UTM and ITMG are both coordinate systems, like the standard 6 digit grid used for military navigation. In that case it is merely 6 digits in what is in reality a 15 digit grid system, which contains the grid zone designator (first 3 digits, 2 numbers and a letter), 100,000 square identifier (next 2 digits, 2 letters) and 10 digits (all numbers, 10 digits gives a 1 meter accuracy). The PLGR (AN/PSN-11) can display 7 different coordinate systems and convert coordinates input in one to another within limits.
Both the ITMB and the OBL are based are only applicable in the British Isle and areas surveyed by them. It is based on the Airy Spheroid of 1830 are is being replaced with WGS-84
MGRS is based on the UTM/UPS coordinate system, the only difference being the UTM coordinates is that the 15 digits are all numbers. UTM is used within previous mentioned limits, the UPS system is used in polar regions
Don’t get Datum and Coordinate systems confused though. The coordinate system defines locations within a datum. The datum defines the starting point for the measurements that are used in the surveying of the earth within a spheroid. In most cases you can switch between coordinate systems while using the same Datum. There is 2 types of MGRS, the new and old. The New normally is used with the WGS 84 datum and the Old with NADs 27 and other non-US datums.
Spoofing is a form of electronic warfare, in which a false signal is transmitted in order to cause confusion or in the case of GPS receivers a decrease in accuracy or an inability to solve a position solution.
In normal conditions, the U.S. Government guarantees that these errors do not exceed 100 meters horizontal, 140 meters vertical, and a time accuracy of 340 nanoseconds(ns) 95 percent of the time. A position derived from a CA code however can be much worse in accuracy than 1 KM depending on various conditions. With SA turned off a non-survey GPS receiver can obtain a 12.1 m average error just using the CA code.
The P code or precision code is used by the military to increase accuracy. It is transmitted in a coded signal called the P(y) code, and is referred to as the Y code. The use of the Y code is one of the anti spoof capabilities of the PLGR. The exact methods used by US equipment cannot be discussed though. That info falls under the current definition of OPSEC.