The swath of space borne optical sensor is an important index in many space missions. For satisfying the swath requirement of optical payload

besides a reasonable design of optical system

a suitable design of orbital elements is also needed. Sun-synchronous orbit (SS-O) is one of the most frequently used orbits for earth observation missions

and orbital altitude is a challenge attention index in SS-O design. The key point of satisfying mission requirement is designing a suitable SS-O. The relationship between swath width and orbital elements (especially the orbital altitude) is discussed in this paper; a method of SS-O design satisfying the swath width requirements of space borne optical sensor is also given. Firstly

this paper discusses the theory of non-spherical earth perturbations and the definition of SS-O. Afterwards

the relationship among nodal period

fundamental interval and orbital altitude will be educed

with deriving the definition of fictitious node. Finally

we get the restriction condition of orbit altitude satisfying swath width requirement

based on the discussion above. The definition of fictitious node

which is very helpful to comprehend the theory on SS-O

is first put forward. Furthermore

the mathematical relationship between SS-O nodal period and orbital semimajor axis is deduce

which is available for SS-O design.