I've noticed in some videos,that there are two IRS switches on the 737NG. Why is that so and what is the primary purpose of IRS? Also, what is the alignment of the IRS mean?
Inertial Reference System is a solid state set of 3 ring gyros which navigate using dead reckoning. The system measures accelerations in and rotation around each of the 3 axis so can calculate where it is by adding how much it has moved to the initial position.
A simple example: The aircraft is stationary pointing North. Over a 10 minute period, it measures the accelerations in the X and Y axis and calculates that is has moved +100 kms in the X axis (i.e. North) and +100 kms in the Y axis (i.e. West). By simple trigonometry, we can calculate that the aircraft is approximately 141.4 kms on a bearing of 45 degrees from it's initial position.
The gyros must be aligned with the aircraft frame of reference when it is stationary. This is the "alignment" process and generally must be completed before the aircraft moves. It may also take inputs of the current position from a manual keyboard or from some other system, e.g. GPS.
More modern IRS fits can align when the aircraft is moving, taking an updated position from GPS.
There are 2 systems on the 737, simply for redundancy.
There are 2 selectors, one for each unit. There are 2 units for redundancy and certification.
Purpose of the IRS
The IRS (inertial reference system) function is to provide the aircraft location/attitude in flight, computed from the position/attitude sensed during its alignment and rotation/acceleration variations accumulated since the alignment.
An IRS provides a reference to the true north (and often the magnetic north equivalence by reading in a table).
In airliners, inertial references are generally included in a broader computing chain, the ADIRU, which also provides air references calculated from air probes (a.g. airspeed and angle of attack).
How it works
The purpose of the IRS alignment is to detect the starting latitude and attitude, by sensing the Earth acceleration. It must be done when the a/c is at rest. To minimize the sensor error, the values sensed are integrated over a period of time (10 mn or so).
The longitude which cannot be sensed using Earth acceleration is provided through other means (like GPS or last position).
Attitude only mode
If the IRS is restarted in flight, then only the attitude can be sensed again. The location reference will be lost (this is what means the ATT mode on the selectors).
Use of IRS
IRS are a complement of the GPS, but IRS has the advantage of being independent of any ground or space equipment. This is why there are still found in the avionics of modern aircraft.
Sensing and integration errors accumulates with time after the alignment.
Precision of IRS compared to other systems: