Wednesday, January 4, 2017

GPS Anti-Jamming

We’ve discussed the vulnerability of US military platforms and weapons to GPS denial (see, "GPS Jamming").  Now, here’s the flip side of the discussion – the GPS anti-jamming capability which may enable GPS signal utilization even in the face of GPS jamming.

Here’s an interesting summary of the GPS problem.

“On Earth, the GPS satellite signal is received about 30 decibels below the background noise level. This translates to a signal strength of about 1,000 times weaker than that of thermal noise normally inherent in electronic equipment. Conventional GPS digital signal processing allows receivers to pluck these signals out of the background noise. When interference raises the level of background noise, however, a receiver may be unable to track the GPS signal.

This problem is especially acute with low-end receivers using omnidirectional antennas. These antennas lack the ability to provide directional discrimination away from sources of interference and toward satellites. Interference sources can be narrowband—affecting only a small part of the overall GPS frequency spectrum—or they can be broadband, affecting the full GPS spectrum. The civilian GPS arena is particularly susceptible to narrowband interference, as it occupies only 2 megahertz of spectrum. Military GPS is spread across 20 megahertz.” (1)

Thus, the fairly concentrated and very low signal power of GPS allows for easy, low power jamming.  So much for the problem.

One solution to defeating GPS jamming is to enhance the receivers signal processing, allowing it to directionally align with GPS satellites and to process the signal to enhance it.  An example of this approach is Lockheed Martin’s GPS spatial temporal anti-jam receiver (G-STAR).

“G-STAR currently is effective against a wide range of GPS jamming environments. Its software-driven nature also permits easier upgrades as new threats emerge. The version that is equipping JASSM consumes only 52 watts of power. It weighs 25 pounds and measures 10 inches by 15 inches by 2 inches.” (1)

“G-STAR also will incorporate the so-called selective availability and anti spoofing module (SAASM), which the Defense Department has made mandatory-beginning in October 2002-for all military GPS receivers that receive the encrypted precision satellite signal.

The SAASM module is a microelectronic device with a large number of digital components that allows an authorized user to receive the precision encrypted GPS signal.” (2)

Here’s a little more detail on GPS anti-jamming for those of you who are interested in a slightly deeper dive.

“Most anti-jam devices currently in use are either "nulling" or "beamforming systems," explained Kelly [James J. Kelly, director of advanced engineering at Telephonics Command Systems]. These two technologies refer to exploit techniques that can be used to counter jamming signals. Some GPS receivers have an antenna array, with up to seven receiving elements arrayed in a geometric pattern. Upon detection of jamming interference, part of the antenna pattern can be turned down, so the noise from that particular direction does not interfere with the rest of the system. That is called nulling the signal.
"A null means that I will not look in the direction in space that the jammer is coming from," Kelly said. The electronics protect the receiver by eliminating the interference signal. One problem with this nulling technique, however, is that "as you eliminate jammers, you eliminate your ability to receive signals from the GPS satellites," he said. "You could have a nulling system that kills off the jammers, but you no longer have enough satellites available for you to navigate."
The beamformer, meanwhile, "doesn't care where the jammers are." The beamformer selects and receives signals from at least four satellites and provides four anti-jam solutions. "The beamformer algorithm is more aggressive and you get a better result," Kelly said. Recent simulations conducted by Telephonics, he said, showed that beamformers performed better than nullers.
But these systems cannot be used with older GPS receivers. Because a beamformer produces four outputs directed at four selected satellites, it cannot interface with a standard GPS receiver that only has one input. "You need a receiver that is customized to accommodate the beamformer interface," said Kelly.
New missiles typically feature a tight package composed of an anti-jam device, antenna and GPS receiver. This makes them more adaptable for beamforming anti-jammers, Kelly asserted.” (2)

That the military is working on GPS anti-jamming is good news.  It offers the possibility that we may be able to continue to use GPS even in the face of electronic countermeasures.  Of course, we also have to recognize that the entire GPS satellite system is vulnerable.  Russia and China both claim to have anti-satellite weapons and China has pretty well demonstrated their capability.  If an enemy can destroy our satellites then having an anti-jamming capability won’t mean anything.

