Jäggi et al. (2015) related the systematic errors to large ionosphere changes, which may be extracted by analyzing epoch differences of the geometry-free linear combination Figure 2 (right) shows for the November–December period of the year 2011 that the systematics can be largely eliminated by discarding measurements with ionosphere changes larger than 5 cm/s. Despite the exclusion of all observations related to large ionosphere changes, this merely corresponds to 94.4% of the total set of available GPS observations. On average 93.8% of the kinematic positions can still be determined, which implies a small reduction of about 6.2% for the set of kinematic positions used for gravity field recovery. For the time span of days 300–365 in the years 2009, 2010, and the 2012, the reduction is even significantly smaller, amounting to 0.1%, 0.2%, and, 3.7%, respectively. A clear reduction of the systematic errors was also observed in gravity field solutions by Jäggi et al. (2015) when using the improved set of kinematic positions.
Jäggi, A., Prange, L., Hugentobler, U. (2011a): Impact of covariance information of kinematic positions on orbit reconstruction and gravity field recovery. Advances in Space Research, 47(9): 1472-1479, doi: 10.1016/j.asr.2010.12.009.
Jäggi, A., Bock, H., Prange, L., Meyer, U., Beutler, G. (2011b). GPS-only gravity field recovery with GOCE, CHAMP, and GRACE. Advances in Space Research, 47(6): 1020-1028, doi: 10.1016/j.asr.2010.11.008.
Jäggi, A., Bock, H., Meyer, U., Beutler, G.,van den Ijssel, J. (2015): GOCE: assessment of GPS-only gravity field determination. Journal of Geodesy, 89(1): 33-48, doi: 10.1007/s00190-014-0759-z.