Comment on 'Simultaneous gravity and gradient measurements from a   recoil-compensated absolute gravimeter'
  The article (Niebauer et al. 2011 Metrologia 48 154-163) reports on the important innovations enhancing the ability of absolute gravimeter to measure vertical gravity gradient along with the gravity acceleration. This comment suggests experiments to further assess the improvements and the results obtained with the modified instrument, considers some limitations of non-linear models in metrology and ways to overcome them, and discusses possible applications of the described instrument. 
V D Nagornyi
physics.ins-det physics.geo-ph