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What Offset is this?


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Hi Pete,

I was wondering if you could give me an answer here:

What is the degrees value (heading, 0-359) for the FSX offset for the "Inner Circle" of a HSI Instrument?

This is the circle that rotates according to course adjustment and the position of the nearest VOR Station.

I am talking about this:

hsi-2.jpg

I'm just really not sure what it would be called. Of course, it won't be called "Inner Circle" or anything like that.

Regards,

Jack

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What is the degrees value (heading, 0-359) for the FSX offset for the "Inner Circle" of a HSI Instrument?

This is the circle that rotates according to course adjustment and the position of the nearest VOR Station.

It is the "Course" setting on an MCP or a VOR/ILS, not just on an HSI. But it was always known in FS as the "OBS" (Omni bearing selector, a VOR being an omni-directional radio).

There are two, one for NAV1 at offset 0C4E, and one for NAV2 at offset 0C5E.

There are FS controls to change them too of course. Those use the abbreviation OBI (omni bearing indicator):

VOR1_OBI_DEC 65662

VOR1_OBI_INC 65663

VOR2_OBI_DEC 65664

VOR2_OBI_INC 65665

Regards

Pete

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Hi Pete,

Thanks for that.

Is that it then? Just 04CE, Length 2? I mean, anything FSUIPC/FSX associated with headings usually comes with some very annoying calculations.

Does 04CE literally just output a normal reading between 0-359?

Regards,

Jack

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Is that it then? Just 04CE, Length 2?

Yes, just the integral number of degrees. You can't set fractional a values on those things.

I mean, anything FSUIPC/FSX associated with headings usually comes with some very annoying calculations.

Well, that's not FSUIPC or FSX. It arises from the units adopted originally by FS98 way way back in time. FSUIPC's interface derived from a previous program called FS6IPC by Adam Szofran (who later worked for Microsoft Aces). The whole point of FSUIPC through out its long life has been to maintain compatibility, through FS98-FS2000-CFS1-FS2002-CFS2-FS2004-FSX-ESP-Prepar3D, so I could never change things applications already used, only add new things. The offsets for OBS date right back, but so do the more complex ones like latitude and longitude, to FS6IPC which merely provided a window into a memory area of FS98 itself, no manipulations, no cloak and dagger maneuvering.

The usual reason for the odd looking units is efficiency. PCs weren't as fast back then, and were certainly nowhere near as efficient as they are now at large floating point computations. Most of FS's calculations were performed using fixed point arithmetic, so values were stored in fixed point ways. The typical heading and other angular values stored in 16 bits are stored with as much precision as possible by making the maximum value you can possibly fit just a tiny bit less than 360 degrees. Hence 65535 = 359.99999... degrees, and you get the formulae you see in the FSUIPC references (multiply by 360.0 and divide by 65536.0 for degrees in floating point).

04CE and 05CE only ever need to contain a simple small integer, so they do. They would be only 1 byte each is 0-255 were enough, but of course it isn't.

These days, internally, FS9 and FSX are relatively wasteful in memory -- because they can be, memory is so much more available. They use 32 bit values just for a 1 bit indicator (1 = "TRUE" or 0 = "FALSE"), and full 64-bit floating point for most all computations because it is now as fast as, or even faster, than fixed point. A lot of those new values are exposed in FSUIPC's offsets. Additions have been made through the years as other values become useful to applications. But the old values still remain, for compatibility.

Regards

Pete

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