GPS from getting started to giving up (XV), DCB differential code deviation

One 、 Concept

DCB（Differential Code Bias Differential code deviation ） It's the global satellite navigation system （GNSS） in , From different signals Systematic deviation between observations .DCB It is caused by the hardware delay of satellite and receiver . Generally speaking, at the receiver DCB It can be solved together with the receiver clock error , Absorbed by receiver clock error ; And the satellite side DCB It must be compensated in precise positioning .

DCB There are two main types

1. The deviation between different codes at the same frequency （ Such as P1-C1、P2-C2 etc. ）;
2. The deviation between different frequencies （ Such as P1-P2）;

Two 、 Data sources

at present DCB There are two main sources of data ：IGS and CODE.

IGS（International GNSS Service） The organization is the most authoritative in the world GNSS One of the precision application service organizations . With GPS Modernization and BDS and Galileo Wait for the construction of navigation system ,IGS On 2011 In, multimode was established GNSS Experimental tracking network (MGEX, Multi-GNSS Experiment), For multimode GNSS Research on navigation signal monitoring and related technologies . be based on MGEX Multi system observation data , German Aerospace Center (DLR) since 2013 Since the year of IGS Organizational submissions include BDS、GPS、GLONASS And Galileo Including multiple systems DCB product . since 2015 From the year onwards , Institute of Surveying and Geophysics, Chinese Academy of Sciences, Wuhan (IGG of CAS) Become the second company in the world to IGS Submit multiple systems DCB Product organization . The data can be found in  ftp://cddis.nasa.gov/gnss/products/bias/  download . European orbit determination center of Bernier University in Switzerland （CODE：Centre for Orbit Determination in Europe） Also provide DCB Product downloads , Its history is even longer . The following is its FTP Download address ：ftp://ftp.aiub.unibe.ch/CODE/ .
 

3、 ... and 、 Application implementation

For dual band receivers , stay Ionospheric delay In, we get that the pseudorange after ionospheric correction is

RTKLIB The single point positioning in is calculated in this way . You can refer to the following code ：

gamma=SQR(lam[j])/SQR(lam[i]); /* f1^2/f2^2 */
P1=obs->P[i];
P2=obs->P[j];
P1_P2=nav->cbias[obs->sat-1][0];
P1_C1=nav->cbias[obs->sat-1][1];
P2_C2=nav->cbias[obs->sat-1][2];
 
/* if no P1-P2 DCB, use TGD instead */
if (P1_P2==0.0&&(sys&(SYS_GPS|SYS_GAL|SYS_QZS))) {
    P1_P2=(1.0-gamma)*gettgd(obs->sat,nav);
}
if (opt->ionoopt==IONOOPT_IFLC) { /* dual-frequency */
 
    if (P1==0.0||P2==0.0) return 0.0;
    if (obs->code[i]==CODE_L1C) P1+=P1_C1; /* C1->P1 */
    if (obs->code[j]==CODE_L2C) P2+=P2_C2; /* C2->P2 */
 
    /* iono-free combination */
    PC=(gamma*P1-P2)/(gamma-1.0);
}
else { /* single-frequency */
 
    if (P1==0.0) return 0.0;
    if (obs->code[i]==CODE_L1C) P1+=P1_C1; /* C1->P1 */
    PC=P1-P1_P2/(1.0-gamma);
}