The CTBT/IMS I55US array was laid out with 8 microphones on the Ross Ice Shelf surface with 5 microphones in an outer pentagonal pattern. Each of these five microphones is 1000 meters from the center of the pentagon. An inner equilateral triangular pattern of 3 microphones was placed at the center of the array. Each of these three sensors is at a distance of 100 meters from the array center. The I55US microphone array has East-West symmetry about the geographic North-South line. Model 5 Chaparral microphones, each with its own Geotech digitizer, are used at each microphone site.

The microphone array was installed using surface-laid wires from a centrally located hybrid diesel/photovoltaic generator that supplies 65 volts dc to each individual microphone site. A map is shown in Figure 1 of the locations of the microphone sites in the I55US array. The coordinates of the microphone sites are given in the table below in kilometers with respect to the location of microphone H1 at the origin of the coordinate system, at zero Easting and zero Northing.

The circular symmetry of the I55US array is nearly perfect because we were able to install the sensor sites very close to the exact locations of the apexes of the pentagon and those of the centered equilateral triangle. The fact that Windless Bight is a flat plane of floating Shelf ice with no significant topographic features to obstruct the installation of the array led to an array with a nearly perfect impulse response diagram for the I55US array as can be seen in Figure 2. The central maximum is larger by a factor of two than any other maxima in the wave-number K plane.

At each microphone site there is an insulated yellow vault to house the Chaparral microphone, the Geotech digitizer, the Free-Wave radio and the Dc to Dc power panel. The heat dissipated by the electronics in each vault is enough to keep the equipment within its operating specifications at the coldest temperatures experienced in Windless Bight. The equipment vaults are covered with snow to insulate them from the most severe cold.   At each site there is a 30 foot guyed steel tower for mounting the Free-Wave antenna for the telemetry of the data to McMurdo Station. The site GPS receiver unit is mounted on top of the tower. A typical microphone site is shown in Figure 3. The yellow instrument vault at the base of the antenna tower can be seen before it was buried by drifting snow.

At each microphone there is a noise-reduction system that consists of 16 surface-laid pipes in a radial pattern from the microphone at its center. The pipes are made of high density polyethylene. They are  1.34 inches in inside diameter, 8.5 meters long, and are vented every meter with a small hole about 0.5 mm in diameter. The summing manifolds, connectors, and sector feel hoses add additional length so the outer diameter of the pipe ends is 18 meters. The acoustic impedance of the holes is matched to that of the pipe itself. Thus there are 96 vents in the entire pipe- array at each microphone.  The system is designed such that, during the annual servicing of the array the 8.5 meter  pipes can be pulled out from under the accumulated snow by using special pipe connectors at the end of the pipe. The noise-reduction pipe system as well as the microphone vault can be seen in Figure 3. A view into an open microphone vault is shown in Figure 4 with the portable calibrator attached to the top of the Chaparral microphone. The digitizer can be seen in the lower left on the bottom of the vault. The sixteen black noise-reducing array pipes can be seen on the surface of the snow.

The hybrid power supply for I55US was custom built by Northern Power Systems in an Arctic-type self-contained trailer with sled runners on its base so that it could be towed out across the ice shelf to Windless Bight by a tractor after it was off loaded from the ship at McMurdo station. The power trailer contains 2 diesel-electric generators, a large bank of batteries, and all the electronic controls for the generators and the photovoltaic systems. The two 2700 watt photovoltaic panels are installed on the top of the trailer facing east and west for maximum solar illumination. In Figure 5 the power trailer is shown on location at the center of the microphone array during the installation of the two solar panels.

The digital output of all eight Chaparral Model 5 microphones as well as the meteorological data and the hybrid power supply operational data are all telemetered to McMurdo Station from each microphone site. The data flow from I55US to UAF in Fairbanks and CTBTO in Vienna began on February 1, 2001. The quality of the infrasonic data as well as that from the hybrid power supply in Windless Bight is monitored at the McMurdo CTBT infrasonic hub site.   Infrasonic and meteorological data as well as power supply operational data are sent by satellite in real-time to the CTBTO in Vienna and to the Geophysical Institute in Fairbanks for analysis and operational quality-control monitoring.