Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Problems and obstacles in their use are represented by a complex pre-processing, the acquisition geometry that can affect the quality of the images over mountainous areas where landslides are likely to occur, a certain difficulty in using the SAR signal in traditional statistical classification approaches (mainly due to speckling) [19], and perhaps a certain attitude against using this kind of product in the landslide mapping community.
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