Re: question on parity

Hello Chuanyu,


yes, of course you're right: bits 23 and 24 are chosen such that
the parity bits 29 and 30 are both zero. As far as I remember that
perculiarity is not implemented in OpenGPSSim.


Regards

Georg

At 13:50 27.07.2004, chuanyu si wrote:
>Thanks Georg,
>
>I'm sorry for i'm still puzzlling on that.
>I think the process like this: ephemris data -> struct NAV -> sf[6][11] -> sfp[6][11] ->msg[1500];
>So, sf[6][11] was encoded data, and sfp[6][11] was encoded data adding parity.
>what was described in the reference book is: last two parity bits of Word #2 of every subframe should be zero.(James Bao-Yen Tsui, Fundmantal of Global Positioning System Receivers....P86)
>So, at the receiver, when checking this two bits, the subframe must be encoded and parity added and should not be _decoded_ HOW.
>also when HOW was decoded, it only has 24 bits, the last 6 was parity.
>
>Thanks for answering..
>
>
>
>
>>From: Georg Beyerle <[email protected]>
>>To: [email protected]
>>CC: [email protected]
>>Subject: Re: [OpenSource_GPS] question on parity
>>Date: Mon, 26 Jul 2004 12:25:32 +0200
>>
>>Hello Si Chuanyu,
>>
>>
>>I haven't looked at OpenGPSSim for a long time. I'll try to
>>answer your question from what I still remember...
>>
>>I assume what you did was something like that:
>>
>>    ./bin/ogsencdec nav-01-template-01-06.txt
>>
>>and then
>>
>>    ./bin/ogsxmit -n nav-01-template-01-06
>>
>>However, the 'transmitter' program 'ogsxmit' inputs
>>the _encoded_ navigation data stored in the file
>>'nav-01-template-01-06'. So the last two bits of sf[1][2]
>>could be anything. As Cliff already said it's the _decoded_
>>HOW word that should end with 00 (or 11).
>>
>>
>>Regards
>>
>>Georg
>>
>>At 16:34 21.07.2004, you wrote:
>>>It looks like you are using Georg Beyerle's data and code which may be
>>>somewhat different from OpenSource GPS.
>>>
>>>Georg, can you take this question?
>>>
>>>Thanks,
>>>
>>>Cliff
>>>----- Original Message -----
>>>From: "si chuanyu" <[email protected]>
>>>To: <[email protected]>
>>>Sent: Tuesday, July 20, 2004 12:53 AM
>>>Subject: Re: [OpenSource_GPS] question on parity
>>>
>>>
>>>> thanks Kelley.
>>>>
>>>> yes, you said it.
>>>> but in my side, when i use nav-01-template-01-06.txt as input, after
>>>> encoding and parity adding, sf[1][2]'s last two bit was "ff 01" in the
>>>> memory. so it is 01 or 10(polarity is unkown)
>>>> I've tried several time and get the same result. I used Visual Studio 
>6.0
>>>> for development.
>>>>
>>>> I added a break point in ogsxmit.cpp 's main function after read the 
>nav
>>>> bit from nav-01-template-01-06 into buffer NavBit[], and watched the
>>>memory
>>>> contents in the debug window, and get NavBit[58]=ff NavBit[59]=01;
>>>> so I am wondering why this ocurr? could you please give me some
>>>information
>>>> on that?
>>>>
>>>> Thank you very much!
>>>>
>>>>
>>>> >From: "Clifford Kelley" <[email protected]>
>>>> >Reply-To: "Clifford Kelley" <[email protected]>
>>>> >To: "si chuanyu"
>>>> <[email protected]>,<[email protected]>
>>>> >Subject: Re: [OpenSource_GPS] question on parity
>>>> >Date: Tue, 8 Jun 2004 21:48:17 -0700
>>>> >
>>>> >I'm not sure which reference you are referring to but the last two 
>bits
>>>of
>>>> >HOW when decoded should be 00.  If the polarity is unknown it could 
>be
>>>11.
>>>> >The reference to check with is ICD-GPS-200.
>>>> >
>>>> >Cliff
>>>> >----- Original Message -----
>>>> >From: "si chuanyu" <[email protected]>
>>>> >To: <[email protected]>
>>>> >Sent: Monday, June 07, 2004 2:37 AM
>>>> >Subject: [OpenSource_GPS] question on parity
>>>> >
>>>> >
>>>> > > hi
>>>> > >    I found in the reference that the sum of the last two bits of 
>HOW
>>>in
>>>> > > each subframe should be +2 or -2. but in the reference code, after
>>>> >encoding
>>>> > > the subframe and adding parity, the last two bits of subframe[][2] 
>was
>>>> 1
>>>> >or
>>>> > > -1. thus in the receiver, we can't pass parity check. what's the
>>>> problem
>>>> > > with that? do I missed something else? thanks a lot!
>>>> > >
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