I had a LORAN receiver in my GA airplane. It was always pretty amazing how well that worked unless you were in one of the known gap areas.
We had a few 727s with OMEGA Omega (navigation system) - Wikipedia nav systems. We used 'em for Gulf of Mexico & Caribbean ops. They were perfectly adequate for enroute, usually fixing us within a couple miles of true position. Not something you’d want to use for an approach, bootleg or otherwise.
The Feds finally shut down OMEGA and we carried the deadweight receivers around for a year or two before the jets went to their final reward as well.
We are flying more and more purely GPS-based approaches now. Our receivers have RAIM, and the IRS position serves as a second source of gross error check. I have to say it’s a little disconcerting to be letting down into a mountain bowl knowing that if GPS goes apeshit we loose pretty much all our computer-aided situational awareness all at once. Yes, we have backup plans. No, it’s won’t be comfortable.
I wanna go with you when you try the backup plan to see if it really will work and how tight you can do it. That will be a ride.
Q? Can the current airliners do back to back Immelmans, got the power, etc.? Tight spiral climb? Can the range of the attitude instrumentation be exceeded & could using basic instruments could you continue a couple of turns if needed …? Of course if you get in a full 360 you should not need to be tighter nor need to climb faster… ??
If last position known, turn to the best heading for the most room & balls to the wall up?? What can be done more or less safely?
Rate and/or angle of climb? ( understand weight etc. make a big difference) But in general??
The general plan for total loss of geo-SA is climb like mad mostly straight ahead with only, say, +/- 30 degree heading changes. During a two-engine approach (IOW at end-of mission weight) any jet can climb better than almost all the rocks found near any airport. Unless, like these poor guys American Airlines Flight 965 - Wikipedia you inadvertently turn 90 degrees to the canyon’s axis first.
Note that this mishap was before the advent of EGPWS which supplies a complete terrain elevation / contour map around the airplane. Even if it suddenly disappeared you’ll still have a pretty good intuitive feel of which ways are safer than others for the next few miles it’ll take to get above all the local rocks. Those guys didn’t have that. Which is a large part of why they followed what they thought was a revised ATC clearance that was actually a mistaken turn directly towards the canyon wall.
The nav failure scenarios that concern me most are takeoffs or missed approaches that involve complicated threading through mountain passes. Many of which don’t have a VOR-based backup plan or the one they do have is implausibly complex for unexpected emergency use.
With full nav capability and with all engines running they’re almost boring. Though it is fun to watch the rocks & trees go by out the side window at or above your level.
With partial or no nav or with an engine out they’re plenty busy but safely doable with the expected 99th percentile crew.
But if an engine failure somehow also blows up the nav solution so now you’re single engine using seat-of-pants DR until finding VMC on top, well it just wasn’t your day, was it?
OTOH, the statistics show that this risk, although easy for an old pilot to identify isn’t the one that’s hurting people today. As always, smart risk analysis is about knowing what’re actually the big risks, not just what’s an obvious risk. Kinda like anti-vaxxers: it’s easy to see or imagine “my kid had an allergic reaction”. It’s hard to see or imagine “measles kills 100x (my WAG) the kids that measles vaccines do.”
With umpteen level redundant everything it truly takes a very, very rare combo of very bad events to reduce us to DR. If both engines fall off taking their electrical generators with them we still have triple IRS, dual GPS, single FMS, single moving map, & single EGPWS w terrain contour display. Sully had 100% of that going for him to touchdown. Not that he needed it; looking out the window was more than enough. Fortunately he was at an airport near a river, not one in the Rockies. Even at pancake flat places like O’Hare, DFW, or Oklahoma City, the Sully scenario does not end nearly so prettily.
How can GPS be the only system in regular use? It’s not too tough for weather conditions to block enough satellites to shut down GPS. Yes, I know, you’d rather not be landing in those kind of weather conditions at all, but you might not have a choice.
