I find it hard to find information on the so-called "Gaussian null coordinates", which [Wikipedia](https://en.wikipedia.org/wiki/Construction_of_a_complex_null_tetrad) says is used to describe "near horizon geometries". Can someone provide a reference where I can read about the basics and usage of them?



Gaussian null coordinates
The [M.Sc. thesis](http://www.theorie.physik.uni-goettingen.de/forschung/qft/theses/dipl/Morfa-Morales.pdf) of Eric Morales dicusses the basics of Gaussian null coordinates in appendix D and gives an application in chapter 3.


