Apparatus comprising: an implantable neural stimulator (i) configured to be advanced through a greater palatine canal to a sphenopalatine ganglion of a subject and (ii) comprising an implant-impedance-sensing electrode; an auxiliary-impedance-sensing electrode; first and second wires (562, 564), electrically coupled respectively to the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580); and impedance-based navigation circuitry comprising: a voltage generator configured to apply current between the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580) through the wires (562, 564); an impedance sensor configured to measure an impedance between the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580) based on the applying of the current; and a disposition tracker configured to determine, based on a change in the measured impedance, a disposition of the implantable neural stimulator in the greater palatine canal, wherein the apparatus is characterised in that the disposition of the implantable neural stimulator includes a shape of the implantable neural stimulator in the greater palatine canal, and wherein the disposition tracker is configured to determine the shape of the implantable neural stimulator in the greater palatine canal based on a change in the measured impedance. an implantable neural stimulator (i) configured to be advanced through a greater palatine canal to a sphenopalatine ganglion of a subject and (ii) comprising an implant-impedance-sensing electrode; an auxiliary-impedance-sensing electrode; first and second wires (562, 564), electrically coupled respectively to the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580); and impedance-based navigation circuitry comprising: a voltage generator configured to apply current between the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580) through the wires (562, 564); an impedance sensor configured to measure an impedance between the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580) based on the applying of the current; and a disposition tracker configured to determine, based on a change in the measured impedance, a disposition of the implantable neural stimulator in the greater palatine canal, a voltage generator configured to apply current between the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580) through the wires (562, 564); an impedance sensor configured to measure an impedance between the implant-impedance-sensing and auxiliary-impedance-sensing electrodes (560, 580) based on the applying of the current; and a disposition tracker configured to determine, based on a change in the measured impedance, a disposition of the implantable neural stimulator in the greater palatine canal, wherein the apparatus is characterised in that the disposition of the implantable neural stimulator includes a shape of the implantable neural stimulator in the greater palatine canal, and wherein the disposition tracker is configured to determine the shape of the implantable neural stimulator in the greater palatine canal based on a change in the measured impedance.