back to article Iran spy drone GPS hijack boasts: Rubbish, say experts

Doubts that Iran managed to bring down an advanced US drone over the country last month using an advanced GPS spoofing attack have been raised by experts, who say that attacks of this type would be extremely tough to pull off. Iran announced on 4 December that it had captured an advanced American remotely piloted spy drone, …


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  1. Steve Evans


    So spoofing is complex, but what would the drone do if it didn't have any valid GPS signal?

    Without it it would have to rely on dead reckoning calculations, with only air speed, bearing and duration to work with to attempt to find its location. It would be off track and lost very quickly indeed.

    In such a situation it could have a fail safe procedure, such as throttling the engine back and performing a controlled descent, or does it just keep zooming along in a lost style, hoping for a GPS lock, until it finally runs out of fuel and drops from the sky?

    If it is the former, then spoofing the GPS signal with all its accurate timing would not be required, you just need to drown the signal out with garbage which should be a great deal easier given the weak signal sent from the GPS satellites.

    1. SkippyBing

      It may not get lost as quickly as you'd think. Assuming the autopilot works with an inertial reference unit, if that was used as a back up nav system it should be enough to have you within a few tens of miles after crossing the Atlantic. Even if it was just working of the Air Data Unit and the last known wind that's normally enough to make sure you land in the right country*.

      I'd have assumed the fail safe procedure for a sensitive drone is to blow yourself up commit a terminal dive rather than risk being captured. After all they only got Gary Powers because he didn't take his cyanide pill.

      *Having been made to do this for 'practice' on a few occasions it's generally good enough to get you near something large and obvious. So a drone should be able to get near enough to a radio beacon it can see.

    2. Raz

      If I would design a drone, it will not be 100% dependant on GPS. Gyroscopes are very accurate these days and it should be able to know where it is based on that as well. When the GPS and gyroscope are sending conflicting information, it should trust the gyroscope. Tampering with it is a lot harder than jamming GPS, which must be considered as a probability in all conflict zones.

      1. Anonymous Coward
        Anonymous Coward


        You're forgetting changes in wind speed and direction.

        Without any external references points navigation quickly becomes unreliable.

        1. Martin Gregorie

          Thw wind vector is easy enough to calculate

          Holding a constant heading and airspeed for 20-30 seconds and comparing the result computed result with the GPS or an equivalently accurate inertial system, i.e. better than 5 metre accuracy, will also give a good enough windspeed to navigate by and is a method that can be fairly continuously updated. Alternatively, if either the inertial system or the GPS is working, you just fly 3-5 circles at a constant turn rate and measure the drift. Depending on turn rate, that need not take more than 5-10 minutes.

          All sailplane navigation programs use the circling method to calculate wind. They update the wind vector every time you stop to circle in a thermal, and at least one of them uses the straight line method and can get an acceptable accuracy from 10 seconds of hand-flown straight, constant speed flight.

          Don't forget the UAV will know its wind vector up to the point when the GPS signal gets jammed, so it should be able to get near enough to home to pick up an NDB or TACAN from its home field.

        2. Error Message Silver badge

          Norfolk 'n' Goode: So bad, you're not even wrong.

          Inertial Navigation Systems (INS) don't care about wind, etc.

    3. Error Message Silver badge

      What it does

      First of all, these systems don't rely entirely on GPS. They also have an Inertial Navigation System (INS) . INS's cannot be jammed, as they rely on nothing external to the aircraft to deduce location. While INS's are subject to what is called "integration drift" which means the uncertainty in it's location (according to the INS) increases with time, even after 24 hours of operation the uncertainty is enough for the drone to navigate back to base using INS. The INS can tell the unit if the GPS is being spoofed even if someone manages to spoof it, as the GPS location will differ from the INS location, and it's programming should give priority to the INS. These drones, if they lose the Command & Control link for a certain period of time, are programmed to return to base, and spoofing the GPS signal would not stop it from navigating its way back to base using the INS. One would presume that by the time it got back to base via INS, it would be out of GPS jamming range anyway. The fact that this drone did not fly back to base, assuming it did lose the C&C link, implies something malfunctioned.

    4. SDoradus

      Never mind spoofing

      Why is it assumed that GPS spoofing is the only way to hack a drone? The use of unencrypted control channels and even Windows as a control interface has been widely reported.

      1. Anonymous Coward

        Inertial Navigation.

        I didn't think of that.


  2. William Boyle

    Difficult != impossible

    This sort of exploit may be very difficult, but with some luck, may not be impossible. After all, think about all of the computer system exploits that have been declared "next to impossible" that have been successfully carried out...

  3. Tom_


    Speculation from me, that is...

    I believe GPS works by listening to a synchronised time signal from multiple satellite sources. You work out where you are by how far out of sync the signals are when you hear them. They're out of sync because you are different distances from the various sources.

    Does this mean that the GPS spoofer could send the drone multiple signals from just one single transmitter, deliberately out of sync by the required amount to fake the drone's position?

    ie. The drone doesn't know what direction the GPS signals are coming from, only that they are out of sync. It reverse engineers the delays to work out its location relative to each signal source.

    It seems there would be no need for the attacker to use multiple antennas.

    1. SkippyBing

      GPS works by comparing the difference in transit time for signals from multiple satellites. This requires really accurate clocks in the satellites, really accurate knowledge of where the satellites are and fairly accurate clocks in the receivers.

      Spoofing requires the transmitter to pretend it's a satellite and send out suitable rogue information, this isn't that hard and in fact some systems that improve GPS accuracy do exactly that just without the rogue information bit. The problem is even a lot of commercial GPS systems now have redundancy built in so that they use more satellites than required and switch between them in turn to check for a jump in position that would indicate an error in the signal.

      For more on how GPS works try google, it really is very clever, especially the bit about using the special and general theories of relativity.

    2. Flocke Kroes Silver badge

      All correct, and ...

      Stone age GPS units had a tiny number of correlators and antennas that rejected left hand circular polarised signals. The satelites transmit right hand circular polarised signals, which get reversed if they reflect off something. Reflected signals arrive late, and made the GPS units report a position further from the satellite than reality. Also, when first switched on, the GPS would pick two satellites on opposite sides of the plannet, divide the correlators between those two, and give each a different guess at the time delay. If they found nothing, they would try other guesses or other pairs of satellites until they eventually guessed right.

      Bronze age GPS units had lots of correlators. They could assign dozens to each satelite and find them from a cold start in seconds - unless the satelites were hiding behind buildings.

      Modern antennas accept both left and right hand signals. This allows the GPS unit to get a signal when the direct path to a satellite is blocked. Modern GPS units expect to spot the satelite signal and multiple reflections of it. They assume the earliest signal is the right one, and the others are reflections. This allows them to remain locked on even if a satelite is temporarily hidden. Even if the GPS does lose its lock on a satelite, it has hundreds of correlators available to get it back again in under a second.

