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.

More information on the RINEX format.

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.