GNSS interference detection and localization using a network of low cost front-end modules (2007)

The expanding fields of usage for global satellite navigation systems (GNSS) have been made possible thanks to the general technological progress. The forthcoming advent of the European Galileo system increases the availability to the user of possible GNSS ranging sources, which even further will increase the dynamic interest in satellite navigation applications. Even so, fundamental problems about satellite navigation persist. One primary issue is that the signals are weak and thus subject to interference, intentional as well as unintentional, especially under delicate conditions. Indoors navigation cannot be said being an original design criteria of the GPS system, however has become actualized by technological achievements making this possible. In metropolitan areas the availability increment from Galileo is welcome; however the main advantage from this second system, for single L1 frequency users, will come in the urban canyon environment where a major issue for GPS users is low availability. Within this urban environment interference pose threats to availability and the ability to achieve accurate position solutions. This paper will discuss different kinds of interference within the GNSS L1 band, their characteristics, and ways of detecting their presence and location. The primary tool that will be utilized for this task is an L1 band front-end ASIC module with a USB interface to a computer. With this low cost sensor module it is possible to deploy a larger number of these over a selected area to monitor for interference. The key idea is to synchronize measurements and post-process collected data at a central server in order to detect, classify and locate the source of interference. Since multi-bit front-ends use an automatic gain control (AGC) to optimize usage of dynamic range with respect to the incoming signal sampling this control level is the primary metric for the measure of the absolute incoming power level (thermal noise or thermal noise plus interference). The ability to read out this AGC metric in parallel with the sampled IF data gives the possibility to make absolute measurements regarding power levels. Since the front-end based module is built up from low-cost integrated circuit components, calibration is useful to obtain individual characteristics. At a first stage, calibration is made against a noise generator providing a Gaussian noise over a wide band and a signal generator providing a continuous wave at different frequencies within the L1 band. This is to examine bandwidth limitations of the instrument. A second stage calibration using a spectrum analyzer as a reference will ultimately provide a reference to absolute measurements. This paper will provide the following: (1) the design of a low cost GNSS L1-band ASIC front end with USB computer interface capable of provide both AGC and raw IF samples; (2) calibration process for this module to obtain absolute input levels; (3) data and testing results from the utilization of an individual module as an interference detection resource; (4) data and testing results from a network-based approach utilizing multiple sensors with a common server to provide detection and localization of interference sources. The final result, the network based sensor grid, will demonstrate how such low cost modules can be deployed over a wide geographic area and be used to quickly detect and isolates sources of interference which GNSS operation would be considered critical.