AIDJEX Bulletin #40 - Polar Science Center - University of Washington

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POSITION MEASUREMENTS OF AIDJEX MANNED CAMPS USING THE NAVY NAVIGATION SATELLITE SYSTEM Pat Martin, C. G. Gillespie, Alan Thomdike AIDJEX D. Wells Bedford Institute of Oceaxography Dartmouth, Nova Scotia ABSTRACT The Navy'Navigation Satellite System was used during AIDJEX to make precise measurements of the positions of the four manned camps. Satellite signals were acquired under computer control to standardize and maximize data collection. Single-channel (400 MHz) translocation (relative positioning) was used to measure changes in distance between camps and a reference azimuth at each camp. Special attention was given to the frequency and time references for Doppler counting, and an efficient editing algorithm was used to ensure that translocation Doppler data matched. Statistical filtering after the experiment produced smoothed and evenly spaced estimates of position, velocity, acceleration, and aeimuth. An ensemble of 500 fixes from a period of virtually no ice motion was examined for position errors. Of these, 68% from a single station had errors of less than 75 m. For translocation between two stations separated by 100 km, 68% of the errors were less than 20 my and between two receivers separated by 100 m at the same station 68% of the errors were less than 5 m. INTRODUCTION During AIDJEX, precise position measurements were needed to determine large-scale sea ice deformation. These measurements were taken from March 1975 through May 1976 with the Navy Navigation Satellite System (NavSat) at four manned camps in the Beaufort Sea, which were deployed at approximately 73'N, 156OW. (NavSat measurements made at unmanned buoys are described by Brown and Kerut [1975 and this Bulletin, pp. 15-20].) Following the field experiment, time sequences of the position of each camp and the distance between the camps were used as input to a statistical filter, which 83
incorporated the positioning accuracy information to give smoothed time series of position, velocity, acceleration, and azimuth. This paper describes the positioning system and the changes made to improve its performance, among them the removal of timing errors by correcting Doppler counts; automatic selection, acquisition, and break-off from satellite passes; and definition of a simplified editing procedure suitable for realtime fix processing which gives the best translocation matching possible. The positioning system at each camp contained two receivers, their antennas separated by 100 m, to provide azimuth data and receiver redundancy. REQUIREMENTS FOR POSITION ACCURACY The motion of a piece of sea ice adjusts to balance the forces exerted by the air and the ocean, the Coriolis force, inertia, and stress gradients within the ice. An objective of AIDJEX has been to study this response and to collect data against which theories could be tested. Motion is used here in several senses. Generally, the motion of a piece of ice is the time series of the positions of that piece of ice. The term is also used loosely to refer to quantities derived from one or several of these time series--in particular, the ice velocity, acceleration, and deformation. These quantities figure into the balance-of-force equation: the velocity appears in the Coriolis force, the acceleration in the inertia term, and the deformation is related to the ice stress by a postulated constitutive law. The need to estimate each of these quantities for the force balance determined the sampling and accuracy criteria for the position measurement p r o g ram. On the basis of earlier work [Thorndike, 19741, it was decided that the highest frequencies of interest were about two cycles per day (approximately the inertial frequency at the latitude of the AIDJEX main experiment). This made it necessary to obtain at least four position measurements per day at each camp. On somewhat less evidence a space scale of 100 km was selected for study. Assuming approximately linear changes in velocity over that distance, three measuring points suffice to estimate deformation. Additional points provide 84
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DATA BUOYS AIDJEX BULLETIN No. 40 J
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NOTE TO FRIENDS, AND BYSTANDERS . .
