Two part question about the FMA Ir Sensor

[Deron Douglas]

Hi,
I just ordered a AP-2000i and something occurred to me, is the FMA ir sensor necessary for it to work? I thought I'd give it a try first before I ordered it.

Second... how does this sensor actually work. What I mean by this, how does it detect the horizon based on IR? Won't the surrounding atmosphere be roughly the same temperature all around?

[MarkWebber]

For stabilization(the AP2000i does more), the FMA IR sensor is required along with the interconnecting cable.

The sensor works by compairing the opposing readings of sensor pairs and feeding the resultant value off to the AP2000i for processing. Except in extreem conditions, i.e. very heavy overcast skys/following rain, the differential between the sky and ground is usually quite significant. If I remember correctly, Angelos once pointed out that the sky measured with and IR thermometer will show something like 5 degrees. Whereas the ground will measure much higher.

For instance, the system will perform wonderfully over snow covered ground despite the snow being such a cold surface.