tag:blogger.com,1999:blog-5848437078181345610.post5757551407458303496..comments2024-08-01T05:17:53.182-07:00Comments on Tailhook Topics: Modex NumberTailspinhttp://www.blogger.com/profile/17837863895661437038noreply@blogger.comBlogger5125tag:blogger.com,1999:blog-5848437078181345610.post-62203122029529102612024-07-19T22:49:07.928-07:002024-07-19T22:49:07.928-07:00“The 50's/60's-era IFF systems used 8-bit ... “The 50's/60's-era IFF systems used 8-bit data systems (Base 8). Digits available were 0 through 7 (a total of 8 bits)."<br /><br />Just to be the pedantic one here, these Octals/Base Numbers are THREE bit numbers.<br /><br />My understanding of modex's were that they were four lots of three bits, that were 'bit set' by the 'host unit'. With 'big endian' numbers, they are the sum of set bits where the first position is worth FOUR, the second position TWO, and the third position ONE. Thus for each Octal the following bits were set:<br /><br />000 - 0<br />001 - 1<br />010 - 2<br />011 - 3<br />100 - 4<br />101 - 5<br />110 - 6<br />111 - 7<br /><br />Just to extend on this, 4 bits gives us 16 'numerals' (which we would enumerate as {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F}) - 8 bits gives us a total of 256 possible different 'numerals', which now renders the point of the single 'digit' moot (these are usually enumerated as Hexi-decimals, a 'glyph' created with two digits of 0-F, for example 15 is '0F' and 26 is '1A')<br /><br />But back when this was first being done, things were kept much simpler - As you've pointed out, the first octal was AIR WING, the second was SQN, and the remaining two were AIRCRAFT - what this gives us is a VERY early digital means of encoding an aircraft (using only 12 'bits') back in the days when the digital memory to store this sort of thing, was VERY expensive, AND computing power was VERY low.<br /><br />Now it is true that this is not the most efficient way to store a 'full modex' - four Octals gives a total of 8 x 8 x 8 x 8 unique identifiers (4096, as above), with the range 0000 to 7777; and this allows us to bit mask really easy too. OK, new term, Bit Mask - in this case, it is just the formalisation of what what have already been talking about - first numeral is AIR WING, the second is SQN, and the remaining two are AIRCRAFT; so to determine air wing, we only need to look at the first digit; to determine SQN within the air wing, we only need to look at the second digit, etc.<br /><br />But this is where it gets interesting and the real smarts start to come through... If we want a format that (as above) first numeral is AIR WING, the second is SQN, and the remaining two are AIRCRAFT, then to cover '7777' we would have needed 13 bits (as 12 bits gets us from 4096 unique number, 13 bits allows us to get to 8192), plus the added complexity of NOT being able to Bit Mask near as easily. <br /><br />To use the example from the article, the CAG jet '7100' using FOUR Octals is:<br />111 001 000 000<br />that same '7100' using 13 bits is:<br />1101110111100<br /><br />And as you can see, there is no easy way to 'mask' that 13 bit number to extra the same info from, as from the FOUR Octals. AND to do that, requires a LOT more computing power; but I think I have gone PLENTY far off topic here!<br /><br /><br />Dan<br />Danhttps://www.blogger.com/profile/09294577743166339195noreply@blogger.comtag:blogger.com,1999:blog-5848437078181345610.post-351151731258638052024-06-01T10:06:35.205-07:002024-06-01T10:06:35.205-07:00Nicely done Tommy! This question comes up frequent...Nicely done Tommy! This question comes up frequently and you thrown some light into those dark corners of history. Mark AldrichAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-5848437078181345610.post-66067165426682414132024-05-31T17:47:20.208-07:002024-05-31T17:47:20.208-07:00Navy and Marine aircraft also commonly used the ta...Navy and Marine aircraft also commonly used the tail code plus either side number or flight schedule event number as callsigns when dealing with civil ATC. eg. "Delta Charlie 32", A VMFA-122 F-4 tail code "DC" plus event number) or "November Kilo 205", A VF-143 F-4 tail code "NK" plus side-number (modex). Nice write up Tommy, cheers Peter Greengrass.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5848437078181345610.post-68025931189682820612024-05-31T10:57:56.144-07:002024-05-31T10:57:56.144-07:00I've added links in the text to a couple of ol...I've added links in the text to a couple of older posts that summarize air group tail code markings practice. Note that in the mid 1950s, aircraft in detachments from shore-based composite squadrons that provided them like VAH and VP began to be remarked with the tail code of the air group they were temporarily assigned to for a deployment.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5848437078181345610.post-59552248637855332612024-05-31T06:22:05.607-07:002024-05-31T06:22:05.607-07:00Wow. Now I have a headache. You also might want ...Wow. Now I have a headache. You also might want explain the air group codes on the vertical stab, what they signify, how they changed and why some squadrons wore fleet codes beginning with N or A sometimes and completely different shorebased codes other times. RA-5Cs and other heavies are a great example. This may be a post for another day, however.Anonymousnoreply@blogger.com