GPS L1C and GALILEO E1 OS are two of the current navigation open service signals being developed nowadays. GPS L1C is the US open signal and is being conceived in order to provide enhanced capabilities to civilian users in relation to previous GPS signals and in order to achieve interoperability with Galileo?s Open Service signal [1, 8]. GALILEO is the European system and is being developed in order to obtain independence from any other navigation system, while searching to reach maximum interoperability with other GNSS systems, for instance with GPS. Nevertheless, GALILEO, being a navigation system of a new generation, also searches to provide a very high positioning performance by means of its open service signal E1 OS. One of the aspects being improved by both systems is the data message demodulation/decoding process, which has proven not to provide a sufficient level of performance in low C/N0 environments such as indoor and narrow urban ones. However, each signal has been conceived in a very different form and some of the design differences such as the signal channel relative power distribution between the data and pilot channel, the symbol rate, channel code, etc, affect directly the performance of the demodulating/ decoding process. Therefore, since they are different and thus their behaviour is not the same with respect to channel impairments, an arising question is to know the pros and cons of each design. The aim of this paper is to make a first simplified performance comparison of the GPS L1C and GALILEO E1 OS data message decoding process by observing the BER, the WER (Word Error Rate) and the EER (Ephemeris Error Rate) as functions of the C/N0. More specifically, this paper seeks these figures of merit in the subframes (GPS L1C) and words (GALILEO E1 OS) containing the ephemeris and clock corrections.