Abstract: To address the limited analysis of observation noise in domestically developed low-cost multi-frequency GNSS receivers, this study introduces a zero-baseline double-differencing approach to recover single-differenced residuals. The elevation-dependent behavior of signals from a UM960-based receiver is examined, and the pseudorange and carrier phase noise characteristics are evaluated. Results indicate that the BeiDou Navigation Satellite System (BDS) exhibits lower overall satellite observation noise than the GPS, with the difference being more pronounced in pseudorange measurements. Specifically, BDS pseudorange noise remains at the centimeter level across all frequencies, with B2a the lowest at 1.49 cm, rising through B2I, B1C, and B3I, and reaching a maximum on B1I at 2.87 cm. Carrier phase noise remains within the sub-millimeter range, with B1I at 0.24 mm, B2I and B1C at similar levels, and B2a and B3I slightly higher at 0.37 mm and 0.62 mm, respectively. Noise levels also vary by orbit type, with medium Earth orbit (MEO) and inclined geosynchronous orbit (IGSO) satellites performing similarly and GEO satellites exhibiting higher noise. For GPS, pseudorange noise differs significantly across frequencies, with L2 reaching 9.40 cm, approximately six times that of L5, while carrier phase noise remains relatively stable at 0.31 mm, 0.75 mm, and 0.35 mm for L1, L2, and L5, respectively. The results demonstrate that the receiver achieves observation performance comparable to geodetic-grade equipment, meeting the requirements of typical navigation and positioning applications.