A functional block diagram of the system is shown in Figure 3. Table 1 provides a list of the system specifications. The imaging capability is obtained from a 16-element antenna array and a Butler matrix phasing system. Each of the 16 beam-forming outputs from the Butler matrix feeds an individual radio receiver tuned to 38.6 MHz. The receiver output voltage, which is proportional to the received power, is digitized for transmission to the data acquisition computer for display and recording. An RF switch assembly between the Butler matrix outputs and the receiver inputs permits calibrated noise levels to be input to the receivers so that the data can be calibrated against a reference.
The 16-port Butler matrix phasing system, the 16-receivers system, and the calibration system (consisting of a stable RF noise source, a precision programmable attenuator, and the RF switch assembly) are constructed as modular units. This will enable the full-scale HAARP imaging riometer diagnostic to be built up from multiple modular units.
The 16-element antenna has been installed as a 1x16 linear array oriented in approximately a north-south magnetic direction. Figure 4 shows the projection onto a flat ionosphere at 90-km altitude of the antenna beam pattern (-3 dB contours) and the orientation of the beams with respect to the north geographic and geomagnetic poles. The straight lines labeled 61 through 65 are segments of contours of constant magnetic invariant latitude. The range of latitudes covered by the array permits an investigation of the subauroral region of the polar ionosphere.
Because the antenna is phased in one dimension (north-south) only, the prototype instrument is not a true imager, but it does offer a meridional view of ionospheric disturbances, similar to that of a meridional scanning photometer. The proximity of the riometer to the HAARP RF transmitter (it is only a few hundred yards away) often results in significant interference to the riometer signal, making the data unusable during some modes of heater operation. It is likely that a more remote location, as well as the full imaging capability of the proposed instrument, will be needed in order to observe small-scale modifications of the ionosphere during heater operations. However, during off times of the heater, the riometer has provided high-quality data on naturally-occurring auroral activity, including some surprising observations. An example is presented below.