1. It is a DR Navigation System which gives Great Circle Tracks/ Distances and True Direction. It consists of two accelerometers which measure aircraft accelerations in N-S and E-W direction.
2. It has a Gyro Stabilised Platform, horizontally stabilised, to ensure accelerations are measured in the horizontal plane only. Three torque motors, two accelerometers and three rate gyros sensitive in each axis are mounted on the horizontally stabilized platform. The Zero position denotes the present position of the aircraft from which the georef coordinates are taken to initialize the system. A total of 9 way points can be fed into the system.
3. Control and Display Unit (CDU).
Fig 1 – Control and Display Unit
4. TK/GS(Track and Groundspeed). The INS computed track, usually referenced to magnetic north, is displayed to the nearest tenth of a degree in the left display and the groundspeed in knots in the right display. For example, a current track of 135 M and a groundspeed of 467 knots would appear as 135.0 and 0467.
5. HDG/DA(Heading and Drift Angle). The heading obtained from the angle between the platform frame and north reference is displayed to the nearest tenth of a degree in the left display. The angular difference between heading and track (drift angle) is displayed to the nearest tenth of a degree in the right display, preceded by the letter R or L to indicate whether drift is right or left. Thus, a heading of 137 M on a track of 135° M would be presented as 137.0 and L 02.0.
6. XTK/TKE (Cross Track Distance and Track Error Angle). Cross track distance is the distance by which the aircraft is displaced right or left of the desired great circle track and is displayed in the left display to the nearest tenth of a nautical mile. The track error angle is the angular difference, right or left, between the desired great circle track and the actual track being made, to the nearest tenth of a degree. If the aircraft were displaced 1½ nm to the left of the desired track of 135°M, the left display would read L 01.5. If the track being made good happened to be 130°M, the right display would read L 005.0.
7. POS (Present Position). The aircraft’s current latitude and longitude are shown in terms of Longitude and Latitude in the left and right displays, respectively. For example, 24°15’07’’N and 78°23’36’’W,
8. WPT (Waypoint Positions). The position of each inserted waypoint is shown as latitude in the left display and longitude in the right display by selecting WPT on the rotary selector switch and scrolling through the waypoint numbers with the waypoint selector wheel.
9. DIST/TIME (Distance and Time to the Next Waypoint). The distance in nautical miles from the present position to the next waypoint is shown in the left display and the time at present groundspeed to the nearest tenth of a minute in the right display.
10. WIND (Wind Speed). The INS is able to compute wind direction and speed and these are displayed in the left and right windows, respectively, to the nearest degree of arc and knot.
11. DSR TK/STS (Desired Track and Status). The Great Circle track from one waypoint to the next changes as the aircraft progresses between the two and INS computes the present desired magnetic track based upon distance from the waypoints, magnetic variation and the assumption that the aircraft is on track. This will appear in the left display to the nearest tenth of a degree and the right display will be blank. The status function is for use only whilst the INS is in ALIGN mode and it shows a numerical display in the right window that indicates the status of the alignment procedure. The display typically shows 99 at the start of alignment and counts down to 0, when alignment is completed and READY NAV is illuminated.
Q. Way Point(WP) 4 is 60°N 90°W, WP3 is 60°N 70°W. Find distance shown at WP3 for WP4 (a) 605 Nm, (b) 600 Nm (c) 594 Nm (d) None.
A. Dep = dlong x 60 x Cos 60 = 20 x 60 x 0.5 = 600 Nm (Rhumb Line Distance), remember GC distance is slightly lesser than Rhumb Line distance, Hence 594 Nm is the correct answer.
Q. What is the track shown by INS on leaving WP3 in the above question (a) 270° (b) 279° (c) 261° (d) 099°.
A. Conversion Angle = ½ dlong x Sin Mean Lat = ½ x 20 x Sin 60 = 9°. RL Track =270 + 9° = 279° which is the GC track.
Q. What is the track shown before arrival at WP4 in the above question? 270° A. It will be 9° less than 270 = 261° and curving left 261°
Q At 80°W INS indicates 0 XTKE, what is the latitude shown by INS?
A. Distance off from rhumb line can be calculated by taking half the CA i.e 4.5° and applying 1 in 60 rule. Distance at 80°W=300Nm =R, θ = 4.5° hence s = 22.5 Nm = 22.5’ (1’ = 1 Nm). Lat = 60°30’N
Q. WP7 is 52°27’S, 017°45’W, WP8 is 52°S27’S, 004°15’E. Find on leaving WP7,(a) DSR/TRK (b) Distance (i) 804 nm (ii) little less than 804 (iii) 494 Nm (iv) little less than 494 Nm (c) Initial true Hdg on leaving was 080 assuming constant drift find true hdg on reaching WP8.
A. (a) CA =½ dlong x Sin Mean Lat = ½ x 22 x Sin 52° 27’ = 9°. RL Track =090 + 9° = 099° which is the GC track. (b) Dep = dlong x 60 x Cos 52° 27’ = 22 x 60 x 0.6 = 800 Nm (Rhumb Line Distance), remember GC distance is slightly lesser than Rhumb Line distance, (ii) is correct. (c) Initial Track was 099 but ac is flying 080 hence stb drift of 19°. At WP 8 Track would be 090 -9 -19 =062°
INS Error. Distance from final ramp position to final INS position divided by time is termed as INS error
Q. After 6h24m flight when at parking bay position 52°18.5’N, 4°45.9’E the ins shows a position of 52°20.7’N 4°40.3’e. Find INS Error
52° 20.7’N Dep = dlong x Cos Lat = 5.6 x Cos 52°18.5’ =3.42 Nm, 04° 40.3E Similarly Dist covered in Northerly Hdg = 2.2 Nm, Tan(2.2/3.42) = Tan (.643) or = 33° Hypotenuse distance 2.2Nm 33° 52° 18.5’N = 4.066 Nm hence error =4/6:24 = 0.64 Nm/hr in 303° 3.42 Nm 04° 45.9’E