Bit Offset on Galileo Message Modifications

Skydel Discussion
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While trying change the week number on Galileo independently from Skydel, I found that the bit index was off by 3. I was only able to change the week number when I included the offset. I know one of the bits can be accounted for due to Skydel being one based and the index bits being zero based, but why is there still an offset of 2 bits?

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Hi @mallory.flynn ,

Would it be possible to obtain a little more information?

  1. We’re not sure what you mean by “independently from Skydel”. Are you using Skydel messages modifications?
  2. Which Galileo signal are you working with?
  3. Which field exactly are you trying to change?

Your answers will help us better understand the problem.

Thanks!

-AD

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  1. Yes, we are using message modifications, essentially overriding the given week number assigned by Skydel.
  2. We are using Galileo E1 I believe.
  3. We are trying to change the week number.
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Hi @mallory.flynn ,

Ok, our dev team has an answer for you, and it’s not a quick one :wink:

I/NAV messages are split into a subframe. A subframe is essentially a set of pages.

A page counts 250 bits. 10 sync + 240 for page parts:

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Message modifications in Skydel allow you to change the 240 bits for page parts.
These bits are comprised of even (120 bits) and odd (120 bits) page parts as shown here:

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For I/NAV, the Week Number is broadcast in the following fields:

  • GST WN (Word Type 0)
  • GST WN (WT 5)
  • GST-UTC WN0t and WN LSF(WT 6)
  • Almanacs WNa (WT 7 and 9)
  • GST-GPS conversion parameters WN0G (WT10)

Let’s, for example, change almanac WN in WT 7 and 9.
WNa are transmitted in nominal page 4 in part 1 in even page (so the first 120 bits of message modification).

Refer to Tables 36 and 46 below :

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So, in order you will have:
1 bit for even
1 bit for page type (alert or nominal)
6 bits for page type
4 bits for IODa
2 bits for WNa
…and, of course, 226 other bits.

So, to modify WNa, you will have to modify the 2 bits starting at the 12 bits.

In Skydel, the message modification will be as follows:

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I hope this helps. Please feel free to reach out if you need clarifications on any of the above.

For reference, the Galileo OS SIS ICD can be found here.

-AD

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