I claim the ornamental design for a mobile field station, as shown and described.
FIG. 1 is a front right perspective view of a first embodiment of a mobile field station;
FIG. 2 is a rear left perspective thereof;
FIG. 3 is a front left perspective thereof;
FIG. 4 is a left side view, the right side being a mirror image thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a front left perspective view, with the tray door in an open position, and the lid in an open position;
FIG. 8 is a front perspective view, with the tray door and tray in open or extended positions, and the lid in an open position;
FIG. 9 is a front right perspective view, with the tray door and tray in open or extended positions, and the lid in an open position;
FIG. 10 is a partial left front perspective view with the lid in an open position; and,
FIG. 11 is a bottom left perspective view with the tray door and tray in open or extended positions, and the lid in an open position.
The broken line showing of some elements in the figures is included for the purpose of illustrating environment and forms no part of the claimed design.