In the summer of 1983, the Earth Physics Branch of Energy, Mines and Resources Canada through a contract with GEO/HYDRO of Rockville, MD conducted MacrometerTM V-1000 GPS carrier phase measurements on the Ottawa Geodetic Test Network. In the spring of 1984, parts of the network were reobserved by Nortech Surveys (Canada) Ltd. of Calgary for the Geodetic Survey of Canada using Texas Instruments 4100 GPS receivers. The baselines observed in both campaigns vary in length between 13 km and 66 km. Both sets of observation records were processed using software developed independently at the Dept. of Surveying Engineering of UNB. Results of the analysis of the data in a 'network mode' will be presented and compared with "ground truth" from terrestrial adjustment. Differences between the results from the single frequency V-1000 and dual frequency TI-4100 observations will be discussed. The effect of ionospheric refraction on GPS phase difference observations will also be presented.