Inertial Reference Systems - GPS Spoofing and Jamming

Recently there have been reports in the aviation press concerning GPS spoofing in the Middle East. Further complicating the issue was the temporary loss of the EGNOS system due to human error around the same time period as the spoofing events. During two periods on 9/10/2023 EGNOS declared all GPS satellites as unusable. EGNOS Outage Report. Honeywell has been working with OEMs to describe the expected behavior so that operational procedures can be developed. This article will help explain some of the misconceptions.

Additional information is provided in Honeywell SIL D202311004193.

Key Points

• Jamming/spoofing does not take out the “Pure-IRS” position
• Jamming/spoofing does not take out radio-based navaids
• Normally there will be multiple clues alerting you to possible jamming/spoofing

Jamming

GPS jamming is as simple as generating a radio frequency signal to interfere with or overpower the signal from the GPS satellite constellation. The FMS is designed to handle the loss of GPS due to jamming and operate in a degraded mode.

The website below collects GPS accuracy reports from ADS-B transmissions and creates a global GPS quality map: http://gpsjam.org

Spoofing

Spoofing can be more concerning because the aircraft position may be affected. Spoofing is more complicated and involves an entity sending a signal that results in a false position. Several of the recent reports are along a busy airway in the eastern part of Iraq near the Iranian border. Spoofing can include GPS position jumps of 50 to several hundred miles. How you recognize spoofing depends on the type of spoofing and the installed navigation equipment.  This article and the associated Service Information Letter aim to describe, at a high level, how the avionics behave in these situations.

Now that we understand the differences between jamming and spoofing, we can look at the different systems and understand the indications and alerts associated with each.

Inertial Reference Systems (IRS)

An Inertial Reference System is conceptually similar to dead reckoning. If the system knows its initial position, and acceleration, direction, and speed are precisely measured, a new position can be accurately calculated even without an external source of input. They allow the crew to input an initial position and subsequently begin point-to-point navigation. Inertial systems prove extremely accurate as the initial position is entered, but due to several external factors, typically associated with drift, that accuracy degrades over time. Typical drift rates fall in the range of 1-2 miles per hour. This is often called the “Pure-IRS” position solution because the IRS is self-contained, and therefore is not affected by jamming or spoofing.

Hybrid IRS

A Hybrid IRS is often described as a “tightly coupled” position sensor which provides extremely accurate position calculations. Better explained, Hybrid IRS uses components in the Inertial Reference Unit to receive GPS data from the GPS receiver.  “Why is this good” you ask?  The idea (and benefit) of the hybrid system is that it uses both position sources, IRS and GPS, to remove error from one another.   Consider IRS – it is extremely accurate for short periods of time yet accumulates drift error that compounds over several hours.  GPS position is accurate in most parts of the world but does have some errors introduced due to atmospheric conditions, satellite coverage and geometry, and interference. Hybrid blends the two, using sensor-specific algorithms to filter out errors in each. The Hybrid systems provide for better overall position from the sensors and, in the case with later systems (referred to as “High Step II”), provide more filtering that produces a RAIM like function and extends integrity protection levels. This allows the system to “coast” using the Hybrid Inertial Position in an environment that for many reasons, may lose GPS. This makes RNP navigation, especially approaches using low RNP, more robust to protect against GPS outages that would otherwise lead to unexpected, missed approaches. Hybrids provide several advantages over conventional systems, but the primary ones are:

• Capability for rapid (in-flight) alignment
• Use as a high integrity back-up for GPS outages including loss of signal, RAIM holes, etc
• Ability to continuously reduce the effects of noise in the system

The tightly coupled (Hybrid) IRS systems provide greater accuracy but use the GPS data for updating position. How the GPS position is impacted when a GPS position divergence is introduced depends on the system. In newer hybrid systems the hybrid position is not impacted initially and will provide some indications to the crew as the GPS and IRS position solutions diverge. Eventually however the hybrid solution will be affected by the erroneous GPS position. Older systems do not detect the jump and will be affected immediately. Hybrid IRS systems calculate both a hybrid position and a “Pure-IRS” position. The “Pure-IRS” position solution is subject to normal drift but is not impacted by spoofing. When spoofing is suspected the operator can deselect the GPS and hybrid sensors. This inhibits the FMS from using the GPS and hybrid positions. This procedure varies by aircraft type.

