GNSS Vulnerability and Alternative PNT

As NextGen air traffic management increasingly relies on GNSS for safety-critical functions, some form of backup is needed in the event of GNSS signal loss, whether due to intentional jamming or other causes.

A group working under the auspices of the Federal Aviation Administration (FAA) Navigation Services Directorate recently prepared a study assessing non-GNSS navigation system architectures to provide alternate positioning, navigation, and timing (APNT) services for aviation users, to mitigate GNSS vulnerability to radio frequency interference (RFI). The APNT architecture would be based on selected elements of today’s terrestrial navigation network, possibly upgraded, plus new elements anticipated for the 2025 timeframe.

This article summarizes the scope and initial results of the study; to download the full paper, visit www.gpsworld.com/alternativePNT. As a result of the 2001 Volpe Vulnerability Study and subsequent U.S. government policy on PNT services provided by GPS, the FAA has begun investigating APNT concepts by which the safety, security, and efficiency of the U.S. National Airspace System (NAS) can be maintained in the event of a loss of GPS-provided PNT services. The sought-after APNT network should be cost-effective based on likely aircraft equipage in the 2025 timeframe.

The FAA recognizes that during migration from the current NAS to the Next Generation Air Transportation System (NextGen), reliance on PNT services will increase to support area navigation (RNAV), digital communications, and enhanced surveillance services. This paper, presented by the FAA to the International Civil Aviation Organization’s (ICAO’s) Navigation Services Panel in May, identifies three major areas of research and analysis. The APNT work represents a constructive response to concerns raised by the simultaneous 9/11 terrorist attacks and the Volpe Report on GPS vulnerability.

The first area of research proposes to investigate current distance measuring equipment (DME) to see if better RNAV services can be provided to current and future users, and to mitigate the possible problem of over-interrogation as demand on the system grows. The second area will investigate multi-lateration to see how the services based on systems currently being planned and fielded could be expanded or enhanced by synergy with other ground-based navigation systems such as DMEs.

The third area of interest will investigate the use of the current and future DME network, and potentially other ground-based equipment, to provide a robust RNAV pseudolite system broadcasting in the current DME L band. This third alternative receives the bulk of the attention of this two-page digest of the full paper.

Background

The United States is pursuing the NextGen air traffic modernization program to support a predicted increase in operations by a factor of 2–3 by 2025. Many of the new capabilities depend on PNT services provided by GNSS. Specifically, performance-based navigation (PBN) and automatic dependent surveillance broadcast (ADS-B) will be based on GPS with satellite-based augmentation systems (SBAS) and ground-based augmentation systems (GBAS). PBN and ADS-B will, in turn, support trajectory-based operations, area navigation (RNAV), required navigation performance (RNP), precision approach, closely spaced parallel operations, and other operational improvements.
As NextGen modernization and implementation progresses, U.S. NAS dependence on GNSS services will increase. Appropriate mitigations for GNSS vulnerability to RFI also must be assessed and implemented where necessary.

APNT Assumptions

The study group established a set of assumptions to guide the analysis activity. Key among the 13 assumptions were:

• In 2025, there will be “RNAV everywhere and RNP where beneficial.” There will likely be many different variants of RNAV and RNP that are yet to be defined.
• APNT is a means to continue RNAV and RNP operations to a safe landing during periods when it is discovered that GNSS services are unavailable, due to interference.
• Users equipped for APNT will be able to continue conducting RNAV and RNP operations (dispatch, departure, cruise, arrival) during the GNSS outage after the transition to APNT.
• Users not equipped for APNT may not be able to continue RNAV and RNP operations in areas where GNSS is required during the GNSS outage.
• APNT service performance may not be equivalent to GPS performance.
• At least one instrument landing system will be retained at airports wherever required for safety or economically justified.

Pseudolite Multi-Lateration

This article passes over the paper’s discussion (see link cited earlier for full version) of DME network optimization and passive wide-area multi-lateration (WAM) to take a brief overview of the pseudolite-based multi-lateration.

As shown in Figure 1, the pseudolite (PL) architecture allocates the position and integrity functions to the aircraft, similar to how GPS receiver-autonomous integrity monitoring works. The PL alternative would leverage all of the existing 1,100 DME facilities plus the planned ADS-B ground-based transceiver (GBT) facilities to provide a combined network of approximately 1,900 sites.

As shown in Figure 2, the PL architecture requires the GBT and DME sites to be synchronized to a common time standard so each facility can generate and transmit a heartbeat message consisting of the station identification and an accurate time stamp. The ADS-B in avionics would host the position calculation and integrity monitoring functions and pass this information to the aircraft navigation over a new interface, if GPS becomes unavailable.

Figure 2. Multi-lateration (MLAT) alternative block diagram.

The potential advantages of this alternative include a simpler architecture that does not require a ground system to compute the position of the aircraft. A common non-GNSS or robust GNSS time reference is required.

Straw Man Signal Design

The authors propose a straw man signal design for the broadcast of one-way ranging signals from existing DME transmitters. The goal is not to provide a final design for such a signal. They recognize that many modifications and improvements will be required to bring such a function to fruition. Rather, they offer the proposal as a catalyst for the community, and hope that it will serve as a starting point for a vigorous discussion on this critical topic.

Signal design is directed at these goals:

• The new signals should be added to the existing broadcast from operational DME beacons without significant degradation to the two-way ranging accuracy provided by the DME beacon to legacy users. The new signals would overlay the existing replies that complete the traditional two-way DME transactions. More specifically, they could be implemented by triggering existing beacon with requests from a pseudo-aircraft located near the operational DME beacon. Thus, they hope to avoid any changes to existing ground hardware and by so doing realize benefit from the entire set of DME beacons in operation today.
• The new signals should provide one-way ranging to modified avionics. The authors do not wish to modify the ground equipment, but recognize that one-way ranging from a DME station will require new avionics.
• In addition to one-way ranging, the new signals should also support a modest data capability. This data would include the DME location, DME identification, time information, and a parity field to ensure data integrity. The proposal targets a data capacity around 150 bits per second, because similar capacity has served well for other one-way ranging systems such as GNSS and SBAS.
• Finally, the new signal should also enable source authentication. The authors feel that signal authentication is needed, becau
  se radio navigation may be subject to electromagnetic attack in the decades ahead.

The authors then describe and illustrate in seven figures the definition of a DME chip, a do-no-harm criterion, synchronization sequence, data field, data erasures and errors caused by competing channel traffic, data content, and source authentication. They indicate that they are looking at other signal alternatives for the DME band as well. These alternatives would make more liberal use of spread-spectrum technology.

Authors of the APNT study were Leo Eldredge (FAA), Per Enge (Stanford), Mike Harrison, Randy Kenagy, Robert Lilly (all with Aviation Management Associates), Sherman Lo (Stanford), Robert Loh (ISI), Mitch Narins (FAA), and Rick Niles (MITRE CAASD).