(Gps) Adjacent Band Compatibility Assessment
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UNITED STATES DEPARTMENT OF TRANSPORTATION GLOBAL POSITIONING SYSTEM (GPS) ADJACENT BAND COMPATIBILITY ASSESSMENT FINAL REPORT APRIL 2018 I EXECUTIVE SUMMARY The goal of the U.S. Department of Transportation (DOT) Global Positioning System (GPS) Adjacent Band Compatibility Assessment is to evaluate the maximum transmitted power levels of adjacent band radiofrequency (RF) systems that can be tolerated by GPS and Global Navigation Satellite System (GNSS) receivers. The results of this effort advance the Department’s understanding of the extent to which such adjacent band transmitters impact GPS/GNSS devices used for transportation safety purposes, among numerous other civil applications. The assessment described in this report addresses transmitters in bands adjacent to the 1559-1610 MHz radionavigation satellite service (RNSS) band used for GPS Link 1 (L1) signals that are centered at 1575.42 MHz. The assessment includes two primary components: • One component, led by the DOT Office of the Assistant Secretary for Research and Technology (OST-R), focused on all civilian GPS devices and their applications, apart from certified aviation. Through this component of the Study, categories of receivers were evaluated that included aviation (non-certified), cellular, general location/navigation, high precision, timing, and space-based receivers. An element of this effort was to determine equipment susceptibility to adjacent band interference to support analyses for deriving compatible power levels. • The other component, led by the Federal Aviation Administration (FAA), focused on certified GPS avionics, and was conducted by analysis to determine the adjacent band power levels that conform to existing certified GPS aviation equipment standards. The DOT GPS Adjacent Band Study is the product of an extensive process to gather stakeholder views and input. OST-R and FAA benefited significantly from feedback received via governmental and public outreach on equipment use cases, interaction scenarios, propagation models, and transmitter characteristics. Certified GPS avionics meet their performance requirements when operating within the RF interference (RFI) environment defined in appropriate FAA Technical Standard Orders (TSOs). For civil GPS/GNSS receivers other than certified avionics, receiver testing needed to be conducted to determine the Interference Tolerance Masks (ITMs) for various categories of receivers. ITM defines, for a particular receiver, the maximum received aggregate interference power that can be tolerated by the corresponding tested GPS/GNSS receiver. To accomplish this testing, OST-R sought to include a broad range of devices used in rail, aviation, motor vehicle, maritime, and space applications, among a number of other civil uses of GPS/GNSS including timing, surveying, precision agriculture, weather forecasting, earthquake monitoring, and emergency response. The GPS/GNSS receivers for this test effort were provided by U.S. Government and industry partners and represented the diverse nature of GPS/GNSS applications and services. II GPS/GNSS receiver testing, led by the OST-R/Volpe Center, was conducted at the U.S. Army Research Laboratory (ARL) at the White Sands Missile Range (WSMR) facility in New Mexico in April of 2016 with 80 civil GPS and GNSS receivers tested, as shown in Figure ES-1. The Air Force GPS Directorate conducted testing of military GPS receivers the week prior to the civil receivers being tested. Figure ES-1: GPS/GNSS Receivers in WSMR Anechoic Chamber In determining the transmit power level analysis, it is important to understand real-world scenarios and the proximity those applications of GPS/GNSS may come to adjacent band transmitters. A graphic of various emergency response uses is shown in Figure ES-2. First responders are increasingly using GPS/GNSS to locate patients both during emergencies and as a normal course of duty. As shown in the figure, there are multiple uses of GPS/GNSS for navigation of emergency service response vehicles, as well as asset tracking, including increased situational awareness of where response personnel and vehicles are located. An unmanned aircraft system (UAS) or drone, which also has a GPS/GNSS receiver incorporated also plays a role in this scenario, supporting the response effort. Drones are becoming of increasing importance in collecting imagery and sensor data in response to natural disasters and other incidents. This scenario illustrates that use of a GPS/GNSS receiver can be quite close in distance -- within tens of meters of a base station transmitter and potentially very close to a handset as well transmitting in the adjacent band. The GPS/GNSS receiver also could be located vertically above the base station. III Figure ES-2: Emergency Response Use Case Results for the high precision receiver category for an emitter at 1530 MHz based on results of analysis and testing are presented in Figure ES-3. These results are for a typical cellular base station power level of 29 dBW (794 watts) with the base station antenna 25 m above the ground. In