Wakes behind spacecraft caused by supersonic drifting positive ions are common in plasmas and disturb in situ measurements. We concentrate on observations of the electric field with double-probe instruments. When the equivalent spacecraft charging is small compared to the ion drift energy the wake effects are caused by the spacecraft body and can be compensated for. We discuss examples from the Cluster spacecraft in the solar wind, including statistics of the direction, width and electrostatic potential of wakes, and compare with an analytical model. When the equivalent positive spacecraft charging is large compared to the ion drift energy, an enhanced wake forms. In this case observations of the geophysical electric field with the double-probe technique becomes extremely challenging. Rather, the wake can be used to estimate the flux of cold (eV) positive ions. We discuss such examples from the Cluster spacecraft in the low-density magnetospheric lobes. For an intermediate range of parameters, when the equivalent charging of the spacecraft is similar to the drift energy of the ions, also the charged wire booms of a double-probe instrument must be taken into account. We discuss an example of these effects from the MMS spacecraft near the magnetopause. We find that the observed wake characteristics provide information which can be used for scientific studies. An important example is the enhanced wakes used to estimate the outflow of ionospheric origin in the magnetospheric lobes to about 10^26 cold (eV) ions/s, constituting a large fraction of the mass outflow from planet Earth.