7 ms·

I don't understand this article. If the GPS signals are jammed, what purpose does it serve to have an atomic clock on board your plane? You still need accurate

I don't understand this article. If the GPS signals are jammed, what purpose does it serve to have an atomic clock on board your plane? You still need accurate signals with time data to measure against. Am I missing something?

BenjiWiebe 2y ago
I didn't read the article, but: a GPS receiver must calculate/find both it's time and position to get a fix. So maybe by having the time already available really accurately it makes the job of finding position easier?
- gorbypark 2y ago
  From my (very basic) understanding of GPS you need at minimum four satellites to calculate the time. If you had a local atomic clock in sync with the GPS satellites, you'd only need three satellites to get a position fix. It would (probably, maybe?) also speed up the time to first fix / time to a precise position fix.
simne 2y ago
As I read from book about gyroscopes, most sensitive achieve so fine accuracy, they detect daily Earth rotation and even yearly Earth rotation. But when they speaking about near zero temperatures, looks like they talking about something like Rydberg atoms - extremely sensitive matter, which could be considered as nuclear scale gyroscopes, or quantum gyroscopes, or read more about quantum accelerometer. And current inertial navigation could be used to calculate relative coordinates like automobile odometer, but from integrating accelerations. But classic accelerometer is just not fine enough, and at this place appear quantum accelerometer and quantum gyroscope. And I agree, article is terrible. I don't know why they use so abstract language, when could just say, navy already tested quantum navigation.
- simne 2y ago
  To be more concrete, space rockets nearly all fly with inertial navigation, but they are extreme case, because most use only inertial navigation just few minutes (so all those classic gyros/accelerometers integrated errors are small enough to successful enter stable orbit, and then using some sort of radio or optical fine measurements and making corrections with fine engines). Planes flights are much more lengthy than rockets - I think, typical ~40 minutes or more (most long I hear 20 hours), so INS could integrate huge mistake.
  - p_l 2y ago
    INS essentially was expensive and AFAIK once GPS became available started to drop off in use outside of military. And with GPS availability coinciding with switching to more modern integrated Flight Management System/Computer, a lot of planes simply don't have INS installed.
    - scrlk 2y ago
      > a lot of planes simply don't have INS installed Perhaps in general aviation, but I can't think of any modern commercial airliner without an INS via the air data inertial reference unit.
      - simne 2y ago
        Many small planes don't have INS in typical meaning, but their pilot is INS computer, calculated approximate nav from air data (air speed + weather data + compass or radio compass).
      - p_l 2y ago
        Except ADIRU isn't INS. The INS unit is separate and often has its own set of gyros and has to be connected as separate input to FMS or other navigational computers, same as connecting GPSes or other radio nav components. For example the current model of popular Universal Avionics UNS1 series of NCU (navigational computer part of FMS) come with built-in augmented GPS receiversz but do not mention INS functionality at all even in extended models. Don't have access to manuals at the moment, but I'd expect to see INS as optional to connect over one of the external connectors on the MCU, as it was on the older models without integrated GPS
- magnetometer 2y ago
  > most sensitive achieve so fine accuracy, they detect daily Earth rotation and even yearly Earth rotation Daily rotation is 360°/23.934h, so 0,25°/min, which is acutally quite a lot if you want to use a device to track your orientation.
  - simne 2y ago
    Unfortunately, these numbers considered state of art for modern classic gyroscopes. Better are quantum navigation systems, using quantum matter as sensor, but they was too bulky to be used on planes, only last years appear more compact systems, sized like common home fridge.
- RandomBacon 2y ago
  > daily Earth rotation and even yearly Earth rotation. Minor FYI: the earth rotates daily, but it revolves around the sun yearly. revolve /rĭ-vŏlv′/ intransitive verb To orbit a central point. "The planets revolve around the sun."
  - HPsquared 2y ago
    I took it to mean "able to measure a rotation rate of 1 turn per year".
- lupusreal 2y ago
  > they detect daily Earth rotation This is the principle gyrocompasses work on; when left running for a while they align themselves with true north; the axis of Earths rotation.
touisteur 2y ago
You can get a very accurate timestamp from GNSS. What lots of people do then is slave a PLL based on a local oscillator, to be able to get time between two GNSS captations. Or to be able to extrapolate when they have no GNSS signal. Now suppose someone is spoofing your GNSS signals, it's pretty hard to replace a constellation with another one whilst maintaining time consistency for you. One way to detect spoofing is comparing what a local clock is saying to whatever the GNSS is giving. A local, unfudgeable, stable, accurate clock is a good reference for this.
- thesh4d0w 2y ago
  Ahhhhh, that makes sense. Treating this as security mechanism rather than an anti-jamming one.
- DannyBee 2y ago
  The article is seriously confused. What you are talking about is easy - chip scale atomic clocks are easy to get. I can have one shipped to me today. Hell, i have one on a time card in my basement. Assume you want it even super accurate. Great, 3k for an SA65 https://www.microchip.com/en-us/products/clock-and-timing/components/atomic-clocks/embedded-atomic-oscillators/csac https://www.microchip.com/en-us/products/clock-and-timing/co... Holdover would be fine for even a very long flight. Hell, even a good rubidium oscillator doing 1PPS will stay within 200 nanoseconds over 12 hours. If you are trying to do navigation while jammed, none of these help you. You still need good reckoning, which is the hard part We done solved the clock problem enough already :)
  - touisteur 2y ago
    I was under the impression (and from experience too) that the very stable oscillators were finicky and sensitive to temperature swings and in general costly to use properly in "hard" environments. I'm happy to learn this is not the case for every good oscillator. TIL.

