Clarification about the term "GPS Shutdown"

Discussion in 'General GPS Discussion' started by Sam Wormley, Dec 16, 2004.

  1. Sam Wormley

    Mxsmanic Guest

    Which is pretty much never.
    Provided that all aircraft have reliable navigation methods.
     
    Mxsmanic, Dec 20, 2004
  2. Sam Wormley

    jon Guest

    that don't depend on just GPS.
    Be sure to reach harder for the ring this time 'round the carousel.
     
    jon, Dec 20, 2004
  3. Sam Wormley

    Iolaos Guest

    It does if you're in RNP 10 airspace more than 6 hours from land.
     
    Iolaos, Dec 20, 2004
  4. Sam Wormley

    Alan Browne Guest

    He does have a good sense of up and down... from falling on his ass a lot.

    ;-)
     
    Alan Browne, Dec 21, 2004
  5. Sam Wormley

    Alan Browne Guest

    Does that mean they really drive on the right in Aussie after all?
     
    Alan Browne, Dec 21, 2004
  6. Sam Wormley

    Alan Browne Guest

    In trail dist is 10 - 15 minutes (60 NM or so)
    Lateral sep is 1 deg of latitude (60 NM).
    To be in low vis flight, you must be IFR qualified and the aircraft IFR
    equipped... it's been like this since before WW II (with ever increasing
    sophistication, of course.

    0 vis approaches are the exclusive domain of CAT IIIc aircraft, runways and
    aircrew. Most low vis approaches take place between much less stringent CAT II
    and CAT I (or non-prec which is even less stringent).

    Not needed. INS is quite capable. eg: MD-11's run by KLM do not currently have
    GPS. The costs of integration and certification would be horrendous ... to
    enhance a system that is highly capable.

    I'm tired of repeating all this Mx, so I hope it will sink in soon, GPS is not
    the linchpin of aviation guidance. It is a preffered primary source for
    accuracy. It is not currently on all transport aircraft and the older ones like
    747-200's will, in the main, never be so equipped.

    Cheers,
    Alan.
     
    Alan Browne, Dec 21, 2004
  7. Sam Wormley

    Alan Browne Guest

    See what I said about adjusting to emergencies. What's up stays up w/o change,
    and what wants to up might be delayed to increase spacings during the emergency.
     
    Alan Browne, Dec 21, 2004
  8. Sam Wormley

    Alan Browne Guest

    Fortunately for all concerned that is not how air navigation is set up. Many of
    my replies to you yesterday refer.
     
    Alan Browne, Dec 21, 2004
  9. Sam Wormley

    Alan Browne Guest


    Oh dear. First of all, TSO-C129a compliant GPS' have an algorithm called RAIM
    (Receiver Autonomous Integrity Monitor) which will quickly exclude sats that are
    broadcasting screwy data. The GPS signals to the INS when its own solution is bad.

    Secondly, the INS does automatically accept large changes in position, velocity
    or fot that matter Range/Range-rate data from the GPS.

    When the GPS goes down, the INS will be in safe, valid, accurate state.
    Read what I've written about spacing and INS drift rates when unaided.
    There heading doesn't change (after a while the INS HDG error builds but it
    takes a very long time). The airspeed of the aircraft reamians constant if the
    power setting is unchanged (actually it slightly powers back as fuel is burned,
    the aircraft is lighter and less lift is required and hence less induced drag
    occurs, but that's fine detail for this discussion, in short, the auto throttle
    maintains a constant Mach no.)

    It's there. Atlantic Oceanic routes are controlled from Gander and Shannon.
    Pilots make position reports at every 10° longitude crossing (as there is no
    radar).

    [As CNS/ATM develops, this reporting will be automated via SATCOM (Some routes
    have this in limited test already). The enabler here is mainly SATCOM]

    Any speed change would be at ATC request via HF on SELCAL. But as I said
    elsewhere, in such an emergency, the best course would be to moniotr, not to
    begin changing things. Only new aircraft entering the system would be delayed
    to transition to a lower density of traffic in the system.
    See above. Most Oceanic routes are covered. There remain areas that hardly
    need it as traffic density does not merit it.

    Cheers,
    Alam
     
    Alan Browne, Dec 21, 2004
  10. Sam Wormley

    Alan Browne Guest

    See other replies I've given you in this respect. GPS is not the sole means of
    navigation.
     
    Alan Browne, Dec 21, 2004
  11. Sam Wormley

    Alan Browne Guest

    Coincidence: I ran into one of the engineers from my old company below and he
    confirmed the theta/rho (VOR/DME) was not used as an RNAV solution for the
    reason cited above.

    Cheers,
    Alan
     
    Alan Browne, Dec 21, 2004
  12. Sam Wormley

    Fafnir Guest

    nowadays...

