A continuing development effort focuses on implementation of a Global Positioning System (GPS) waveform under the Software Communications Architecture (SCA). [As used within the special technological discipline of the SCA, "waveform" signifies not only a waveform in the commonly understood sense of the word, but also subsystems and components for receiving and transmitting the waveform; subsystems for processing the information conveyed by the waveform; subsystems that perform ancillary communication and control services relevant to the role of the affected software-defined radio (a transmitter, receiver, or transceiver) as a node in a data-communication network; and any or all of the aforesaid information and services.] The intent is to optimize GPS services by providing position and time information as an embedded waveform within a software-defined radio (SDR), rather than using additional GPS chip sets to provide the information. It is further intended that the GPS waveform first will be used to provide position and time information in Joint Tactical Radio System (JTRS) radios. [The JTRS is a family of military SDRs, waveforms, and cryptographic algorithms designed under the SCA.] The JTRS radios are reprogrammable to run a family of special waveforms that utilize carrier frequencies from 2 MHz to 2 GHz — a frequency range that includes the 1.2- and 1.5-GHz GPS frequencies.

This GPS Waveform (essentially, a software-defined GPS receiver) is initiated as a single aggregate device within an SDR.
An inherent aspect of SDR is the ability to redefine radio functionality by interchanging waveform software on one set of radio hardware. This ability supports interoperability of different radio-communication systems (e.g., systems used by allied military forces of different countries or different military and civilian government agencies of the same country). An industry demand for a clearly defined hardware, software, and network architecture to provide a standardized environment for the deployment of interoperable waveforms has been the driving force for formulation of the SCA, which is an open-architecture standard that tells designers how elements of hardware and software are to operate in harmony within an SDR. The SCA governs the structures and operations of SDRs, enabling programmable radios to load waveform software, run application programs, and become nodes in a network that constitutes or is part of an integrated data-handling system. Through adherence to standards detailed in the SCA definition document, both hardware and software designers know what equipment and programs to design. These specifications ensure that software written according to SCA guidance will run on SCA-compliant hardware. The core conceptual framework of the SCA provides an abstraction layer between the waveform application software and an SDR, enabling porting of application software to SDR products of multiple vendors.

{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


This Brief includes a Technical Support Package (TSP).
Implementing a GPS Waveform Under the SCA

(reference NRL-0008) is currently available for download from the TSP library.

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This article first appeared in the April, 2008 issue of Defense Tech Briefs Magazine.

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