【卫星】 GNSS（比如 GPS）接收机在多径环境下的伪距测量误差附matlab代码

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🔥 内容介绍

GNSS（如 GPS、北斗、Galileo）伪距测量的核心是通过接收机与卫星之间的信号传播时间计算几何距离，进而解算载体位置。但在多径环境（信号经建筑物、地面、水面等反射后到达接收机）中，反射信号与直接信号叠加，会导致伪距测量值偏离真实几何距离，形成多径误差，这是 GNSS 定位中最主要的误差源之一，严重影响定位精度与稳定性。

⛳️ 运行结果

📣 部分代码

function acqResults = acquisition(longSignal, settings)

%Function performs cold start acquisition on the collected "data". It

%searches for GPS signals of all satellites, which are listed in field

%"acqSatelliteList" in the settings structure. Function saves code phase

%and frequency of the detected signals in the "acqResults" structure.

%

%acqResults = acquisition(longSignal, settings)

%

% Inputs:

% longSignal - 11 ms of raw signal from the front-end

% settings - Receiver settings. Provides information about

% sampling and intermediate frequencies and other

% parameters including the list of the satellites to

% be acquired.

% Outputs:

% acqResults - Function saves code phases and frequencies of the

% detected signals in the "acqResults" structure. The

% field "carrFreq" is set to 0 if the signal is not

% detected for the given PRN number.

%--------------------------------------------------------------------------

% SoftGNSS v3.0

%

% Copyright (C) Darius Plausinaitis and Dennis M. Akos

% Written by Darius Plausinaitis and Dennis M. Akos

% Based on Peter Rinder and Nicolaj Bertelsen

%--------------------------------------------------------------------------

%This program is free software; you can redistribute it and/or

%modify it under the terms of the GNU General Public License

%as published by the Free Software Foundation; either version 2

%of the License, or (at your option) any later version.

%

%This program is distributed in the hope that it will be useful,

%but WITHOUT ANY WARRANTY; without even the implied warranty of

%MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the

%GNU General Public License for more details.

%

%You should have received a copy of the GNU General Public License

%along with this program; if not, write to the Free Software

%Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,

%USA.

%--------------------------------------------------------------------------

%CVS record:

%$Id: acquisition.m,v 1.1.2.12 2006/08/14 12:08:03 dpl Exp $

%% Initialization =========================================================

% Find number of samples per spreading code

samplesPerCode = round(settings.samplingFreq / ...

(settings.codeFreqBasis / settings.codeLength));

% Create two 1msec vectors of data to correlate with and one with zero DC

signal1 = longSignal(1 : samplesPerCode);

signal2 = longSignal(samplesPerCode+1 : 2*samplesPerCode);

signal0DC = longSignal - mean(longSignal); %%Problems here....

% Find sampling period

ts = 1 / settings.samplingFreq;

% Find phase points of the local carrier wave

phasePoints = (0 : (samplesPerCode-1)) * 2 * pi * ts;

% Number of the frequency bins for the given acquisition band

numberOfFrqBins = round( (settings.acqSearchBand*1000) / settings.acqSearchBin) + 1;

% Generate all C/A codes and sample them according to the sampling freq.

caCodesTable = makeCaTable(settings);

%--- Initialize arrays to speed up the code -------------------------------

% Search results of all frequency bins and code shifts (for one satellite)

results = zeros(numberOfFrqBins, samplesPerCode);

% Carrier frequencies of the frequency bins

frqBins = zeros(1, numberOfFrqBins);

%--- Initialize acqResults ------------------------------------------------

% Carrier frequencies of detected signals

acqResults.carrFreq = zeros(1, 32);

% C/A code phases of detected signals

acqResults.codePhase = zeros(1, 32);

% Correlation peak ratios of the detected signals

acqResults.peakMetric = zeros(1, 32);

fprintf('(');

% Perform search for all listed PRN numbers ...

for PRN = settings.acqSatelliteList

%% Correlate signals ======================================================

%--- Perform DFT of C/A code ------------------------------------------

caCodeFreqDom = conj(fft(caCodesTable(PRN, :)));

%--- Make the correlation for whole frequency band (for all freq. bins)

for frqBinIndex = 1:numberOfFrqBins

%--- Generate carrier wave frequency grid -------------------------

frqBins(frqBinIndex) = settings.IF - ...

(settings.acqSearchBand/2) * 1000 + ...

settings.acqSearchBin * (frqBinIndex - 1);

%--- Generate local sine and cosine -------------------------------

sigCarr = exp(1i*frqBins(frqBinIndex) * phasePoints);

%--- "Remove carrier" from the signal -----------------------------

I1 = real(sigCarr .* signal1);

Q1 = imag(sigCarr .* signal1);

I2 = real(sigCarr .* signal2);

Q2 = imag(sigCarr .* signal2);

%--- Convert the baseband signal to frequency domain --------------

IQfreqDom1 = fft(I1 + 1j*Q1);

IQfreqDom2 = fft(I2 + 1j*Q2);

%--- Multiplication in the frequency domain (correlation in time

%domain)

convCodeIQ1 = IQfreqDom1 .* caCodeFreqDom;

convCodeIQ2 = IQfreqDom2 .* caCodeFreqDom;