this figure, the horizontal axis is the lateral distance between the GPS/GNSS receiver and the base station. The vertical axis is the height of the GPS/GNSS receiver above the ground. Note the high precision category of receiver exceeds a 1 dB signal-to-noise density (C/N0) interference protection criteria at a distance beyond 14 km from the transmitter. When this occurs, the behavior of the GPS/GNSS receiver can become unpredictable in its ability to meet the accuracy, availability, and integrity requirements of its intended application and a receiver in a mobile application may not be able to reacquire GPS positioning as the mobile application encounters multiple, closely-spaced emitters in an urban scenario. Furthermore, this category of receiver experiences loss of lock for low elevation GPS/GNSS satellites at distances up to 3 km with loss of lock on all satellites at approximately 1 km from the transmitter. ≥ 1 dB C/N degradation 2000 0 Loss of Lock on Low Elevation Satellites 1000 Loss of Lock on All Satellites 0 Height (m) 0 2000 4000 6000 8000 10000 12000 14000 Distance from Tower (m) Figure ES-3: Impact of a 29 dBW Cellular Base Station Transmitting at 1530 MHz on a High Precision GPS/GNSS Receiver IV Further analysis was performed to determine the maximum tolerable power levels for various categories of civil GPS/GNSS receivers for deployments of a macro urban and micro urban cellular network at frequencies within 100 MHz of GPS L1 (1475 – 1675 MHz). As an example, the results for 1530 MHz are shown in Table ES-1 for general location and navigation (GLN), high precision (HPR), Timing (TIM), and cellular (CEL) receivers. The transmit power level as quantified by the effective isotropic radiated power (EIRP) that can be tolerated is a function of distance from the transmitter. Two distances were chosen for evaluation (10 m and 100 m). The results demonstrate that other than the cellular devices, the other categories of GPS/GNSS receivers are sensitive to adjacent band power and can tolerate levels in the milliwatts or microwatts range as described below, depending on the separation distance to the transmitter. Table ES-1: Maximum Tolerable Power Level for GPS/GNSS Receivers at 1530 MHz Table ES-2 depicts the maximum tolerable power levels of space-based receivers used for performing scientific measurements. A future NASA mission, COSMIC-2, fitted with a TriG receiver built by NASA/Jet Propulsion Laboratory, was modeled, simulated, and analyzed using various cellular network deployment scenarios. The COSMIC-2 mission will be operating at an orbit of 800 km. V Table ES-2: Maximum Tolerable Power Level for Space-Based Receivers at 1530 MHz For certified GPS avionics, the FAA analyzed a number of scenarios including: 1) Inflight Aircraft with a Ground-based Handset 2) Inflight Aircraft with a Ground Base Station 3) Inflight Aircraft with an Onboard Handset 4) Aircraft on the ground with an Onboard Handset 5) Aircraft at Gate / Single Handset Source on or near Boarding Stairs or Jetway 6) Aircraft at Gate/Users Inside Airport 7) Terrain Awareness Warning System (TAWS) / Helicopter TAWS (HTAWS) Scenarios with Ground-based Mobile Broadband Handsets 8) TAWS and HTAWS Scenarios with Broadband Base Station The analysis for certified avionics is based on the concept of an “assessment zone” (see Figure ES-4) inside of which GPS performance may be compromised or unavailable and GPS-based safety systems will be impacted accordingly due to the elevated levels of RFI. Under the described engineering and operational assumptions, helicopter operations are the limiting factor in the analysis. These analyses indicate that protection of certified avionics, operating under the assumption of the described 250 foot (76.2 m) radius assessment zone, requires that the ground station transmission not exceed 9.8 dBW (10W) (cross-polarized) at 1531 MHz. This limit is obtained from the HTAWS scenario which was found to be the most restrictive of the certified aviation scenarios examined. VI Figure ES-4: Candidate Assessment Zone (Not to Scale) This concept generated a number of comments and questions from the aviation community when vetted through RTCA, Inc. One rotorcraft operator stated that its pilots use visual reference within the assessment zone and the assessment zone would have no negative impact on their operation. However, there were unresolved concerns expressed by several, though not all, operators about the assessment zone and its impacts to aviation operations and safety. These concerns include: technical and human factors issues associated with re-initialization of GPS after loss of the signal or when the signal reception is intermittent; workload and human factors impacts on pilots to monitor and track assessment zone locations; the possibility that pilot workload, confusion, or error could lead to aircraft inadvertently entering an assessment zone and losing needed GPS functionality; and impacts to onboard and ground systems that are dependent upon GPS, such as Automatic Dependent Surveillance (ADS) Broadcast/Contract (B/C), or fixed-wing and helicopter terrain awareness warning system including obstacle alerting.