,1-\frac{(2p^{2}-1)}{6(T-1)p}\left(\frac{T}{N}-\frac{1}{NT}\right),\\ \omega_{2}&=\,\frac{p^{2}(p^{2}-1)}{24\rho^{2}}\left(\frac{T}{N^{2}}-\frac{1}{(NT)^{2}}\right)-\frac{p^{2}(T-1)}{4}\left(1-\frac{1}{\rho}\right)^{2} \end{aligned} \end{equation*}
$^1$ Anderson, T. W. (1962). An introduction to multivariate statistical analysis (No. 519.9 A53). New York: Wiley.
Sometimes the Gaussian hypothesis is not accurate !
To model this kind of distribution the family of elliptical distributions have been introduced$^1$
$^1$E. Ollila, D. E. Tyler, V. Koivunen and H. V. Poor, "Complex Elliptically Symmetric Distributions: Survey, New Results and Applications," in IEEE Transactions on Signal Processing, vol. 60, no. 11, pp. 5597-5625, Nov. 2012.
Probability distribution:
\[p_{\mathbf{x};g;\mathbf{\Sigma}}(\mathbf{x};g;\mathbf{\Sigma}) = \mathfrak{C}_{p,g}|\mathbf{\Sigma}|^{-1}g(\mathbf{x}^{\mathrm{H}}\mathbf{\Sigma}^{-1}\mathbf{x}) \]
where $g$ is a density generator function with some rgularity conditions, $\mathbf{\Sigma} \in \mathbb{S}^{\mathbb{H}}_p$ is the scatter matrix and $\mathfrak{C}_{p,g}$ is a normalisation constant.
Example:
Multivariate Student-t: ${{\Gamma \left ({{\nu+g}\over{2}}\right)\vert{{\Sigma}}\vert^{-1/2}}\over{(\pi \nu)^{{1}\over{2}}\Gamma \left ({{\nu}\over{2}}\right)\left \{1+{{\delta ({\bf y}, {\mu},{{\Sigma}})}\over{\nu}}\right \}^{{{1}\over{2}}(\nu+g)}}}$
Problem: How do we test the equality of scatter matrix in this model ? In peculiar, can we test scale and shape separately ?
Since the distribution of Gaussian-derived detector is known$^1$ under $\mathbb{C}ES$ model it is possible to correct it in order to obtain one keeping the CFAR property in $\mathbb{C}ES$ context.
Hallin proposed to use the following test$^2$:
$\mathcal{Q}_\mathcal{N} = 2\displaystyle\sum_{1\leq t\le t'\leq T} \mathcal{Q}_{\mathcal{N};t,t'} \geq \chi^2\left(\dfrac{(T-1)p(p+1)}{2}\right)_{1-\mathrm{P}_{\mathrm{FA}}}$, where:
\begin{equation} \begin{aligned} \mathcal{Q}_{\mathcal{N};t,t'}=\dfrac{1}{4(1+\hat{\kappa}_p)}\left\{\mathrm{Tr}\left[(\hat{\mathbf{\Sigma}}_0^{\mathrm{SCM}})^{-1}(\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}}-\hat{\mathbf{\Sigma}}_{t'}^{\mathrm{SCM}})^2 \right]\right. - \\ \left.\dfrac{\hat{\kappa}_p}{(p+2)\hat{\kappa}_p+2} \mathrm{Tr}^2\left[(\hat{\mathbf{\Sigma}}_0^{\mathrm{SCM}})^{-1}(\hat{\mathbf{\Sigma}}_t^{\mathrm{SCM}}-\hat{\mathbf{\Sigma}}_{t'}^{\mathrm{SCM}}) \right] \right\}, \end{aligned} \end{equation} and $\hat{\kappa}_p = p(p+1)/2\sum_{t=1}^T\sum_{k=1}^N d^4(\mathbf{x}_k^{t}, \hat{\mathbf{\Sigma}}_0^{\mathrm{SCM}})-1$ and $d(\mathbf{x},\mathbf{\Sigma}) = \|\mathbf{\Sigma}^{-1/2}\mathbf{x}\|$
$^1$ Yanagihara, H., Tonda, T., and Matsumoto, C. (2005).The effects of nonnormality on asymptotic distributions of some likelihood ratio criteria for testing covariance structures under normal assumption. Journal of Multivariate Analysis, 96(2):237{264.
$^2$ Marc Hallin, Davy Paindaveine, Optimal tests for homogeneity of covariance, scale, and shape, Journal of Multivariate Analysis, Volume 100, Issue 3, 2009, Pages 422-444,
Problem: We need to know $g$ !!!
The statistic reads: \begin{equation} \hat{\Lambda}_\mathcal{R}^{1/x^p} = \frac{|{\hat{\mathbf{\Sigma}}_0^{\mathrm{M}}}|^{TN}}{\displaystyle\prod_{t=1}^{T} |{\hat{\mathbf{\Sigma}}_t^{\mathrm{M}}}|^N} \displaystyle\prod_{t=1}^T \displaystyle\prod_{k=1}^N \dfrac{\left({\mathbf{x}_k^{t}}^{\mathrm{H}}\{\hat{\mathbf{\Sigma}}_t^{\mathrm{M}}\}^{-1}\mathbf{x}_k^{t}\right)^p}{\left({\mathbf{x}_k^{t}}^{\mathrm{H}}\{\hat{\mathbf{\Sigma}}_0^{\mathrm{M}}\}^{-1}\mathbf{x}_k^{t}\right)^p}\underset{\mathrm{H}_0}{\overset{\mathrm{H}_1}{\gtrless}} \lambda, \end{equation}
Properties$^1$: - This statistic is valid for testing the Shape matrix for any elliptical distribution.
- The distribution of $2\log(\hat{\Lambda}_\mathcal{R}^g)$ under $\mathrm{H}_0$ is assymptotically that of a $\chi^2 \left( (T-1)p(p+1) \right)$
Problem: We can't test a change in the scale !
$^1$ A. Mian, J. Ovarlez, G. Ginolhac and A. M. Atto, "A Robust Change Detector for Highly Heterogeneous Multivariate Images," 2018 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Calgary, AB, 2018, pp. 3429-3433.
$^1$ A. Mian, G. Ginolhac, J. Ovarlez and A. M. Atto, "New Robust Statistics for Change Detection in Time Series of Multivariate SAR Images," in IEEE Transactions on Signal Processing, vol. 67, no. 2, pp. 520-534, 15 Jan.15, 2019.
$^1$ A. Mian, G. Ginolhac, J. Ovarlez and A. M. Atto, "New Robust Statistics for Change Detection in Time Series of Multivariate SAR Images," in IEEE Transactions on Signal Processing, vol. 67, no. 2, pp. 520-534, 15 Jan.15, 2019.