What is the difference between two models of situation awareness? The most famous definition of situation awareness (SA) is the three-level model proposed by Endsley (1995):


  the perception of environmental elements with respect to time or space [(Level 1 SA)], the comprehension of their meaning [(Level 2 SA)], and the projection of their status in the near future [(Level 3 SA)]. 


However, the definition of Smith and Hancock (1995) is based on Neisser's perceptual-cycle model (Neisser, 1976) and states that SA is: 


  adaptive, externally-directed consciousness that has as its products knowledge about a dynamic task environment and directed action within that environment. 


How do these models differ from each other, and do these differences have practical implications?

Ps. Many more definitions of SA exist. These are all summarized in this document.



References

Endsley, M. R. (1995). Toward a theory of situation awareness in dynamic systems. Human Factors: The Journal of the Human Factors and Ergonomics Society, 37(1), 32-64.

Neisser, U. (1976). Cognition and reality: Principles and implications of cognitive psychology. WH Freeman/Times Books/Henry Holt &amp; Co.
ISO 690 

Smith, K., &amp; Hancock, P. A. (1995). Situation awareness is adaptive, externally directed consciousness. Human Factors: The Journal of the Human Factors and Ergonomics Society, 37(1), 137-148.
Short answer

Endsley considers SA to be a product. Smith and Hancock argue that SA is a process. Deciding for a definition is important, since it can determine the focus of the research and methodology. 

Long answer

The Three-level model proposed by Endsley considers SA to be a product. In other words, SA is a mental picture that can be tested by testing an operators knowledge about the current situation. On popular mean of doing is the Situation Awareness Global Assessment Technique (SAGAT; Endsley, 1988). With the SAGAT, researchers freeze the task (in a simulator) and present the operator with a set of queries, that cover each of the three levels. Examples of these questions in the domain of train traffic controlling is shown in Table 2 (taken form Lo, Sehic and Meijer, 2013). 



The perceptual-cycle model argues that SA is a cyclical process of attention, in which perception, knowledge, anticipation and action-taking take place concurrently, in order to generate a `product of consciousness', that is, an understanding of the current situation (see Figure 3, taken from Smith and Hancock, 1995). The product can not be not be seen separately from the process, however; The perception of relevant information will lead to a better understanding, but a better understanding will also direction your attention. 

Measuring one's SA as a process implies measuring how well the acquisition of SA is over time. This has, among others, been done with tools such as eye tracking and EEG (For a review, see Salmon, 2006). The perceptual-cycle model does acknowledge the existence of a product of consciousness (and the testing of it with the SAGAT), but this provides an incomplete picture.