The 285 won't work with a spektrum receiver unless you use the Spektrum CPPM adapter cable, the 285 ony accepts CPPM natively, which the Spektrum Rx does not output. Here's more info on hooking up a Spektrum Rx to the GPS port:

And here's the Spektrum CPPM adapter cable:

So these are the two options you have available, a third one would be to use a Orange R620 CPPM receiver, which outputs CPPM and is compatible with DSM/DSMX.