    A gyroscopic compass has no concept of either true or magnetic
    north.

    It simply stays pointed in the direction to which you set it.
     
    Fafnir, Dec 21, 2004
  13. Sam Wormley

    Mxsmanic Guest

    It's a problem for the military, whence the motivation for GPS.
     
    Mxsmanic, Dec 21, 2004
  14. Sam Wormley

    Mxsmanic Guest

    Try it and see.
     
    Mxsmanic, Dec 21, 2004
  15. Sam Wormley

    Mxsmanic Guest

    It is the only means of navigation that will allow an aircraft to
    navigate indefinintely without land radio contact and with precision
    great enough to allow commercial traffic levels at modern densities
    while doing so.

    If GPS is so unnecessary in so many situations, why did the military
    spend billions of dollars to develop it?
     
    Mxsmanic, Dec 21, 2004
  16. Sam Wormley

    Mxsmanic Guest

    Famous last words.
    That was the basis for my statement.
    If the INS error is building, the heading is changing.
    Gander and Shannon don't reach the Pacific. Line of sight transmission
    at 30,000 feet is about 300 nm, as I recall.
    What ATC?
    Ah ... most, therefore not all?
    Circular reasoning, no? It matters a lot to the aircraft in those
    areas.
     
    Mxsmanic, Dec 21, 2004
  17. Sam Wormley

    Iolaos Guest

    It does if you're in RNP 10 airspace more than 6 hours from land.
     
    Iolaos, Dec 21, 2004
  18. INS doesn't go bad THAT fast, unless you're feeding it wrong
    information. This thread is about turning GPS off, however, not about
    turning SA back on (with ludicrously huge errors, too)

    Juergen Nieveler
     
    Juergen Nieveler, Dec 21, 2004
  19. Sam Wormley

    jon Guest

    You need to go learn what "Integrity" means. Perhaps start with what it
    meas in the aviation navigation realm, before tackling any other realms
    ;)
    solution is bad.

    There's that word "Integrity" that keeps popping up. Alan, you seem to
    have a thing for that word, don't you? ;)
    state.

    Oh, so if I understand you correctly, a GPS/INS hybrid operates with
    INS serving as the primary and receving updates from GPS (when
    available sufficient integrity). And this allows for periods of GPS
    unavailability.

    Thanks for clearing things up for me, Alan. I was about to fly (heh)
    off the handle. :p
    I recall something like 1/100th of a degree per hour (gyro) which
    translates to something like around 1 NM per hour?
    Just a shot in the dark here Alan (most of the trans-oceanic flight
    I've been on have been at night ;), but let me see if I grasp the
    concept:

    So, the lateral separation standards as well as the in-trail separation
    standards are set sufficient high enough to account for the worst-case
    error rates... and then some? I think I remember seeing an acronym TSE
    (Total System Error).

    So, assuming I'm flying Logan -> Frankfurt for a visit with the good
    folks over at www.dfe.de , and one of my colleagues is returning home
    on the 10:30AM (I bet it's still Flight #LH422 ), there's enough pad
    built into the design of the routes and the air traffic requirements
    and procedures such that if we were crossing the ocean at the same time
    in 0 visibility AND we both lost GPS (Bush had a brain synapse and it
    shut down, so backup receiver not an issue) for the entire time AND we
    both lost every other navaid except INS....<takes a deep breath> AND we
    both lost all of our voice comms after going out of radar contact so
    both ATC was plotting increasingly less confident estimates of our
    positions AND we couldn't boradcast any kind of "May Day" to any other
    aircraft in the area who might be within range but far enough away to
    engage evasive maneuvers AND we both lost our Collision Avoidance
    goodies....... so, both of our INS' had been drifting the most it
    possibly could before we get "near" each other (i'm assusming somewhere
    around 3 or so hours each, given head/tail winds)..... that we STILL
    wouldn't close enough to each other?

    I'm not a pilot, but I want to be able to eat my little snacks on board
    with some degree of comfort level, Alan. Please advise.

    ;)

    Ah, radio comms. Isn't technology great?
    Ok, so what you're saying is the they make noises with their vocal
    chords using a communications link to another human. And the controller
    is able to know last reported position, heading, speed, etc. and
    determine future position (to w/in a different set of tolerances than
    GPS or INS, granted), yes?

