NavList:
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Re: Silicon Sea: Leg 72 -NEW
From: Michael Wescott
Date: 2001 May 26, 13:43 EDT
From: Michael Wescott
Date: 2001 May 26, 13:43 EDT
> Sight Reduction and Revised > Dead-Reckoning problem Leg 72 052301 Test your knowledge! > ---------------------- ------ ------ -------------------- > > Our Last position FIX was (13d 00.0'N 61d 30.1'W) at 08:50:10UT on > 30/05/2001. We are bound for a point on chart INT 401 marked as Aruba Gap > (13d 50.0'N 70d 00.0'W). Aruba Gap provides a favorable westerly current of > 1.5 Knts. > > We have had overcast skys and have missed some possible sights. The winds > have been fluky and we have had to tack several times since our last FIX > position. > > Tack 1: CC 333.0d - 04Hrs @XXX.XXX5 Kts - Var 12dW Dev 1dE > Tack 2: CC 242.0d - 11Hrs @XXX.XXX5dE > Tack 3: CC 262.0d - 08Hrs @XXX.XXX5dE > Tack 4: CC 307.0d - 10Hrs @XXX.XXX0dE > > 1) What is the DR position at the end of the Tack 4? > -- ------------------------------------------------- TC Dist D.Lat dep Tack 1: 319.0 30.0 22.6 -19.7 Tack 2: 231.5 93.5 -58.2 -73.2 Tack 3: 252.5 40.0 -12.0 -38.1 Tack 4: 297.0 90.0 40.9 -80.2 ============= -6.7 -211.2 D.Lo = -211.2/cos 13.4 = -216.8 = 3d 36.8' DR Position = 12d 53.3'N 65d 06.9W > 2) What is the DR position local Zone Time(ZT) and Date? > What is the DR position GMT/UT time and Date? > -- ----------------------------------------------------- 4 + 11 + 8 + 10 = 30hrs = 1d 6h DR UT = 17:50:10 31/05/2001 DR ZT = 13:50:10 31/05/2001 > 3) From the DR position, what is the TC to Aruba Gap > (13d 50.0'N 70d 00.0'W)? > -- ------------------------------------------------- D.Lat = 13d 50.0 - 12d 53.3 = 56.7 D.Lo = -70d 00.0 - -65d 06.9 = 4d 53.1 = 293.1' dep = 293.1 * cos 13d 20 = 285.2 TC = 270 + atan(56.7/285.2) = 281.2d > Our speed is 8.5 Kts, Var 10dW Dev 1.0dE. Current Drift 0.7 Kts Set 265.0d > > 4) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap > from the DR position? > -- ------------------------------------------------------------ CC = 281.2d + 10 - 1.0 = 290.2d correction for current (by simple plot) adds a fraction more than one degree: CC = 291.5 and gives a speed over ground of about 9.0 > The following morning the navigator see stars through breaks in the > clouds. He takes the following sights. Same data as for Q3 & Q4. > > Body ZT Hs > > UNKNOWN 05:39:00 13d 53.6' (Bearing 349.7 M) > Mars 05:39:11 18d 14.5' > UNKNOWN 05:40:03 54d 58.9' (Bearing 327.3 M) > Vega 05:41:06 40d 10.3' > Venus 05:43:33 35d 25.8' > > 5) What is the FIX position? > -- ------------------------- 9 knots @ 281 for 16 hrs gives AP of about 13d 20'N 67d 30W Plotting the sights gives us a crossings all in the vicinty of 67d 00'W and 13d 30'N. A correction of 1.0' T gives a very small set of crossings at 13d 30.5'N 66d 59.5'W Was the IC really -1.5' rather than the -2.5' given? > 6) What are the names of the two unknown bodies? > -- --------------------------------------------- Unknown #1: 349.7 + 10 - 1 = 358.7d almost due north and altitude is about same as our latitude: got to be Polaris Unknown #2: 327.3 + 10 - 1 = 336.3 This is visually in the angle between Polaris to the north and Vega to the northwest. GHA Aries is about 34d and Long is 67W so zenith has SHA of about 33d. Looking at the starchart in the almanac, there is only one navigational star between Vega and Polaris: Deneb > 7) What is the TC and Distance to the Aruba Gap (13d 50.0'N 70d 00.0'W) > waypoint from our FIX? > -- --------------------------------------------------------------------- dLat = 13d50.0' - 13d30.5' = 19.5' dLo = 70d00.0 - 66d59.5' = 180.5' dep = 180.5*cos 13d40' = 175.4 TC = 270 + atan(19.5/175.4) = 276.3 Dist = sqrt(19.5^2 + 175.4^2) = 176.5 <PRE> -- Mike Wescott Wescott_Mike@XXX.XXX </PRE>