Originally Posted by AdamFrisch
My question is twofold:
1. Do you see his altitude on your primary return on the radar?
2. If you don't see altitude, are you relying on the mode C return solely for altitude? Because a mode C return can obviously be altered and be incorrect - I've many times flown with them being well off or have the wrong pressure altitude input.
How does this work?
1. The primary return is merely a reflection of electro magnetic waves. (cf. light waves from a torch.) Limited capability existed in the sixties and seventies using stacked multi-beam primary radars eg. Marconi Type 82 height-finding primary radars, but these were rendered obsolete by SSR Mode C)
2. Mode C is used for vertical reference. Mode C provides altitude information from encoded altimeters. The altitude is encoded at 1013.2 Hpa. It is unaffected by subscale setting. The ATSU will convert the display as required eg. by inputting QNH to display altitudes below the TA. Yes, sometimes altitude encoders send information out of tolerance (which I recall is +/- 200ft) hence ATC endeavours to avoid unknown traffic by a minimum 5nm or 5000ft because intentions and Mode C accuracy are unknown.
Heathrow CTR busts cause carnage - huge increase in workload, huge delays, expense and waste of time and fuel, and are a serious risk to flight safety, hence zone-busters are not at all popular. The Controlled Airspace Incursion Tool highlights intruders in order to provide a bit of early warning.