Let me begin with a quote worth repeating — “Do we really need to wait for a catastrophe before taking action against GNSS vulnerabilities ?” — and follow with an extension of scope beyond.

It’s encouraging to see LinkedIn discussions recognizing ADSB limitations that preclude dependable collision avoidance capability – but that recognition needs to be far more widespread. The limitations are both severe and multifaceted including, in addition to vulnerability from inadequate security,
* accuracy goals based on present position instead of the monumentally more important relative velocity — ADSB allows 10 meter/sec velocity error (!), without characterization as vectorial or relative or probabilistic.
* the glaring but near-universal flaw of sharing coordinates, thereby failing to exploit what made differential operation spectacularly successful: work with individual measurements separately.
Note that these deficiencies existed long before the emergence of unmanned vehicles. The need to correct them is as fundamental as it is urgent. I’ve communicated these concerns over and over, most recently receiving a gratifying response from my June 11 presentation to the satnav National Advisory Board, with details available from URLs at the end.
In that presentation I cited a successful flight validation achieving accuracy on the order of cm/sec, for the crucially important relative velocity between vehicles that can be on or near a collision course. That is a thousand times less error than the 10 meter/sec allowed by ADSB. Furthermore, reduction by a thousand in each of three directions translates into a billion times less volume of uncertainty — or, in just two dimensions at fixed altitude, a million times less area. To realize this crucial safety improvement no new discoveries are needed and no new equipment needs to be invented; only the content of transmitted data needs to change: measurements rather than coordinates. Yet usage of the method is not being planned. After initially proposed before 2000, a limited support program started within the past few years is the only step taken toward this direction.

No claim is made that the last word has been spoken or that introduction of the needed modifications — nor accompanying regulation — would be trivial.  The intent here is not criticism and complaints for the sake of criticism and complaints.  Emphasizing unwelcome reality always caries risk of drawing wrath.  Nevertheless, especially now with growing usage of unmanned vehicles, sounding an alarm is better than passively waiting for a calamity. So here’s an alarm: Inadequate preparation for collision avoidance is a microcosm of a much wider overall flaw in today’s decision-making process. For years substantial numbers of qualified people have spent extensive effort trying to prevent cataclysmic failures in one area or another involving PNT (position/navigation/timing).  They definitely deserve attention and action.

Anything approaching a thorough compilation of worthy advocacy would require considerable length; just a few recent examples are cited here.  Explanations tracing inaction to current shortcomings can logically include a diagnosis of dissatisfaction expressed at a pinnacle of authority within DoD. An even more current offering is only the latest expression of regret over insufficient support for satnav, describing a highly relevant chain of inaction over a multiyear period. Near the beginning of that period, a cover story for Coordinates magazine repeated a quote from the previous month’s cover story   The quote worth repeating, cited at the start of this, is a perfect expression of the frustration prevalent over a decade following the universally acclaimed 2001 Volpe report. Now, almost a decade-and-a-half after that report, partial progress toward a solution coexists with minimal progress toward collision avoidance — while unmanned vehicles are already threatening passenger flight safety. Now to extend the quote: “Do we really need to wait for a catastrophe before making better use of measurements — GNSS or otherwise — to prevent collisions in the presence of increased manned and unmanned traffic?”

Schuler cycles distorted — Here’s why

1999 publication I coauthored took dead aim at a characteristic that received far too little attention — and still continues to be widely overlooked: mechanical mounting misalignment of inertial instruments.  To make the point as clearly as possible I focused exclusively on gyro misalignment — e.g., the sensitive axes of roll, pitch, and yaw gyros aren’t quite perpendicular to one another.  It was easily shown that the effect in free-inertial coast (i.e., with no updates from GPS or other navaids) was serious, even if no other errors existed.

It’s important here to discuss why the message took so long to penetrate.  The main reason is historic; inertial navigation originated in the form of a gimbaled platform holding the gyros and accelerometers in a stable orientation.  When the vehicle carrying that assembly would rotate, the gimbal servos would automatically receive a command from the gyros, keeping the platform oriented along its reference directions (e.g., North/East/vertical for moderate latitudes).  Since angular rates experienced by the inertial instruments were low, gyro misalignment and scale factor errors were much more tolerable than they are with today’s strapdown systems.  I’ve been calling that the “Achilles’ heel” of strapdown for decades now.  The roots go all the way back to 1966 (publication #6) when simulation clearly showed how serious it is.  Not long thereafter another necessary departure from convention became quite clear: replacement of the omnipresent nmi/hr performance criteria for numerous operations.  That characteristic is an average over a period between 83 and 84 minutes.  It is practically irrelevant for a large and growing number of applications that depend on short-term accuracy. {e.g., synthetic aperture radar (SAR), inertial aiding of track loops, antenna stabilization, etc.}, Early assertions of that reality (publication #26 and mention of it in still earlier reports and publications involving SAR) were essentially lost in “that giant shouting match out there” until some realization crept in after publication #38.