ADAHRS/AHARS/GPS Equipped Aircraft

On these aircraft without an IRS sensor the FMS will follow the GPS but typically aircraft operations in this area of the world are IRS equipped. Manual radio NAV (DME/DME or VOR/DME) is available as are ATC vectors.

Radio NAVAID Tuning/Updating

There were reports of radio nav impact, but this was likely an auto-tune issue due to an erroneous FMS position. The FMS keeps a table of nearby NAVAIDs for auto-tuning based on location. If the location is off significantly the auto-tuning may be trying to tune NAVAIDs out of range, and this was interpreted as a radio nav malfunction. Manually tuning nearby NAVAIDs will provide a position update usable by the FMS, likely with increased Estimated Position Uncertainty (EPU).

FMS Sensor logic

A related topic that often gets confused is sensor logic (i.e. what position sensor the FMS will use). The different systems output multiple position solutions (GPS, Hybrid, Pure IRS, DME/DME, VOR/DME). Sensor logic has changed from legacy platforms to Epic platforms; and with hybrids in the equation, this can cause some confusion. A quick bit of history to explain the architecture should help to clarify. In the legacy FMS lines (NZ prior to FMS 6.1 and EPIC prior to FMS 7.1), the system used the linear hierarchy below:

1. GPS

2. DME/DME

3. VOR/DME

4. IRS

5. DR

Starting with software versions 7.1 (EPIC) and 6.1 (NZ 2000), the hierarchy was modified to a more logical format. Rather than only a linear hierarchy based on the probability of best position, the sensor logic for FMS position is choosing the available sensor with the best accuracy. The incorporation of this new logic also means that crews are more likely to encounter Hybrid Navigation mode because the newer Hybrid Inertial Reference Units are constantly being updated by the GPS and are generally more accurate than DME/DME.

Indications of Jamming

One of the first indications of a loss of GPS will be the synthetic vision reverting to the basic blue-over-brown mode. Other indications may include:

• GPS position is blanked out
• GPS mode goes to ACQUISITION from the normal NAVIGATION modes
• The FMS degrades to other available sensors for position like IRS, DME/DME or VOR/DME
• As this degradation occurs the EPU will increase to that of the next sensor
• ADS-B is unable to broadcast a GPS position

Alerts associated with GPS jamming may include:

• GPS X FAILED
• UNABLE RNP (if EPU exceeds the required leg RNP)
• UNABLE RNP NEXT WPT (same as above for the next leg)
• ADS-B FAIL messages associated with being unable to transmit GPS position
• FMS/GPS POSITION MONITOR UNAVAILABLE

Indications of Spoofing

• CHECK GPS X POSITION (FMS and GPS position differ)
• CHECK IRS X POSITION (FMS and Pure IRS position differ)
• CHECK VORDME X POSITION (FMS and IRS position differ)
• FMS – GPS POS DISAGREE
• FMS 1-2-3 GPS MISCOMPARE
• DEGRADE Indication
• UNABLE RNP (when EPU exceeds the RNP of the current flight path leg)
• UNABLE RNP NEXT WPT (when EPU is expected to exceed the RNP of the next flight path leg)
• ADS-B FAIL/ATC ADS-B TRANSMITTER FAIL

There may be additional indications and/or different messages that vary by OEM. In the image below the pilot had three indications of spoofing on the GPS Status page. The GPS position was suddenly more than 100nm away from the FMS position, the groundspeed was 27 knots, and the GPS altitude was 3,607’ when the aircraft was at FL410. In addition, on the Predictive RAIM page the GPS Almanac indicated “EXPIRED” indicating a GPS date/time shift providing additional clues.