Parallel observations with three superconducting gravity sensors 2014-2015 at Metsähovi Geodetic Fundamental Station

The new dual sphere superconducting gravimeter (SG) OSG-073 was installed to Metsähovi Geodetic Fundamental Station in February 2014. Its two gravity sensors are side by side, not one on top of another as in other earlier dual sensor installations.
One sensor (N7) is the standard iGrav, with a lightweight sphere (5 grams), which has a low drift rate. The second sensor (N6)uses a heavy 20-gram sphere which gives ultra low noise with a much higher quality factor Q. SG T020 has been recording continuously since 1994 to 2016. This instrument is situated in the same room at a distance of 3 metres from the dual sphere SG. We show noise level calculations of these instruments and comparison to NLNM. Levels of N7 and T020 are quite similar when compared. The sensor N6 has a very low noise, comparable to the best instrument in the world, OSG-056L at BFO. OSG-073 was send back to GWR in May 2015 for warranty maintenance due to problems in the feedback system of N6. T020 observed simultaneously with N6 for one year and with N7 for 15 months. Gravity behaviour was very similar between N6 and N7 sensors, excluding size of the initial exponential drift. We observed clear, 1-2 microgals difference between the seasonal gravity variations of OSG-073 and T020. We present explanations for this behaviour using models and different hydrological sensors.
OSG-073 will be reinstalled as two separate instruments iGrav-013 and iOSG-022 at Metsähovi in 2016.