OK blip I'll bite....
How can you use a wx radar successfully as a primary surveillance radar?
I think you can't. And if you can't, you will be giving you and your pax a false sense of security.
But I would be most interested if you could prove me wrong.
I'm not trying to wind you up, I just think you might be wasting time that might be better used another way to avoid unknown traffic.
First problem: Wx radar unit design v primary surveillance radar.
Wx radar is 'tuned' to a particular band of the microwave range of the e/m spectrum. Unsurprisingly, the engineers that designed it used a band that gives best reflections/scatter/returns when striking precipitation. Not moisture as cloud, which is orders of magnitude smaller than precip, nor light aircraft, which are several orders of magnitude larger than precip, with different reflectivity qualities.
You will notice that primary surveillance radar receiver/transmitters on major airports are not also used as wx radar rx/tx for RAPICs. They do entirely different jobs. The PSR head is rotating happily on a gantry on one part of the airfield, whilst the BoM Wx radar head is in a golfball on a gantry on another part of the airfield.
So for a start we are using the wrong sort of radar energy to spot unannounced aircraft.
Secondly, the radar display in your cheiftain/B737 has some software in it. This software is designed to present you with the best wx information. It might be designed to cut out unnecessary clutter. It certainly will not be optimised to show a rogue C402 or smaller.
Thirdly, resolution in azimuth. Say you wind down your radar to the lowest range, perhaps 10nm. Half way along is 5nm. Maybe a blip at 5nm ahead is warning you of possible conflict less than 30 seconds from closest point of approach (impact!) if you are doing 200 knots and he/she is a very slow 100 knots.
So how big would the blip be if it were a C402, five miles ahead? Say the radar display displays 120 degrees arc of azimuth. One degree of azimuth at one mile is a target 100 feet in width. One degree of azimuth at five miles is a target 500 feet, or roughly 180 metres, in width. What was the wingspan of that C402? Or a better question -- what was the frontal area of that C402? The width of the smallest "blip" able to be displayed by wx radar (and visible to an operator that is intently looking at only the wx radar display) might be 3 degrees of arc, so we are only going to see objects that are greater than 1500 feet in width?
Fourth, coverage in elevation. You tilt your wx radar down 3 degrees. In many turboprops like a metro, your beam width in elevation is 8 degrees, and in aircraft with larger wx radar antennae, probably 6 degrees top to bottom.
Best case, 8 degrees of coverage in elevation for a smaller wx equipped aircraft top to bottom of beam. Tilt the centre of the beam down 3 degrees. So your radar is now scanning an elevation 7 degrees below level to one degree above level.
You then descend on a 3x profile, which is a very rough 3 degree slope. You then have protection against an aircraft that is level with you, or climbing at less than 400'/nm.
A C210 climbing at 90knots showing 1000 fpm climb is climbing steeper than 400'/nm. Your radar would not show him, even if the other problems above were solved. He/she is 'underneath your radar!'
And have you done a quick rule of thumb check to ensure that your radar is REALLY tilted 3 degrees down, rather than just what the display says? What if it is really only tilted 2 degrees down?
Fifth. even if your radar had the right bandwith to detect metal/ragwing/composite construction objects the size and dimensions of Jabirus, Luscombes and C402's, had a high enough resolution in azimuth to actually paint it on the screen, and was within your tilt in elevation range without being obscured by ground returns....... so it would actually be displayed 30 seconds or so before you hit it..... were you watching the screen as you descended into the MBZ, called company, centre and other traffic and did all the other things you had to do? There is a reason why TCAS units issue audio commands and warnings!
Sixth, if you are doing what you recommend when you are inbound to a class C 'drome, then what is the problem? Class C is usually out to 30nm, where you would be not below A100 anyway, and barring some absolute dill being at F130 near a location that has PSR coverage, most of the lighties you would be trying to avoid would be nowhere near A100. Melbourne, Sydney, Brissie, Adelaide have PSR, Nowra, Willytown and Pearce have S-Band PSR, Darwin Amberley and Townsville have L-band air surveillance radar, and I watched a Darwin RAAF APP ATC overlay primary paints on his screen in 1996.
Class D out of Class E, well that's different, could be overlflying at 4500, but how will you see him in the ground return, even if all my above thoughts are wrong?
Now I'm not trying to be (too much) of a smarty, I just think you are wasting your time.
Scene from Sesame Street.....
Bert: Hey Ernie, what are you doing with a banana in your ear.
Ernie: Its to keep the elephants away.
Bert: There aren't any elephants in Sesame Street!
Ernie: See! It works!
Please, by all means, show me if I am a woolly thinker. But I suspect you may get equal protection from rogue lighties in class E by sticking a banana in the Captain's ear. You may as well tune the ADF's to 200 and see which way they point as you descend.
No offence intended. The above is just my picking holes after half a bottle of Tanqueray Gin with tonic!