      If you try to spoof a modern GPS, it will have enough information available to know something is wrong, but may not be programmed to recognise the problem or do something sensible about it. After all, the Americans know beyond all possible doubt that spoofing will always be well beyond Iran's technical abilities no matter how much GPS technology has changed since the first attempts to spoof them.

    3. Gary B.

      Title not required

      The GPS receiver needs to have some idea of (a) which satellites it is receiving timing information from, and (b) where those satellites actually are in the sky, so it knows how to triangulate its position. I would think that is why multiple antennas are required, to one for each of the GPS satellites that are being faked.

      Also, I would find it hard to believe that a SPY drone wouldn't be designed in such a way to be able to continue being useful even in the event of a localized disruption of GPS. If I were designing such a drone, I would certainly take into account that just maybe the people I'm spying on might not be too keen on the idea and want to try and disrupt my spying in some way.

      1. Charles Manning

        Spoofing only needs one antenna

        The receiver is receiving everything over one antenna and has no notion of direction etc. GPS rebroadcasters (used by GPS developers) receive the signal from all the satelites and just amplify and rebroadcast from a single antenna.

        Having dabbled a bit in the GPS signal arena, I was quite surprised with the initial claims. While it is simple to swamp a GPS signal (because they are so weak), getting the timing right is a huge challenge. Some commercial level receivers designed to handle huge multipath (like the one in your Tomtom or phone) would perhaps be more forgiving, but I'd expect drones to use a top-shelf GPS or even a military grade GPS which have far tighter timing restrictions.

        Spoofing commercial GPS is not impossible, but would be a hell of an accomplishment. Much simpler to chase the drone with a fast helicopter and chuck a net over it.

        1. a_been
          Thumb Down

          "Spoofing commercial GPS is not impossible, but would be a hell of an accomplishment. Much simpler to chase the drone with a fast helicopter and chuck a net over it."

          Your taking the piss now i assume. Forget about how fast the drones average speed is for a moment (hint: to fast) and just think about altitude, then take a brick to your head and see if you can fly that high.

          "The unnamed Iranian boffin told Christian Science Monitor that Iran developed the attack after reverse-engineering previously captured or shot down US drones, and by taking advantage inherent weaknesses in the GPS navigation system."

          This should tell us all we need to know about the GPS spoofing and Iran and by that I mean, the Iranians never made the claim.

        2. Field Marshal Von Krakenfart
          Black Helicopters

          @Charles Manning

          I’m inclined to agree with you, I can’t see how it would be possible to capture a drone by tampering with the GPS signal, first you would have to capture the signal from same GPS satellites (at least 4)that the drone is using to establish position, rebroadcast the signal quickly enough so that the drone does not break lock, then you would have to start 'adjusting' the signal to change the drone's location (a bit like range gate stealing).

          Remember that in order to force the drone down you would then have to move it’s reported position a couple of hundred miles _relative_ to it’s proposed landing site, and you have no guarantee that the drone will be programmed to land at the same site it took off from.

          Another consideration is what would the drone be programmed to do if its GPS position suddenly shifted a couple of hundred miles, I know I would build in some sort of cross reference between the INS and the GPS to ensure the two system are in agreement with each other.

          All in all, the GPS spoofing attack seems highly unlikely, I suspect that some other attack vector was used to bring down the drone and the GPS story is just FUD.

          1. Field Marshal Von Krakenfart

            In fact as I am typing this I recall several incidents where commercial aircraft suddenly departed from their planned route because one of the pile-its in cockpit programmed the INS with the wrong waypoint coordinates.

            Tech-grunt: "bomb ayatollah assahola airfield, sorry I though you said land ayatollah assahola airfield"

  4. Mondo the Magnificent

    Rock the Casbah!

    This entire drone loss/capture thing has been quite interesting

    The U.S. are playing it down, whilst the Republican Guard use it as a mighty propaganda tool.

    I wouldn't be surprised if the Iranians did cause it to crash with some sort of jamming device, they're a fairly canny bunch and quite self sufficient militarily.

    Due to sanctions the Iranians have had to develop a lot of home brewed tech (A little like South Africa during the Apartheid / Sanctions era) however we know the Iranians do get more than a little help from the Russians and Chinese in respect to military technology, as long as it doesn't break the UN sanction laws.

    Needless to say the Russians and Chinese are probably jumping up and down to get a look at the RQ-170 "if" it's genuine.

    Looking at the drone on the video, it's hard to make out whether it's genuine or not, however a lot of sceptics claim that the drone on display is a fake. One comment I recall was it "looks like a carnival float"

    One site I visited had extracted decent resolution pics from the HD video of the Iranians showing the drone off.

    Notable observations were the fact that the wings swoop the wrong way, the video seemed to been filmed in a gym (wooden floors, high windows and small double doors in the video) and also that the top access panel seemed to have been "drawn with a sharpie" or marking pen as we know them.

    But the most noted observation was the condition of the craft itself. Remarkably good indeed, although there was signs of scraping along the leading wing edges and tape on the wing joints, where some believe the wings may have been shorn off in the "crash", or perhaps removed to fit the RQ-170 into the building where it was filmed.

    Lastly why did the Iranians go to such lengths to conceal the underside of their captured prize?

    On the tech side, the RQ-170 is not a new aircraft, from what I've read it's been in use for a number of years, so it's not as if the Ayatollah's men are in possession of something that is blazing with the latest U.S. technology, or are the Pentagon trying to water down the importance of their drone?


    1. Paul Johnston

      If it's a fake?

      A couple of points come to mind,

      Why would the US ask for it back and not just say it's a con job?

      How are you sure the high resolution pictures proving this were not themselves faked?

      Just a thought

      شب بخیر

      1. a_been

        @Paul Johnston

        Don't confuse people with logic, it always ends badly.

      2. FrankAlphaXII

        The whole story doesn't make sense

        Ive been thinking the same thing since we handed them, erm excuse me "lost" this "RQ-170". They base the RQ-170 out of the place the mainstream press refers to as "Area 52" because the security and secrecy around that facility and the 3rd Reconnaissance Squadron which operates from there is about the same as Area 51 was before World+Dog found out about it.

        Because of the circumstances surrounding the RQ-170 and Sentinel project, I find it very hard to believe that we just let one of these things fall into enemy hands. The US Air Force has been known to bomb crashed secretly configured Aircraft in places that said bombing was very likely to have repercussions, like Laos and Cambodia before Vietnam expanded.

