Innovation: One Year in Orbit

September 1, 2009 By: Christian Kurzhals, Maktar Malik, Manuela Rapisarda, Matthias Söllner, Michael Meurer, Stefan Erker, Steffen Thölert, Wolfgang Kogler GPS World

GIOVE-B E1 CBOC Signal Quality Assessment

GIOVE-B has now been in orbit for just over one year. How well is it performing? In particular, what can we say about one of GIOVE-B’s pioneering features: its E1 CBOC signal? In this month’s column, we take a detailed look at a particular monitoring and assessment program set up to examine the GIOVE-B signals and discuss some of its initial CBOC results. The successful operation of this program bodes well for its use in future validation campaigns.

Richard Langley

INNOVATION INSIGHTS with Richard Langley

THE SECOND GALILEO TEST SATELLITE, GIOVE-B, was launched on April 27, 2008, and began transmitting navigation signals a few days later. It joined its older sibling, GIOVE-A, which was placed in orbit over two years earlier. Standing for Galileo In-Orbit Validation Element, the GIOVE satellites constitute the first in-orbit test phase in the development of the Galileo navigation system.

In addition to securing the frequencies for the system, the satellites are being used to assess key technologies for the full Galileo constellation. The GIOVE test phase will be followed by the In-Orbit Validation (IOV) phase during which four IOV satellites will be launched, two at a time, aboard Soyuz rockets from Europe’s spaceport in French Guiana. Together with a preliminary ground network, the IOV satellites will be used to validate the Galileo system as a whole, using advanced system simulators. The launches are expected to occur by the end of 2010.

But before the IOV phase can begin, a thorough analysis of the performance of the GIOVE satellites must be carried out to minimize any difficulties with the IOV satellites. This includes monitoring and assessing the different signals broadcast by the satellites.

The GIOVE satellites can transmit on all three Galileo frequencies, E5, E6, and E1 (also known as L1) but only on two simultaneously (either E1-E5 or E1-E6). A variety of modulation types can be transmitted on the different frequencies by both satellites to test their use for the different Galileo services to be implemented for the operational constellation. These include alternative binary offset carrier (BOC) and quadrature phase shift keying on E5 and cosine BOC (BOCc) and binary phase shift keying on E6. On E1, the satellites have different capabilities. Although both satellites can transmit BOCc on this frequency, GIOVE-A can additionally transmit a single BOC signal with a subcarrier frequency of 1.023 MHz and a spreading code chipping rate of 1.023 MHz (BOC(1,1) ) whereas GIOVE-B transmits a more versatile multiplexed composite BOC or CBOC, which linearly combines BOC(1,1) and BOC(6,1). The CBOC signal is being transmitted by GIOVE-B to explore its performance, usability, and any possible side effects including its use in receivers designed to track a BOC(1,1) signal.

GIOVE-B has now been in orbit for just over one year. How well is it performing? In particular, what can we say about one of GIOVE-B’s pioneering features: its E1 CBOC signal? In this month’s column, we take a detailed look at a particular monitoring and assessment program set up to examine the GIOVE-B signals and discuss some of its initial CBOC results. The successful operation of this program bodes well for its use in future validation campaigns.

“Innovation” is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick.

The first measurements of the navigation signals transmitted by the second Galileo test satellite, GIOVE-B, were recorded during the night of May 7, 2008, following the successful launch from Baikonur a little over a week earlier on April 28. During the In-Orbit Test (IOT) phase of the mission, which lasted about three months, a program of intensive measurements was carried out by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt or DLR) and the Astrium subsidiary of the European Aeronautic Defence and Space Company (EADS) using the 30-meter high-gain antenna at Weilheim/Lichtenau, near Munich, Germany. These and follow-on activities were performed under a bilateral agreement between the European Space Agency (ESA) and DLR’s Institute of Communication and Navigation on the exploitation of the GIOVE satellites.

The first measurements indicated that the navigation payload suffered no damage or degradation with regard to power and signal-in-space (SIS) quality during the launch of the satellite. After the successful completion of the IOT phase, the satellite configuration was maintained for long intervals to allow stability and long-term quality assessments within the ESA GIOVE mission activities and to stabilize ground segment operation. DLR and Astrium continued their measurements and analysis on GIOVE-B signals, focusing on modulated power and on modulation and correlation quality. The observation intervals included an eclipse season, when GIOVE-B spends a fraction of its orbit in the Earth’s shadow. Any impact of the corresponding temperature variations on signal transmission characteristics and key navigation parameters was of special interest.

