Validation of Swarm satellite magnetic data using observatory measurements

The scientific use of Swarm magnetic data and Swarm-derived products is greatly enhanced through their combination with geomagnetic observatory data and indices. The strength of observatory data is their long-term accuracy, with great care being taken to ensure temperature control/correction, platform stability and magnetic cleanliness at each site. Recent work to improve the coverage and timeliness of observatory data has been encouraged and now over 60 INTERMAGNET observatories and several other high-quality observatories are providing close-to-definitive data within 3 months of measurement. An effort is being made to use these measurements to ground-truth Swarm data as part of the Calibration/Validation phase. These observatory data are gathered and homogenised on a regular basis by BGS and those collected during overhead passes of the Swarm satellites are identified. For each pass, we remove an estimate of the main field from both the data collected at altitude and that collected on the ground. Both sets of data are then normalised relative to the data variance during all passes since launch. The absolute differences of the two sets of normalised data can be used as a metric of satellite data quality relative to observatory data quality. When we analyse the results obtained for each Swarm satellite alongside those found when a complementary analysis is carried out on CHAMP, the findings are similar.