        It just sounds an awful lot like a deception operation in the same vein as Operation Mincemeat during World War II. We give the enemy a ruse and let them think its the real thing so they wont believe it when they actually do find something later.

      3. Also McFly


        >Why would the US ask for it back and not just say it's a con job?

        Didn't the US report a UAV lost to a malfunction? Even if the video is faked to make Iran look like super hax0rs, there's still the matter of the *actual* wrecked drone.

        I thought most drones had pilots flying them remotely, shouldn't someone have noticed one of them drifting off course and given it a destruct signal of some sort?

    2. Matt Bryant Silver badge

      RE: Rock the Casbah!

      "......they're a fairly canny bunch and quite self sufficient militarily....." No, they're anything but. When it comes to tech, they are just into copying or modifying old tech or buying it in from Russia, China or North Korea. All their much-hyped "developments" are obvious jokes to anyone with a clue.

      The perfect example is the Iranian "Saequeh" jet fighter, which is very obvioulsy just an old Northrop F-5A with the single-fin replaced with twin-fins. The F-5A was designed in the '50s for the US military aid program, and was deliberately limited in capability to give friendly Third World countries a barley supersonic fighter that was capable of reasonable air-def, without giving them the strike ability to threaten neighbours with. It has been ridiculed as "having the range to carry a matchbox the length of a football field", yet the Iranians claim their Saequeh has a mission range of "3000km" and can match the ability of an F/A-18! F-22 pilots should have no trouble clearing them from the skies.

      Their indigenous MBT, the Zulfiqar, is nothing more than a hodge-podge of old US M-60 and Soviet T-72 parts, neither being tanks of particular note and definately not viable on the modern battlefield. The fire control system is also an import from Slovenia and about twenty years behind NATO designs. In a fight against even the 90's tanks that kicked Saddam out of Kuwait, the Iranian tanks would be cannonfodder. Against modern NATO tanks they would be little more than target practice.

      Their missiles are all imports - the Shahab ballistic missile family they hope to put a nuke tip on is nothing more than a licensed copy of the North Korean Rodong 1, and they had the Norks set up their missile factories for them. The Rodong in turn is just a developed copy of the ancient Soviet SS-1 Scud. The Fajr missile the Iranians claim has MIRVs with radar-avoiding technology has never been seen to launch successfully, let alone deliver MIRVs, and is again a copy of Soviet '60s tech sold to Iran by the Chinese.

      Iranian drone tech? Their best would be the Karrar, another hodge-podge of Western and Soviet tech in a frame copied from the MQM-107 target drone from the '70s. Again, the Iranians have made many wild claims for capability but have never demonstrated any of them. At best, the Karrar would provide amusement for NATO interceptors and SAMs. Other Iranian drones include the Ababil, a poor copy of the Predator, only with less than half the capability. Of the three Ababil drones launched by Hezbollah all three were shot down by the Israelis in short order. The Iranians did attempt an Abibal flight into Iraq in 2009, but it was shot down by an F-16. The other "mass-produced" Iranian drone is the Mohajer, a toy that has only been recorded making an operational flight once, when one was flown into Israel by Hezbollah, but crashed into the sea before the Israelis could shoot it down, so hardly a success story.

      The Iranians like putting out propaganda about amazing Iranian military developments, but they all prove to be nothing more than copies and old tech. A perfect example of this blatent lying is the Hoot "supertorpedo", which is just a copy of the Soviet VA-111 Shkval, again more 60's/70's tech. The Iranians flatly refuse to admit it is a copy, even though Russian experts have pointed out design points that could only have come from the Shkvall. This propaganda has two purposes - to fool the Iranians into thinking they have a technologically capable and proficient economy, and for fodder for those in the West that want to believe.

  5. Anonymous Coward

    Ofcourse its "rubbish"

    Because if you say it isn't rubbish you're basically admitting "defeat" and we can't have that, can we?

    Learned absolutely nothing here. Same as in Iraq and Afghanistan when locals (for crying out loud) shared stories how they managed to bring down aircrafts and helicopters. "Nonsense", the boffins said deeming it "impossible" and then the pictures came out.

    The first situation (not ridiculing it right away) could have some negative effects (morale perhaps?) but then again most people and countries realize that they're not invincible nor immortal. So I'd say its neglectable.

    The second scenario on the other hand seems much worse to me. Experts and such are totally convinced that the news is fake and use strong words to brush it all away. And then it turns out to be true.

    Maybe I'm seeing things a bit too simplistic, but to me it makes those "experts" look like a bunch of idiots right then and there. And I'd immediately start to question every other thing coming out of their mouth.

    Personally I think that scenario is much more damaging; it doesn't necessarily affect morale (perse) but sure puts dents in ones credibility. And sometimes that's just what you need to keep morale up.

  6. Yet Another Anonymous coward Silver badge

    Counter rotating spin machines

    The quoted document is correct in that it's very hard to spoof GPS in such a manner that you can undetectably take over a genuine GPS signal and shift it without anyone being able to tell - as in a Bond movie plot.

    Simply sending a fake GPS position is trivial - we rent a test range to test our equipment that can generate a GPS position to be anywhere on earth (they tend to be very careful about not having the signal leak out!)

    The drones do have an IMU in addition to the GPS - it's very difficult to land if you don't know pitch/roll/yaw - but they drift and so you update them from the GPS. You can put a super duper accurate IMU but it's expensive and is exactly the sort of tech you don't want the otherside to get their hands on when you do inevitably lose one. For the same reason this only has fisher-price levels of stealth.

    There are a couple of reasons for not putting a massive self-destruct charge onboard triggered whenever it loses GPS. One is that effect on the life expectancy of USAF squaddies servicing it, the other is that it does tend to look a wee bit threatening. If a 'peacefull' Iranian drone exploded over the London olympics because it lost GPS that might result in slightly more than a letter to the Telegraph.

    1. Anonymous Coward
      Thumb Up

      Interesting supposition that it is essential to prevent the drone losing GPS lock, as if there were a human on board who would think "that's strange, before we lost lock we were 14 miles from the coast of China, now it says we're 20 miles" (*)

      Setting aside that it probably was incompetence, if an "attack" were to be made then It may well be enough to *force* a GPS loss-of-lock with any old jamming and then broadcast a fake Ephemeris at a higher strength than the actual signal.

      If the receiver has not been programmed appropriately with how to handle a temporary loss of lock, it would happily start using the "reacquired" spoof signal and never know ...

      Oh, by the way, I was under the impression that the military signal *was* encrypted, which lends credence to the "incompetence not conspiracy" angle.

      (*) That was the plot of the Tomorrow Never Dies Bond film ... GPS spoofing by hacking the receiver itself I think ...