It’s also not enough to simply develop an anti-jamming device that can theoretically work.  We need to subject whatever is developed to rigorous testing – far more rigorous than the testing we’ve subjected our other weapon systems to.  We need to throw the best jamming capability we possess against it and see if it actually works in a combat environment.  Ideally, we would clandestinely test it in the real world in places like Ukraine, against actual Russian ECM.

Finally, we can’t simply develop a device, congratulate ourselves, and think that our navigation is secure.  Our enemies are continually working to develop new methods of GPS denial and we may find out the hard way that our anti-jamming isn’t as effective as we thought.  This means that we have to keep working on alternate navigation methods.  Every ship, plane, and missile should have multiple navigation systems so that if one is denied we have an alternate available.  In other words, we have to plan for failure – something we have not done in recent decades.

Inexplicably, not everyone is searching for GPS alternative capabilities.  Notably, the Air Force has limited interest.  Terry Little, Air Force program manager for JASSM, has this to say,

“In the JASSM program, we are not interested in an alternative guidance technology to GPS.” (2)

Well, that’s about as plain a statement as you could want.  Hopefully, that philosophy is not indicative of the military as a whole.

I’m encouraged that the military is recognizing the vulnerability of GPS and is working to protect the capability.  Given the ease of jamming and the physical vulnerability of the GPS satellite system to anti-satellite destruction, we need to continue working on alternate guidance systems. 


(1)Signal AFCEA website, “Jam-Proof Signals To Guide Navigation”, Robert Ackerman, November 2001,

(2)National Defense website, “Threat to Satellite Signals Fuels Demand for Anti-Jam Products”, Sandra I. Erwin, June 2000,


  1. The GPS system should be pretty safe from any tactical ground based attack due to its average altitude being roughly 20,000 km. The Chinese anti-sat mission in 2007 was conducted at only 900 km.

    1. An excellent point. On the other hand, China is pursuing co-orbital anti-sat, micro-satellite kinetic kills, and energy weapons so we should still be very concerned. Presumably, a single rocket could release dozens of micro-satellites at the GPS orbit height.

    2. Minor point: there are six GPS orbit planes, and I think you need about 3km/s of delta-V to get from one to another, so one flock of micro-sats couldn't get them all. Would probably need six launches. Still, risk is there.

  2. That JASSM article is 16 1/2 years.

    I found a more recent article at Defense Industry Daily that quoted Air Force Captain Phillip Atkinson as saying, "One of the emphasis items is to be able to operate in contested and degraded environments… and we have demonstrated the ability to operate with intense GPS jamming. Even without GPS, the JASSM can find its target due to its internal sensor."

    JASSM uses a combination of GPS/INS for guidance en route to the target area with Imaging Infrared (IIR) for final targeting.

    1. I think you may be missing the point. INS is not, generally, a sufficiently accurate guidance system espeically given the range of the JASSM/JASSM-ER which is 200+/500+ miles. If it were, we wouldn't even have GPS. Unfortunately, tiny INS errors add up over hundreds of miles.

      The AF apparently believes that their GPS is secure. That's the kind of overconfidence that loses wars!

    2. It's been over 20 years since I last worked with INS, but I'll bet today's are accurate enough to get JASSM to the target area for the terminal guidance system to take over. And, I'm sure they will be even more accurate in the future.

    3. I'll repeat my question: if INS is so good, why are we bothering to develop GPS anti-jamming capability? The logic seems inescapable: INS is not sufficiently accurate.

      Here's a more specific question: if INS is so good, why are the Russians bothering to develop GPS jammers and employing them as terminal defenses?

      You can do all the betting you want but the scant evidence we have strongly suggests that all military parties know that INS is insufficient for today's targeting purposes.

  3. Actually as I understand it INS works better with fast movers. These things tend to go out over time rather than distance. And the motion and “change in motion” of a fast moving object is more easily detected than the steady slow drift of a slow cruising ship ( or similar ).
    Nowadays most duel navigation is Inertial with GPS update when available, this works well with a long range missile as the GPS is unlikely to be jammed continuously along the flight path and will update INS periodically along route.
    Any inaccuracy will likely just be in the terminal phase, And at 500kts +, this likely just equates to 10 minutes or so of pure INS navigation to target.
    Then many systems will have a terminal guidance system separate to GPS and INS to ensure the last phase precision we are looking for.