    So, could that whole link with human and another part of the RF
    spectrum, taken togehter, be considered another navaid?
    I was under the impression that oceanic separation standards are
    greater for some reason, Alan? Do you think it has anything to do with
    contingency planning for outages (of any of one or more "navaids")?
    Regards,
    Jon
     
    jon, Dec 21, 2004
  20. Sam Wormley

    Alan Browne Guest

    Again, while GPS functions, yes, higher densities will be attainable but the
    seperation distance will not shrink in proportion to the nav accuracy
    improvement. If this fantasy GPS outtage occurs, the drift rates in the INS'
    will still keep everybody happily separated for 10's of hours, as I've
    illustrated to you in other posts. New aircraft entering the system (during the
    proposed emergency) may be delayed to increase separations for the duration of
    the outtage.

    This whole discussion is also missing the CNS/ATM evolution of which SATCOM is
    another enabling component. In a nutshell, one part of this allows for the data
    linking of position reports to oceanic controllers automatically and at a higher
    rate than the HF voice relayed reports. This will allow oceanic controllers a
    near real time view of the air traffic, and hence narrower (but by no means
    'real close') separations between aircraft. The positions of aircraft are
    reported by the FMS, which in turn is using the INS or GPS PPOS. And (again),
    unbounded INS errors would take tens of hours to result in a lateral collision,
    requiring perfectly opposing drift directions... not at all likely on many
    counts. Regarding traffic fore and aft, well, they're traveling at the same
    speed and direction and separated by 60NM or so, so impossible to colide.

    When approaching land many variations on the theme can occur. One is flying a
    radial towards a single VOR station using the VOR receiver in which case the
    error is well within airways tolerances. VOR's can be tuned at 35,000 from
    about 150-200 NM away. Another is that as soon as an acceptable geometry occurs
    a DME-DME update to the position will reduce the error well within acceptable
    accuracy.
    They didn't develop it for commercial use, but they were politically astute
    enough to allow the C/A to be non-encrypted for civil use in order to secure
    funding. Prior to GPS commissioning as a full constellation, commercial
    airlines were doing quite fine with INS and VLF/Omega for long range navigation,
    the airways system (NDB, VOR) for short range over land, and the Microwave
    Landing System (MLS) was in development to improve upon the post WW II ILS
    system that was (and still is) the precision approach system.

    Why did the military develop GPS? In the mid 70's, when the whole idea got
    started, there were dozens of navigations systems from short range radio
    navigation (VOR, DME, TACAN, NDB, RNAV, LOC, ILS, ...), INS, emerging VLF/Omega,
    Doppler Radar, LORAN-A/C/D and probably a few others I can't remember. The Brits
    had other systems such as ... hmmm, can't remember, but similar to LORAN.
    Decca, I think.

    The systems all had their own data formats and performace differences according
    to their principle of operation. There were wonderfully complex integrations
    (Doppler Radar to bound INS velocity drift; Omega/VLF to bound INS position
    drift). Datalinks had been emerging sicne the 60's, but inter service system
    integrationw as extremely difficult. (It was bad enough within groups within
    the same service). This doesn't even begin to touch on the needs of the surface
    Navy, grunts, artillery, cavalry and the emerging army aviation needs for
    accuracy. Systems like DME, TACAN and Doppler radar are also emmisive,
    requiring the aircraft to transmit to use the system, and this is taboo in
    modern warfare: be passive. (TACAN, was a double jeopardy as ships at sea had
    TACAN stations, as well as tanker aircraft. "I'm here, shoot me")

    DOD recognized the problem and decided to get "above it" and go for one system
    that could satisfy almost all needs for all services. GPS. It would provide
    accurate navigation and timing and help all services evolve to a common
    communications format for everything navigation related. (They're still not
    there... a destination that won't be reached fully for another 10 years, I
    expect). And all 'users' have passive receivers.

    (ILS with only 40 channels was experiencing congestion in some high density
    regions like LA and certain parts of Europe and Asia. Further, MLS was much
    more precise than ILS and could cover a much larger volume of airspace... but,
    the promise of what has become LAAS has put MLS to sleep... there are dozens of
    MLS appraoches commissioned out there, and the US Mil has portable MLS ground
    stations to setup approaches where needed. About 1100 airborne receivers that I
    know direclty about were sold to the USAF. There are a few commercial operators
    still using it as well, usually with privately owned ground stations).

    GPS is of course the wunderkind of navigation for everyone. The expensive stuff
    is done by the US taxpayer (Thanks!) in the space and ground segment. The
    receiver end is relatively cheap. (Even when second generation 5 channel GPS
    receivers cost over $20K, it was considered cheap). Today, for under $1000 a
    grunt in the field can have his keyed receiver courtesy of the DOD.... and any
    civilian can have a Wal*Mart special for $100 or so.

    That's why GPS came about. Not as a grand design to help aviation. That was
    secondary and a small part of justification to get funding. (Initial GPS
    thought was that only the US+Allies would have access.)

    Cheers,
    Alan
     
    Alan Browne, Dec 21, 2004
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