The ground network is useful for observing large-scale disturbances. Dr Essex has focused on the south because ionospheric events begin at the poles and move towards the equator. In the southern hemisphere there is a larger offset between the geomagnetic and geographic poles than in the northern hemisphere. So southern Australia is almost high latitude in geomagnetic terms. Hence we are well placed to study these events.
Tomography is the technique of studying hidden structures with a sequence of cross-sectional images.
The technique has already been established using pre-GPS navigation satellites, called Transit satellites, which orbit at 1000 km. So you have all these intersecting rays between the whole array of satellites, so you're basically sampling the ionosphere in different directions as the satellite moves over. Tomographic technique for studying the ionspheric structure using ground-based GPS receivers. The main difference is medical scanners use X-rays, compared to Transit satellites' radio signals, and medical scanners have an uninterrupted 360 o view. Though we don't have 360 o imaging as in CAT scans, we can use a reference ionosonde on the ground to get a vertical fix.