Mitigation of multipath is important to a wide variety of users of CORS data. It would be optimum to have all CORS stations equipped with antennas having maximum resistance to multipath and to have these antennas mounted in such a way as to minimize multipath. However, it is unrealistic to expect that operators of stations of the CORS network will fund new antennas that are not required for their application. Scientific users of CORS data should consider funding the implementation of choke ring antennas at existing sites as an inexpensive way of gaining additional stations for use in scientific applications. In so far as mounting of antennas are concerned there are often operational constraints. For example, the USCG has placed its stations, whenever possible, at existing facilities where there is access to support and broadcast antennas. The USCG antennas are placed on 3 to 10 meters above the ground to allow tracking down to 7.5 degrees above the horizon. NGS is working with the USCG to determine the magnitude of multipath at the USCG sites, to determine the effectiveness of improved antenna types in decreasing multipath and to develop multipath models on a sit specific basis.

Antenna stability is of great interest to scientific users of CORS data. Many scientific users of CORS data would like antenna stability at the millimeter level. Practical considerations may make this difficult if not impossible to achieve at many sites. The large distance above the ground required by the USCG for their antenna locations would make millimeter stability extremely expensive, if not impossible. Certainly the scientific community could be expected to fund the incremental cost of achieving millimeter stability. At the USCG sites there are ways of evaluating antenna stability. Because there are two antennas at each site the differential position between the two antennas can be monitored and used as a measure of antenna stability. Also, NGS has established two ground monuments at the USCG sites and positioned them relative to the CORS antennas. This provides another means of monitoring antenna stability.



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