GNSS is the primary enabler for Performance Based Navigation (PBN) and Automatic Dependent Surveillance Broadcast (ADS-B) applications and is becoming an increasingly essential technology used in air navigation. GNSS outputs are also used in various other Communication, Navigation and Surveillance (CNS) applications. Unfortunately, in some regions, Radio Frequency Interferences (RFI) affecting aviation has become a widespread challenge. EUROCONTROL monitors GNSS RFI impact through a number of means, including pilot reports and ADS-B data. Thanks to alternative CNS capabilities, these situations can generally be managed. Nonetheless, EUROCONTROL is investigating how GNSS RFI impact zones can be detected and how this information can be used by operational centres to improve the management of air traffic when subject to GNSS RFI. EUROCONTROL has conducted a GNSS Receiver Interference Testing (GRIT) study with a GMV-led consortium to better understand the behavior of currently fielded aircraft GNSS receivers when subject to RFI. The test campaign used three commonly used aviation receivers. The objective of the testing was to see how receiver observables could be used to help detect RFI. Based on the simulation results, a set of RFI detection techniques have been defined and tested against different types of RFI, including CW, chirp and Noise-like jammers. The proposed techniques focused on the use of C/N0 derived metrics: detection of RFI based on the entropy of the C/N0 mean, Classification of RFI versus non-RFI events using Support Vector Machine method and Random Sample Consensus method applied to C/N0 versus elevation. These methods will be further explained in the paper and their performances will be discussed in terms of detection, complexity and possible ways of improvement. Finally, a dataset was generated to obtain receiver tracking profiles with a simulated RFI source to see how ADS-B data can be used to detect and localize the RFI source.

Susceptibility to Radio Frequency Interference (RFI) of aviation receivers in the GPS L1 band has been investigated in trials flights. With 200mW, the equivalent isotopically radiated power was comparable to common GPS jamming signals. For the first time in Switzerland, civil and military aircraft had the opportunity to investigate the performance of their GPS receivers in realistic jamming scenarios. The analysis of recorded GPS data allows investigating performance parameters. From the transmitting power of a GPS jammer the calculation of the field strength at the GPS receiver is straight forward. Since the GPS antennas are normally located on top of the aircraft and the jamming signal is presumed to be transmitted from the ground, the received power of the RFI can only be estimated. Experiments in the laboratory might not reflect the situation in real environments correctly. In addition, investigations of dynamic effects are costly and are most often not performed. Thus, field trials are required to investigate the effect of jamming signals under real conditions. This paper describes the experimental set-up. Four different types of interference signals were radiated at a biconical broadbandwidth antenna during predefined times. The bandwidth of the interference signals was limited to 2 MHz around the center frequency of the GPS L1 band. Civil and military organization participated with different aircraft, fixed wing and rotorcraft. Operators with multi-band GPS receivers were instructed to use the L1 band only. Several flights were conducted with active in-

GPS becomes more and more important for approach and departure procedures. Aircraft rely on GPS on and close to the ground, where intentional or unintentional jamming signals may occur. Because of the low power, the received GPS signal is susceptible to RF interference, even for low power jammers. While theoretical calculations and laboratory experiments are essential for the understanding of jamming effects, they cannot reflect reality in all the details. From the known transmit power of a jammer the calculation of the field strength at the aircraft in distance d is straightforward. The directional gain of the GPS antenna with its low noise amplifier and the cable that is connected to the GPS receiver are specified. However, the total receiver gain, which includes the influence of the fuselage of the aircraft, is unknown. This is because the GPS antenna is normally mounted on top of the aircraft, whereas the interference is transmitted from the ground. Without the total receiver gain, the calculation of the interference power at the GPS receiver, and thus the interference-to-signal ratio ( J/S) or the carrier-to-noise ratio ( C/N-0) is not feasible. By laboratory experiments, the critical thresholds of J/S or C/N-0, where the tracking of the C/A code is lost, has been determined for different types of interference signals for the aviation receiver CMA-5024. Comparing those results to the ones of field trials reveal critical distances to the interference source and thus reliable values for the total receiver gain. This paper describes tests of civil and military aircraft performed in live jamming scenarios. The analysis of the recorded aviation GPS and flight management system ( FMS) data reveal the effects of the jamming signals on different aviation receivers and aircraft types. An equivalent isotropically radiated power of 200 mW has been chosen for the jamming signal. This is comparable to low power jamming devices. Four different types of interference signals were radiated from a biconical broad bandwidth antenna during predefined times, namely a pseudo random noise ( PRN) sequence, a continuous wave ( CW), a frequency hopping ( FH) and a radar like signal with high pulse repetition frequency ( PRF). The bandwidth of the interference signals was limited to 2 MHz around the center frequency of the GPS L1 band. Civil and military organizations participated with two fighters, four helicopters, two business aircraft and one flight calibration aircraft. Three helicopters and one of the business aviation aircraft are equipped with specific data recorder units that collect data from the aviation GPS receiver, the FMS and attitude data of the aircraft. In addition, independent multiband GNSS receivers record reference tracks in the GPS and GLONASS bands. Several flights have been conducted while the four interference signals were successively radiated. As results, most of the GPS L1 receivers were susceptible to three of the four transmitted jamming signals, namely the PRN sequence, the CW and the FH. The high PRF signal seemed not to impact the GPS reception. Data of the four aircraft equipped with additional data recorder units is analyzed in the position and range domain. GPS tracks are compared to the FMS position solution and the track retrieved from an independent multiband GNSS receiver. Performance parameters like the horizontal integrity level ( HIL), number of used satellites and position differences are discussed. The evaluation of the critical tracking threshold C/N-0 with respect to the distance to the jammer yields reliable values for the total receiver gain for one specific receiver type. Analyses of further recorded data show that the detection of jammer events by monitoring C/N-0 is generally reliable. The field trials complement findings of theoretical calculations and laboratory experiments and support the understanding of realistic jamming scenarios.

