Data Demodulation and Processing - Revision history

Filipe.Pelica: Included editor logo.

2014-09-18T16:36:11Z

Included editor logo.

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Carlos.Lopez at 11:00, 23 February 2012

2012-02-23T11:00:46Z

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Carlos.Lopez at 10:34, 23 February 2012

2012-02-23T10:34:01Z

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	While the tracking loops extract code and carrier information to synchronize the locally generated PRN code with the incoming signal, the Data demodulation and Processing block aims at extracting “useful information” to be used by the applications processing block, when generating a navigation solution. The core of this information consists on the navigation message and the observables, namely pseudorange and Doppler frequency.		While the tracking loops extract code and carrier information to synchronize the locally generated PRN code with the incoming signal, the Data demodulation and Processing block aims at extracting “useful information” to be used by the applications processing block, when generating a navigation solution. The core of this information consists on the navigation message and the observables, namely pseudorange and Doppler frequency.

Carlos.Lopez at 14:43, 8 August 2011

2011-08-08T14:43:06Z

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	Finally, this information is passed to the Applications Processing block, which:		Finally, this information is passed to the Applications Processing block, which:

	*[[GNSS ~~measurements and datapreprocessing~~\|Processes the GNSS observables]].		*[[GNSS Basic Observables\|Processes the GNSS observables]].

	*[[Solving navigation equations\|Computes the navigation solution]].		*[[Solving navigation equations\|Computes the navigation solution]].
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	*[[Generic Receiver Description]]		*[[Generic Receiver Description]]
	*[[Digital Signal Processing]]		*[[Digital Signal Processing]]
	*[[GNSS ~~measurements and datapreprocessing~~]]		*[[Combination of GNSS Measurements]]
			*[[GNSS Basic Observables]]
	*[[Solving navigation equations]]		*[[Solving navigation equations]]
	*[[GNSS Applications]]		*[[GNSS Applications]]

Rui.Pereira: moved Work in Progress:Data Demodulation and Processing to Data Demodulation and Processing over redirect

2011-05-27T15:54:39Z

moved Work in Progress:Data Demodulation and Processing to Data Demodulation and Processing over redirect

← Older revision	Revision as of 15:54, 27 May 2011
(No difference)

Rui.Pereira: moved Data Demodulation and Processing to Work in Progress:Data Demodulation and Processing

2011-05-11T14:50:01Z

moved Data Demodulation and Processing to Work in Progress:Data Demodulation and Processing

← Older revision	Revision as of 14:50, 11 May 2011
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Rui.Pereira at 13:45, 11 May 2011

2011-05-11T13:45:50Z

← Older revision		Revision as of 13:45, 11 May 2011
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	===Cyclic Redundancy Checks (CRC)===		===Cyclic Redundancy Checks (CRC)===
	Cyclic Redundancy Checks, commonly known as CRC<ref>[[~~wikipedia~~:~~Cyclic_redundancy_check~~]]</ref> use pre-defined polynomials to compare check values computed locally (at the receiver) with the ones conveyed in the message and generated at transmission.		Cyclic Redundancy Checks, commonly known as CRC<ref>[[Wikipedia:Cyclic redundancy check\|Cyclic redundancy check in Wikipedia]]</ref> use pre-defined polynomials to compare check values computed locally (at the receiver) with the ones conveyed in the message and generated at transmission.

	===Forward Error Correction (FEC)===		===Forward Error Correction (FEC)===
	FEC encoding<ref>[[~~wikipedia~~:~~Forward_error_correction~~]]</ref> consists in representing bits with more than one symbol: this way, redundant information is transmitted, allowing the receiver to detect and correct some potential errors, hence improving the performance of the channel. As an example, Galileo Open Service use forward error convolutional codes with a constraint length of 7 and a coding rate of 1/2 to spread the navigation message, i.e. one bit is encoded with two symbols. Receivers may use Viterbi decoders<ref>[[~~wikipedia~~:~~Viterbi_decoder~~]]</ref> to recover the transmitted navigation message.		FEC encoding<ref>[[Wikipedia:Forward error correction\|Forward error correction in Wikipedia]]</ref> consists in representing bits with more than one symbol: this way, redundant information is transmitted, allowing the receiver to detect and correct some potential errors, hence improving the performance of the channel. As an example, Galileo Open Service use forward error convolutional codes with a constraint length of 7 and a coding rate of 1/2 to spread the navigation message, i.e. one bit is encoded with two symbols. Receivers may use Viterbi decoders<ref>[[Wikipedia:Viterbi decoder\|Viterbi decoder in Wikipedia]]</ref> to recover the transmitted navigation message.

