PPK
PPK turns up in 2 fields: hydrography, statistics. Here is what it means to the people in each one.
PPK in hydrography
Post-processed kinematic: GNSS positions computed after the survey from logged rover and base data, at centimetre level.
Post-processed kinematic positioning runs the same carrier-phase solution as RTK, but in the office, using raw observations logged on the vessel and at a base station or reference network. Processing forward and backward through the data recovers ambiguities lost in real time, and combined with the inertial data it yields a smoothed trajectory for the whole day. PPK removes the dependence on radio links and rescues lines run in float.
"No radio link past the headland, just log raw and we'll PPK it tonight."
Ppk in statistics
Process performance index: a Cpk-style figure computed with the overall standard deviation, capturing long-term variation.
Process performance index, built like Cpk but using the overall standard deviation of all the data, so shifts between subgroups, lots and days count against it. Its partner Pp is the matching version of Cp. Ppk reflects what the customer actually received, while Cpk shows what the process could deliver if those between-subgroup shifts were removed. Automotive initial process studies commonly look for a Ppk of 1.67 before launch.
"Cpk looks great at 1.8, but Ppk is 1.1. Lot-to-lot variation is eating us, go look at the raw material."