Getting to M: direct acquisition of the new military signal

GPS World, April, 2005 by John W. Betz, John D. Fite, Paul T. Capozza

This article describes the first integrated circuit (IC) designed, fabricated, and tested to perform direct acquisition of the new M-code signal, scheduled to begin broadcasting from Block IIR-M satellites this year. The prototype IC, named DirAc, takes advantage of the M-code signal's binary offset carrier (BOC) modulation to reduce acquisition processing complexity.

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M-code, the modernized GPS military signal designed in the late 1990s, is scheduled to be first transmitted by a Block IIR-M satellite in 2005. The M-code signal's revolutionary design includes a novel modulation, new data message, and new security architecture.

M-code signal acquisition relies primarily upon direct acquisition, where in effect the receiver correlates (over time and frequency shifts) a locally generated replica of an M-code signal with the received waveform. When there is a match between the replica and a received signal, initial synchronization is achieved, and the receiver commences signal tracking, data message demodulation, and position calculation. Since the M-code signal, like the current GPS military Y-code signal, uses very long spreading codes, signal acquisition cannot take advantage of the short spreading codes that simplify acquisition processing in civilian signals such as the GPS C/A-code signal.

While direct acquisition circuits were developed for Y-code receivers in the 1990s, these circuits provided much less capability than would be needed for direct acquisition to be the primary mechanism for acquiring M-code. During M-code signal design, studies demonstrated that a combination of factors would allow direct acquisition to surpass the M-code design requirements. But the results of these studies did not lead to consensus that integrated circuits (ICs) ready for receiver production in the latter half of this decade could meet the performance requirements while providing adequately low complexity, low parts cost, low peak and average power consumption, and low thermal dissipation. Functioning silicon was needed to remove remaining doubts.

A team with expertise in systems engineering, digital signal processing, and IC design took on the challenge of developing a prototype IC for direct acquisition of the M-code signal. The team identified ways to exploit the unique characteristics of the M-code signal, evaluated processing architectures that balanced risk and capability, developed predictions of performance and of IC complexity, designed and applied algorithms, developed detailed simulations, and traded off processing implementations, yielding design files that were sent to the foundry in June 2002, only 12 calendar months after the design effort began. The resulting "DirAc" ICs have been packaged and tested, confirming that they provide full functionality and meet or exceed performance predictions. Software and hardware development is underway to integrate the IC into a test receiver for further testing.

After discussing direct acquisition and describing the first DirAc IC design, built using 180 nanometer (nm, 1 X [10.sup.-9] meter, or one billionth of a meter) lithography readily accessible in 2001, we outline a second version DirAc that could be developed using 130 nm technology available in 2003, and give fundamental performance characteristics.

Direct Acquisition

Typically, an acquisition circuit (we comply here with common terminology calling the circuit that performs initial synchronization an "acquisition circuit") crosscorrelates a locally generated signal replica against the received waveform containing multiple signals, interference and possibly jamming, and noise. In concept, the locally generated reference is shifted in time and frequency, forming a segment of a cross ambiguity function (CAF) between the replica and the desired received signal. The time duration of the signal segments used in the crosscorrelation is called the coherent integration time. Noncoherent integration can be accomplished by adding the magnitudes of multiple CAFs, computed over the same time delay and frequency offset values. This noncoherent integration enhances performance in the presence of noise and jamming, but consumes additional time to collect and process the longer segment of received waveform.

Digital processing actually searches discrete values in time and frequency space, called time-frequency cells. The time span and frequency span searched in parallel by an acquisition circuit may be called a time-frequency tile, composed of multiple cells in a rectangular array. If the initial time uncertainty (ITU) or (initial frequency uncertainty) IFU is larger than the span of the tile, sequential tiles are computed serially to compute the CAF over the entire ITU and IFU. Figure 1 shows how cells and tiles fit into the ITU and IFU region. Although for signals with short periodic spreading sequences, the largest ITU that must be searched corresponds to the period of the spreading sequence, signals whose spreading sequences have much longer periods require search of the entire ITU.