An interesting trick is possible with a balloon or other flying craft (say a solar plane). If the craft has a GPS receiver, all it need do is receive signal from a set of visible satellites, and then retransmit the same signal on another frequency. It should be possible for the end user’s receiver to combine this signal with any other GPS signals they may receive (or indeed just signals from other relay craft) and still resolve their position to GPS precision. For a fully non-satellite solution, ground based transmitters could replace the GPS satellites, so long as they provided line of sight to the flying craft. It might even be viable to have multi-jump relays occur, but there will be a point where the system becomes too messy to converge. Altitude resolution would likely be poor for many users. Not many would find a transmitter overhead enough to get a good reading. But the altitudes mooted for long lived solar planes would make such a nav system viable.
Citation needed. The GPS signal was specifically designed to resist degradation from weather conditions, since it’s based on a center frequency that corresponds to a “transparency” in the atmosphere’s spectral absorption profile. Which is driven mostly by water and water vapor.
Rain, snow, sleet, cloud cover, fog… all transparent. Maximum signal degradation is less than 2db in a radiative budget of -160db. The only real weather-related risk is if a fairly solid layer of water comes between the receiver and the satellites, but we’re not talking about trying to navigate underwater, are we? And very heavy snowfall can cause a degradation in lock time and accuracy, by causing multi-path reflection, but that’s not “shut down”.
The only part of the atmosphere that has any real chance of affecting the GPS signal is the ionosphere, and it’s accounted for. The kind of space weather that can affect the ionosphere enough to threaten GPS operations is also the kind of space weather that can directly threaten on-orbit satellite health and operation, so that’s the only kind of “weather” that can “shut down” GPS, and then only for a few hours and with plenty of advance notice.
I didn’t say that as clearly as I might have. In the name of simplicity most of the baby ended up on the floor. It’s a bit of a hijack to clarify more thoroughly. A decent qualitative gloss is about like this. I’ll leave the TLAs unexplained; wiki can fill in the background as needed.
Modern airliners mostly use wholly internal inertial gyro- or equivalent systems (IRS in the argot) with continuous GPS fine tuning to fix their instant position. In many areas the enroute, arrival, and final approach routes are constructed from ground-based radio beacons of various sorts (NDB, VOR, DME, ILS). But in normal practice rather than actually following the signals from the ground based beacons, the airplane navigates from the known lat/long of one beacon to the known lat/long of another. Or to the lat/long of points officially defined as X distance in Y direction from beacon Z.
If the beacons are somehow inoperative we fly unhindered since normally we’re not actually referencing them. If GPS is somehow inoperative the airplane can use some of the beacons as an alternate and not quite so precise fine-tune for the IRS. And if the GPS & IRS is kaput we can still fly the old-fashioned way by actual reference to the radio beacons themselves.
This is the transition state the industry is mostly in now. Some operations are purely beacon based, some are purely aircraft internal + GPS, and some are a blend.
To realize the full benefits of space-based nav the end state (circa year 2050) is to get rid of all of the existing ground beacons. And to supplement current-tech GPS with LAAS, WAAS, as well as receivers able to use Russia’s GLONASS & the EU’s Galileo as well as the USA’s GPS and eventually China’s BeiDou-2. These are collectively the GNSS.
Ultimately what matters is the statistical reliability of position fixes in general and your confidence in your specific position at this specific moment. If you have the ability to reliably draw an uncertainty circle around where you think you are, you have the ability to reliably avoid bumping into known obstacles. The required precision varies from a +/- a couple miles in cruise to +/- a few feet for an automatic touchdown in thick fog. The current state of IRS + raw GPS easily supports getting within a mile-ish of the end of a runway aligned well enough to land visually/manually. Tightening it up the rest of the way to permit autolands in thick fog is what LAAS & WAAS and some other still-experimental alphabet soup are for.
Today the comprehensive permanent loss of GPS would be chaos for a couple days and reduced system throughput UFN. Some areas, e.g. Alaska, have run farther into the GPS-only future than have other areas. So they’d be harder hit.
