GPS spoofing is posing a threat to aircrews, especially those flying near war zones. Mentour put together this 22-minute video to explain a growing issue in aviation.


  1. Is inertial nav (IN) still an option? If so, it would be impervious to outside interference. The old IN systems were electro-mechanical Rube Goldbergโ€™s, complex and expensive. Could modern solid state gyros, accelerometers and computers be used to make a cheaper, more reliable and accurate IN system? Anyone know?

    • There are inertial reference systems (IRS) and there are inertial navigation systems (INS) and they’re cousins, but not exactly the same thing. The IRS knows where you are (lat/long) and what direction you’re pointed (and what direction you’re moving) but it doesn’t really know where you’re going. The INS has everything the IRS has, but it will also guide you along a line to a next waypoint.

      For a lot of years, IRS in transport aircraft was completely separate from other navigation sources, but in the last few years, the IRS has been modified to accept a GPS input to improve its accuracy and ease of alignment.

      • Yes. Iโ€™m wondering if the IRS technology of solid state accelerometers and gyros can be used to make a much superior INS. it depends on a computer to integrate cumulatively each time interval of acceleration to a velocity at a specific direction. Then integrate again to get incremental distance in that direction. The accelerometers and gyros must be very accurate, since error will be highly cumulative with this double integrating process. The idea is to create a relatively cheap system, immune to external jamming or need for GPS except for initialization and updates to offset long term drift. The old INS systems were very complex, bulky and expensive electro mechanical devices.

      • The serviceable drift limit is something like 2 miles for every hour of operation. On a flight across the Pacific, even a good low-drift example could easily be off by ten miles by the time you’re approaching the coast. That means that IRS/INS isn’t good enough to fly an approach at your destination, but it’s certainly more than good enough to get you into the service volume of a VOR that could bring you to the start of an ILS. In reality, it’s not the VOR but rather the DME that the airplane needs. Modern jet flight management systems can use a collection of DME ground stations in about the same way your GPS receiver uses satellites. Get your distance from three or four (or eight) DME ground stations, and see where the circles intersect, and there you are. This works great at altitude, over land, when you are in sight of many DME ground stations, and like the VOR it’s good enough to get yourself to the start of an ILS. Even if you lose some stations in the descent. The FMS can remember the offset between the “radio fix” from the DME and the “IRS fix” at top of descent, and the IRS won’t drift by much during the descent. So the FMS can bring you to the start of the ILS even if all DME signals are lost on the way down.

        And then there’s terminal radar service…

    • INS is a very outdated navigation system, particularly those with an electromechanical sensor platform. They could not do performance calculations and were only two-dimensional. Additionally, the electromechanical sensor platform was a point of failure, and much larger than modern alternatives.

      An Inertial Reference System (IRS) with a Flight Management System (FMS) does everything that an INS, but with MUCH more capabilities. The IRS provides the raw data to the FMS, which turns it into usable navigation data. However, Inertial Systems tend to have quite a bit of drift, and FMS requires additional navaids for accuracy/RNP

      Most IRUs in use today have Ringl Laser Gyros (RLG) and pendulous accelerometers to update position. The RLG is MUCH more reliable than an old electromechanical system. About 30 years ago, Delco came out with a Hemispherical Resonating Gyro (HRG), which was more accurate and more reliable than RLG. It never made it, because RLGs already had a very long service life, and the cost difference made the increased service life a moot point.

      Since then there have been several significant improvements made to Inertial Systems. There are no Fibe-Optic Gyros (FOGs), that replace the HeNe filled, precision glass block with fiberoptic lines using laser diodes. This significantly reduces size, weight and cost. When I was an engineer at Raytheon, we had a system that used a FOG Inertial Measurement Unit (IMU).

      The IMU is like the sensor package inside an IRU, and does not supply refined data, such as Heading, Pitch, Roll, Acceleration, etc. The IMU only provides raw data that must be converted by an onboard computer.

      Another technological improvement is the use of ‘gyro chips’, which are solid-state accelerometers. These have become very common on modern aircraft that use a solid-state, Integrated Standby Flight Display (ISFD). It works on the same principle of an IRS in the ‘ATTITUDE mode’, supplying raw pitch and roll acceleration that is used by an internal processor to provide attitude data to the built-in Liquid Crystal Display (LCD).

      These ISFDs are used as a third attitude source for Cat IIIb-equipped airplanes. Although the attitude information is accurate, accelerometers are unreliable for determining heading.

    • Yes, but LORAN required multiple land-based transmitters to determine position. The stations required large amounts of real estate for the HUGE antenna arrays required for Very Low Frequency (VLF) transmissions. Additionally the transmitters were EXPENSIVE to maintain and operate.


      • The US Navy still uses VLF to communicate with underwater craft. Those signals can be used for navigation, but are not certified as stand-alone navigation sources. Some OMEGA Navigation Systems (ONS), such as Canadian Marconi were able to use VLF communications to augment the position fix, but they could not be used without at least three ONS stations.

        One problem with OMEGA/VLF is that the wavelength is HUGE (4 Miles), and they have a very high Required Nav Performance (RNP), even when compared to an inertial only system

  2. I tested three military grade INS units with GPS upgrades in a DC10-30 with an FAA certification manager on board in1995. The units were integrated so that they “TALKED” to each other so that the two that agreed would disregard the third. After 2 1/2 hrs. of flying (In Mexican airspace) and visually flying low over a VOR I landed and the position was verified. Two of the units were within feet and the third disagreed by 500 feet. I agree with navigation awareness by all available means, but there are close to non sabotage units available.