    2. Anonymous Coward
      Anonymous Coward

      There are a number of reasons to use drone surveillance aircraft; one of those reasons is that they are relatively expendable. I would hope that the military wouldn't put highly classified equipment in a drone for the simple reason is that they are susceptible to being lost and captured -- they were designed so that nobody would cry over a lost drone.

      Whether the item that the Iranians have been crowing about is an actual US drone or not probably makes little difference. If it is a real drone, the US now knows that they have a vulnerability that needs to be remediated. If the Iranians did spoof the GPS, whatever security classification the navigation equipment may have had was compromised before it was lost.

  7. MrT

    Interesting tactic...

    ...flood the area around a highly sensitive potential/likely target for, perhaps, cruise missile attack with the spoofed GPS signature of a US airbase miles away. Missile won't commit blue on blue, and comms will likely be disrupted, so cruise will loiter whilst other on-board systems (visual etc) confirm target area, by which time the thing has probably been taken out by SAM.

    Lobbing dumb iron on a ballistic trajectory would be an option I suppose - bring on the rail guns!

  8. LarsG


    Sgt Duffer and His ground crew forgot to top up the fuel tank. It ran out of super unleaded.

    Sgt Duffer is now a private stationed in Alaska. You wouldn't want to admit to that error would you.

    It's so simple it is painful.

    1. theModge

      You have to assume that they'd have some sort of fuel level sensor, however being as that it's maintained by Sgt Duffer's mate, who was drunk at the time, it's perfectly possible it was broken...

    2. Mark 62

      Sgt Duffer may have forgotten to top up the tanks but it would be Lt McDufferson flying the bird and ultimately responsible.

  9. Homer 1
    Black Helicopters



    Translation: "Doubts that Iran managed to bring down an advanced US drone over the country last month using an advanced GPS spoofing attack have been injected into the mainstream media by US government-led propagandists, in a damage limitations exercise (a.k.a. the "war of perception").

    Conspiracy theory?

    Think again:


    A secret CIA report, brought to light last month by Wikileaks, reveals the cynical battle plans for the "war of perception" being waged over public opinion in Europe about NATO's war in Afghanistan. The four-page document is well worth reading, mainly to see exactly how cyncial the powers-that-be are when assessing the public.

    The report's subheadings tell the story: 'Public Apathy Enables Leaders To Ignore Voters ... But Casulaties Could Precipitate Backlash', 'Tailoring Messaging Could Forestall of At Least Contain Backlash', 'Appeals by President Obama and Afghan Women Might Gain Traction'.


    1. Bumpy Cat

      Sure the CIA wrote it ...

      ... but don't you support womens right in Afghan anyway? Or is it the case that, because supporting womens rights means supporting NATO operations in Afghan, the correct attitude is "sod the women"?

      This is unfortunately the case with a lot of the Stop the War lot ... NATO is inherently bad, so anyone who opposes them is naturally good, so yay Taliban!

  10. Volker Hett

    Back in the days

    when I was in the german navy, we weren't allowed to track Tornados with our WM25 Fire Control System. (Mk. 92 in the US on the Perry Class frigates) because one was lost due to electronics malfunction when hit by the pulse.

    I do think that there are ways to down a drone short of GPS jamming, just use some very high powered land based STIR like Radar and burn out it's brains.

  11. Peter Simpson 1
    Thumb Up

    And satellites are usually located *above* the aircraft

    while spoofer transmitters are almost always (unless airborne) located below it. I suspect the drone's GPS antenna pattern is optimized for reception of satellites, rather than ground transmitters...

  12. Nux Vomica

    Flat spin = unscathed? (even relatively!)

    Hmm not sure about a flat spin leaving you unscathed once you hit. Your rate of descent in a spin is still far in excess of anything you would ever want to approach a solid object at. Have done one of these in a Chippy and the VSI was down against the stops (300 fpm), even though it didn't feel like it.

    Any real pilots care to comment?

    1. Martin Gregorie

      Flat spins have a lower descent rate than than a 'normal' spin, but I wouldn't know by how much. My gliding club requires that I spin a glider for recovery practise at least once a year, but I have never been in a flat spin and hope I never am: they are much more difficult to recover from.

      The only aircraft I've seen in a flat spin, using only my Mk1 eyeball to observe it, has been an F1A class model glider, which are deliberately spun to get them down out of the thermal at the end of a flight. I've seen these go into a flat spin many times when the spin settings are not quite right. As it transitions from a 'normal' spin to a flat spin its descent rate is reduced a lot while the rotation rate increases dramatically. After a short while the flat spin destabilises and becomes a violent tumble which may, if you're lucky, transition back into a normal spin. The tumbling descent rate is a lot higher than during the initial spin, and if its tumbling when it hits the ground you get a lot of damage.

      Flat spins, etc. may well do more damage on impact than normal spins because the rotation speed is so much higher.

  13. csumpi

    Trojan Drone

    aka 21st century Trojan Horse.

  14. John Smith 19 Gold badge

    How it *might* be done.

    For starters you'd need a radio base *above* the drone (It's GPS aerial/s are likely above the fuselage for best access to the sky). Note the receiver power levels (IIRC -160dBmw, that's about 2^53 below the reference power level) so "swamping" it from say a couple of Km above it is not going to take too much *real* power. Think car battery, not truck engine.

    Switch on the jamming and cut off the drone from GPS reception.

    Switch on the GPS simulator channels (You'll need at least 4 of them, on 2 channels) with a position *plausibly* close to where the drone thinks it is (from its internal IMU package) and start walking.

    Note the initial data string, tap positions and shift register length for the military code have (AFAIK) *never* been published. It has been stated (in the USG design docs) that each satellite uses a different section (IE different point in the sequence) of the *same* code with the same taps. The code per satellite repeats over roughly 1 week at 10.24MHz and is roughly 1/10 of the full capacity of the shift register generator. You have to think in terms of early 1970s TTL chips. Calculating its likely length is none of my business.

    The military code *can* be encrypted but I'm not sure how often it is (like Selective Availability for the C/A code it is *optional*). There is at least one paper in the open literature about it but the USG has never officially released it. Give how widespread GPS is the key is likely to be transmitted in the more frequently repeated parts of the 300 bit frames that make up the full 12.5min GPS data stream, allowing potential lock up in 6 secs rather than 12.5 (which is a *long* time in guided weapons).

    With fast enough logic it might not even be necessary to have your own atomic clock. Use the *real* GPS stream to work out your position, calculate the numeric offsets needed to push your target in the right direction and re-encode for for the drone to receive it.

    In an era of GHz logic a country with sufficient resources could get something like this made on a reasonable budget in the way the EFF finally built a DES cracker to *prove* once and for all DES was no longer secure. A fairly modest 2Ghz processor could run 200 instructions to process each bit sent.

