Range spectral shift這個問題在斜坡效應中其實已經說過了。理解這個問題的關鍵在於：
- 知道頻率是相位的微分（但這個（干涉數據的）頻率跟載頻、採樣頻率等其他的頻率並不能混爲一談）。這樣就能夠很好地理解“斜坡效應”部分根據頻譜偏移的公式解釋條紋密度隨着地形坡度的關係，因爲頻譜偏移本質上就對應相位變化；
- 觀測視角的變化對應頻率的變化；
- 這個問題和斜坡效應、失相關以及critical baseline都是相通的
頻譜偏移和相位梯度那些事兒（由此導出了一個條件限制）
Other sources of decorrelation are more significant and non-reversible. The two most important conditions are related to the phase gradient and the temporal variation in the physical distribution of the elementary scatterers.
The phase gradient condition can be conveniently described in the spectral domain. The temporal bandwidth of the SAR images in range corresponds with a spatial bandwidth due to the projection on the earth’s surface. A phase gradient in range of n cycles/pixel corresponds with a spectral shift between the spectra of both acquisitions of n*f Hz, where f is the sampling frequency.
The spectral shift results in a decreased overlap between the corresponding parts of the spectrum (the signal) and an increasing non-overlapping part of the spectrum (the noise). Due to the limited bandwidth, a phase gradient larger than B/f cycles/pixel (approximately 0.822 for ERS) results in a zero overlap between the spectra, hence a complete loss of correlation. The occurrence of this situation is dependent on: the length of the perpendicular baseline, the steepness of the topographic slopes, and/or the gradient of the surface deformation.
說明
理一理這裏的邏輯：
這個頻譜偏移會導致失相關 是個壞東西 應該消滅掉
他代表的是相位隨着斜距的變化 是由於視角差造成的
如果沒有視角差 就沒有干涉相位（或者說干涉相位是零，這裏不考慮形變） 自然也就沒有干涉相位的變化
這個視角差使得我們能夠通過干涉相位進行地形測繪，但她同時導致了干涉相位的變化這個“副產品”。我們應該消除掉這個副產品，他和干涉相位不是一回事兒。
以上說明了Range spectral shift的由來
那麼怎麼進行濾波呢？
參考文獻
Hooper, A., Bekaert, D., Spaans, K., & Arıkan, M. (2012). Recent advances in SAR interferometry time series analysis for measuring crustal deformation. Tectonophysics, 514, 1-13.
Zhong, L., & Dzurisin, D. (2014). Insar imaging of aleutian volcanoes. Springer Praxis Books, 2014(8), 1778–1786.
Ketelaar, V. (2009). Satellite radar interferometry : subsidence monitoring techniques.