# Two-Line Element Sets, OMMs, and SGP4
API reference for the SGP4 analytic propagator and its inputs. For background, see:
- [TLEs, SGP4 & OMMs](../guide/tle.md) — theory and format descriptions
- Primary sources: [Hoots & Roehrich (1980)](../guide/references.md#hoots1980) and [Vallado et al. (2006)](../guide/references.md#vallado2006) for SGP4; [Vallado & Crawford (2008)](../guide/references.md#vallado2008) for TLE fitting; [CCSDS 502.0-B-3](../guide/references.md#ccsds502) for OMMs
- Tutorials: [Two-Line Element Set](../tutorials/Two-Line Element Set.ipynb), [TLE Fitting](../tutorials/TLE Fitting.ipynb), [SGP4 vs Numerical Propagation](../tutorials/SGP4 vs Numerical Propagation.ipynb)
::: satkit.TLE
::: satkit.sgp4
::: satkit.omm_from_url