During approach and landing of airplanes, the Ground Based Augmentation System (GBAS) is used to augment the Global Positioning System (GPS) positioning by delivering differential corrections. The ionospheric delays are typically considered to be removed by generating differential observations if the baseline between the GBAS station and airplane is relatively short. However, the situation may change in case of strong or stormy ionospheric activities. The remaining differential ionospheric residuals that are not eliminated might cause large slant pseudorange errors and degrade the positioning results significantly, especially in the vertical direction. In this work the Global Positioning System (GPS) data of the swisstopo's Automated GNSS Network for Switzerland (AGNES) were processed over 15 years from 1999 to 2013. We used the double-difference phase observations with reliably resolved ambiguities to establish the station-pair single-difference ionosphere residuals in the slant and vertical direction. An epoch-wise zero-mean condition over all satellites and per baseline was applied to avoid singularities and the estimated single-difference slant ionospheric residual was corrected with the 2-hour global ionosphere maps provided by the Center of Orbit Determination in Europe (CODE). The spatial gradients were calculated using the Ionosphere Pierce-Point (IPP) distances or the baseline length depending on the concrete case. It was found that the absolute maximum slant ionosphere gradients calculated from the slant differential ionospheric residuals and the baseline length during these 15 years are below the slant ionosphere gradient bound of the Conterminous United States (CONUS) ionospheric anomaly threat model.At last, the so-called overbounding vertical ionosphere gradients sigma(overbound)(vig), which confine all the non-Gaussian tails on a daily basis, were calculated for all days from 1999 to 2013 except the days with very stormy ionosphere activities. Ignoring the gradients with an IPP distance shorter than 20 km, over 96% of the. values are smaller than 4 mm/km.

Noise protection has an increased importance for approach und departure procedures in vicinity of airports. A possible option is to concentrate flight tracks over sparsely populated areas to the greatest extent possible. Switzerland has many congested and extensively fragmented airspace due to the various user requirements. ICAO's latest update on performance-based navigation will allow Radius-to-Fix (RF) legs during initial and intermediate segments of an approach. The latter are a major enabler to minimize airspace use and to optimize fuel consumption.In the frame of the Swiss-wide implementation program to promote GNSS procedures and applications (CHIPS), test flights were carried out to analyze the use of various RF leg applications in an approach procedure. Two different approach scenarios were developed. The first scenario was designed according current ICAO standards with one RF leg and a straight stabilization segment of 2 NM before the final approach segment. The second scenario included three RF legs, the first one to the right, followed immediately by a second and third leg, both, to the left. The latter ended directly at the start of the final approach segment. A maximum bank angle and wind conditions without safety margins tailored to the deployed aircraft were applied, pushing the system to the envelope limits. Both scenarios were designed with an ILS and RNAV final approach segment.Test flights have been carried out with a Super King Air of the Swiss Air Force. This aircraft is equipped with a Pro Line 21 system having RF leg capability and a Rockwell Collins GPS4000S GPS/SBAS receiver.A total of 13 approaches have been flown. The aircraft has been equipped with a geodetic GNSS receiver in order to record raw GPS measurements. These measurements have been post-processed to determine the true flight path at decimeter level accuracy. The navigation system flight trajectory has been determined by recording ADS-B messages.The test flight results have been analyzed concerning compliance to the ICAO RNP 1 specification. The flights have been performed with different autopilots and flight director settings. Furthermore, configurations with deselected GPS and deselected DME have been tested.The performance boundaries of RNP 1 were maintained for all flights. Even abnormal conditions with dead reckoning (DR) navigation only, resulted in an acceptable performance. The transition with a single RF leg did never show any significant deviation from the designed flight track. The transition with three RF legs in an S-shape, however, showed significant lateral deviations of up to 0.3 NM.

Performance-based navigation allows implementing approach procedures in areas, where conventional procedures are not a feasible solution. This holds especially true for difficult topographic terrain. In case of the Meiringen Air Force base in Switzerland, which is located in a valley surrounded by mountains up to 10'000 ft, a cloud break procedure for rotary wing operations has been developed. This procedure will allow the Swiss Air Force and the local HEMS (Helicopter Emergency Medical Services) operator to fly under adverse meteorological conditions. Due to topographic constraints a required navigation performance (RNP) value of 0.3 NM is envisaged for the whole approach, starting at the initial approach fix until the end of the missed approach.One of the most critical points is the loss of GNSS navigation capability combined with a one-engine-out situation. In the first part of this analysis, the approach availability for a 30-satellites GPS constellation has been determined. A GNSS simulator has been used to assess different avionics GPS receivers envisaged for this approach.Based on the mean time between outages (MTBO) of GPS satellites, the probability of a satellite outage, and the effect thereof, has been determined. The discontinuity probability of the received GNSS signals have been considered together with other main technical error sources such as malfunction of navigation equipment and engine problems. Furthermore, statistics on meteorological conditions in the area of Meiringen have been taken into account.A failure model has been used to determine the probability of the hazardous situations for each helicopter type individually. The resulted probability has then been compared with the required target level of safety. For all helicopter types, the achieved probability values are better than the required TLS, meaning that the minimum level of safety can be maintained.