Note that the aircraft position is IRS primary with GPS fine tuning. So the total loss of GPS to any given aircraft doesn’t mean its position fix suddenly changes from “<here> +/- 50 feet” to “I have no friggin’ clue where we are.” Instead the position fix continues to evolve validly over time as the airplane moves while the error circle starts to grow at a mostly predictable rate. Typically well less than 1/10th mile per hour of fine-tuneless operation = very roughly 10 feet per minute.
As that error circle continues to grow, various operations become incrementally untenable.
First to go are the low-weather pure-GPS approaches to a landing; if you’re doing that procedure and are more than a few seconds from touchdown you’re going around. Not Gus’s “OMG, we’re lost! Claw for the sky!” Simply an ordinary go-around and circle back around for a try at a different beacon-based approach.
Next to go are are GPS-enhanced approaches used in somewhat better weather. Eventually we can’t do some complicated off-ramp and on-ramp procedures that connect the enroute structure to the airport environment. So we’d fall back to the older beacon-based on- and off-ramps. etc.
The GPS receivers also incorporate RAIM, which continuously validates that we’re receiving enough satellites and they’re in a good enough geometry to ensure adequate fix quality. As well fixes are continuously derived from various subsets of the received satellites and compared to detect outliers promptly enough for our purposes.
The speed with which RAIM can trigger a warning for a drifting/failing satellite is one of the limiting factors to how close to a fully blind approach can be flown off GPS.
I was thinking that E-2 Hawkeye, E-8 JSTAR and other early warning/surveillance aircraft with large arrays must have that capability. The Russians seem big on GPS jamming equipment, I presume primarily to degrade the performance of cruise missiles. An airplane could have considerably more transmitter power to burn through the jamming, right?
How easy is it to do a soft kill on a satellite? What are the different kinds of soft kill?
How easy are hard kills?
I remember NATO bombing TV and radio stations in Yugoslavia. Part of it must have been about countering Serb propaganda but could it also have had more direct military value?
I don’t get this. If a single relay aircraft re-transmits signals from 3 GPS satellites to a user, that will allow the user to resolve the position of the relay aircraft. Not the position of the user. Because the time delay between the 3 signals is only dependent on the position of the relay aircraft.
LSL Guy Are navigators still taught dead reckoning and are they still taught to “shoot the stars”? I’ve always wondered if old school navigation is still taught now that we have gps. Do bombers such as the B52 still carry a dedicated navigator who plots the course with a map like the B17 navigators of WWII?
I should note that my point above won’t work with existing GPS receivers. You need some extra capability in them to make the idea work. So anyone using the idea would need to develop their own receiver technology.
You don’t need big arrays of anything for GPS signals - the wavelengths are pretty small, and you are going to want fairly omnidirectional transmissions. Certainly not needing a lot of gain. A big disadvantage of a big plane with a nav transmitter, flying nearby a theatre of war, is that the first thing the opponent does is launch a missile straight up the signal. The surveillance planes tend to be passive. The big transmitters are things like the Growler, an F-18. Fast and able to keep pace with the assets they are protecting, they can deny use of radio frequencies across a wide band.
GPS satellites are in a pretty high orbit. Killing one will need a launch with a powerful rocket. There are lots to kill. Such an attack would be pretty much on the same level as a first strike nuclear attack. Not something any nation is going to be doing unless they are ready and capable of escalating to all out MAD levels of war.
Jamming GPS attracts a lot of interest. Anti-jamming capabilities are known to exist. One thing in favour of the GPS signal is hat it comes from the sky. Planes need little more than upward facing antennae to exclude terrestrial jamming signals. You can get more sophisticated quite quickly. Also, your jammer is going to attract a guided missile pretty fast.
I mean that the re-transmitted signal becomes a surrogate for a single GPS satellite. The retransmitted signal contains all four (or more) GPS signals the relay sees. (You always need four sats minimum for GPS, as you must resolve time in order to determine position. Which is why a GPS receiver can deliver a time signal to atomic clock precision once it got its position.) The retransmission thus codes both the time and location of the relay, which is all that is needed to act as a single GPS satellite surrogate. It is the time information that is carried along with the location information that makes this possible.