    1. None of us know enough about the actual performance of weapon guidance systems to comment authoritatively. That just leaves semi-informed speculation. Nothing wrong with that - I do it all the time! However, I would pose this one, simple question: if INS is so good, why did/are we pursuing GPS? The logic seems inescapable: INS is not sufficiently accurate.

      Here's the same logic question from a different angle: if INS is so good, why are we bothering to develop GPS anti-jamming capability? The logic seems inescapable: INS is not sufficiently accurate.

      Yet another version of the same question: if INS is so good, why did we bother to develop laser guided weapns? The logic seems inescapable: INS is not sufficiently accurate.

      The last question/answer also suggests an explanation. If the few miles of travel for a bomb can't be made accurate enough by INS then that suggests that sufficient drift occurs (wind, turbulence, thermals, etc.) in the terminal approach to warrant a more accurate guidance system and that the use of INS/GPS to just get into the vicinity of the target is irrelevant.

      Also, remember that fixed targets are generally hardened to some extent or are co-located with civilian facilities that we don't want to damage. A miss of only a few meters is often a complete failure due to lack of damage or unacceptable collateral damage. INS seems inadequate for the degree of accuracy we seem to want.

      Also, you're undoubtedly correct that GPS isn't going to be jammed the entire length of a thousand mile flight path. Of course, no one cares about jamming the entire length. It only needs to be applied towards the end to be effective. At what point that is, I don't know.

    2. Perhaps the interest in GPS stems from a requirement to re-target a missile in flight and or maneuver around threats as they pop up.

      Pure conjecture on my part, but I bet current INS is great for point A to B to C, but can't handle dynamic changes to flight plans well.

    3. This is how I look at it.

      GPS gives you precise P(t), position at any time.

      And, we know,

      P(t) = P_0 + V*t.
      V(t) = V_0 + A*t.

      INS gives you precise A and t, but you need to know what V_0 (initial launch platform speed) and P_0 (initial launch platform position) to back out V(t) and subsequent P(t). Thus, when you have both GPS and INS, the initial position P_0 can be coordinated, or even position updated/redundant checked at realtime. Without GPS, its all 'map, math and stopwatch' projection with INS.

  4. So the question seems to be come down to what the detection window for a modern IIR seeker looks like. What is the lateral spread that the seeker can search when it arrives at the range at which it can detect its target.

    Assuming a stationary target, is that window five miles wide? 20 miles? 2 Miles? And how far from the last GPS update will it take the missile to drift that distance on just INS.

    Looked at that way, it seems to me that the problem is not insurmountable.

    1. I think you're oversimplifying a bit. Or, maybe you're not but you're just not saying it explicitly. The issue is not the field of view - it's the ability to identify a legitimate (THE legitimate) target out of a continuous spectrum of infrared signatures. Everything has an infrared signature. Some things are hot, some are cold, and some are in between. A missile seeker has moments to identify its intended target from among a huge, continuous field of signatures. Is that a flare, a fire from a previous hit, a vehicle engine, a person, a building, hot sand? How does that compare to the valid target? A missile can arrive exactly at its target and still pick out the wrong IR signature or fail to find the right one.

      If the target is a ship and it's the only one in the area, then the identification is easy. If the target is a particular building on a street full of buildings, vehicles, etc., then the identification is a challenge even if the missile arrived at the exact place it needs to be. Ten feet to the side and maybe the missile picks out the wrong building and we blow up a hospital or apartment. There's a reason why we use laser guidance with a continuously painted target. GPS/INS/IR just isn't accurate enough in some circumstances.

      Do you see that the problem is a little different and more challenging than you have described it?

  5. I do see the point, but it makes me wonder wonder what happened to the old tercom guidance, which seems much better suited to the scenario you describe. Did they just believe that GPS plus IIR was enough? And if so, how hard would it be to go back?

    1. The major disadvantage of TERCOM is that it requires pre-planned navigational routes. If a sudden target appears that is not part of a pre-planned route, there is no ability to engage it.

      Today, we want the flexibility of retargeting in flight and rerouting. TERCOM can't handle that.

      TERCOM also assumes that we can actually map out the planned flight path. If we want to launch missiles into Russia or China, for example, we may well not have the ability to generate a pre-planned route because we don't have access to the required mapping.

      Under the right circumstances, TERCOM was quite effective but it just wasn't flexible enough to be generally useful.

