This paper describes the earth terminal (ET) interoperability demonstrations that were conducted with the wideband global SATCOM (WGS) block I payload engineering model testbed (PEMT) in the factory, and with the WGS-1 satellite on orbit. In June 2005, during the course of initial interoperability demonstrations with the PEMT, the results showed that, although it met the integrated phase noise specification, the payload phase noise power spectral density (PSD) was sufficiently elevated in some frequency offset ranges to cause unacceptable negative impact on communications performance for 8-PSK and 16-QAM waveforms of interest to DoD. Further testing revealed that the high phase noise originated in some of the payload's synthesized reference generators (SRGs). Boeing and the government ultimately determined that the integrated phase noise specification that was levied on the payload was not sufficient to ensure good performance for higher order modulation schemes, and that a mask would need to be imposed on the payloadpsilas phase noise PSD. Boeing modified the SRGs on all Block I satellites prior to launch in order to meet this new phase noise mask. On June 6, 2006, interoperability testing recommenced with an improved SRG installed in the WGS PEMT. Further testing, concluded in November 2006, showed that the reduced phase noise of the modified SRG improved communications performance to satisfactory levels. WGS-1 was launched on October 10, 2007 with the improved SRGs. The WGS-1 On-Orbit Terminal Interoperability Demonstration (OOTID) was successfully concluded on January 16, 2008. The OOTID, which employed the same ETs used during the PEMT tests, confirmed that the communications performance improvements are also present in the operational satellite. WGS Block II phase noise performance is expected to be even better than Block I as a result of a decision to implement a new SRG design for these follow-on satellites.