Measurements of energetic particles obtained by the two geosynchronous satellites (1991-080 and LANL-97A) are performed to investigate the plasma injection boundary and source region during the magnetospheric substorms. The measurement method is developed to allow remote sensing of the plasma injection time and the radial distance of injection boundaries by using measured energy dispersion and modelling particle drifts within the Volland–Stern electric field and the dipole magnetic field model. The radial distance of the injection boundary deduced from a dispersion event observed by the LANL-97A satellite on 14 June 1998 is 7.1RE, and the injection time agrees well with the substorm onset time identified by the Polar Ultraviolet Imager. The method has been applied to an event happened at 22.9 UT on 11 March 1998, when both the satellites (1991-080 and LANL-97A) observed the dispersionless character. The results indicate that the radial distance of injection source locates at 8.1RE at magnetotail, and particles move earthward from magnetotail into inner magnetosphere at 22.5 UT.