%--- Perform inverse DFT and store correlation results ------------

acqRes1 = abs(ifft(convCodeIQ1)) .^ 2;

acqRes2 = abs(ifft(convCodeIQ2)) .^ 2;

%--- Check which msec had the greater power and save that, will

%"blend" 1st and 2nd msec but will correct data bit issues

if (max(acqRes1) > max(acqRes2))

results(frqBinIndex, :) = acqRes1;

else

results(frqBinIndex, :) = acqRes2;

end

end % frqBinIndex = 1:numberOfFrqBins

% figure;

% mesh(results);

%% Look for correlation peaks in the results ==============================

% Find the highest peak and compare it to the second highest peak

% The second peak is chosen not closer than 1 chip to the highest peak

%--- Find the correlation peak and the carrier frequency --------------

[~, frequencyBinIndex] = max(max(results, [], 2));

%--- Find code phase of the same correlation peak ---------------------

[peakSize, codePhase] = max(max(results));

%--- Find 1 chip wide C/A code phase exclude range around the peak ----

samplesPerCodeChip = round(settings.samplingFreq / settings.codeFreqBasis);

excludeRangeIndex1 = codePhase - samplesPerCodeChip;

excludeRangeIndex2 = codePhase + samplesPerCodeChip;

%--- Correct C/A code phase exclude range if the range includes array

%boundaries

if excludeRangeIndex1 < 2

codePhaseRange = excludeRangeIndex2 : ...

(samplesPerCode + excludeRangeIndex1);

elseif excludeRangeIndex2 > samplesPerCode % PHAHN was >=

codePhaseRange = (excludeRangeIndex2 - samplesPerCode) : ...

excludeRangeIndex1;

elseif excludeRangeIndex2 == samplesPerCode % PHAHN was >=

% codePhaseRange = (excludeRangeIndex2 - samplesPerCode) : ...

% excludeRangeIndex1;

codePhaseRange = [excludeRangeIndex2, 1 : ...

excludeRangeIndex1];

else

codePhaseRange = [1:excludeRangeIndex1, ...

excludeRangeIndex2 : samplesPerCode];

end

%--- Find the second highest correlation peak in the same freq. bin ---

secondPeakSize = max(results(frequencyBinIndex, codePhaseRange));

%--- Store result -----------------------------------------------------

acqResults.peakMetric(PRN) = peakSize/secondPeakSize;

% If the result is above threshold, then there is a signal ...

if acqResults.peakMetric(PRN) > settings.acqThreshold

%% Fine resolution frequency search =======================================

%--- Indicate PRN number of the detected signal -------------------

fprintf('%02d ', PRN);

%--- Generate 10msec long C/A codes sequence for given PRN --------

caCode = generateCAcode(PRN);

codeValueIndex = floor((ts * (1:10*samplesPerCode)) / ...

(1/settings.codeFreqBasis));

longCaCode = caCode((rem(codeValueIndex, 1023) + 1));

%--- Remove C/A code modulation from the original signal ----------

% (Using detected C/A code phase)

xCarrier = ...

signal0DC(codePhase:(codePhase + 10*samplesPerCode-1)) ...

.* longCaCode;

%--- Compute the magnitude of the FFT, find maximum and the

%associated carrier frequency

%--- Find the next highest power of two and increase by 8x --------

fftNumPts = 8*(2^(nextpow2(length(xCarrier))));

%--- Compute the magnitude of the FFT, find maximum and the

%associated carrier frequency

fftxc = abs(fft(xCarrier, fftNumPts));

uniqFftPts = ceil((fftNumPts + 1) / 2);

[~, fftMaxIndex] = max(fftxc);

fftFreqBins = (0 : uniqFftPts-1) * settings.samplingFreq/fftNumPts;

if (fftMaxIndex > uniqFftPts) %and should validate using complex data

if (rem(fftNumPts,2)==0) %even number of points, so DC and Fs/2 computed

fftFreqBinsRev=-fftFreqBins((uniqFftPts-1):-1:2);

[~, fftMaxIndex] = max(fftxc((uniqFftPts+1):length(fftxc)));

acqResults.carrFreq(PRN) = -fftFreqBinsRev(fftMaxIndex);

else %odd points so only DC is not included

fftFreqBinsRev=-fftFreqBins((uniqFftPts):-1:2);

[~, fftMaxIndex] = max(fftxc((uniqFftPts+1):length(fftxc)));

acqResults.carrFreq(PRN) = fftFreqBinsRev(fftMaxIndex);

end

else

acqResults.carrFreq(PRN) = (-1)^(settings.fileType-1)*fftFreqBins(fftMaxIndex);

end

acqResults.codePhase(PRN) = codePhase;

if(abs(acqResults.carrFreq(PRN)-settings.IF)>=10000)

%warning(['carrFreq for ' num2str(PRN) ' exceeds 10kHz. Skipping for now. May be bug in code?'])

acqResults.peakMetric(PRN) = -1.;

end

else

%--- No signal with this PRN --------------------------------------

fprintf('. ');

end % if (peakSize/secondPeakSize) > settings.acqThreshold

end % for PRN = satelliteList

%=== Acquisition is over ==================================================

fprintf(')\n');

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