    Easier than just shooting it down or jamming the GPS and hoping it goes into some sort of emergency landing safe mode. No.

    Within the financial and intellectual resources of a nation state with a substantial oil revenue. Definitely.

    It was said no way could Afghanistani Jihadists read drone video (despite it not being encrypted). This proved incorrect.

    Sun Tzu noted that those that show contempt for their enemies are likely to underestimate them until they ultimately loose.

  15. FreeTard

    "...conceding it was carrying out a covert spy operation over Iran. The US has asked for the return of the drone..."

    Love that tactic - just like when the Iraq war was being staged, when the "Allies" asked to allow spyplanes fly over Iraq - definitely not finding the tanks, artillery and stuff. :)

  16. brainwrong

    Mindless speculation

    The Iranians transmit some kind of gibberish at the command channel of the drone. Something trips a bug, and the computer resets. When the computer restarts, it thinks "FUCK! What's going on!" and promptly lands.


  17. Big Al

    Let's underestimate the opposition again!

    So... By its own admission the US lost a spy drone over Iran.... but continues to make the Iranians out to be technologically incompetent despite their recognised successes in satellite launching, stem cell research etc etc.

    Might be good enough for people who don't want to think, but it's not an attitude that will be very helpful going forwards.

    1. The last doughnut


      All it takes it the combination of a lot of clever hard-working and well-motivated Iranians, prolonged and repeated drone incursions, some unknown vulnerabilities in the drone systems, and a bit of good luck.

  18. Local Group

    What about Iran's BFF?


    " Moscow also wants to improve diplomatic and military ties with some of Iran’s rivals. The Russian Ministry of Defense is interested in purchasing sophisticated weapons from abroad that its arms industry either does not produce or produces poorly. IT HAS ALREADY BEGUN BUYING UNMANNED AERIAL VEHICLES FROM ISRAEL." (2010)

    Just sayin'.

  19. a_been

    Can we stop the BS

    So the CSM says "someone told us something" and some people say it's not true. Did the Iranians ever mention GPS spoofing? NO, how about the US? NO. All we have is someone claiming to have spoken to some Iranian dude in a pub.

  20. Neoc

    You're just not thinking fourth-dimensionally...

    Love that quote.

    I have a nasty mind, so it came up with this scenario:

    General: We know Iran has boffins in a secret lab near this area. How do we find this lab?

    Corporal: Sir, we could "lose" a seemingly high-tech piece of technology in the target zone and track it to see where it's taken.

    General: What do you propose?

    Corporal: Gut one of our spy planes from anything we don't want them to have, fill it with useless but interesting-looking tech, and have it develop a "fault" over Iran - a leaky fuel tank should do it.

    General: But would they fall for it?

    Corporal: If we go on record to say the Drone is not worth anything but proceed to ask for it back via the usual channels, yessir I think it would.

    1. JohnG

      Look here, don't look there

      Another possibility is that they wanted to keep the Iranians looking in one direction while something else happened nearby. Maybe somebody wanted to get some people in or out of a sensitive area in Iran and needed a diversion.

  21. JaitcH

    The more important thing is China and Russia have ...

    a working, almost undamaged drone with some of the most sophisticated sensors on board now sits in Tehran.

    Regardless of whether a desk-bound pilot or some Russian jamming equipment brought it down, it is Iranian hands and will likely be made available to the highest bidder. Likely something to advance their nuclear ambitions.

    I saw reports that over 30 drones have crashed in the past two years which demonstrates a systemic weakness.

    The U.S. is embarrassed over this loss and no doubt much of the chatter is an attempt to play down the significance of this loss and therefore is as reliable as Iranian claims.

  22. JMB

    Drones seem to fly mainly in level flight so won't the GPS antenna be directional to pick up signals from above. They could even incorporate a simple test for stronger signals from below which would indicate jamming and interference.

    I wonder if one crashed (presumably innards would self-destruct) and the iranians used the wreckage to make a replica?

    1. Ken Hagan Gold badge

      Re: Directional

      Last time I looked, you need to be listening to several GPS satellites to get a decent fix, and simple trigonometry says you want those to be dispersed around the available hemisphere. Then your drone might want to be able to perform banking turns in flight without losing a satellite for the duration of the turn, so that's "a hemisphere plus a little".

  23. Hollow
    Thumb Up


    ...Sadly the most likely circumstance, it's guidance computer was likely coded in Ruby, making it that much easier to induce a bug :D

  24. John Deeb

    integration drifts

    Some are claiming here the integration drift of INS's would not be enough to divert a drone being fed modified GPS signals. This all depends on the quality of the INS sensors which might be a cost issue too. But what people forget in the calculation is the cumulative effect of a maximum (false) GPS correction on all parameters (velocity, acceleration, speed) one that doesn't trigger any alarm.

    I think one would find that within a few hours or less of constant false GPS signals the cumulative drift would be significant enough to get lost. The drones are likely not designed to withstand a really long duration accurate false GPS signal. How likely it is Iran was able to feed the drone false but convincing GPS for any length of time remains open to debate of course. For sure they will not weaken their defenses by telling how they did it or if they did it at all!

  25. love not war

    Sgt. Duffer and the Storm

    Agree with LarsG, there are many plausible Sgt. Duffer scenarios that explain this. As he says, fuelling error, or something like a legitimate, but wrong, command and control signal from the US controllers easily explain this.

    It is even plausible that a real mission went awry, for example, maybe the drone was deliberately landed in an attempt to re-supply a special-forces unit, but was intercepted by Iranian ground-forces.

    Lots of other "natural" explanations work too --- hit by lightning or buffeting from turbulence causing some sort of system disconnection, for example.

    If it did crash, as long as it lands in relatively flat terrain a fairly unscathed body is not that surprising either. The drone will probably be designed to glide --- as this saves on fuel for high altitude, long-duration missions. And it is surely designed to be rugged enough to experience a hard-landing without disintegrating into thousands of pieces. It will be designed such that it can land on an improvised/poor run-way when being used by a unit in the field, for example.

    On the other hand, it would be foolish to assume that the Iranians hadn't figured out some method of causing a malfunction. And it would be equally foolish to assume that a propaganda statement (from either side) actually contained any truthful information about what this exploit was.

    1. Anonymous Coward
      Anonymous Coward

      Balance of Probabilities

      "If it did crash, as long as it lands in relatively flat terrain a fairly unscathed body is not that surprising either. The drone will probably be designed to glide --- as this saves on fuel for high altitude, long-duration missions. And it is surely designed to be rugged enough to experience a hard-landing without disintegrating into thousands of pieces. It will be designed such that it can land on an improvised/poor run-way when being used by a unit in the field, for example."

