Ink jet printer droplet height sensing control

A ink jet printer having a reciprocating printhead with a single jet to print full pages of information on a recording medium by printing contiguous swaths of information. The recording medium is stepped a distance of one swath height after each swath is printed. A height control sensor is located on one side of the recording medium to receive periodically one or more sweeps of test droplets. The height control sensor has upper and lower pairs of photodetectors to detect droplets passing thereby and to produce differential sensing signals which identify the droplet having a trajectory closest to the desired trajectory. By determining the number of droplets between the identified droplets, the interdroplet spacing and drift of the droplet trajectories is monitored. In response to the height control sensor signals, the printer controller with associated circuitry adjusts the operating parameters of the printer to correct and to maintain the desired droplet trajectories and interdroplet spacings.