Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Relativistic Positioning Systems: The Emission Coordinates   This paper introduces some general properties of the gravitational metric and
the natural basis of vectors and covectors in 4-dimensional emission
coordinates. Emission coordinates are a class of space-time coordinates defined
and generated by 4 emitters (satellites) broadcasting their proper time by
means of electromagnetic signals. They are a constitutive ingredient of the
simplest conceivable relativistic positioning systems. Their study is aimed to
develop a theory of these positioning systems, based on the framework and
concepts of general relativity, as opposed to introducing `relativistic
effects' in a classical framework. In particular, we characterize the causal
character of the coordinate vectors, covectors and 2-planes, which are of an
unusual type. We obtain the inequality conditions for the contravariant metric
to be Lorentzian, and the non-trivial and unexpected identities satisfied by
the angles formed by each pair of natural vectors. We also prove that the
metric can be naturally split in such a way that there appear 2 parameters
(scalar functions) dependent exclusively on the trajectory of the emitters,
hence independent of the time broadcast, and 4 parameters, one for each
emitter, scaling linearly with the time broadcast by the corresponding
satellite, hence independent of the others.

23
The categories of this paper are: solv-int
32
The categories of this paper are: atom-ph
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex
33
The categories of this paper are: plasm-ph
0
The categories of this paper are: hep-ph
17
The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
3
The categories of this paper are: physics
35
The categories of this paper are: mtrl-th
6
The categories of this paper are: astro-ph
22
The categories of this paper are: chao-dyn