Only because there isn't an approach from the south to Sinop - you'd have to make a longer route to hit it from West Or East... While you can just head north in a straight-ish line via DETOS UW-99 ODIRA.

Departure practically due south. Of course ~ with relations being poor between Israel and Syria/Lebanon, there might routinely be a large detour over the Med, making SINOP slightly more probable... but I'd still favour DETOS/UW-99.

Another consideration indicating a turn at ODIRA is that the plot has the aircraft someway south of the nominal corridor ~ this could be the result of an early turn intended to make up time?

Plane, say local experts from turkish Flight Services Control, at 12:29 local time (13:29 MSK) on Thursday passed the airport flight information zone Samsun on the Black Sea coast, in 12.40 - left the flight zone of Turkey

OFFICIAL ANNOUNCEMENT of Siberia
Flight details SBI1812
The Tu-154 (RA-85693) was Flight SBI 1811/1812 from Novosibirsk, Sochi-Tel Aviv-Novosibirsk. On the way to Israel, landing in Sochi was performed in order to refuel.
The actual timetable (time UTC):
October 3, 2001
SBI - 1811 Novosibirsk - Sochi - Tel Aviv (DST 0909 - Vol 1338/1500 - TLV 1700).
October 4, 2001
SBI - 1812 Tel Aviv - point Rabit (TLV 0800 - RABIT 0944).
Estimated flight time based on actual weather:
Novosibirsk - Sochi = 4 hours 38 minutes, Sochi - Tel Aviv = 2 hours 08 minutes.
Tel Aviv - Novosibirsk = 6 hours 00 minutes.
The actual flight time:
Novosibirsk - Sochi = 4 hours 29 minutes.
Sochi - Tel Aviv = 2 hours 00 minutes.
Tel Aviv - RABIT = 1 hour 44 minutes.

Official Statement airline "Siberia"

NOVOSIBIRSK, October 6, 2001 (12:00 local time).

Unfortunately, the media may be full of inaccuracies and distortions. Most often distorted by the following:

1 The Tu-154 (RA-85693) Campaign followed the flight number 1812 from Tel Aviv to Novosibirsk landing directly without intermediate in Bulgaria. Flight plan did not envisage such a landing.
2 Flight number 1812, adopted in accordance with the Russian civil aviation terminology, is a "charter on a regular basis." Departure of this flight from Novosibirsk is year round on Wednesdays. The first flight of "Siberia" in Novosibirsk - Tel Aviv was executed in 1995.
3 The exact title of Victor Alexeev, located in the crew of Flight number 1812 - The head of flight safety inspection airline "Siberia." It was not the deputy director general of airline "Siberia."
4 The exact title of Vladimir Tasun, head of the operational headquarters in Novosibirsk - the head of the West Siberian District Interregional Territorial Administration air transport ministry.

The airline "Siberia" will provide further information as it becomes available.

2. Press release from "Siberia" 1:



Source

I'd personally favour SIV-DETOS-UW99-earlyturn-UW96, but if there is specific information confirming being out of track position on the inbound leg to ODIRA, as well as the range indication that it was south of track 'outbound' towards Sochi.

The strongest contributors to RCS are engines, so Tu-154, with three of them, should be somewhere between Tu-16 and Il-76. Strictly my IMHO.

I would guess the Tu-154 RCS around 42sqrm.
That gives 280km of 10dB lockon range for the GSN...

Here some info for the Tu-143... :

http://www.valka.cz/clanek_10610.html

RCS stated is 0.25 -0.35 m2...(!)