A self-propelled irrigation machine including an elongated main boom pivoted at one end to a center pivot and supported on a plurality of self-propelled, non-steerable drive towers. An extension boom is pivotally secured to the other end of the main boom for irrigating corner areas of the field. A steerable drive tower supports the extension boom and includes a pair of steerable drive wheels. The guidance and control for the extension boom utilizes GPS receivers as position sensors. The system is comprised of two GPS units, two microprocessor based smart boxes, a communications link running from the pivot to the steerable drive unit, and an angle sensor to monitor the angle of the corner arm steerable wheels.