Misalignment: mechanical mounting imprecision

Whenever this topic is discussed, certain points must be put to rest.  The first concerns terminology; much of the petinent literature uses the word misalignments to describe small-angle directional uncertainty components (e.g., error in perception of downward and North, which drive errors in velocity).  To avoid misinterpretation I refer to nav-axis direction uncertainty as misorientation.  In the presence of rotations, mounting misalignment contributes to misorientation.  Those effects, taking place promptly upon rotation of the strapdown inertial instrument assembly, stand in marked contrast to leisurely (nominal 84-minute) classical Schuler dynamics.

The second point, lab calibration, is instantly resolved by redefining each error as a residual amount remaining due to calibration imperfections plus post-cal aging and thermal effects — that amount is still (1) excessive in many cases, and (2) in any event, not covered by firm spec commitments.

A third point involves error propagation and a different kind of calibration (in-flight).  With the old (gimbal) mechanization, in-flight calibration could counteract much overall gyro drift effect.  Glib assessments in the 1990s promoted widespread belief that the same would likewise be true for  strapdown.  Changing that perspective motivated the investigation and publication mentioned at the top of this blog.

In that publication it was shown that, although the small-angle approximation is conservative for large changes in direction, it is not extremely so.  The last equation of its Appendix A shows a factor of (pi/2) for a 180-deg turn.  A more thorough discussion of that issue, and how it demands attentiveness to short-lived angular rates, appears on pages 98-99 of GNSS Aided Navigation and Tracking.  Appendix II on pages 239-258 of that same book also provides a program, with further supporting analysis, that supersedes the publication mentioned at the top of this blog.  That program can be downloaded from here.

The final point concerns the statistical distribution of errors.  Especially with safety involved (e.g., trusting free-inertial coast error propagation), it is clearly not enough to specify RMS errors.  For example, 2 arc-sec is better than 20 but what are the statistics?  Furthermore there is nothing to preclude unexpected extension of duration for free-inertial coast after a missed approach followed by a large change in direction.  A recent coauthored investigation (Farrell and vanGraas, ION-GNSS-2010 Proceedings) applies Extreme Value Theory (EVT) to outliers, showing unacceptably high incidences of large multiples (e.g., ten-sigma and beyond).  To substantiate that, there’s room here for an abbreviated explanation —  even in linear systems, gaussian inputs produce gaussian outputs only under very restrictive conditions.

A more complete assessment of misalignment accounts for further imperfections in mounting: the sensitive axis of each accelerometer deviates from that of its corresponding gyro.  As explained on page 72 of Integrated Aircraft Navigation, an IMU with a gyro-accelerometer combo for each of three nominally orthogonal directions has nine total misalignment components for instruments relative to each other.


This set of blogs will not be considered complete until at least seventy (or possibly a hundred) are available for visitors to download and/or print.  Each individual blog, with links to references (which in some cases can also be downloaded and printed from this site), summarizes a specific aspect from a chosen set of topics.  A smaller number of these “one-pagers” will address topics from my earlier, more fundamental, book Integrated Aircraft Navigation.  An additional few (very few) will deal with topics not covered in either of those two books.  An example of the latter publicizes some useful facets of the ultra-familiar classical low-pass filter which (believe it or not – after all these years) have remained obscure.