I’m on my phone at a conference so I won’t try to find this part, but IIRC all US Coast Guard Stations were to have DGPS installed to aid in SAR missions. Also, the FAA installed two DGPS stations, one on each coast, and their own GPS satellite right above the central part of the USA to aid in navigation.
So, if you have a GPS that will list out the transmitters and types you’ll see a whole lot lock on in a coastal area. Like in the twenty-some and you’ll readily have accuracy down to several feet.
Also, the space based GPS sats move around in space quite a bit. The term gel-stationary is a type of orbit in that the satellite is relatively in the same place over the ground. But, all satellites have a “wobble” and move in all three axis throughout the day. Plus, I believe the expansion and contraction of the atmosphere (maybe the ionosphere layer?) as the earth rotates from sunlit to dark causes them to move dramatically. You can actually measure this with a GPS device if you lock onto satellite only and log your location info. You’ll notice throughout the day and over many days the accuracy changes in a consistent manner.
Um, it was mentioned in post #2 and a few times after.
GPS sats are not in geostationary orbit. Their orbits do wander, but in a very predictable way over the validity time of their ephemeris. They get new ephemeri uploaded as needed and as determined by ground-station measurement of their orbits.
As I mentioned earlier, the P channel of GPS is designed to explicitly correct for this. What is annoying is that the P channel is restricted to military users. Differential GPS gets around this of course.
As to airborne (or any moveable) GPS surrogates: The device needs to transmit the highly synchronized time signal and what amount to its own 3D coordinates. Anyone listening to that surrogate signal can use it as just another GPS source to triangulate off of.
But that also means that the position fix of any receiving “client” device can be no better than the position fix of the transmitting “server” device. The challenge is getting and maintaining the precise location of the moving transmitter while its moving around.
If the transmitter can only figure out where it is +/- half a mile, than anyone listening to it can also only fix their own position +/- half a mile.
As an aside, ref my earlier posts in this thread. Today I had to work after I posted those. So since I was thinking about it I took note of our position uncertainty en route. The airplane was confident it knew its position +/- 300 feet almost the entire 3 hour flight. Sometimes it was a little tighter, other times it was slightly looser.
Which is to say if you imagine 5 airplanes touching tip to tip arranged like the spots on the 5 side of a ordinary 6-sided dice, the actual airplane position would be almost entirely somewhere inside that tight formation. Not too shabby a fix for enroute, but being 300 feet off to the left of a 150 foot wide runway would make for an unsuccessful autoland.
The military has aerial navigators; the civilian FAA rating still exists, but I can’t imagine anyone getting it or using it for employment since the early 1960s. Last I knew they still shot stars in training, although it wasn’t something they used after training except in very rare circumstances.
Military pilots and navigators have always been taught DR = flight by compass and stopwatch and also finding our way by map reading= “pilotage”. That was exactly how we expected to fight WWIII in my era. GPS didn’t exist yet and IRS / INS / LORAN / OMEGA / Doppler wasn’t that reliable. You’d hope to have it help you find your way to the missile complex at Laputa, but if it failed you were still expected to finish the job.
The other big nav skill both attack pilots and navigators have is using ground mapping radar to navigate vs. the ground and special purpose radar maps. At least in favorably varied terrain it was possible to pick your way along using radar to identify the various mountains, ridges, lakes, and valleys.
Life for either DR, pilotage, or radar nav gets vastly easier if the IRS / INS / GPS / etc. still knows roughly where you are, so you’re using the other skills to corroborate, not to generate a position fix *ab initio *in real time. It is far, far easier to avoid becoming lost than it is to regain positional awareness after you did get lost.
I have no doubt that the newest military aircraft today have pretty fancy nav systems. Aging B52s? Not so much.
I dunno, a navigation system is probably one of the simplest systems on an airplane to retrofit onto an old frame. There just might be B52s out there with up-to-date nav systems.
Well rats. You said uncomfortable and a straight ahead climb or max climb is not uncomfortable… ::: sigh ::: I thought you got to play fighter pilot in big Iron & I wanted to go with…
