Antennas at two older BSL stations, CMBB and HOPB were replaced with Dorne-Margolin design choke-ring antenna that minimize the effects of multipath signals and are the accepted standard for the IGS global permanent GPS network. The older microstrip Ashtech antennas still used at two BSL stations (BRIB and TIBB) will be replaced in 1998-99 with choke-ring antennas when the hemispherical radome designed by SCIGN becomes widely available. Antenna radomes cover the antennas to provide security and protection from the weather and other natural phenomenon. Many BSL stations do not currently use radomes, particularly if they are in isolated areas with temperate climates. FARB uses a prototype hemispherical radome designed by UNAVCO to protect the antenna from birds during nesting season. Other stations, such as CMBB and MUSB, use the Ashtech conical design radome to minimize snow buildup on the antenna during the winter season. However, because radomes cause additional delays to the GPS signals, they can alter the antenna phase pattern and its average phase center. The CMBB radome was damaged in December 1997 when vandals apparently kicked a large (10 cm) hole into one side of the conical dome, which introduced a 2-cm apparent offset in the east coordinate of the CMBB position (Figure 5). The new SCIGN radome designed for the Dorne-Margolin antennas minimizes differential radio propagation delays by being hemispherical about the phase center and uniform in thickness at the 0.1 mm level. It is also far more resistent to damage and will completely cover the dome and cable connections for added protection. The BSL will purchase these radomes for all of the choke-ring antennas after their first production run in late 1998, and will install them at all the sites to ensure the most uniform and internally consistent antenna configuration for the BARD network.