GPS C1, P1 and P2 Codes and Receiver Types

Fundamentals
Title	GPS C1, P1 and P2 Codes and Receiver Types
Author(s)	J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain.
Level	Intermediate
Year of Publication	2011

As explained in GPS Services, under Anti-Spoofing (A/S) conditions, P-codes (P1, P2) are encrypted to (Y1, Y2) for non-authorised users. Nevertheless, many commercial receivers are able to provide P1, P2 measurements among the C1. The generation of such P1, P2 codes under A/S conditions depends of the code tracking technology, which must be taken into account for a correct DCBs handling.

For instance, Cross Correlated receivers (e.g. Rogue and Trimble 4000 models) provide a synthetic P2 code generated from C1 code and the cross correlation of encrypted Y2-Y1 codes (roughly speaking: P2:=C1+[Y2-Y1]). This leads to a P2 code measurement associated to C1, not P1, and thence, it must be corrected by the Differential Code Bias [math]\displaystyle{ { DCB}_{ P1-C1} }[/math] to have a consistent P2 measurement.

On the other hand, these receivers do not provide the P1 code, but the C1 code. Thence, the [math]\displaystyle{ { DCB}_{ P1-C1} }[/math] must be added to this C1 code to emulate a consistent P1 code measurement. In this way, the interfrequency bias [math]\displaystyle{ { DCB}_{ P2-P1} }[/math] will cancel when computing the Code ionosphere-free combination (PC) of such P1 and P2 codes.

Three different kind of receivers are usually considered (see more details in [Schaer, S. and Steingenberger, P., 2006] ).

• Type 1: Cross correlated receiver:

The C1 and P2 measurements must be corrected by the [math]\displaystyle{ {DCB}_{P1-C1} }[/math]:

[math]\displaystyle{ { C1_{raw}+{ DCB}_{ P1-C1}} \longrightarrow { P1} }[/math]

[math]\displaystyle{ { P2_{raw}+{ DCB}_{ P1-C1}} \longrightarrow { P2} }[/math]

• Type 2: Receivers reporting C1 in place of P1:

The C1 must be corrected by the [math]\displaystyle{ {DCB}_{P1-C1} }[/math]:

[math]\displaystyle{ {C1_{raw}}+{DCB}_{P1-C1} \longrightarrow {P1} }[/math]

• Type 3: Receivers reporting L1,L2,P1,P2 as consistent set

No bias removal is needed.

A RINEX conversion utility cc2noncc.f is provided by IGS to easily make code measurements consistent with P1/P2 data by applying satellite-dependent P1-C1 bias corrections. This tool is available at https://goby.nrl.navy.mil/IGStime/cc2noncc/cc2noncc.f

These DCBs corrections are only required for geodetic (high accuracy) surveying, as its impact on the coordinates accuracy is at the level of few millimetres.