      What you are describing is a CRASH landing, emphasis on crash, impacting the ground at any speed good enough to generate lift will leave a significant dent in an aircraft with a multi-G airframe, let alone an oversized RC model plane.

      Factor in the US Gov will be wanting to play down any possibility of EW working against their UAV and UCAV's, and the balance has to come out as it was under at least partial control by the iranians (airframes do not land themselves, they tend to leave long large dents in the ground, for example Souix City, or any other ideal situation crash landing)


      if you think gliders are different, have a look at the state of the gliders from the "flat ground" at Arnhem, and they had pilots flying them.

  26. ARaybould
    Thumb Down

    What the Paper Actually Claims

    "We will show, for example, that any number of receivers can easily be spoofed to one arbitrary location".

    The problems considered in this paper are harder than simply persuading a naive GPS user (one that is not attempting to determine if it is being spoofed) that it is somewhere other than it actually is. They include such problems as doing so without losing lock, and spoofing a number of spatially-separated receivers such that their apparent mutual separation conforms with reality. They conclude that "spoofing detection based on lock loss has two disadvantages: (i) strong attackers can achieve a seamless satellite-lock takeover, and (ii) lock loss can occur due to natural causes" and so propose the use of multiple, separated receivers as the basis of a spoofing-detection mechanism.

    The actual content of this paper, therefore, does not justify the claim in the Register article's headline.

  27. alien anthropologist

    So the question has been answered...

    as to what happens when a drone gets a BSoD over enemy airspace...

  28. Jerry

    GPS 'n stuff

    I've started working on very precise GPS systems as part of a safety system. It's been an awakening. GPS is not that good.

    I use GPS & GLONAS combined and I see constantly how the receivers pitch the location all over the place. "20cm" units can have 5 metre excursions no problem as the constellation changes and the system has to start recalculating based on a new constellation.

    Hijacking the drone doesn't need 75ns accuracy. It simply needs a local transmitter that transmits high power signals with a known phase relationship - which is varied to suit the purpose.

    The receiver will keep on adjusting to the new constellations and in the meantime use inertial navigation to tide over the outages.

    All that's needed is the occasional burst of multispectral/phase signal to allow the drone to resync and point its inertial system in the required direction. That's why the landing was flubbed. Precise control is not possible - though why it didn't use a microwave ground sensor I have no idea.

  29. Grease Monkey Silver badge

    Discussing the technicalities of this supposed attack is missing the point. The Iranians initially claimed to have shot the drone down then changed their story. Any observer of propaganda shite will tell you that this means that it's extremely likely that neither story is true.

    Furthermore it doesn't take a genius to spot that were the Iranians capable of this sort of attack they would be doing it all the time.

    1. Ramazan

      @Grease Monkey

      "The Iranians initially claimed to have shot the drone down" [citation needed]

  30. Dave Bell

    "Tomorrow Never Dies"

    The James Bond movie with a plot that uses GPS spoofing (and a bit of hand-waving over encrypted GPS signals) is "Tomorrow Never Dies".

    The cast includes Michelle Yeoh, so maybe the McGuffin doesn't matter that much.

  31. peterb

    Shield the GPS antennas

    Couldn't they sort this problem out by shielding the underneath of the drone so that the GPS antennas can't receive signals from below (i.e. the ground) and only from above (i.e. space).

    1. Anonymous Coward
      Anonymous Coward

      @Pointing Antenna upwards

      It is probably impossible to shield the antennas sufficiently, as the angle to the satellites varies wildly and the receiver needs to read as many signals as possible to make a proper fix.

      That applies to fixed antennae. If you added moving, highly directional antennae (which track "their" satellite), your idea could work to some degree. Still, all antennae have more or less strong sidelobes which could be used for spoofing. I am also not sure anyone does "moving/directional" GPS antennae.

      To make a proper GPS fix you to receive need multiple satellites at the same time, so your idea would require multiple moving apertures. Probably too heavy and bulky...

      1. Anonymous Coward
        Anonymous Coward

        Electronically Steered Directional Antennae

        Actually, my post is not wholly correct. Lower-gain antennas can be made out of "electronically steered" receiver arrays (also used in electronically steered radars). Still, the signal from the satellites is very weak and as I said, every antenna has sidelobes, electronically steered or not.

        Said antennae would be much more compact and probably less heavy.

    2. Anonymous Coward
      Anonymous Coward

      "Couldn't they sort this problem out by shielding the underneath of the drone so that the GPS antennas can't receive signals from below (i.e. the ground) and only from above (i.e. space)."

      This is a country that has an airforce, it's own drone programme, and a enough of a space programme to be on the way to building credible IRBM's

      They can get higher than the drone if they want to, or force the operators to fly at such an altitude that they cant collect intel. either way they win.

      The US drone programme is based on the premises that the other side lives in wood huts, and doesn't have radio's let alone an airforce. For further details as to the incredible levels of stupity in US drones ops, look no further than the predators broadcasting un-encrypted live feeds in Afganistan.

  32. Anonymous Coward

    Disingenious TR Article

    Assuming there are not even five competent electrical engineers in Iran is quite stupid. Delaying a GPS signal by 75 ns can be done with some standard parts either ripped out of commercial electronic devices or bought in a country not under embargo.

    Just "losing track" of (say) twenty microwave transistors and twenty FPGAs would not be noticed in any Chinese or Vietnamese factory. Actually Iranian agents could buy it in Germany and send it by parcel to Iran with a high chance of success. Also, Chinese oscilloscopes can nowadays surely perform 1 Gigasamples/second.

    Finally, Iranian engineers are surely able to use a modern oscilloscope, a soldering iron and I am sure they understand the concept of a "nanosecond".

  33. Anonymous Coward

    Thinking About GPS Spoofing

    Apparently the GPS signal is not enciphered, so the practical problems boil down to this:

    A) A Microwave signal generator which produces the GPS signal and can be modulated to any desired GPS timestamp.

    B) An antenna+amplifier which will (locally) overpower the true signal from the satellites by a factor of ten. That will make sure the automatic gain control of the receivers will reduce receiver amplification and consequentially "see" only the bogus signal

    C) A control computer which will generate all the "proper" timestamps for the different "bogus" satellites which are simulated by A) and B).

    D) To test/debug the spoofing system the team ventures out into the next supermarket to buy a commercial GPS receiver. They have all the time they need to perfectionize/debug the spoofing system

    E) Then the whole thing is mounted into an F-14 stripped of all non-essential heavy stuff (guns, external tanks, second seat etc) to perform jamming onto drones which are optically acquired.

    That all sounds more like a graduate student project of a proper university than "leading-edge Science". One has to assume Iran does indeed have this ability.

