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

some redundancy. Four points were used in the central AIDJEX array: three
camps (Caribou, camp 1; Blue Fox, camp 2; Snow Bird, camp 3) initially forming
a triangle 100 km on a side with a fourth camp (Big Bear, camp 0) in the
center.
Typically, sea ice moves about 2 km in 12 hours (Fig. 1). Thus, measurements
of geographical position accurate to 100 m or better will resolve the
motion quite well. However, the relative position of one camp with respect to
another typically changes by about one quarter of one percent of the distance
between them. For camps 100 km apart, then, typical changes in relative positions
are 250 m in 12 hours (Fig. 2). The design criterion for the positioning
system was set at 510 m for relative positions.
Estimates of ice motion were required to support other parts of the AIDJEX
field program, but the requirements did not make the sampling and accuracy
criteria more severe. Briefly stated, these other requirements were: to
provide geographical reference for the meteorological and oceanographic observations;
to correct ocean current measurements by removing the velocity of the
ice itself; to correct measurements of ice tilt by accounting for the acceleration
of the ice; and to support aircraft and submarine operations.
SYSTEM DESIGN
In defining the system configuration we sought to combine high reliability
with low maintenance and repair, improve accuracy in distance measurement,
have automatic operation with standardized selection and acquisition of the
maximum number of passes, obtain measurements of floe rotation, and keep costs
as low as possible. Figure 3 is a block diagram of the NavSat equipment
installed at each of the four camps. Hardware details are given by Wasilew
and Vivian [1976].
Simple, redundant, commercially proven hardware was used to achieve our
objectives of reliability and low cost. Spare components for major systems
were kept at the main camp so that a system could be repaired quickly by
replacing an entire component. We sacrificed some geographic accuracy by
using single-channel (400 MHz) receivers which cannot correct for refraction
effects. On the other hand, because of the low cost of single-channel
85