# Detection of RINEX-2 Files With Mixed GPS L2P(Y)/L2C Carrier Phase Observations
## 1 Introduction  
In 2005, the first global positioning system (GPS) Block IIR-M satellite was launched. It became the first satellite in the GPS constellation to broadcast an additional signal on L2 frequency (1227.60 MHz), the so-called L2C signal [1, 2]. With this new signal, receivers had the choice to observe L2 code and carrier phases either on the legacy L2P(Y) signal, the new L2C, or both.  
In 2018, approximately 60% of the active GPS satellites are capable of transmitting L2P(Y) and L2C (L2C-satellites: Block IIR-M and IIF). However, there are still 12 older Block IIA and IIR (non-L2C) satellites that are restricted to L2P(Y) (Figure 1). The youngest IIR satellite was launched in 2004 and has been in operation for almost 14 years.  
The two L2 carrier components of the IIR-M and IIF satellites are fixed in phase quadrature, with L2C lagging L2P(Y) by 90 degrees [1]. Thus, a \(-\)0.25 cycle phase correction should be applied to L2C carrier phase measurement to obtain alignment [3]. If all measurements on the same frequency are aligned, any common quarter cycle phase shift is eliminated by differencing or can be estimated as a frequency-specific delay. If, however, L2P(Y) and L2C phase measurements are mixed without being aligned, severe problems in precise positioning applications occur. This mix of unaligned observations is referred to as the \"L2 quarter cycle problem\".  
Even if the two L2 carrier components are aligned on the level of quarter cycles, there are still satellite and receiver induced delay differences between L2P(Y) and L2C [4]. On the satellite side, they should not exceed 100 mrad [1], which equals 3.9 mm on L2. They can be determined from simultaneous L2P(Y) and L2C carrier phase observations. [PERSON] (2011) confirmed that the satellite induced delays are on the mm level [5]. We determined the receiver induced delay differences, and thelargest differences were on the level of some mm. Since the delay differences of the aligned signals were so small, this paper only focuses on the L2 quarter cycle problem.  
The RINEX-2 data format defines just one carrier phase observation on each frequency, whereas two different GPS code observations (C and P) can be stored [6]. It cannot be indicated whether carrier phase observations originate from L2P(Y) or from L2C. There is no data field that informs about any quarter cycle alignment of the carrier phase observations. The mix of L2P(Y) and L2C carrier phase observations is not considered in RINEX-2 format definition and is not explicitly prohibited. Therefore, RINEX-2 observation files are prone to containing a mix of unaligned L2P(Y) and L2C carrier phase observations.  
RINEX-3 can overcome many of RINEX-2's shortcomings [3], but for the price of more complexity, especially by defining a much larger number of observation types. Therefore, acceptance of RINEX-3 has risen only slowly, and there is still some hesitation to adopt this more complex data format.  
The next section describes the effects of mixed, unaligned L2P(Y) and L2C carrier phase observations on ambiguity estimation. Subsequently, algorithms are presented that allow the identification of RINEX-2 files being corrupted by L2 quarter cycle biases. In a further section, detection results based on the RINEX-2 observation files of more than 2600 individual stations are shown and analyzed. The last section demonstrates the effect of the L2 quarter cycle problem on coordinate estimation.