There's a separate crystal for each scan, generally not near each other. Once you've got one hooked up, your HUD finder won't detect it anymore so it's easier to find the remaining ones. If it says "No Signal" you're simply really, really far away from any remaining crystals, just wander around until you pick up the next one's signal. The crystals also have distinct names on the HUD finder (Alpha, Bravo, etc.) so you can at least be sure you know which you're closest to and when it jumps to something else.