Unintentional Interference
All communication systems with carrier frequencies close to the GNSS band could be a potential source of interference for a receiver. It is likely that some out of band energy from signal frequencies located near the GNSS bands could interfere with GNSS receivers, due to harmonics or power leakages that may be negligible for the communication system, whereas threatening for a GNSS receiver. Thus, interference sources must be distinguished in the case in which they fall in the GNSS frequency range (in band RFI) or far from it (out of band RFI). There are almost no in-band authorized emissions in L1 frequency; interference signals mainly come from out of band or spurious emissions and are usually located in small portions of the frequency band. Several real cases have been experienced in the past due to different systems, such as analog and digital TV channels, other VHF channels the harmonics of which fall in the L1/E1 band, portable electronic devices, or Ultra-wideband systems (Savasta, 2010; Wildemeersch, Rabbachin, Cano, & Fortuny, 2010).
Particular focus has been recently given to the potential interference generated by 4G LTE (Long Term Evolution) satellite broadband communication network, developed by LightSquared. The radio band in which the system will operate is close to the L1 band and it was proved by a TWG (Technical Working group) (Federal Communication Commission, 2011) that LightSquared transmissions, assured by a new network of 40 000 ground stations, can produce adverse effects on GPS receivers (Boulton, Borsato, Butler, & Judge, 2011).
Depending on the nature of the interfering signal, the effect on the receiver can range from a worsening of the position accuracy to the appearance of biases in the measurements, and in some cases to a loss of tracking.