scope:

INTRODUCTION

Background

The RTCA Task Force #1 Report on the Global Navigation Satellite System (GNSS) Transition and Implementation Strategy (September 1992) recommended that "the FAA should base GNSS initial operational implementation on the use of the U.S. GPS (Global Positioning System) national resource and appropriate augmentations". The Task Force also recommended that RTCA develop initial standards that will be used to certify GNSS equipment for operational use. The accuracy, integrity, availability and continuity of satellite-based navigation must be assured before it can be adopted into safety of life aviation use. Radio frequency interference (RFI) is a factor that potentially degrades system performance and one that must be examined with care.

RTCA SC-159 Working Group 6 was created to investigate radio-frequency interference issues, and appropriate mitigation alternatives, relevant to GNSS. The original Terms of Reference for this group were to:

Develop quantitative assessments of the current and projected RF interference (RFI) environment

Recommend measures to reduce RFI at the source where it is technically feasible and cost-effective to do so

Develop updated interference rejection criteria for future GNSS receivers that will enable them to satisfy required navigation performance in the near future RFI environment

Develop an out-of-band emissions mask for adjacent-band Mobile Satellite System mobile Earth terminals

The original GNSS L1 RFI environment assessment by RTCA (RTCA DO-235), completed in late 1996, identified potential sources of RFI and assessed the vulnerability of GNSS receivers (particularly GPS) to that interference. The original assessment was motivated by a number of factors. These included sporadic GPS interference trouble reports from various RFI sources, the potential RFI from new sources in adjacent frequency bands, and the need to identify appropriate mitigation measures for flight-critical GNSS applications. Out of the original assessment work came susceptibility requirements for RTCA DO-229 GPS/SBAS airborne receiver standard, two proposals for "Big-LEO" Mobile Satellite Service transmitter emission requirements, and several other key findings.

In October 1999 at Department of Transport (DOT) request, the RTCA undertook to investigate the RFI environment near the GPS L5 frequency (1176.45 ± 12 MHz) and determine appropriate receiver susceptibility criteria and related RFI unwanted emission limits for the use with new civil signal. Aviation-related issues were acknowledged to be of primary importance, but the group was to seek significant involvement and input from non-aviation public safety applications (e.g., maritime, E-911, police, fire fighting). By June 2000 the regulatory and business activity pace had intensified on ultra-wideband (UWB) transmission technology. As a result, the DOT requested the RTCA enlarge the study to explicitly treat UWB RFI effects and operational scenarios for the GPS L1 frequency as well as L5.

Additional terms were added in mid-2000 to address certain issues with UWB and other new RFI sources and proposed new operational uses. They were:

Develop RFI encounter scenarios for new aeronautical and non-aeronautical public safety operations with GPS receivers, review and revise current scenarios as required;

Assess the potential for RFI to GPS receivers from proposed unlicensed UWB transmitters and recommend suitable emission limits and other associated mitigations for use by the FAA and DOT in FCC rulemaking proceedings;

Update, as required, the assessment of existing and new L1 RFI sources, the existing GPS receiver susceptibility standards, and other RFI mitigation means to aid evolving aeronautical GNSS development.

As a result, WG-6 put its L5 work aside temporarily in order to deal with the more urgent issue of UWB effects on GPS L1. That decision focused the work on both UWB and as well as other L1 issues added in mid-2000. This work was captured in the publication of RTCA DO-235A in December 2002. That report addressed aeronautical GPS RFI encounter scenarios, risk modeling, began to address Cat II/III scenarios, and the effects of personal electronic devices carried on-board aircraft. It also evaluated GPS intrasystem interference from C/A, Y and M codes, utilizing the concepts of spectral separation and critical satellite.

The work next shifted to GPS L5, which resulted in the publication of RTCA DO-292 in July 2004. Pursuant to the development of DO-292, WG6 performed an update to DO-235A and this addressed (for GPS L1) new operational scenarios that were patterned after the DO-292 link analyses , improved aggregate terrestrial RFI modeling, updated analyses of OOB pulsed RFI effects on GPS receivers at L1 and addressed new RFI sources (such as transmitting portable electronic devices (T-PED's)) and GPS re-radiators. All these updates were captured as part of DO-235B.

Since 2008, there have been multiple planned aviation GNSS developments/updates that necessitated yet another update to DO-235 and this body of work was commissioned to address the same. Details regarding the scope of this update can be seen under report objectives in Section 1.2.