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#21 rmgill

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Posted 22 September 2017 - 1022 AM

Stuart, I don't think that the Loran-C frequencies are re-allocated yet. Moreover newer tech means that the necessary bandwidth for such signals is much smaller. I suspect that more modern tech would allow transmitters to transmit their own positions and work much like the timing signal of GPS but with the added provision of being terrestrial sources that one should be able to detect based on specific bearing and with more signal power. 


Edited by rmgill, 22 September 2017 - 1022 AM.


#22 shep854

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Posted 22 September 2017 - 1031 AM

Stuart, Cessna 172s don't have astrodomes. ;)
----
I've been out of the loop for a while, but as GPS was being introduced, there was a consensus that some ground-based navaids should be maintained as backup for US aviation, especially for instrument approaches to airports, where precision is critical.

#23 Stuart Galbraith

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Posted 22 September 2017 - 1034 AM

Stuart, I don't think that the Loran-C frequencies are re-allocated yet. Moreover newer tech means that the necessary bandwidth for such signals is much smaller. I suspect that more modern tech would allow transmitters to transmit their own positions and work much like the timing signal of GPS but with the added provision of being terrestrial sources that one should be able to detect based on specific bearing and with more signal power. 

 

Oh, fair enough then. I was looking something that said they had some of the same frequencies for Amateurs, but that would I guess be Loran A which was a bit different.

 

Could an INS system backed up by an echo location worked? At that point you have something similar to TRN as on the early Cruise missiles.

I guess the obvious negative is that sea beds in most litoral zones tend to be a lot flatter...



#24 Stuart Galbraith

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Posted 22 September 2017 - 1036 AM

Stuart, Cessna 172s don't have astrodomes. ;)
----
I've been out of the loop for a while, but as GPS was being introduced, there was a consensus that some ground-based navaids should be maintained as backup for US aviation, especially for instrument approaches to airports, where precision is critical.

 

Bit of a design flaw that really. :D

 

Been spending a bit more time on Microsoft flight simulator, and been using a Naviad course that I bought on steam, and it struck me how  remarkable the kind of flight accuracy you can get from even relatively primitive systems like NDBs, if you use them right.



#25 rmgill

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Posted 22 September 2017 - 1119 AM

Stuart, Cessna 172s don't have astrodomes. ;)
----
I've been out of the loop for a while, but as GPS was being introduced, there was a consensus that some ground-based navaids should be maintained as backup for US aviation, especially for instrument approaches to airports, where precision is critical.

Well, you could always put it down in a random field to shoot the sun. 



#26 rmgill

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Posted 22 September 2017 - 1122 AM

 

Bit of a design flaw that really. :D

 

Been spending a bit more time on Microsoft flight simulator, and been using a Naviad course that I bought on steam, and it struck me how  remarkable the kind of flight accuracy you can get from even relatively primitive systems like NDBs, if you use them right.

 

Given the rather amazing level of accuracy companies like Apple get out of little micro Accelerometers, I have to wonder at a modern INS system's ability to use more recently designed accelerometers for movement tracking that aren't for a consumer grade unit. 



#27 2805662

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Posted 22 September 2017 - 1559 PM

Aren't the military channels encrypted? Wouldn't you need to be able to crack the code to be able to spoof the satellites timestamp?

 
But what happened if some of the ships AROUND the warship, were not using an encrypted channel? It wouldnt do much on the open ocean, but around choke points, say Gibraltar, Suez, the possibilities of a pile up in poor visibility can be envisaged.
 
I mean yes, I dare-say most responsible maritime operators have people on watch in poor visibility, and everyone would slow down in bad weather. Some however likely wont.
I'm not saying it isn't a danger, especially to civilian ships running into each other, I just wanted to better understand the vulnerability. A warship minimally should also be running an inertial nav system that should be able to catch any major discrepancy.

This is what I mentioned at the time of the USS McCain collision: attack (in this case spoof) the weak link - the civilian ship's GPS.

