An apparatus and methods are disclosed for determining a navigational constraint for a portable device using an ultrasonic sensor. The navigational constraint may be used to supplement other navigational solutions or may be used independently. The ultrasonic sensor may provide a plurality of samples to be processed to determine the constraint. Processing the ultrasonic samples may include performing a flow analysis regarding detected external objects. Determining the constraint may include any or all of determining a context for usage, distinguishing usage modes, estimating relative heading changes, and estimating misalignment angle between device and platform needed to determine direction of motion of the platform and determining a speed of the portable device from the samples.