An exemplary radio-based navigation system uses a small multimode direction-finding antenna and a direction-finding receiver capable of determining platform position, velocity, attitude, and time while simultaneously providing protection against narrowband and broadband sources of interference. Global Navigation Satellite System (GNSS) signals such as those from the Global Positioning System (GPS) provide attitude measurements with a compact multimode direction-finding antenna (e.g., a small two-arm spiral with improved angle-of-arrival performance over the entire hemisphere enhanced through the use of a conductive vertical extension of the antenna ground plane about the antenna perimeter and/or conductive posts placed evenly around the antenna perimeter) which provides simultaneous protection against jammers. The multimode spiral may be treated as an array of rotationally-symmetric antenna elements. The GPS receiver architecture also may be modified for direction-finding and thereby attitude determination by increasing the requisite number of input signals from one to at least two while minimizing the required number of correlators and mixers.