    Regarding a possible Inertial Navigation System, I guess the Americans simply omitted it, as that would reduce payload seriously and of course generate significant cost. I guess they simply did not think rigorously about GPS spoofing.

    1. Charles 9

      Sounds interesting...

      ...but how do you pull off (C)? There's more than just timestamps involved. VERY precise timing would be needed to ensure each timestamp fires at precisely the right nanosecond (and I DO mean nano--electromagnetic energy can travel almost a foot--a good 25cm or so--in that short a time). Otherwise, the signals won't match up, confusing the receiver.

      1. Anonymous Coward
        Thumb Up

        @How To Do Timing

        Modern PCs operate at 3 GHz or more frequencies and the mainboards are nearly all made in capitalist China (aka. Taiwan).

        Creating a 1 GHz counter which triggers a signal generator inside an FPGA is clearly in the skillset of every competent EE engineer.

        Also, I am really not sure you need to be highly exact, as the receiver always has to cope with all sorts of errors and consequentially has lots of error-correction features which will be of great help - to the spoofer.

  34. Anonymous Coward

    Locating "Stealth" Drones Electronically

    I guess what the Americans also sharply underestimate is Iran's capability to locate their "stealthy" drones by the electronic emissions of the control uplink to communications satellites.

    All antennae do have sidelobes and even if a microwave antenna is pointed upwards, it will emanate significant energy sideways. That energy can be picked up by sensitive (ie. large) antennas on hilltops or in elint aircraft. Two bearings make a fix and then a fighter jet will be sent to investigate.

    Surely the drone could be programmed to be completely silent while in or near "hostile" airspace, but considering previous mishaps I would say chances are they did lots of chattering with the drone via satcom.

  35. Tom 7

    Well I dont know how the drones navigation works

    but I guess the eyeranians do now.

  36. Ben Holmes

    SPB Request

    Maybe the SPB could do some high-level research into GPS spoofing and report back to us?

    1. Error Message Silver badge

      Iranian disinformation

      The whole GPS spoofing thing is Iranian disinformation, just as the initial claim of shooting it down was, or the Iranians constantly releasing Photoshoped photos of their weapons tests. GPS Spoofing is irrelevant, even if is can be done. These drones all contain Inertial Navigation Systems (INS) specifically to tell how reliable the GPS is, and they can navigate entirely on INS.

      1. Anonymous Coward
        Anonymous Coward


        So you are just releasing information about a classified drone or what ? Or is it just your assumption that they contain an INS ? Whatabout weight and cost ?

        1. Error Message Silver badge

          No assumptions, no classified material

          Even cheap Joint Direct Attack Munition (JDAM) strap on kits that turn dumb bombs into "smart bombs" contain BOTH an integrated inertial guidance system (an INS) coupled to a Global Positioning System (GPS) receiver specifically to deal with GPS spoofing & jamming. And that's 20th Century technology. These drones are 21st Century technology.

  37. Atonnis

    Interesting... everyone seems to think it's extremely unlikely for a US-controlled aircraft to miss the target...

  38. scub


    any reason the drone can`t compare current position with magnetic north, thus giving a little added extra feedback? Or am I missing the point? Did the drone "if compromised" think it was somewhere it wasnt?

    Err, I seem to remember back when Iraq-USA were buddys, that Iraq protected the Gulf state from the Iranians. Someone like to forward a history lesson as to why they were a problem back then, and possibly a problem now. Not sure I`m up to date as to why Iran is seen as a problem. I`m guessing they are a little hard on their female population, buts thats just a guess, any other reason not to like them? Do they not conform to some form of imposed market model or something?

    1. Anonymous Coward
      Anonymous Coward


      Iran is A) developing nukes, B) making belligerent statements towards Israel, C) supporting anti-Israel militias like Hamas by arms and other things and D) has tried to disrupt the oil flow from the persian gulf.

      A) and B) and C) make them the #1 enemy for Israel and we all know they have lots of influence in US politics. Also, they fought lots of wars against their neighbours and are understandably nervous about any threat in the region.

      It has to be noted though, that Israel does have lots of nukes and delivery systems themselves. Iran's diplomatic position is just not improved by Ahmadinjad threating to destroy all of Israel more than once. If they were silent, they could get away with that. Their non-diplomatic rhetoric and their support of anti-Israel militias is their main problem.

      1. Anonymous Coward
        Anonymous Coward

        Iran-Iraq War

        That one was started over territorial disputes by Saddam Hussein when he thought Iran would be weak due to their revolution. It turned out that they were much stronger than expected and Iraq needed lots of help from their arab neighbours (who also can't stand Iran, has something to do with Sunnism/Shiitism) and Uncle Sam. The soviets also sold lots of heavy weaponry.

  39. Chris 96

    Shhot drone down ...

    its crashes into a million bits, then build a model from the countless pictures from the internet.

    Then you claim you are a world-leader in hacking (which is the current military meme) and proclaim that the world should bow to your 1337 5killz.

  40. Anonymous Coward
    Anonymous Coward

    “experts, who say that attacks of this type would be extremely tough to pull off.”

    Like more difficult than setting up and running your own nuclear weapons and rocket programme, using the best educated scientists and engineers in the Middle East?

    Or is it more likely the “experts” are the same DoD people who left gaping hole in flight control system?

  41. Neil Barnes Silver badge

    A thought...

    ...based on *no* knowledge of the plane in question whatsoever.

    It occurs to me that while it is probably theoretically possible - using a single transmitter of sufficient complexity - to recreate a valid fix on a single receiver, this falls over if multiple receivers on the same craft are used: consider for example a receiver at the end of each wingtip, and therefore several metres apart.

    In that case, in normal operation, suppose that two satellites in the constellation are aligned with the wings (by chance) but on opposite horizons. For one wing tip, satellite 'A's signal will arrive before that of satellite 'B' but for the other, the opposite will hold. Each will calculate a close but slightly different position - ideally, the 'true' position of its own wingtip.

    But this occurs because multiple satellites are used. If a single transmitter is used - or, significantly, multiple transmitters which are not in a similar orbital constellation and speed - then while one receiver can be spoofed, the other will *always* be in an error condition; because the signal it sees is simply the same signal as the other, delayed by some nanoseconds depending on orientation. The two receivers will always produce the *same* position.

    To be fair this is based on a number of assumptions:

    - that two receivers (or more) are installed

    - that they are capable of sufficient resolution

    - that they are sufficiently far apart to discriminate

    But with such assumptions, it would seem that it is easy to tell when someone is trying to spoof you and to take appropriate action (e.g. fly on EMU/scream for help/aim at the ground).

    Or am I ignorant of some critical fact?