	===Block Interleaving===		===Block Interleaving===

Rui.Sarnadas at 15:01, 6 May 2011

2011-05-06T15:01:54Z

Show changes

Teresa.Ferreira: /* Generation of Basic Observables */

2011-05-06T08:14:24Z

Generation of Basic Observables

← Older revision		Revision as of 08:14, 6 May 2011
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	The pseudo-range to a given satellite, m, can be computed as ~~(neglecting all propagation and equipment delays as well as noise)~~:		The pseudo-range to a given satellite, m, can be computed as:
	<math>\rho^m=c(T_U^m - T_S^m)</math>		<math>\rho^m=c(T_U^m - T_S^m)</math>

	Where		Where
	*<math>T_S</math> is the transmission time using the satellite clock, which has an error relatively to an absolute time reference (e.g. GPS time)		*<math>T_S</math> is the transmission time using the satellite clock, which has an error relatively to an absolute time reference (e.g. GPS time)
	*<math>T_U</math> is the reception time using the receiver clock, which has an error relatively to an absolute time reference (e.g. GPS time)		*<math>T_U</math> is the reception time using the receiver clock, which has an error relatively to an absolute time reference (e.g. GPS time), plus all propagation delays, equipment delays and noise)
	*c is the speed of light		*c is the speed of light

	In reality, most GNSS receivers extrapolate the transmission time (at satellite clock) for all satellites at each receiver time epoch. Figure 2 illustrates this concept: <math>T_S’</math> are extrapolated for satellites m and n, where TOW is the time stamp recorded by the satellite at the beginning of each subframe.		In reality, most GNSS receivers extrapolate the transmission time (at satellite clock) for all satellites at each receiver time epoch. Figure 2 illustrates this concept: <math>T_S’</math> are extrapolated for satellites m and n, where TOW (Time Of Week) is the time stamp recorded by the satellite at the beginning of each subframe.


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	Figure 3 illustrates the computation of the extrapolated transmission time at the receiver, using the example of GPS L1 C/A, where 1 bit lasts 20 ms and each symbol lasts 1 ms. The code delay is the value output by the [[Delay Lock Loop (DLL)\|DLL]].		Figure 3 illustrates the computation of the extrapolated transmission time at the receiver, using the example of GPS L1 C/A, where 1 bit lasts 20 ms and considering that each pseudo-symbol lasts 1 ms (correlators are being integrated over one single PRN code). The code delay is the value output by the [[Delay Lock Loop (DLL)\|DLL]].

Teresa.Ferreira: /* Data Demodulation */

2011-05-06T08:10:53Z

Data Demodulation

← Older revision		Revision as of 08:10, 6 May 2011
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	Some of the techniques used are:		Some of the techniques used are:

	*<b>Parity Decoding</b>: the parity of pre-defined sets of bits (e.g. words) is checked against a field containing the parity computed at transmission and sent within the navigation message. Any discrepancies would imply that there was at least one error in the ~~transmission. This same concept is the basis for Cyclic Redundancy Checks (CRC)~~.		*<b>Parity Decoding</b>: the parity of pre-defined sets of bits (e.g. words) is checked against a field containing the parity computed at transmission and sent within the navigation message. Any discrepancies would imply that there was at least one error in the message.

			*<b>Cyclic Redundancy Checks</b>: CRC<ref>http://en.wikipedia.org/wiki/Cyclic_redundancy_check</ref> use pre-defined polynomials to compare check values computed locally (at the receiver) with the ones conveyed in the message and generated at transmission.

	*<b>Forward Error Correction (FEC)</b>: FEC encoding<ref>http://en.wikipedia.org/wiki/Forward_error_correction</ref> consists in representing bits in more than one symbol, such that additional information is transmitted, allowing the receiver to detect and correct some potential errors, hence improving the performance of the channel. As an example, Galileo Open Service use forward error convolutional codes with a constraint length of 7 and a coding rate of 1/2 to spread the navigation message, i.e. one bit is codified as two symbols. Receivers may use Viterbi decoders<ref>http://en.wikipedia.org/wiki/Viterbi_decoder</ref> to recover the transmitted navigation message.

			*<b>Forward Error Correction (FEC)</b>: FEC encoding<ref>http://en.wikipedia.org/wiki/Forward_error_correction</ref> consists in representing bits with more than one symbol, such that redundant information is transmitted, allowing the receiver to detect and correct some potential errors, hence improving the performance of the channel. As an example, Galileo Open Service use forward error convolutional codes with a constraint length of 7 and a coding rate of 1/2 to spread the navigation message, i.e. one bit is codified in two symbols. Receivers may use Viterbi decoders<ref>http://en.wikipedia.org/wiki/Viterbi_decoder</ref> to recover the transmitted navigation message.

	*<b>Block Interleaving</b>: consists in transmitting the symbols in a different order. As an example, Galileo Open Service assembles the symbols in blocks of M columns x N rows and then transposes the block before sending the symbols in the SIS. The reverse operation needs to be conducted at the receiver in order to recover the ~~transmitted symbols~~. The advantage of block interleaving is that it renders the message more robust to burst errors, since burst errors are now spread over a larger part of the message and may be still recoverable, e.g. using ~~the~~ FEC decoding techniques which have some capabilities to correct errors.
			*<b>Block Interleaving</b>: consists in transmitting the symbols in a different order. As an example, Galileo Open Service assembles the symbols in blocks of M columns x N rows and then transposes the block before sending the symbols in the SIS. The reverse operation needs to be conducted at the receiver in order to recover the original symbol sequence. The advantage of block interleaving is that it renders the message more robust to burst errors, since burst errors are now spread over a larger part of the message and may be still recoverable, e.g. using FEC decoding techniques which have some capabilities to correct errors.

	==Generation of Basic Observables==		==Generation of Basic Observables==