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SUMMARY OF TECHNICAL DEVELOPMENTS I
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milestones, kept the vendor working
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The scene during deployment of HF-N
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The initial AIDJEX scientific plan
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poizr-orbiting satellite, which is
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t@ the spacecraft during each orbit
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BUOY NAHE COMMUN. COMMANDS FIXES TA
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L( 0 I 2 3 4 3 Radial error, kilome
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The buoy will be subjected to tempe
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nics occupy about 12 feet of the 17
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Ill -- - KEY AIUJEX WIN WW AEBa 0 4
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event that a melt pond should form
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chute supplied by Paranetics Inc. i
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6'" IqORGANIC LITHIUM BP.TTEI?Y IMP
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OM (AD RAMS) UREMEN~ \ AIRDROP N m
Page 36 and 37: provides a grid from which geostrop
Page 38 and 39: words; a four-bit air temperature w
Page 40 and 41: PERFORMANCE OF MET/OCEAN BUOYS IN A
Page 42 and 43: Sensor samples were taken every thr
Page 44 and 45: strengths are known, were used with
Page 46 and 47: the ice (causing reduced shear). An
Page 48 and 49: (McPhee, Mangum, and Martin) , TABL
Page 50 and 51: 70°N 75'N n 3 P 17OOW (D Y 170°W
Page 52 and 53: 12OOW * U Station 25. Xeasurements
Page 54 and 55: (McPhee , Mangum, and Martin) 2701
Page 56 and 57: (McPhee, Mangum, and Martin) a 0 a
Page 58 and 59: "i BUOY 4 I 43 360- 309- 257 206 15
Page 60 and 61: a30gl 251 I 60 511 r 43 BUOY 4 I
Page 62 and 63: n 4, (McPhee, Mangum, and Martin) 8
Page 64 and 65: (McPhee, Mangum, and Martin) 33 W c
Page 66 and 67: . '-,..I.. . B A R R O W W I N D S
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Page 70 and 71: I 25 23 t h--:!li[ NCI PiHCEl4l 22
Page 72 and 73: .5 .4 .3 0 A R 2 R 0 W J - 0 B U -
Page 74 and 75: 9 1.5 1.0 c 8 w .5 R 930 $ 565 1000
Page 76 and 77: Clock corrections were applied rout
Page 78 and 79: Q) u o z z i 4- 4- Z + + z f n 9 Ef
Page 80 and 81: TABLE 1 DIFFERENCES IN DAILY TEMPER
Page 82 and 83: Laboratory calibration of the ADRAM
Page 84 and 85: 3.20- - 1 t W o? 2 3.051 ...:-.;..
Page 88 and 89: some redundancy. Four points were u
Page 90 and 91: 75.5 75.: - E75.1 LLI % y75.c - 1 1
Page 92 and 93: The translocation principle removes
Page 94 and 95: POSITION ACCURACY To evaluate the q
Page 96 and 97: TABLE 1 STAND-ALONE ACCURACY 68th P
Page 98 and 99: Az i mu t h s When both receivers a
Page 100 and 101: DATA PROCESSING Following the field
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Page 104 and 105: FREC 1 INCY / DOPPLER / FREQUENCY /
Page 106 and 107: 35 30 25 v) > 0 2 20 LL 0 r= E m 15
Page 108 and 109: An example of a large-scale buoy ar
Page 110 and 111: observations are needed in remote a
Page 112 and 113: useful for determining how various
Page 114 and 115: as well as velocity fields, we must
Page 116 and 117: Since small changes in the yield su
Page 118 and 119: The transformation of the system of
Page 120 and 121: e able to judge how well such a mod
Page 122 and 123: then the flow rule becomes Q = +A -
Page 124 and 125: [-B' -F Finally, we eliminate Carte
Page 126 and 127: differential equation. 2. Discontin
Page 128 and 129: Substituting (36) and (37) into (34
Page 130 and 131: arbitrarily assign the values of 1
Page 132 and 133: The special case when advection can
Page 134 and 135: TABLE 1 STRETCHING, STRESS, ASP CHA
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ate of deformation, we have normali
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CHARACTERISTIC EQUATIONS The ordina
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One way to circumvent the problems
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The equations governing solutions a
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It is readily seen that when body f
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which may be substituted into equat
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a the stress derivative dCldS remai
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Although we have not studied in thi
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Courant, R., and D. Hilbert. 1962.
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A RESEARCH PLAN TO TEST THE AIDJEX
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the barges managed to reach their d
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APPROACH A baseline simulation 0-f
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SENSITIVITY OF ICE RESPONSE TO DIFF
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Since free drift of ice is computed
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and what is observed becomes less i
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stresses appear to be relatively un
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ehavior is directly related to accu
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TABLE 1 COMPARISON OF ESTIMATED ERR
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a) Base line b) 24-hour prediction
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\ / a) Base line b) 24- hour predi
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a) Base line b) 24- hour predi ct i
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\ / a) Base line b) 24-hour predict
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.. a) Base line b) 24- hour predict
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J2 a) Base line b) 24-hour predicti
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) 24-hour prediction c) 36-hour pre
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8 Figure 17. AIDJEX surface pressur
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APPENDIX 1 AIDJEX DATA FILES 1. Pos
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9A. Surface-1 eve1 ai r pressure (d
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19. Surface pressure (Val i dated)
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APPENDIX 2 AIDJEX DATA TRANSFERRED
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I -7-71327GEO A Cards (continued) 7
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B Cards (continued) 201
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