This article focuses on modulated spectral power analysis of GIOVE-B signals for the detection of frequency and elevation-angle-dependent variations in the transmitted signal spectrum and power. Also, a detailed in-phase/quadrature-phase (I/Q) sample analysis is presented, focusing on modulation correctness and correlation distortions. These assessments are based on sample files of a few seconds of navigation signals as received by a calibrated high-gain antenna and recorded with the BayNavTech Signal Experimentation Facility (BaySEF). We have evaluated, in particular, correlation loss and S-curve bias since these parameters are very sensitive to onboard signal distortions although their reliable evaluation is quite challenging. In this article, we concentrate on the analyses of the E1 composite binary offset carrier (CBOC) modulation.

We discuss the GIOVE-B measurement and evaluation parameter results from the initial IOT phase and from later phases, including those from an eclipse period. This comparison of measurement results — spread over the first year of operations — demonstrates the excellent stability of signal power, modulation, and correlation quality of the GIOVE-B signals.

GIOVE-B E1 Signal

GIOVE-B can transmit navigation signals either simultaneously in the Galileo E5 and E1 bands or in the E6 and E1 bands. At E1, with a center frequency of 1575.42 MHz, GIOVE-B transmits three signal components called E1-A, E1-B, and E1-C. The E1-A signal has a BOCcos(15,2.5) modulation, with 2.5575 MHz code chip-rate and a binary cosine-type subcarrier modulation of 15.345 MHz. The B- and C-components have CBOC(1,6,1,10/1) modulation. Within this type of multiplexed BOC implementation, the code chips are provided at a constant rate of 1.023 MHz, modulated with a composite quaternary subcarrier with rates of 1.023 and 6.138 MHz. The latter part, called the BOC(6,1) subcarrier, has a relative power of 1/11 and is added to the BOC(1,1) binary subcarrier in CBOC-B and subtracted for CBOC-C. From the beginning of May 2008 until July 2009, GIOVE-B transmitted signals 96.8% of the time. Considering the experimental nature of the satellite, this represents a very successful operation.

Signal Quality and Relevance

In GNSS operations, signal quality assessment generally refers to the behavior of the navigation signals as transmitted from individual satellites. In this article, we assess two major aspects: the transmitted signal power and the frequency-transfer distortions of the satellite relative to the ideal signal definitions.

Why should we consider these aspects in particular? For transmitted signal power, the answer is obvious. In addition to proof of compliance with regulatory declarations, verification and monitoring of signal power is relevant for the system provider to commit to certain navigation service performance.

Concerning frequency-transfer distortions, the answer may be less obvious. The relevance is obtained via the impact of distortions on a receiver’s correlation function. First, the available maximum correlation power for a receiver binary replica may be affected. This is due to changes in the power sharing of the signal components within the complete signal and due to reduced matching of distorted transmitted signals with the ideal receiver replica. Second, the shape of the correlation function may suffer from asymmetric distortions, leading to receiver-dependent biases in the discriminator lock point. The most relevant receiver parameters are the input bandwidth and the discriminator type, especially the discriminator spacing (early-late spacing). With respect to this, user receivers and receivers in the control system ground segment (used to derive the satellite orbit and clock parameters) may differ. This leads not only to timing but also to positioning errors, if the distortions are different for different satellites.

These are the main reasons why we need to analyze and control the corresponding distortions for the Galileo system.

For an accurate assessment of transmitted signal power and frequency distortions, we had to obtain measurements with a highly directive antenna. This is essential in order to lift the instantaneous signal far above the noise floor and to avoid environmental distortions from interference as well as multipath.

But why don’t we consider also other aspects of signal quality such as the correctness of the navigation message or the stability of hardware delays and clocks? Because measurements with the highly directive antenna are not so appropriate for an assessment of these parameters. Instead, continuous monitoring over weeks or longer is preferred, based on measurements from a network of distributed navigation receivers. Such monitoring likely will be discussed in other publications.

Evaluation Parameters

For the assessment of the satellite’s radiated power in the individual navigation bands, absolute calibrated spectral measurements are evaluated for spectral flux densities integrated over frequency. These parameters provide a first insight into spectral asymmetries and variations over time.

Other parameters are used to evaluate the distortions in the instantaneous transfer characteristics of the satellite. They are derived from wide-band recorded baseband-signal samples of up to a few seconds duration.

Initially, we consider the I/Q probability density of the signal after Doppler frequency shift removal, well known from communication system analysis as scatter plots. Secondly, as introduced in the previous section, we want to quantify the impact of transfer distortions on the navigation performance obtained for ideal navigation receivers. The corresponding acquisition and tracking performance is based on the correlation function.