    1. Anonymous Coward
      Thumb Up

      @Neil Barnes

      Your reasoning is correct if you only have a single signal source to perform the spoofing. But the spoofer could (at probably significantly higher cost) put several aircrafts into the air, with each simulating just one GPS satellite. The spoofing aircraft would have to be located a few kilometers off the drone to create an "innocuous" set of signals at the drone's multiple, spatially separated receivers. Also, the spoofing aircraft would have to move at the same angular velocity as the simulated satellites to make the signals consistent. A major problem of this approach would be proper coordination of the multiple a/c, especially without them crashing into each other.

      Also, the number of a/c to execute this would probably be in the order of 20 or so, because there are constantly "satellites rising above the horizon and disappearing beyond the horizon". A typical GPS fix consists of five GPS satellites or more. The complexity could be reduced by simply jamming some satellites with noise, but a minimum number of satellites must indeed be simulated or the receiver will lose its ability to generate coordinate/height vectors.

      I assume the drone simply had a single antenna, maybe accompanied by a backup 30 cm away of the first antenna, being in idle mode.

  42. Anonymous Coward
    Anonymous Coward

    @Neil Barnes, 2

    Also, your spatially-distributed antenna countermeasure would need some quite intricate software to detect that the signal sources are actually moving angle-wise. Straightforward GPS software surely does not care about the satellites moving or not. What this (non-spoof-hardened) software does is to take the position of the sates (x,y,z) and the timestamp as input for a quite simply geometry problem (intersection of circles). It will still properly calculate a position even if the sats don't move their x,y,z position at all.

    So if you ever design such a difficult-to-spoof system, make sure you record the position of the sats and check whether their trajectories appear to look like a real sat (and not an F14 doing craty maneuvers).

  43. DanceMan

    Simpler explanation

    After reading many of the highly technical and quite interesting explanations given here, I have a simpler suggestion: some klutz spilled his coffee on the keyboard back at the monitoring station. Or some variation therof.

  44. Anonymous Coward

    Celestial Navigation

    As the drones appear to be made for high-altitude operation, an obvious option should be celestial navigation. It is of course not as precise as GPS, but certainly much better than an internal navigation system, which adds up errors all the time. Also, it is quite hard to spoof the sun, moon and the stars...

    The soviets always built special windows into their long-range a/c to facilitate that.

    1. John Smith 19 Gold badge

      @Looking At The Clouds

      "As the drones appear to be made for high-altitude operation, an obvious option should be celestial navigation. "


      "It is of course not as precise as GPS, but certainly much better than an internal navigation system,"

      A presumption. Star tracking nav systems were developed by NAA Autonetic and their successor companies. They were used on the SR71 for precision fixes on the ground sites being photographed. The B2 also has one in it's upper wing. Some of the stars used are daylight visible (with the right filters) and a computer controlled system developed for US Army ground survey was accurate to c6m.

  45. Anonymous Coward
    Anonymous Coward


    There has to be a GPS or non-GPS authentification signal too no?

  46. Anonymous Coward
    Anonymous Coward

    Does it matter?

    The US have demonstrated not too long ago that they are capable of killing 24 allies in Pakistan. Obviously errors happen, sometimes even really tragic ones.

    Now, there's a drone, in whatever condition, allegedly in enemy's hand. Bad luck. But nobody was hurt and at best the Iranian's will find out how they were spied on and may make it more difficult for the US in the future. That remains to be seen, though.

    Neither the fact that they are being spied on is news, nor the fact that people make mistakes and systems aren't infallible/invulnerable.

    The whole point of drones (aka unmanned planes) is that you can send them into areas where you'd otherwise risk losing life. And you can produce them cheaper and lighter than manned planes. But losing a drone is part of the concept. Not the ideal scenario, but it happens. So what?

  47. Peter Mc Aulay

    Buzz bomb

    I keep imagining an Iranian air force jet flying alongside the drone and simply tipping it over with its wing tips.

  48. Matt Bryant Silver badge

    In summary.

    So, we have an uncorroborated Iranian story, full of holes a twelve-year-old could identify. We then have experts confirm out the holes. Naturally, we then have the usual crowd of anti-Yanks posting any possible way they can carry on believing.

  49. John Smith 19 Gold badge

    A note on GPS antennas

    Most high data rate sat systems are either in geosynchronous orbit (IE *fixed* in the sky) or needing to communicate with 1 sat at a time (LEO comms constellations like Iridium and Orbcomm).

    The usual answer is a *steerable* dish in azimuth and direction to track the *single* target while the platform moves, while receiving or sending a signal across a *very* narrow angle of sky.

    GPS does not have this luxury. Signals could come from anywhere across the whole visible *hemisphere* of sky, 180deg in *either* direction.

    And a GPS receiver has to track *four* satellites for a fix. Not 1 or 2.

    The *simplest* solution is a special *passive* aerial with good coverage in a blister on the top side of the vehicle. Phased arrays have trouble down to the horizon ("endfire") and add complexity. Multiple blisters worsen aerodynamics (and presumably stealth characteristics).

    Tests have been done for using GPS for *attitude* determination on satellites (JPL and IIRC SSTL have done so), which is a similar problem.

    Obviously depending on how seriously people take this capability future designs may choose to go with other aerial and receiver options.Modern DSP based GPS receivers can have enough "channels" to track *all* visible satellites continuously.

    BTW the CSM article *names* a General supposedly involved in the programme (who was reportedly dead of a heart attack shortly afterward) who should be senior enough to speak with some authority on the project. It's not entirely an anonymous informant.

  50. Ben Cosin

    Interesting to learn that Swiss as well as US citizens are contributing to the US aggression against Iran. Since this constitutes JOINT ENTERPRISE, I wonder if they are aware that Iranian defensive and pre-emptive measures against the offending sites would be in accord with international law, and such measures against the whole jurisdiction from which aggression emanates would be in accord with AngloAmerican ideology and practice. Hunker down, geeks.....

    1. Matt Bryant Silver badge

      RE: Ben Cosin

      "Interesting to learn that Swiss as well as US citizens are contributing to the US aggression against Iran...." Que? I'm sorry, but maybe the realities of international politics didn't penetrate your alternate reality, but the Swiss embassy in Tehran is simply providing a channel of diplomatic communication between Iran and the US. This was agreed by the Iranians after they broke off all diplomatic contact with the US. The Iranians maintain a consular official in the Pakistani embassy in Washington under the same agreement.

      ".....Iranian defensive and pre-emptive measures against the offending sites would be in accord with international law...." Wrong, Iran would have no legal basis for any attack on Switzerland or Swiss sites outside Switzerland, regardless of what the US gets up to. Sorry, but that little bit of scaremongering is simply silly.

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