    2. Memory chips are many, many orders of magnitude more capable and cheaper than used in the '80's TERPROM and would not be surprised if current memory chips able to hold detailed TERPROM data maps of continents.

      With appropriate software and algorithms see no problems with flexibility in re-targeting on the fly as LRASM is claimed to be capable of with its IN,GPS,RF, ESM & IR sensors.

      PS Quality of TERPROM data - The ESA Copernicus Sentinel-1 twin satellites are able detect subtle surface changes – down to millimetres. In the news December after confirming size of tilt of San Francisco's Millennium Tower skyscraper:)

  6. The assumption should be that for periods of time, there will be no GPS and perhaps against a major nation state, the loss of GPS satellites (ex: perhaps no GPS).

    I'd imagine that most nations that have launched rockets have thought about anti-satellite warfare at the geosynchronous orbits. It would take multiple launches, but even if you don't get all of the satellites, you might be able to get most of them around the key orbits, rendering the whole system inaccurate.

    Military training and doctrine should revolve around that.

    Then there's the matter of even if the satellites are up, there is another problem. The other problem is that jammers and anti-jammers cannot be around everywhere. My bet is that it is probably easier to create a jammer. The other matter is that the enemy would probably try to counter anti-jammers. You need more anti-jammers than the enemy has jammers and they have to be at the right spot at the right time.

  7. Is it fair to use 15+ year-old articles to judge what our capability is today?

    The state of the art is advancing in GPS and INS systems. For example, the military is upgrading to M-code receivers that use a higher-power signal that is more resistant to jamming and interference. Also, M-code signals have advanced security features to prevent unauthorized access and exploitation by potential enemies.

    1. We use what we have available in the public domain. If you have detailed descriptions of current weapon navigation systems and their resiliency towards electronic warfare, by all means share them and we'll use them. Lacking that, we use what data and logic we have. I note your utter lack of response to my logic questions. I'll repeat them one last time:

      If INS is so good, why are we bothering to develop GPS anti-jamming capability? The logic seems inescapable: INS is not sufficiently accurate.

      If INS is so good, why are the Russians bothering to develop GPS jammers and employing them as terminal defenses?

      And I'll add: If GPS is so secure, why are the Russians bothering to develop GPS jammers and why are we bothering to develop anti-jammers and alternative navigation methods?

      The logic is inescapable. Both Russia and the US believe that GPS is not secure and that INS is insufficiently accurate.

    2. "The state of the art is advancing in GPS and INS systems."

      The state of the art is advancing in GPS jamming. If you're going to credit one side of the equation with advances, you have to credit the other as well.

    3. This is also in the public domain.

      From the website above, "The Precise Robust Inertial Guidance for Munitions (PRIGM) program is developing inertial sensor technologies to enable positioning, navigation, and timing (PNT) in GPS-denied environments. PRIGM comprises two focus areas: development of a navigation-grade inertial measurement unit (NGIMU) based on micro-electromechanical systems (MEMS) platforms, and basic research of advanced inertial micro sensor (AIMS) technologies for future gun-hard, high-bandwidth, high-dynamic-range, GPS-free navigation."

      This article describes different grade IMUs and their capabilities.

      INS are not intended provide end-to-end navigation for any weapon system. They operate in conjunction with GPS to cover gaps in GPS coverage. For brief periods of time (minutes), tactical grade IMU as pretty accurate.

      You're right in that for every system we have, our adversaries develop technologies to counter that system. Then we improve those systems and develop countermeasures to counter our adversaries technology.

      My point was that INS systems are becoming more accurate and I wouldn't so easily discredit their capabilities.

    4. "INS are not intended provide end-to-end navigation for any weapon system."

      "My point was that INS systems are becoming more accurate and I wouldn't so easily discredit their capabilities."

      You just agreed with everything I've said!!!! I didn't "discredit" INS, I simply stated that it is insufficient as a stand alone navigation system - exactly what you said!!!!

      Move on!

    5. I assume you understand how many DARPA (and industry in general) research projects never pan out? Every project sounds good on paper but few ever develop far enough to actually be beneficial.

  8. ComNavOps, are you familiar with the Kessler syndrome? If not its an interesting theory in regards to GPS vulnerabilities.

    1. I assume you're referring to the concept of cascading collisions? As I understand it, it requires an object density threshold and I have no idea whether the GPS orbits have enough objects. Interesting, though.


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