Radial velocity is the rate at which one or 2 objects approach or move apart from each other (aka the old 2 trains on collision course).
I'm trying to find out if there is a similar measurement that will tell the user that both objects are in fact running towards each other and not apart. As in a negative radial velocity means closing in, positive radial means moving apart.
I need to find out if there is a measurement specifically for TWO objects (not one, not three.. two) that would tell the user if moving closer it will be a negative number and apart a positive number.
Its a long shot but I wonder if you guys know any (if any exist).
I am sorry but I fail to understand your question.
The most common way to measure radial velocity is by using the Doppler effect. You shoot a signal with some frequency related to it (wave, series of pulses etc.) at the other object and measure the frequency in the returned signal - This is how radars and lasers are used to measure radial velocity. The other option is to use a signal of known frequency emitted by the other object and measure it frequency when it reaches you - this is how radial velocities are measured in astronomy. The ratio between the "original" and received frequencies is a function of the relative radial velocity. The exact formula depends on the case (returned signal, source emitted signal, sound wave, light etc.).
The positive/negative value of the velocity is just a matter of definition and may be confusing. This is why "blue shift" and "red shift" are commonly used to describe "approaching" and "receding" objects instead.
By the way, an object with a radial velocity toward you does not necessarily mean it is on a collision course.
