This is quasi-nicked from the Enchanced Senses power in Critters where it suggests Enhanced Senses (Motion Detection) meaning the ability to detect magnetic field disturbances.

Essentially: What would it really do? How would it compare to something like ultrasound for lowering modifiers? Could it compliment ultrasound's 1/2ing of vision penalties?

I'd rather compare it to the Thermosense Organ, with the relative movement of whatever you are trying to spot modidying the Signature as the TN for Perception. Obviously the temperature/IR based TN modifiers shouldn't apply. I don't know whether ambient magnetic fields should, I have no idea how the magnetic field based motion detection of animals works.

[Edit]Or do animals even have anything of the kind? If not, ambient magnetic fields should certainly affect it in some way. Less useful near power lines, etc.

I probably confused it with the pressure-sensing motion detection of fishes. I should probably get some sleep...[/Edit]

Can't birds work some electromagnetic magic, or is that only for seagull navigation and pinpoint pidgeon missile strikes?

I think that's only for navigation, and not for detection of objects -- smaller than the earth at any rate. But I'm not exactly a wildlife expert, and my Google-Ki energy is running low.

I think it's that some birds can see magnetic north due to iron in their eyes or something. <reveals his gullible faith in Discovery Channel>

I don't know how it would work either. The only creatures that have it are the Juggernaught (croc), Fideal (Jellyfish), and Bogie (Nasty Dog). There is motion detection described on P99 of SOTA 2063, but that's ultrasound-based.

I like the comparisson with the thermosense organs. Strong EM signals would probably overstimulate the sense.

Actually, the rules for Motion Detectors would work really well for this, I think. Just ignore the fluff description and focus on the rules for it. An Opposed Test pitting your Perception Test against an opponent's Stealth Test. You gain a +1 TN bonus on your roll for every 0.25 Meter beyond 1 Meter/Turn the opponent is moving. Ignore the part about Ultrasonic Emitters.

Since it's an adept power, use Magic (or maybe Magic * 10) meters for the maximum range.

You could just use the Thermosense TN Mod table as basis. In most urban areas you'd be looking at a constant +1 TN (equivalent to "warm" with all the background EM radiation and magnetic fields), near powerful fields +2 or more TN. The base TN to spot could be lower to offset the fact that there are no "EM Cold Zones" -- 5 for all critters between gnome and cyclops size, for example.

BTW, Juggy is an armadillo. Which doesn't explain why it has Motion Detection any better, though.

I forgot actual Motion Detectors existed... Oh well. That probably works better for the effect, at least to make the Power true to its name.

Some fish have electrical sense organs that they use to detect the electrical systems of other organisms around them. Could google for "lateral line". This sense would not work in air, the seawater is required to conduct the faint electrical signals.

I think the birds have some iron crystals in thier brains to give them direction sense. Sea turtles have the same thing.

Could be a magical power, or some cyberware that is basically tiny eyes that detect only motion rather than actual images.

The game effect would be something like -2t# to detect motion within 5 meters but out of your normal field of vision, maybe -1 or 2t#'s against invisible opponants I think ultrasound should override that effect though. Would probably help with anti-stealth tests, for example. Probably would not work at all if you are moving.

The lateral line is exactly what I was thinking of, I just couldn't remember whether it was pressure-related or electricity-related. Obviously I'm not in any condition to continue this discussion, so I'll retire for now. Off to bed...