The Echo 7 sounding rocket experiment injected electron beams on central tail field lines near L=6.5. Numerous injections returned to the payload as ''conjugate echoes'' after mirroring in the southern hemisphere. We compare field line lengths calculated from measured conjugate echo bounce times and energies to predictions made by integrating electron trajectories through various magnetospheric models: the Olson-Pfitzer Quiet and Dynamic models and the Tsyganenko-Usmanov model. Although Kp at launch was 3-, quiet time magnetic models best fit the echo measurements. Geosynchronous satellite magnetometer measurements near the Echo 7 field lines during the flight were best modelled by the Olson-Pfitzer Dynamic Model and the Tsyganenko-Usmanov model for Kp=3. The discrepancy between the models that best fit the Echo 7 data and those that fit the satellite data was most likely due to uncertainties in the small-scale configuration of the magnetospheric models. The field line length measured by the conjugate echoes showed some temporal variation in the magnetic field, also indicated by the satellite magnetometers. This demonstrates the utility an Echo-style experiment could have in substorm studies. ¿ American Geophysical Union 1992