The Impact of Uninformed RF Interference on GBAS and Potential Mitigations

RF interference (RFI) has been and will continue to be a significant worry for GNSS users. This paper introduces several different types of RFI, categorizes them based upon the intent (if any) of the RFI transmitter, and then examines a relatively new and growing source of RFI: Personal Privacy Devices (PPDs) that aim to prevent people and vehicles from being tracked by GNSS within a limited area. Unfortunately, signals from PPDs are not well-controlled and can interfere with GNSS receivers several hundred meters away. The impact of PPDs on the GBAS reference station site at Newark Airport, New Jersey and the WAAS reference station at Leesburg, Virginia are illustrated. While GBAS ground station monitoring prevents PPDs from posing a significant integrity threat, PPDs can force the sudden loss of service and thus harm continuity and availability. The hardware and software modifications made to the Newark GBAS installation to reduce this impact are described, and the future benefits of more-flexible ground-station siting and GNSS modernization are also identified. 1.0 Introduction and RF Interference Categorization Because Global Navigation Satellite System (GNSS) signals are very weak when received by user equipment, they are vulnerable to radio frequency interference (RFI). Signals that overlap with GNSS frequencies are likely to come from transmitters much closer than the GNSS satellites. Therefore, these signals can easily "overpower" the GNSS signals and make them unusable. To protect GNSS, existing ITU and FCC regulations prohibit the intentional broadcast of any non-GNSS signals on or near GPS L1/Galileo E1, while lesser protections apply to the GPS L2 and GPS L5/Galileo E5A frequencies. Despite these protections, RFI affecting GNSS is occasionally observed, and its apparent frequency has increased significantly with the number of civil GNSS users. In order to better understand the many possible sources of RFI and their potential effects on GNSS, this paper suggests a means of classifying RFI affecting GNSS into three categories. These categories are not all-inclusive, nor do their names fit all possibilities, but they help to separate RFI scenarios in a way that makes it easier to forecast impacts and design mitigations. The first category is malicious interference, meaning RFI that is intentionally transmitted to prevent the use of GNSS (or make its use hazardous) for as many users as possible. Coordinated hostile broadcast of RFI, while hopefully very rare, has the potential to make GNSS unusable over large regions and is difficult to defeat. Therefore, it makes sense to provide non-GNSS backup services to support transportation and other critical infrastructure needs [1]. The second category, and the focus of this paper, is uninformed interference, which results from the intentional transmission of signals at or near GNSS frequencies but without the desire to cause harm. At first, it may seem that signals deliberately broadcast on or near GNSS frequencies are likely aimed at harming GNSS users, but this is not true of the vast majority of cases, as will be illustrated in the following sections. Personal Privacy Devices, or PPDs, fall into this category and are of particular concern because they have become numerous in the last few years. The third category is accidental interference, which results from unintentional transmissions at or near GNSS frequencies. This usually is due to malfunctions of equipment that is designed to transmit at other frequencies or not to transmit at all. It is less common than uninformed interference both because malfunctions are rare and because they are more rapidly detected now that many GNSS receivers are likely to be in use nearby. On the other hand, accidental interference is more variable because it is not designed to prevent harm to users. Section 2.0 of this paper provides past and recent examples of accidental and uninformed RFI. Section 3.0 focuses on PPD interference and illustrates the Figure 1: RF Interferer Location on Stanford Campus Figure 2: Area at Stanford where GPS was Unusable characteristics of PPDs. Section 4.0 shows the impact of PPDs on the Wide Area Augmentation System (WAAS) Reference Station (WRS) in Leesburg, Virginia, while Section 5.0 describes their more-severe impact on the Ground Based Augmentation System (GBAS) ground facility at Newark Airport in New Jersey. Section 6.0 describes the GBAS hardware and software modifications being pursued to limit the impact of PPDs at Newark Airport so that acceptable GBAS Category I precision approach service can be provided. Section 7.0 concludes the paper and looks forward to the additional mitigation steps that will be made possible by GNSS modernization. 2.0 Examples of RF Interference to GPS Figures 1 and 2 show an instance of RF interference to GPS that occurred at Stanford University in 1999. At the time, construction was occurring in the Engineering section of the Stanford campus. A camera had been Figure 3: RF Interference at Moss Landing Harbor [2] installed on the Durand Building with a good view of the construction site, and an attached datalink transmitted digital pictures of the site to the construction headquarters trailer to allow progress to be monitored. This proceeded without incident until, for some reason, the datalink transitioned from its primary frequency of 1530 MHz to its secondary one of 1570 MHz, which is very close to the GPS L1 frequency of 1575.45 MHz. The GPS lab at Stanford discovered that GPS was suddenly "gone"  we could not acquire or track any signals. We also discovered that other GPS users in the area were affected, including the helicopters that transported severe cases to Stanford Hospital. This made it clear that the outage zone had a radius of at least 1 km. The cause was not immediately evident, but the use of directional antennas and signal analyzers allowed us to track down the offending device and (manually) remove its power source, after which GPS became usable again. Once the offending data transmitter was discovered, Todd Walter of Stanford communicated with the device's designers, who knew that their secondary transmission frequency was close to GPS L1 but thought that such transmissions were legal as long as they remained below a certain power level. In other words, they had no intention of interfering with GPS and had no idea that they were capable of doing so. In any case, this company's understanding was incorrect no intentional transmissions (regardless of power level) are allowed this close to L1. At this time, the civil use of GPS was relatively new, and it is not surprising that the regulations protecting it were not well understood. Uninformed interference due to misunderstandings of this sort should be less likely now that GNSS is well-established. Figure 3 shows another, well-known RFI incident from 2001 that was previously described in [2]. This was an example of accidental interference caused by amplifiers attached to UHF/VHF antennas for receiving over-the-air Figure 4: RFI Generated by GPS Repeater at German