Abstract: Pulsar provides the world's most secure PNT service by incorporating a Combinatorial Watermark into its ranging service. Under conservative signal conditions, Pulsar's Combinatorial Watermark, where 21 ranging code chips are inverted among the 1023, provides 32-bit authentication security each second. In this work, we present our software-defined radio (SDR) implementation of the Pulsar watermark processor. We discuss various threat models, multiple receiver architectures, implications to the receiver, and results from using Pulsar's SDR processing watermarked signals daily from the Pulsar-0 mission. Our results demonstrate the feasible implementation of Pulsar's watermark verification process in commercial GNSS receivers. This is an important milestone in making Xona's Pulsar service available to customers later this year after deployment of our first four production satellites.