To clarify the "encrypted" nature of military GPS, all the crypto does is enable the receiver to guarantee that an element of the signal being received is from a GPS satellite via the P(Y) Code signal. This is the "anti-spoofing" part of the SAASM chip. Don't have a (crypto) fill loaded in your military GPS? Bad luck, no P(Y) code for you. You'll still have GPS, but are vulnerable to spoofing (introduce a locational margin of error, provide inconsistent results, reduce trust in your device).

The most effective demo I've seen is a side-by-side military & civilian GPS, displaying almost identical information...spoofing device activated, data displayed on military device doesn't change, civilian one? Big changes.

#28 shep854

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Posted 22 September 2017 - 2007 PM

 

Stuart, Cessna 172s don't have astrodomes. ;)
----
I've been out of the loop for a while, but as GPS was being introduced, there was a consensus that some ground-based navaids should be maintained as backup for US aviation, especially for instrument approaches to airports, where precision is critical.

Well, you could always put it down in a random field to shoot the sun. 

 

I'd just use dead reckoning--'If you reckon wrong, you're dead'... :P.

Actually, pilotage--looking at the ground and comparing it to a chart was quite sufficient. ;)



#29 a77

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Posted 23 September 2017 - 0131 AM

If this shit get common we all have to start to use kinematisk GPS measuring. The atmosphere, solar storms etc do "jamming" the signal, so insted of 1 cm we get 10 cm deviation on surveying. To resist natural jamming you use a fixed base station, a GPS on a known point that then transmit corrections in real time to the measuring GPS.

 

One weakness that the bace station must be inside the jamming, not a problem if its only natural "jamming" solar strom and atmosphere is big area effect evvents. 

 

#30 2805662

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Posted 23 September 2017 - 0510 AM

Basically described Differential GPS there.

Edited by 2805662, 23 September 2017 - 0511 AM.


#31 DB

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Posted 23 September 2017 - 1728 PM

My understanding is that the mems accelerometers have very high drift rates, but are very good over short periods. This makes them completely dependent on GPS to stay accurate over timescales longer than a very few seconds.

#32 Josh

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Posted 25 September 2017 - 0836 AM

Military inertial units certainly do better than that, even in throw away ordnance. JDAM is supposed to be good for 40 meters accuracy sans GPS, so long as the parent aircraft has a clear idea of where it is when it launches. I'd have though combining INS with GPS for a civilian nav system would be pretty trivial and that software could integrate the two measurements and alert crew if the two started diverging drastically or if the movement rate exceeded that of the platforms' max speed (or ideally integrate the actual current speed of the platform into the calculation).

#33 Brasidas

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Posted 10 October 2017 - 0226 AM

Commercial receivers even when they receive only L1 C/A have anti-spoofing capabilities. The Horizontal Alert Limit will be rapidly exceeded by the Horizontal Integrity Limit and annunciate loss of integrity. Most receivers usually notice a collective shift of several hundred meters. Due to ITAR, if the apparent position shift exceeds a speed averaging above 800 knots, the GPS receiver then assumes it is guidance for a weapon and "locks up". INS is usually a good back up, if you have a decent installation. Civil aircraft without a hybridized GPS/INS will still be able to maintain an RNP-10 for at least six hours. This doesn't include the latest commercial receivers that will be able to use L5 freqs to determine ionospheric scintillation from each satellite, which will only improve Horizontal Integrity at the receiver. Even with SBAS, there is only a guarantee for 99.5% GPS coverage for en route capability (RNP 2.0 or better) within CONUS. All newly certified aircraft will have to meet this performance requirement. This also affects things like ADS-B and ADS-C, which absolutely require GPS Position/Nav/Time. 

 

So while SAASM is great and all, it's only for the military really and the second frequency from the L2 band, helped in finding the ionospheric scintillation constant for the emitting satellite.

 

Kinematic GPS is ok, but it takes forever to align in high precision mode and typically, it's not made for the rigors of performance based navigation, so the vendors all opt for a very good L1 band solution on civil aircraft. For ships, I would hope they use backup inertials to supplement GPS like some of the higher end new aircraft do.  






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