ATC facilities in the area
CDN ATC at #223 suggests there is little primary radar in the area of interest and what there is has only 100 mile range. Elsewhere she suggests that ATC radar is "mosaiced" together to form a composite picture.
Firstly there is a great deal of radar in the area and most of it is both long range primary RSR with SSR on the same rotating radar head. Range of the PSR is sometimes limited to 180 nm but can exceed 200 nm at the levels of interest.
At the very least, WIHH Jakarta, WIPP Palembang, WIIS Semarang, WARR Surabya have coverage in this area as well as some Malaysian stations. Whilst the Indonesian AIP is not currently available on line in English, a quick look at the source data being used by FlightRadar 24 will confirm this assertion - and you will also find a strange radar called TEST1 in operation too - who knows where that one is located.
The combining of radar data from multiple sources by use of a Mosaic tile system is a North American phenomenon. Most radar in this part of the world is of European origin (because French and Italian companies pay the biggest bribes) and Europe does not normally use the Mosaic system but rather a mullti-radar tracking system whereby the weighted average position from multiple sites are combined to give one "average" aircraft position. Regardless, Indonesian ATC is not exactly a paragon of modernity and is more likely to be using single source radar on any given display.
The reason for the continued existence of PRI radar in the area is that unlike Canada where the biggest threat is your neighbour to the south, life is not always so eternally peaceful in Asia. There are many countries very close to each other here. Indonesia has a population of 253 million, over seven times that of Canada with Malaysia only a few miles distant population 30 million, Singapore 5 million etc. Hence the need to maintain PRI radar for defence purposes as well as civil aviation.
So maybe CDN ATC you are not an ATC expert when it comes to south east Asia no matter what your homeland experience may be? But why worry, the amount of misinformation on this site never ceases to amaze me!