{ntbnoad)The generalized application components of the GPS waveform consist mainly of the following:

• A GPS-receiver digital antenna element (DAE) [also denoted a modem radio-frequency (RF) front end];
• A GPS signal-processing/correlation unit, denoted a correlator accelerator card (CAC), that includes one or more field-programmable gate arrays (FPGAs);
• GPS-receiver, GPS-tracking, and network- assistance application-software components deployed on a general-purpose processor (GPP); and
• In the case of the GPS P(Y) mode of operation [so named because it involves the use of the GPS precision (P) code, which is also known as the Y code], a unit that incorporates the Precise Positioning Service security module that molecuperforms auxiliary-output-chip functions within the JTRS cryptographic resources.

The GPS waveform is distributed among a number of hardware and software components in the system and is flexible in its utilization of system resources in the sense that it can be deployed on various SDR platforms on which different resources are available.

The GPS waveform is initiated as a single aggregate device (see figure) that is divided internally into a modem, a tracking component that is further divided into real-time and non-real-time components, and a navigation component. In the modem component, a CAC driver receives the GPS signal data from the DAE. The CAC performs the carrier and code frequency mixing and correlates the resulting signal against the GPS coarse-acquisition (C/A) code. The driver is loaded when the radio is initiated and forwards data to real-time and non-real-time tracking components. The real-time tracking component optimizes range measurements. The non-real-time tracking component refines computed time in conjunction with the solution message as a part of continuing position calculations. The non-real-time tracking component contains the threads of tracking-executive software, receiver-manager software, and common object request broker architecture (CORBA) communication software for transferring range and navigation data messages. A navigation component generates real-time navigation solutions.

The GPS waveform is configured at initiation by use of Extensible Markup Language (XML)-defined parameters within a profile (.PRF) file. These configuration parameters include the intermediate frequency, the sampling rate of the RF-to-digital conversion, such resource-dependent parameters as the number of satellite channels to be run, and such optimization parameters as Doppler search windows and the horizon limit to be used in detection of signals from GPS satellites.

In a test, a prototype GPS waveform in an SDR was found to be capable of tracking four GPS satellites, using the C/A code. Further optimization is expected to enable tracking of more than four satellites. Upgrading to add P(Y)-code tracking was underway at the time of reporting the information for this article.

This work was done by Alison Brown, Lynn Stricklan, and David Babich of NAVSYS Corp. for the Naval Research Laboratory.
NRL-0008

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