Frankly I find your response rather snarky, remember comments aren't just for your benefit. Hopefully this helps answer your question. You asked why it doesn'

Frankly I find your response rather snarky, remember comments aren't just for your benefit. Hopefully this helps answer your question. You asked why it doesn't have a positioning system: the simple answer is the battery (Philae has only 130Wh or so), cost and whether it's necessary to do the science: it wasn't. If you measure distance by timing light, you're using a LIDAR system by definition. A laser isn't explicitly necessary, but because the power received is proportional to R^-4, using an isotropic emitter is wasteful. A pulsed LIDAR system with cm accuracy over 20km is doable, you'd need to time the light to within around 30ns. However we'd struggle to get that kind of accuracy on Earth, let alone orbiting a comet. Luminosity isn't good enough to measure distance (certainly not cm level), it depends on the efficiency of the emitter and the efficiency of the detector, both of which might have changed during flight. It also depends on the transmission medium, in this case it may well be dusty. It could potentially give you a location, but I'm sceptical Rosetta would see it above the background unless it was an extremely bright source. I think putting the transmitter on the lander would be the backward route. A corner cube reflector would probably be the most viable solution. We're assuming that Rosetta can overfly Philae and fire its laser in the right place, but I think that would ok. LOLA on the Lunar Recon. Orbiter flew as low as 30km and only got down to around 10cm resolution and required around 15-30W. My comment regarding GPS was that many people assume that you can get a 3D fix on an object with a single recieve/transmit pair (like in the movies) and it simply doesn't work like that. A directional antenna could work, perhaps.