Searching for Galileo: reception and analysis of signals from GIOVE-A

We captured the GIOVE-A BOC(1,1) signal using the digital storage receiver illustrated in FIGURE 1. It employed a 20 MHz L1 filter and direct radio frequency (RF) sampling at 41.19 MHz to alias the L1 GPS/Galileo signals to the nominal intermediate frequency [f.sub.IF] = 10.36 MHz.

The two data sets used in our study, captured on March 2 at 14:45:10 UTC and on March 8 at 09:29:00 UTC, were taken when GIOVE-A was visible both from Ithaca and from Europe during European business hours--a measure taken to increase the likelihood that the satellite was broadcasting. The flyovers were predicted using NORAD two-line ephemeris elements.

Two Lobes. The power spectrum of the raw data from the digital storage receiver is shown as the red dashed line in FIGURE 2. The combined power of several GPS/SBAS C/A-code signals is evident in the large hump centered at [f.sub.IF]. These GPS/SBAS signals interfere with codeless acquisition and analysis of the Galileo L1 BOC(1,1) signal. In addition, they obscure the GIOVE-A signal in the power spectrum, thereby precluding a quick test for its presence.

We used a software GPS receiver to acquire, track, and remove the nuisance GPS/SBAS signals. This procedure removes the large central hump in the power spectrum, revealing distinct lobes to each side of the intermediate frequency--the expected signature of the Galileo L1 signal's BOC(1,1) modulation.

Signal Structure

Our analysis required a detailed understanding of the Galileo L1 signal structure. We pieced together a coherent signal description from early, publicly available drafts of the Galileo Interface Control Document (ICD). Our later findings demonstrated that these documents were a generally accurate description of the GIOVE-A signal, with a few important exceptions. In this section, we present the GIOVE-A L1 BOC(1,1) signal structure as we detected it.

The BOC(1,1) signal is composed of two multiplexed channels: the L1-B data channel and the L1-C pilot channel. The sampled Galileo L1 BOC(1,1) signal that exits at the RF front-end takes the form

[y.sub.i] = A[[b.sub.L1-B]([[tau].sub.i])[d.sub.L1-B]([[tau].sub.i]) - [c.sub.L1-C]([[tau].sub.i])[s.sub.L1-C]([[tau].sub.i])] [s.sub.SC]([[tau].sub.i]) cos(2[pi][f.sub.IF][[tau].sub.i] [[phi].sub.0]) [[nu].sub.i] (1)

at receiver sample time [t.sub.i]. The quantities in Equation (1) are the carrier amplitude A, the PRN code of the data channel [b.sub.L1-B]([[tau].sub.i]), the data symbol [d.sub.L1-B]([[tau].sub.i]), the primary PRN code of the pilot channel [c.sub.L1-C]([[tau].sub.i]), the secondary code of the pilot channel [s.sub.L1-C]([[tau].sub.i]), the sine-phased BOC signal [s.sub.SC]([[tau].sub.i]) = sign[sin(2[pi][f.sub.BOC][[tau].sub.i])] where [f.sub.BOC] is the BOC(1,1) modulation frequency equal to 1.023 MHz, the initial carrier phase [[phi].sub.0], the measurement noise [[nu].sub.i], and the signal transmission time [[tau].sub.j]. This latter time can be expressed as a quadratic polynomial in [t.sub.i] with a constant term that depends on pseudorange and linear and quadratic terms that depend on carrier Doppler shift and Doppler shift rate.

The PRN codes and the data time history in Equation (1) take the form