Rocket to launch 8 cubesat to LEO at an equidistant distance reminded me that I'd never asked this question. NASA's TROPICS 3U cubesat constellation hasn't been launched yet, so there's still time to ask.
Gunter's Space Page TROPICS says:
The TROPICS constellation will be formulated with four CubeSats in each of three orbital planes with the following launch parameters: equally spaced RAAN (±30° tolerance), 600 km altitude (550±50 km tolerance), 30° inclination (±3° tolerance). These mission parameters will meet science requirements should the threshold number of CubeSats (eight) be implemented. Analyses have shown orbit lifetime of 9 years, well over the expected mission lifetime of one year and well in advance of the 25-year de-orbit requirement. This configuration yields 30-minute median revisit rates with average spatial resolution better than precipitation and all-weather temperature and humidity (PATH) requirements, all from a low-cost, low-risk CubeSat constellation platform.
and includes the image below.
Question: How will these 3U cube sats be inserted into three different orbital planes and phased within each one? For propulsion Gunter says "None"!
note: I'm looking for answers supported by existing information on this mission, not just hypotheses, thanks!
Nation: USA Type / Application: Earth observation, technology Operator: Massachusetts Institute of Technology - Space Systems Laboratory (MIT SSL) Contractors: Blue Canyon Technologies, Massachusetts Institute of Technology - Space Systems Laboratory (MIT SSL) Equipment: Configuration: CubeSat (3U) Propulsion: None Power: Deployable solar array, batteries Lifetime: 1 year (planned) Mass: 4 kg Orbit: 600 km × 600 km, 30°
†Further reading: Wikipedia and eoPortal