A/ P works well with INS and Doppler. Yesterday, I have to turned off the INS during the flight. Surprisingly,  A/P maintained flight plan with out INS or Doppler active.
During  return flight I loaded only flight plan and engaged  the NAV mode. A/P followed the flight plan " TJSJ-KJFK" with out any problem.
I wonder if  there is a hidden GPS in 707 navigation program. IF there is,  it will take away charm of INS and Doubler navigation system .

Sanal