Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The NRL Program in X-ray Navigation   This chapter describes the development of X-ray Navigation at the Naval
Research Laboratory (NRL) within its astrophysics research programs. The
prospects for applications emerged from early discoveries of X-ray source
classes and their properties. Starting around 1988 some NRL X-ray astronomy
programs included navigation as one of the motivations. The USA experiment
(1999) was the first flight payload with an explicit X-ray navigation theme.
Subsequently, NRL has continued to work in this area through participation in
DARPA and NASA programs. Throughout, the general concept of X-ray navigation
(XRNAV) has been broad enough to encompass many different uses of X-ray source
observations for attitude determination, position determination, and
timekeeping. Pulsar-based X-ray navigation (XNAV) is a special case.

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The categories of this paper are: hep-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-bio
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The categories of this paper are: math-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: chem-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: stat
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: physics
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The categories of this paper are: funct-an
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The categories of this paper are: dg-ga
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The categories of this paper are: gr-qc
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The categories of this paper are: atom-ph
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The categories of this paper are: patt-sol