This study, convened and funded by the U.S. Army, analyzes the tradeoffs involved in designing a local or theater GPS replacement system. Our operating assumption was that such a system would not be constrained by legacy GPS architecture, equipment, or waveforms. It could therefore employ more efficient Z4 cyclostationary ranging codes and reap the benefits of two-way measurement and communication including: elimination of User Device (UD) clock error, dramatic reduction of the UD infill bandwidth by computing the UD position on the Reference Device (RD), code ambiguity elimination, and (perhaps) the use of cognitive radio techniques to resist jamming. The existence of robust cellular telephony with compact handsets in very crowded spectrum shared by many users testifies to the viability of this approach. 508 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 24-26, 2011 We studied a physical architecture in which the RDs were carried by Unmanned Aircraft Systems (UASs). It was shown that such architectures are robust to jamming, and that the Horizontal Dilution Of Precision (HDOP) benefits greatly from two-way ranging. We also described how the use of Mission Level Autonomy (MLA) by a resource limited UAS constellation provides robust location services to dynamically changing ground operations in a near optimal way. For operations over very large areas of denied airspace, we considered geolocation of static UDs from two-way satellite links which are consistent with specifications developed in the Operationally Responsive Satellite (ORS) program. Two-way satellite links that can measure Doppler as well as range enable single-satellite, singleepoch geolocation. This reduces the number of satellites required in the constellation by about a factor of two. A notional satellite design consistent with launch from an SSBN submarine was presented.