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For this reason it is often necessary to recognize and classify which signals can cause any receiver performance degradation.

In [1] and [2] the interference sources are classified with respect to the nature of the undesired signal and its time characteristics (e.g. Continuous Wave -CW- or pulsed signals). Within the investigation of a real interfered scenario and in particular in presence of high power interferers an alternative classification can be introduced.

Within the analysis that is going to be presented, the attention will be focused on non-intentional out-of-band interferers. In such a family of signals, a further significant classification could be identified, according to the actual disturbing signal that is distorting the GNSS signal.

In fact, a classification of the interferers than can be considered ‘external’ or ‘internal’ to the receiver is proposed. The two classes can be defined as following:

• External: Real strong interference harmonics that fall in the GPS L1 band. They can be generated within the transmitters due to the non-linearity and implementation issues of the transmitters;
• Internal: Signals generated within the receiver’s front end as a result of the intermodulation effect due to strong off-channel signals [4].

Such a classification is preliminary to further subsequent steps, as the implementation of detection or mitigation techniques.

As it will be analyzed in the following Sections of the article, the relevance of the degradation factor of both classes of interferers, is dependent on other features. It is important to notice that the classification is strongly dependent on the location of the undesired sources as well as the position of the GNSS receiver antenna.

Furthermore it is likely that some particular receiver architectures could be more sensitive to interference signals. For example, the new upcoming Galileo and GPS signals will require wider front end filter bandwidths in order to exploit the improved multipath rejection capabilities [3]. Since using wider bandwidths, the front end is no longer able to cut off the undesired external harmonics that fall into the GNSS bands. During the receiver architecture design, a trade off between the different core technologies and the overall performance must be considered.

For this reason the classification between ‘external’ and ‘internal’ sources assumes a special importance.

As it will be shown within the article, the whole analysis is based on real data sets collected in a urban area close to transmitting VHF and UHF antennas. In fact, since the Digital Video Broadcasting-Terrestrial (DVB-T) has, in Europe, some channels around 500 MHz (one third of the central L1 frequency), this experiment has been planned with a twofold objective:

• to check if ‘external’ interference represented by DVB-T or analog TV harmonics can be detected in the GNSS band;
• to check if the high signal power transmitted by the antennas and the short distance between them and the GNSS receiver could lead to interference internally generated within the front end.

After the description of the system set up used in the data collection, that is provided in Section II, Section III will show the main results obtained in the observation of the signals trying to classify the interference detected at the front end output as ‘internal’ or ‘external’. Furthermore, the article discusses the effect of the detected interferers on the tracking phase within a conventional GNSS receiver.

Note that, in order to assess the presence of the interference and to have a term of comparison, the same data collection has been repeated on two subsequent days (April 11th and 12th, 2006) at the same time in an interfered and in an interference-free site, respectively.

II. Data Collection Test Campaign
The data collection took place in an suburban area called “Colle della Maddalena”, (Test site 1 at about 45° 01' 52.15'' N, 7° 43' 17.75'' E, h=715 m) which is sited on the hill on the side of Torino downtown and 1.6 Km away from the “Eremo” site where two transmitting antennas are used to broadcast several channels of digital and analog TV. (Antennas site at about 45° 02' 29.48'' N, 7° 44' 06.86'' E, h=630 m). Fig. 2 (a) shows the map of the area where the experiment was performed, while Fig. 2 (b) is a picture of the two transmitting antennas.


Fig. 2: Map of the area where the data collection took place (a), 1.6 Km far from the transmitting antenna (b) for analog/digital TV sited on the hill of Eremo

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