BenjiWiebe 2y ago

I didn't read the article, but: a GPS receiver must calculate/find both it's time and position to get a fix. So maybe by having the time already available really accurately it makes the job of finding position easier?

gorbypark 2y ago
From my (very basic) understanding of GPS you need at minimum four satellites to calculate the time. If you had a local atomic clock in sync with the GPS satellites, you'd only need three satellites to get a position fix. It would (probably, maybe?) also speed up the time to first fix / time to a precise position fix.

simne 2y ago

As I read from book about gyroscopes, most sensitive achieve so fine accuracy, they detect daily Earth rotation and even yearly Earth rotation. But when they speaking about near zero temperatures, looks like they talking about something like Rydberg atoms - extremely sensitive matter, which could be considered as nuclear scale gyroscopes, or quantum gyroscopes, or read more about quantum accelerometer. And current inertial navigation could be used to calculate relative coordinates like automobile odometer, but from integrating accelerations. But classic accelerometer is just not fine enough, and at this place appear quantum accelerometer and quantum gyroscope. And I agree, article is terrible. I don't know why they use so abstract language, when could just say, navy already tested quantum navigation.

simne 2y ago
To be more concrete, space rockets nearly all fly with inertial navigation, but they are extreme case, because most use only inertial navigation just few minutes (so all those classic gyros/accelerometers integrated errors are small enough to successful enter stable orbit, and then using some sort of radio or optical fine measurements and making corrections with fine engines). Planes flights are much more lengthy than rockets - I think, typical ~40 minutes or more (most long I hear 20 hours), so INS could integrate huge mistake.
- p_l 2y ago
  INS essentially was expensive and AFAIK once GPS became available started to drop off in use outside of military. And with GPS availability coinciding with switching to more modern integrated Flight Management System/Computer, a lot of planes simply don't have INS installed.
  - scrlk 2y ago
    > a lot of planes simply don't have INS installed Perhaps in general aviation, but I can't think of any modern commercial airliner without an INS via the air data inertial reference unit.
    - simne 2y ago
      Many small planes don't have INS in typical meaning, but their pilot is INS computer, calculated approximate nav from air data (air speed + weather data + compass or radio compass).
    - p_l 2y ago
      Except ADIRU isn't INS. The INS unit is separate and often has its own set of gyros and has to be connected as separate input to FMS or other navigational computers, same as connecting GPSes or other radio nav components. For example the current model of popular Universal Avionics UNS1 series of NCU (navigational computer part of FMS) come with built-in augmented GPS receiversz but do not mention INS functionality at all even in extended models. Don't have access to manuals at the moment, but I'd expect to see INS as optional to connect over one of the external connectors on the MCU, as it was on the older models without integrated GPS
magnetometer 2y ago
> most sensitive achieve so fine accuracy, they detect daily Earth rotation and even yearly Earth rotation Daily rotation is 360°/23.934h, so 0,25°/min, which is acutally quite a lot if you want to use a device to track your orientation.
- simne 2y ago
  Unfortunately, these numbers considered state of art for modern classic gyroscopes. Better are quantum navigation systems, using quantum matter as sensor, but they was too bulky to be used on planes, only last years appear more compact systems, sized like common home fridge.
RandomBacon 2y ago
> daily Earth rotation and even yearly Earth rotation. Minor FYI: the earth rotates daily, but it revolves around the sun yearly. revolve /rĭ-vŏlv′/ intransitive verb To orbit a central point. "The planets revolve around the sun."
- HPsquared 2y ago
  I took it to mean "able to measure a rotation rate of 1 turn per year".
lupusreal 2y ago
> they detect daily Earth rotation This is the principle gyrocompasses work on; when left running for a while they align themselves with true north; the axis of Earths rotation.

touisteur 2y ago

You can get a very accurate timestamp from GNSS. What lots of people do then is slave a PLL based on a local oscillator, to be able to get time between two GNSS captations. Or to be able to extrapolate when they have no GNSS signal. Now suppose someone is spoofing your GNSS signals, it's pretty hard to replace a constellation with another one whilst maintaining time consistency for you. One way to detect spoofing is comparing what a local clock is saying to whatever the GNSS is giving. A local, unfudgeable, stable, accurate clock is a good reference for this.

thesh4d0w 2y ago
Ahhhhh, that makes sense. Treating this as security mechanism rather than an anti-jamming one.
DannyBee 2y ago
The article is seriously confused. What you are talking about is easy - chip scale atomic clocks are easy to get. I can have one shipped to me today. Hell, i have one on a time card in my basement. Assume you want it even super accurate. Great, 3k for an SA65 https://www.microchip.com/en-us/products/clock-and-timing/components/atomic-clocks/embedded-atomic-oscillators/csac https://www.microchip.com/en-us/products/clock-and-timing/co... Holdover would be fine for even a very long flight. Hell, even a good rubidium oscillator doing 1PPS will stay within 200 nanoseconds over 12 hours. If you are trying to do navigation while jammed, none of these help you. You still need good reckoning, which is the hard part We done solved the clock problem enough already :)
- touisteur 2y ago
  I was under the impression (and from experience too) that the very stable oscillators were finicky and sensitive to temperature swings and in general costly to use properly in "hard" environments. I'm happy to learn this is not the case for every good oscillator. TIL.