With much of the military and civilian communities becoming dependent on GPS techlogy to navigate, it has become imperative that the navigation systems be tested in situations in which GPS does t work. This testing is especially necessary for precise tasks such as landing an aircraft. Currently, research is being conducted into using a pseudolite-based reference system to use as a truth model for the GPS jamming test. Pseudolite systems have been proven to provide sub-centimeter level accuracy in the horizontal plane; however in the vertical plane the position error is still in the decimeter to meter level range. This is largely due to the fact that the geometry of a ground based pseudolite system provides poor slant angles in the vertical plane, which contributes to large positioning errors. The goal of this research is to study the effects of system geometry on the vertical plane solution. The results of this effort show that elevation angles of greater than 20o-30o are necessary to attain reasonably good positioning solutions. Multiple pseudolite deployments, while effective at reducing the geometry errors, are very cost ineffective and the geometries pose significant risks to a landing aircraft. The best geometry involved using an orbiting aircraft, with a pseudolite transmitter and receiver attached, as an elevated pseudolite to create better slant angles and thus better positioning solutions.