Coverage problem (transmitter and receiver)
You can use the Voronoi diagram, together with Kirkpatrick's data structure to solve this problem.

Like Raphael and Syzygy suggested, you can use Fortune's (sweepline) algorithm to create the Voronoi diagram. Worst case time: $\mathcal{O}(n \log n)$.

The Voronoi diagram will have a bunch of polygons, each one containing a transmitter. Any point within the polygon is closest to that transmitter. Thus, if you can find out which polygon contains the receiver, you can find the closest transmitter to it by somehow finding out which polygon it is in. After that, you check if that transmitter is within $1\ \text{km}$.

To determine which Voronoi polygon contains the receiver, you first triangulate each polygon in the diagram. Now you have a triangle mesh. Next you use Kirkpatrick's data structure to locate any triangle containing a given point in $\mathcal{O}(\log n)$ time, worst case. Constructing Kirkpatrick's data structure takes $\mathcal{O}(n\log n)$ worst case. Once you know the triangle, you will know the polygon that contains it, and thus the closest transmitter. Doing this for all the receivers will be $\mathcal{O}(n\log n)$, worst case.


  Each cell in a Voronoi 
  diagram is a convex polygon, possibly 
  unbounded.
  
  ...
  
  The number of vertices [of a Voronoi diagram of n sites] V ≤ 2n-5
  
  ― www.cs.arizona.edu


Each polygon in a Voronoi diagram is a convex polygon. Therefore, since triangulation of a convex polygon takes $\Theta(v)$ time, $v$ being the number of of sides, we can triangulate each cell efficiently. If you fear that triangulation can be pathological (that we might have $n$ cells, each with $n$ sides), then consider this. The Voronoi diagram has $\mathcal{O}(n)$ vertices (see quote above). The triangulation of the Voronoi diagram is planar, and so is a sparse graph, and thus has $\mathcal{O}(n)$ edges and faces. Thus triangulation of a particular cell might take $\mathcal{O}(n)$, but the overall triangulation also takes $\mathcal{O}(n)$. (Basically, we will not encounter many cells with $\mathcal{O}(n)$ sides, this would result too many triangles for a planar graph).


Great explanation/implementation of Fortune's algorithm
Illustration of Kirkpatrick's data structure
Psuedocode for Kirkpatrick's algorithm