Innovation: Searching for Galileo
June 1, 2006 By: Mark L. Psiaki, Todd E. Humphreys, Shan Mohiuddin, Steven P. Powell, Alessandro P. Cerruti, Paul M. Kintner Jr. GPS WorldReception and Analysis of Signals from GIOVE-A
Galileo, Europe's answer to the U.S. Global Positioning System, achieved a milestone on December 28, 2005, with the launch of its first test satellite, GIOVE-A, which began transmitting navigation signals in early January 2006. Since the launch, receiver developers around the world have been anxious to test their Galileo-capable receivers on the GIOVE-A signals. But because the GIOVE-A signal structure documentation had not yet been publicly released, only approved groups involved in validation tests had authorized access to the pseudorandom noise (PRN) codes required to track the GIOVE-A navigation signals.
Eager to study the Galileo signals and to develop Galileo-capable receivers, Cornell University's GNSS research group set out to determine whether the GIOVE-A L1 binary offset carrier - BOC(1,1) - signal could be acquired and the PRN codes uncovered using codeless acquisition and statistical signal processing techniques. The short answer: Yes!
We recorded data using a digital storage receiver connected to an inexpensive roof-mounted patch antenna on Cornell University's campus in Ithaca, New York. We then processed the data offline in several stages. First, the "nuisance" GPS and satellite-based augmentation system (SBAS) - WAAS and EGNOS - C/A-code signals were tracked and removed. Then the GIOVE-A L1 BOC(1,1) signal's carrier phase, Doppler shift, and BOC phase were determined using codeless acquisition techniques. Next, the carrier and BOC signals were removed by mixing, and 1.023 MHz in-phase accumulations were computed. Finally, the code timing, data symbols, and secondary code chips were analyzed and the results were used to accurately determine the primary PRN codes by averaging over many code periods.
The resulting PRN codes can be obtained online at http://gps.ece.cornell.edu/galileo/.
Signal Data
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 IF = 10.36 MHz.
 Figure 1 Schematic block diagram of the data recording hardware and the post-processing software receiver |
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 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.
 Figure 2 L1-band power spectral densities, raw data, and data after removal of GPS and SBAS L1 C/A-code signals |
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