Could the cosmic microwave background radiation (CMBR) map be used as a backup for GPS navigation? Using a sextant and a clock you can navigate by measuring the position of the observable stars and comparing them with the known map of the fixed stars. However, if it was cloudy couldn't you theoretically use the CMBR map instead?
The cosmic microwave background radiation (CMBR) map is a constant pattern in the sky through which the earth travels and rotates. Given an accurate clock, microwave antenna and model of the earth's orbit, shouldn't it in principle be possible to match the patch of the the CMBR pattern above you with the overall CMBR map and therefore calculate where you are located on the earth's surface?
Yes, I appreciate that measuring the CMBR sufficiently accurately at 160GHz is not trivial and needs a large antenna (preferably a phased array), probably with a cryogenically cooled microwave receiver. However, an antenna of this size should be easy to accomodate onboard a large military ship for example. There would be a tactical advantage in the USA turning off the GPS satellites and switching to CMBR-based navigation prior to launching an attack.
Presumably for this application some of the the major sources of microwave interference that scientists normally have to carefully subtract in order to get the classic CMBR map could be left in the data if they are from outside the solar system (e.g. milky-way sources of microwaves)?
What are the other practical issues and how might they be resolved to make a working system?
 A: Not really. In principle, the information is there. In practice, it's impossible to get the kind of precision needed from any device (let alone a consumer grade one) on the surface of the earth.
It takes a scientific instrument in space (see COBE) to measure the minuscule anisotropy in the CMBR. If you were desperate, there are other methods (such as stellar navigation) that would have an actual chance of suceeding.
To give an earthly comparison, that would be like trying to determine your location in a soccer field by measuring the spectrum from half a dozen blades of grass on the ground and comparing it against a previously compiled chemical composition map (but much, much harder). Frankly, there are better ways to do it.
A: Pulsars potentially provide a natural, deep-space GPS. Probably not useful for navigation on or near the earth's surface, but DARPA has funded research at Johns Hopkins under the name XNAV for use by spacecraft: article at Space Review, rather minimal Wikipedia article with useful links.
Mentioned in passing in a presentation to the IAU linked from the Wikipedia article is the interesting point that you can count the pulses from pulsars. This means they provide a stable time-base and would be useful in locating a probe moving at relativistic velocities relative to its starting point. That is, set the count to 0 when the probe departs. As it moves toward (away from) the pulsar, it counts more (fewer) pulses than have been counted at its launch point on Earth, allowing it to be located in space (by the angle to the pulsar) and in time (by pulse count).
