View Full Version : Difference between IRS and INS

15th Aug 2001, 16:35
Hey guys,

Could any one explain the difference between IRS and INS.

I understand the INS cannot work in polar regions.


15th Aug 2001, 21:10
An INS is a self contained navigation system, i.e. you can program waypoints and the system will direct you point to point. It generates its own attitude platform and inertial position information (like an IRS). It also provides attitude reference. Think of an INS as an IRS with a navigational computer integrated into one system.

An IRS is a inertial REFERENCE system only. It provides attitude reference and present position information (along with some misc info like speed, heading etc). The position information is provided to separate flight management systems (FMS) that do the navigational computations with pilot interface.

INS systems are rapidly falling from use due to the increased adaptation and cost effectiveness of Flight Management Systems that use IRS laser-ring gyro systems for basic position/attitude inputs.

INS systems like the Carosel and Sperry are very expensive to maintain due to the complex mechanical components and are now outmoded. Properly maintained they are reliable and provide good accuracy, but cost and reliablilty becomes a major factor.

[ 15 August 2001: Message edited by: Roadtrip ]

16th Aug 2001, 00:22
I understand that an INS is a mechanical, stabalised platform using gyros and gimbals to keep it horizontal (ref. to the Earth).

An IRS is a second generation strapdown system which is bolted to the aircraft, and using laser ring gyros to work out its orientation (and maintains a stable platform mathematically).

16th Aug 2001, 02:15
Thanks a lot. What would a 747-200 and a 747-400 use?

16th Aug 2001, 03:05
I think that the INS using a gyro stabilised platform keeps itself orientated to the north pole so if you go directly over it, the platform would flip and not work properly. A laser ring IRS system would not suffer from this so could be used over the pole. This might rubbish though.

16th Aug 2001, 08:50
747-200 Generally a Triple Carosel or Sperry INS

747-400 Triple IRU w/Dual FMS

Dan Winterland
19th Aug 2001, 01:50
FF, not rubbish. The INS platform comes in two varieties, the North aligned platform and the rotating platform. The North aligned platform (as it's name suggests) constantly corrects it's position to align to north. If you fly over the pole, it can't rotate fast enough and 'crashes'. The rotaing platform slowly rotates to minimise schluer loop or pendulum errors and consequently is (slightly) more accurate. The Carousel is a rotaing platform, hence the name.

20th Aug 2001, 07:38
One can use the INS's over the pole no problem, the issue is that the INS's cannot realign themselves at lattitudes greater than 76'.30" north or south, so should one for some strange reason land at these lattitudes the INS's wont align. to my knowledge this lattitude limit is due to the northern most airfield that can be used by heavies being Thule.
Glaciers dont make ideal landing sites.

20th Aug 2001, 11:05
Might I suggest that in fact they are both part of the same.
INS = Inertial Navigation System, a nav system that uses acceleration information from an inertial reference system to provide through computation in all 4 axes a position in space from a predetermined point. In the terrestrial case the surface of the earth
IRS = Inertial Reference System is the actual inertial platform that measures the acceleration inputs in all axes that provides the means of measurement and computation to the INS. This can be gyro (either free or strapdown) or ring laser. This measurement is relative to a starting point in space, therefore the INS must be oriented and aligned with the grid along which it must provide navigational information. In our case reference earth and Lat and Long. This is why it is necessary for the sytem/aircraft to remain stationary for a period of time on system start up to set reference zero acceleration within the IRS and allow alignment with the navigation grid and reference time. The old gyro systems may have taken up to 60 minutes to align with the modern ring laser only minutes.

FMS use a combination of available land based navaids, IRS and ultimately GPS to arrive at the most accurate position available by crosscheck.
As they all have differing degrees of accuracy over time and area the mode that is available or being used is always annunciated.

20th Aug 2001, 12:14
The latitude limit for inititalizing IRN or INS plantforms is because the platform cannot sense the latitude of the aircraft with sufficient accuracy as you approach the true pole - either North or South.

22nd Aug 2001, 22:06
Ok can some one explain to me what Appolo used? I am currently reading a book about the space programme, and i am interested to know, was it an INS and if it was what was it aligned to? For those smarter than me, I thank you for the answers

23rd Aug 2001, 11:09
The INS system was developed for the Apollo program by MIT. On 09 August 1961 the first Apollo development contract Program was awarded. NASA selected MIT's Instrumentation Laboratory to develop the guidance-navigation system for Project Apollo spacecraft. This first major Apollo contract was required since guidance-navigation system is basic to overall Apollo mission. The Instrumentation Laboratory of MIT, a nonprofit organization headed by C. Stark Draper, has been involved in a variety of guidance and navigation systems developments for 20 years. This first major Apollo contract had a long lead-time, was basic to the overall Apollo mission, and would be directed by STG.

All primary Apollo 7 mission objectives were met, as well as every detailed test objective (and three test objectives not originally planned). Engineering firsts from Apollo 7, aside from live television from space, included drinking water for the crew produced as a by-product of the fuel cells. Piloting and navigation accomplishments included an optical rendezvous, daylight platform realignment, and orbital determination via sextant tracking of another vehicle. All spacecraft systems performed satisfactorily. Minor anomalies were countered by backup systems or changes in procedures. With successful completion of the Apollo 7 mission, which proved out the design of the Block II Crew Service Module (CSM 101), NASA and the nation had taken the first step on the pathway to the moon.

Apollo 10 . During the return trip the astronauts made star-lunar landmark sightings and star-earth horizon navigation sightings.

All cut & paste from this site (http://www.friends-partners.org/mwade/craft/apolocsm.htm).