You need to go learn what "Integrity" means. Perhaps start with what it
meas in the aviation navigation realm, before tackling any other realms
For an air transport GPS receiver (TSO-C129a compliant), integrity comes from
several sources including RAIM (Receiver Autonomous Integrity Monitor). That,
in a simplistic nutshell, validates each satellite pseudo-range as fitting the
current PPOS within a certain tolerance. If it fails to meet that tolerance,
the satellite data is deemed unsafe and not admitted into the PVT solution.
(You would need a minimum of 5 sats to determine which particular one is unsafe;
4 sats would result in just saying the GPS nav solution is invalid if one of
them was invalid ... unless of course the altimeter was also being used as a
virtual ctr-of-the-earth satellite (and this is a common feature of most air
transport GPS'). Given all of the above, when you fall below 4 validated sats
(incl. altimeter) the solution is then invalid and flagged as such.
(I don't recall offhand how 2D solutions are flagged in a TSO-C129a receiver.
If integrating a GPS to an FMS or INS, I would reject such solutions and go INS,
RNAV or even DR (in that order) and wait for the number of available sats
solution to improve.)
SBAS (WAAS) provides further integrity warnings where available.
INS, before accepting the GPS data, verifies that it is close to the predicted
value that it should be. If it is outside of tolerance, it is not used for the
update. This can be quite simple or part of a Kalman filter, which in turn has
many checks before taking in data for the nav solution.
All of the above is occuring at at least 1 Hz.
There's that word "Integrity" that keeps popping up. Alan, you seem to
have a thing for that word, don't you?
Integrity in an Air Transport navigation solution is crown prince, if not king.
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.
Yes. Although "federated" would be the right word, as opposed to hybrid.
Hybrid INS/GPS is a deeper solution with more of the GPS data (including pseudo
range and psudo-range-rate being more directly integrated into the INS. I can't
describe it much further). For shorter range aircraft (non-Oceanic), this means
lower grade INS (lower cost) can be integrated.
Thanks for clearing things up for me, Alan. I was about to fly (heh)
off the handle.
Why?
I recall something like 1/100th of a degree per hour (gyro) which
translates to something like around 1 NM per hour?
Not sure. In any case you have a lot of things drifting in an INS. The
platform drift (HPR angles) and accelerometer integration (I mean literally
stepwise integration of the accelerometers to get velocity data) bias and random
walk, there is a thing called the Schuler oscillation which can be likened to a
pendulum with a lenght of 1/2 earth diameter and this drags the platform around
(I don't know the physics at all of this).
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:
(Most E-W are overnight, most W-E are during the day (N. Atlantic).
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
East and westbound a/c are at different altitudes. Usually the visibility is
quite good at 30 - 40,000'
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
Nope. Why would you lose voice comm. There are two means: the classic HF,
still in use and SATCOM.
There is no radar whasover over the ocean (for ATC, anyway).
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?
Yep. But more like 20 hours. Winds have nothing to do with it. You're all in
the same air and the fact that it is moving relative to the earth has nothing to
do with all the aircraft in flight in that same air. (Two boats on the river
will both drift with the river and not colide due to the river movement).
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.
Enjoy. Though do find business class standards have fallen in the past few
years, and now plastic cutlery!
there is no
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?
Yeah. They're main concern (ATC) is separation and 'feeding' the flights into
the European airspace (or NA coming back), and in feeding the continental
traffic into the Oceanic. Once there, everyone follows the protocol and
everyone is happy.
So, could that whole link with human and another part of the RF
spectrum, taken togehter, be considered another navaid?
No. Since the ATC component doesn't even *know* where you are based on a voice
report, they can't 'guide' you. In the CNS/ATM env., the nav system of the
aircraft is reported to them by datalink, but you have the same data anyway.
ATC will just have a more up to date and accurate position for each aircraft.
This will permit tighter separations, but by no means close.
I'm a commercial pilot (light stuff) and flight instructor (not currently).
When I was 15 and learning to fly, my father (a pilot) said something like
'pilots need to do 3 things, and in this order: "Aviate, Navigate and
Communicate"'.
And that is really how it is all set up.
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")?
I'm not sure what you're referring to specifically. Over land and under enroute
facilities covered by radar, the separation can be closer. For instance, the
RNP in Europe is now 5 NM (nav system accuracy) and the altimeters are required
to be more accurate such that less vertical spacing can be used togehter with
less lateral sep.
Oceanic requires greater spacing because ATC does not 'see' the aircraft, and
because the accuracy of the nav systems is not as great. This is now being
tightened up with SATCOM allowing a closer to real time snapshot of where
everyone is, more accurate nav (GPS) and TCAS which alerts aircraft to other
aircraft in the vicinity along with collision prediction.
Ooops. I left TCAS out of the whole discussion didn't I? In the remotest
chance of two aircraft on a collision course, both crews will be informed, and
in fact told what to do by the TCAS system.
Cheers,
Alan