Radar Contact Classification
An air defence operator tracks an unknown contact. Data over four sweeps: range from the station falls from 180 km to 150 km to 120 km to 90 km at 12-second intervals; bearing is constant at 045°; altitude is steady at 8,500 m; speed over ground is 900 km/h. The station's identification zone extends to 100 km. A civilian airway crosses the sector at 045° at 9,000–10,000 m. Rules: any contact entering the 100 km zone on a constant bearing towards the station must be challenged; a contact that is squawking a valid civilian code may be handed to civil control; an unidentified contact inside 60 km must be treated as hostile until identified.
1. Classify the contact's track and determine at what time it crosses the 100 km and 60 km rings. What action is required at each?
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Track analysis:
- Range decreases 30 km per 12 s → closing speed = 30 km / 12 s = 2.5 km/s = 9,000 km/h?? That is inconsistent with the stated 900 km/h. Recheck: 30 km in 12 s = 2.5 km/s = 9,000 km/h. The stated 900 km/h would give 3 km per 12 s.
Conclusion: the data are internally inconsistent. A contact at 900 km/h closes 30 km in 120 s, not 12 s. The operator must flag the discrepancy - either the range readings or the speed readout is wrong, and a 9,000 km/h closure is physically implausible for an aircraft.
Assuming the 12 s interval is correct (i.e. true speed 9,000 km/h - a missile-like or erroneous track):
- From 90 km, time to 60 km = 30/2.5 = 12 s
- Time to 100 km ring already passed (contact was at 120 km two sweeps ago, crossed 100 km between sweeps 2 and 3).
Assuming 900 km/h is correct:
- Closing rate = 0.25 km/s
- From 90 km to 60 km = 30/0.25 = 120 s
- 100 km ring was crossed before the first reading (180 km) - no, 180 > 100, so crossing occurs at (180−100)/0.25 = 320 s after first contact.
Action: Challenge immediately (constant bearing, closing, entering zone). Do not hand to civil control without a valid squawk.
2. The contact now squawks a valid civilian code but continues on a constant bearing at 045° towards the station. What is the correct judgement call, and why is 'constant bearing' the decisive cue?
Read the worked answer
Constant bearing + decreasing range = collision course. This is the classic constant-bearing-decreasing-range (CBDR) geometry. A genuine civil airliner on an airway would normally pass abeam the station, not track directly at it; a constant bearing to a fixed point implies the track is aimed at that point.
Judgement:
- A valid squawk is evidence, not proof - codes can be copied, spoofed, or mis-set. It lowers but does not eliminate suspicion.
- The contact is inside 60 km and unidentified in intent → rule says treat as hostile until identified. A squawk alone does not satisfy 'identified'.
- Correct action: continue challenge, alert weapons/escort, and prepare to hand to civil control only if the track diverges from the station or the crew responds to challenge.
Why constant bearing is decisive: it is the one cue that distinguishes an aircraft transiting the area from one homing on the station. Speed and altitude can be matched by a civil aircraft; a track aimed at you cannot be explained by airway geometry. The operator who releases on the squawk alone has substituted a procedural token for tactical judgement.