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Pseudorandom Noise Code Assignments


The Global Positioning System is a spread-spectrum system that employs direct-sequence spreading of the spectrum to achieve better ranging accuracy. Although GPS was originally conceived for use by the military in their missions, it has become an important utility for a number of civil and commercial applications. To facilitate improvement in the use of satellite navigation by the civilian users, for example, aviation and terrestrial transportation systems, spread-spectrum signals called L1C, L2C, and L5 are being added to the current GPS signals in addition to the existing C/A code. All these signals use spreading codes to spread the spectrum. Also, a number of GPS augmentation systems already exist or are being planned that use GPS-like spreading codes. Therefore, it has become very important to assign the GPS spreading codes to various GPS augmentation systems in a systematic way to avoid code duplication and using codes with poor properties that may interfere with the GPS receiver performance. These spreading codes are called PRN codes.

An important consideration in the design of a spread-spectrum signal for Code Division Multiple Access applications such as in GPS is to select PRN codes to minimize what is known as CDMA noise. The code selection process can be very tedious, depending on the number of codes needed and the number of codes available in a given code set. Code balance, autocorrelation sidelobe peak, cross-correlation peak, and spectral line distributions are some of the measures available in the code selection process, and these measures are not necessarily independent.

PRN Code Assignment Process

With the advent of domestic and international GPS augmentation systems and other Global Navigation Satellite Systems, it has become critical to manage the issuance and use of GPS PRN codes to prevent unnecessary mutual interference. As part of its acquisition and sustainment effort for GPS, the GPS Directorate is responsible for managing the issuance of GPS PRN sequences. For this purpose, a large set of GPS PRN sequences, that provide good auto- and cross-correlation properties, have been pre-selected and the selection and issuance of PRN codes will be done from this set.

The PRN Code Assignment Process document describes the process for requesting, assigning, and recording the GPS PRN numbers pre-selected and defined in GPS interface specifications IS-GPS-200 and IS-GPS-705.

Many domestic and international radionavigation systems, including satellite-based augmentation systems, seek high levels of interoperability with GPS, including the use of PRN sequences from the same spreading code families developed for GPS signals. Signals that employ PRN sequences from the GPS spreading code families achieve an established level of code performance, including good compatibility with all other signals using GPS codes. In addition, receivers can more readily generate and use all the spreading codes, thus fostering receivers that use all signals. The resulting compatibility and interoperability are mutually beneficial to all systems using GPS codes.

PRN numbers currently assigned and available for assignment can be found at the C/A, L2C, and L5 PRN code assignments tables.

Application for PRN Number Assignment

In order to request one or more PRN numbers, an applicant must provide the applicant and system information by filling out the Application for PRN Number Assignment. Once the request is complete the applicant should mail the request to the GPS Directorate at smc.gp.prn@losangeles.af.mil. During the PRN Number Assignment Process, the directorate will review the submitted data and may request additional information. The assignment process generally will require three to six weeks following proper submission of all required information. If the review results are satisfactory, the directorate will issue a PRN Number Assignment. The assignment may be either Preliminary (expires in 3 years) or Final (expires in 10 years) and the applicant may renew the assigned PRN number prior to the expiration. If there are concerns about mutual interference or other interactions, the directorate will contact the applicant to initiate more in-depth discussions.

Applicants can contact the directorate with any comments or questions at smc.gp.prn@losangeles.af.mil.