THE SOUTHWEST PACIFIC GPS PROJECT - GEODETIC RESULTS FROM BURST-1 OF THE 1990 FIELD CAMPAIGN

The Southwest Pacific GPS Project (SWP) is using the Global Positionin g System (GPS) to monitor crustal motion across and within a plate bou ndary complex between the Australian and Pacific plates. GPS field cam paigns were conducted in 1988, 1989 and 1990, to observe networks of i ncreasing size and complexity. The 1990 campaign consisted of two peri ods, or ''Bursts'', and this paper focuses on the analysis of data col lected during the nine day Burst 1 in July, 1990, a period in which GP S Selective Availability was activated. During Burst 1, baselines that spanned the Tonga Trench and the Lau Basin were observed, and only on e station (Espiritu Santo, Vanuatu) was located west of Fiji in the ne twork. The lengths of the baselines observed fall mainly between 300 k m and 1600 km, but some lines are as long as 3500 km. A total of 78 st ation-days of field site data and approximately 150 station-days of gl obal fiducial data were processed from predominantly codeless receiver s. A global fiducial network of 20 sites was used to provide orbit con trol and accuracy assessment for the 13 available satellites. The dail y solutions for 45 baselines between 10 SWP sites have an RMS scatter in the length of 24 mm plus 6 parts per billion. This scatter provides an estimate of baseline precision for the Burst 1 ''nominal solution. '' Experiments were conducted to investigate a variety of possible eff ects on the SWP Network baseline estimates, including the influence of a reduced global fiducial network for the purpose of assessing the qu ality of results obtained in 1988 and 1989 in which the fiducial netwo rk was smaller than in 1990. These experiments produced results that a greed with the nominal solution at the level of the precision estimate . Furthermore, estimates for selected baselines in Australia, the Cent ral Pacific, North America and Europe, also measured by VLBI and SLR, were used for an external accuracy evaluation. The GPS and VLBI or SLR determinations of length agreed at a level consistent with the nomina l solution precision estimate.