A modern ring Laser Reference Unit is fairly complex, and it is extremely sensitive to movement in all axes - as you would expect. It does not suffer so badly from the mechnical degradation of the gyro systems. It will always give an output relative to True North - it also has a database (which needs updating by the manufacturer from time to time) which stores the Magnetic Variation around the world. Note a unit that does not have the Magnetic Database is referred to as an AHRS unit since it provides Attitude, Heading Reference only. So it is the application of the Database to True North which provides the output to the aircraft.
When in the "ATT" position, the unit is completely immune to wind or any other influence since it senses raw data and is effectively an AHRS unit. Once in "NAV" then the unit will start to be affected by external influences - hopefully at a rate not worse that 3+3T where T is Time and 3 is NM. so after 6 hours an individual unit should not be more than (6x3)+3=21 NM from the actual position.
Civilian units can only be aligned (wind, etc zeroed) on the ground but Military units can be aligned in flight.... and are more accurate.
I have installed several Honeywell Laser reference units in aircraft in the UK.
To do this on modern digital aircraft, the aircraft is jacked (in the hangar) and the units are individually levelled in pitch and roll axis.... normally using a digital spirit level at reference points on the aircraft. By taking the 429 output to the Bus with an ARINC 429 reader, you can achieve accuracy of 3 decimal places if you have the time and effort....... It should be noted that just tightening the rack down on the Avionics shelf is enough to change the reading, such is the sensitivity....
Once aligned in Pitch and Roll the aircraft is wheeled out of the hangar and placed on a surveyed line of True North. A plumb bob is dropped from the nose and tail and a measurement is taken and the aircraft difference from the nose to the survey line and the tail to the survey line is calculated..... back to the classroom and use a little pythagoras and you come up with the adjustment needed to read True North....
Now for a funny story.... a number of years ago, I asked the Ordanance Survey to survey a line for me so that I could perform the above.... having tried to align the aircraft which has only +/- 3 degrees of adjustment, I was disturbed to find I needed much, much more. This was on both Avionics racks - one located on the left and one located on the right of the aircraft with each of the 3 units installed indicating a similar large adjustment...... The man from the Ordanace Survey was called, who in no uncertain terms clearly was unhappy that some spotty individual was questioning the accuracy of his work.... "Don't you know who I am" he railed..... Having checked his line, he embarrassingly pulled up his marker and moved it a bit..... he had perfectly surveyed the reference line to GRID North which of course all UK Ordinance Survey Maps are aligned to but are a few degrees from True North.....