by CHfour:
"The QDM/QDR is known from the VOR receiver info so it's not difficult to calculate the relative brg if the hdg is known."
I don't see what VOR has with magnetic bearing (qdm/qdr)? VOR just receives two signals - one electronicaly directed to cover 360 radials and other none directed - VOR receiver just calculate the time diference betwen two signals - this gives you the info on whic radial you are - then a VOR gauge shows the deviation from the preselected radial. mining that if your VOR receiver calculates that you are on 90° radial and you have selected 100° radial, than you are 10° from the radial, and by DME you can determine right-angled lenght to the selected radial. This is how I understand the VOR.
"Now, how the 737 IRS computes the true hdg without any magnetic input is IMHO the really impressive bit!"
IRS(it is more known as INS) gets true nort by a gyrocompas or directional gyro. you can reed how it works on the web.
by hetfield:
"Die VOR Nadel ist dagegen auf der Kompassrose (RMI) "zementiert"."
HSI - combined VOR and DG and RMI in combined ADF and DG.
VOR or ADF needle on both swings independantly of the rose.
and jet none did answer the question. it was how dual needle RMI - VOR needle show direction towards VOR station. I reayl don't know it dual needle RMI has one VOR needle, because on the web I can only find data about dual ADF needle RMI - one needle for one ADF freq.
how I understand the vorking of VOR receiver it doesn't have directional antenna like ADF, so how can one determine direction towards VOR station. (don't mix radial bearing with relative bearing!)