Ø LHA = GHA + E Longitude or W Longitude
3. Chosen Position (CP)
Ø DR usually shown in degrees and minutes e.g. 48° 10' N, 5° 35' W. We can use an alternate position CP so that a whole degree of latitude and longitude can be used.
Ø For CP's latitude choose round degree of latitude nearest the DR latitude
Ø For CP's longitude, choose one that will result in a whole degree of LHA when subtracted from or added to GHA
o i.e. For W longitude, CP longitude's minutes are made the same as GHA's minutes
e.g. DR long 21° 34'.0 W
GHA 93° 45'.0
CP long W - 21° 45'.0
o i.e. For E longitude, CP longitude's minutes are made the complement of GHA's minutes
e.g. DR long 33° 18'.0 E
GHA 284° 45'.0
CP long E +33° 14'.0
o Note that the CP's longitude must be within 30' of DR longitude, thus it may necessary to change the degree at times.
4. Use Sight Reduction Tables (Appendix 3 Extracts from Sight Reduction Tables for Air Navigation AP 3270 Vols 1-3)
Ø Need: Whole degree of CP latitude, whole degree of LHA, Declination
Ø Find tables for CP's latitude in AP 3270 e.g. lat 50°
Ø If Dec and CP latitude both N or both S, use same table, otherwise use contrary table. Use the one that contains Declination, either the 0°-14° or 15°-29° table (Appendix 3 Figs. A3.1-A3.7)
Ø Look under column with degree of Dec e.g. 3°: Get Hc, d and Z.
Ø Refer to formula at top left-hand corner of table: If LHA > 180°, Zn (azimuth)=Z. If LHA < 180°, Zn=360-Z.
Ø Look at Table 5 Correction to Tabulated Altitude for Minutes of Declination (Appendix 3 Fig A3.8): Find Dec's minutes and d and make correction to Hc
Example 3b: A complete sight
(Use extracts in Appendix A)
On the morning of 19 May at 07h 52m 05s DWT in DR position 50° 15'N 9° 40'W, a sight was taken of the sun's lower limb giving a sextant altitude of 27° 01'.4. Index error 2'.0 on the arc, height of eye 2.0metres. The DWE was 10s slow.