This paper describes the prototype 16-beam, 38.6-MHz riometer system developed by Advanced Power Technologies, Inc., and the University of Maryland for the High Frequency Active Auroral Research Program (HAARP). The prototype system is the forerunner for a full-scale imaging riometer diagnostic instrument for characterizing the ionospheric volume perturbed by controlled radio frequency (RF) heating experiments. The prototype system, installed at the HAARP site near Gakona, Alaska, consists of a 1¿16 antenna array phased in one dimension (beam width of ~6.7¿) and oriented approximately along the magnetic meridian. The system responds sensitively to natural variations of auroral absorption, such as those caused by magnetospheric substorms, and provides clear evidence of its capability to discern spatial structure and motion. A newly observed feature seen near dusk are intense, short-duration absorption spikes accompanied by only weak magnetic signatures. Because the HAARP heating facility is still a work in progress, there have been only limited opportunities to conduct science investigations with it. Consistent with estimates, heater-induced effects in the ionosphere have not been observed with the riometer at the power levels currently available. It has been noted that the proximity of the prototype system to the HAARP RF heater can result in interference to the riometer signal for some frequencies and operating modes of the heater. The full-scale imaging capability of the proposed instrument, and a remote location, may be required to detect small-scale modifications of the ionosphere caused by the heater. ¿ 1999 American Geophysical Union