  JEM--EUSO is an international program for the development of space-based
Ultra-High Energy Cosmic Ray observatories. The program consists of a series of
missions which are either under development or in the data analysis phase. All
instruments are based on a wide-field-of-view telescope, which operates in the
near-UV range, designed to detect the fluorescence light emitted by extensive
air showers in the atmosphere. We describe the simulation software ESAFin the
framework of the JEM--EUSO program and explain the physical assumptions used.
We present here the implementation of the JEM--EUSO, POEMMA, K--EUSO, TUS,
Mini--EUSO, EUSO--SPB1 and EUSO--TA configurations in ESAF. For the first time
ESAF simulation outputs are compared with experimental data.
