;+
; Convert frequency(MHz) to velocity (m/s) using the given rest
; frequency and velocity definition.
;
; @param freq {in}{required} Frequency in MHz.
; @param restfreq {in}{required} Rest frequency in MHz
; @keyword veldef {in}{optional}{type=string} The velocity definition
; which must be one of OPTICAL, RADIO, or TRUE.  Defaults to RADIO.
;
; @returns velocity in m/s
;
; @version $Id: freqtovel.pro,v 1.3 2005/04/08 20:36:06 bgarwood Exp $
;-
function freqtovel, freq, restfreq, veldef=veldef
    compile_opt idl2

    if (not keyword_set(veldef)) then veldef = "RADIO"

    case veldef of
        'RADIO': result = !gc.light_speed * (1.0d - double(freq) / restfreq)
        'OPTICAL': result = !gc.light_speed * (restfreq / double(freq) - 1.0d)
        'TRUE': begin
            g = (double(freq) / restfreq)^2
            result = !gc.light_speed * (1.0d - g) / (1.0d + g)
        end
        else: message, 'unrecognized velocity definition'
    endcase 

    return, result
end