Over time, dozens more will be added from a wide span of topics (all firmly supported by experience as well as theory, ranging from elementary to advanced, in some cases relatively new and therefore largely unknown) that will include

  • Modern estimation in both block (weighted least squares) and sequential (Kalman filtering, with Battin’s derivation – much easier to follow than Kalman’s) form, with their interrelationship developed quite far, enabling “plant noise” levels to be prescribed in closed-form, also providing highly unusual insight into sequentially correlated measurement errors; chi-squared residuals; implications of optimality during transients; need for conservatism in modeling; sensitivity of matrix-vs-vector extrapolation (“do’s and don’ts”); application-dependence of commonality and uniqueness features; quantification of observability and effects of augmentation on it; duality among a wide scope of navigation modes; commonly overlooked duality between tracking and short-term inertial nav error propagation; when “correction-to-the adjustment” terms can and can’t be omitted; suboptimal (equal-eigenvalues) estimation with steady-state performance indistinguishable from optimal; all fully supported by theory and experience
  • Basic building-blocks for attitude expressions: superiority of quaternions and direction cosines over Euler angles, due to singularity (“gimbal lock” at 90-deg for x-y-z sequence) and at 0-deg for z-x-z sequences used for orbits
  • GPS issues related to the top-priority goal of robustness: beyond elementary (4-state and 8-state) formulations; duality of pseudorange and phase ambiguity; exploitation of modern processing capabilities in GPS/GNSS receivers; carrier phase as integrated doppler vs frequency data; 1-sec sequential phase changes (much easier to mix across constellations, negligible sequential changes in IONO/TROPO propagation, ambiguity resolution not needed, instant reacquisition, no-mask angle needed); streaming velocity for dead reckoning with segmentation of position fixes; differential operation – differencing across satellites, receivers, and time; handling correlations from differencing; orthogonalization for simple QR factorization; measurement relocation in time and lever-arm adjustment; E(Extended)RAIM;  D(Differential)RAIM; necessity of weighting in single-measurement RAIM with pseudoranges and carrier phases, concurrently; sample flight test results showing state-of-the-art accuracies in dynamics (e.g., cm/sec RMS velocity error and tenths-mrad leveling) with a low-cost IMU; revisit of the same flight segment, achieving decimeter/sec RMS velocity error without any IMU
  • Tracking (with subdivision into over a dozen topics including a littoral environment operation with hundreds of ships present; orbit determination; usage of Lambert’s laws; surface-to-air (subdivided into ground-to-air and tracking from ships), air-to-surface and surface-to-surface (again with the same subdivision),  air-to-air; reentry vehicles; usage of stable coordinate frames; linearity in both dynamics and measurements; Mode-S squitters for mutual surveillance and collision avoidance in crowded airspace; multiple track output usage (placement of gates, antenna steering, file maintenance); crucial importance of transmitting measurements rather than coordinates (publication #66); extension to noncooperative objects, critical distinction (often blurred) between errors in tracking and stabilization; sucessfully accomplished concurrent track of multiple objects with electronically steered beams; bistatic and multistatic operation; postprocessing to form familiar parameters from estimator outputs; short-range projectiles over “flat-earth” – plus many more)
  • Processing of inertial data – incrementing of position, velocity, attitude; straightforward state-of-the-art algorithms for complete metamorphosis from raw gyro and accelerometer samples into final 3-D position, velocity, and attitude; motion-sensitive inertial instrument errors; coning; sculling; critical distinction between misalignment (imperfect mechanical mounting) vs misorientation; adaptive accommodation of gyro scale factor and misalignment errors; instability of unaided vertical channel; azimuth pseudomeasurement; near-universal misconceptions connected to free-inertial coast
  • Support functions (transfer alignment; SAR motion compensation; stabilization of images; sensor control mechanizations; synchronization; determination of retention probability)
  • Vision-for-the-future with maximum situation awareness for all cooperating participants in a scenario; critical role of interfaces (implications of singularities, RAIM, Differential GPS, etc.), software modularity, reuse, coordination).  Full validation in GNSS Aided Navigation and Tracking.

As a lifelong techie I’m constantly reminded of erratic pacing for changes in our industry. Hardware and software lurch at dizzying rates while advanced concepts, with dramatic potential for exploiting improved technology, languish unused for years. Whether in GPS/GNSS receiver configurations, surveillance, collision avoidance, or various other areas,  needed solutions await industry’s willingness to change the status quo.  A basic function in today’s systems is source-to-destination data transmission. Quite often an urgent need can be met, not by more precision nor higher data rates nor larger capacities, but simply a different selection of information content.

Space limitations preclude full elaboration here; see other parts of this site and the references cited below. Although today’s modus operandi limits both military and commercial systems. I’m not implying that inertia plus oversimplification in methodologies are entirely to blame for “missing the boat” in all instances.  Additional factors are well known (e.g., safety often requires smooth – thus, coordinated – “old-to-new” transitions).  It is striking, though, to witness how much effect the one facet noted above (selection of information content) can exert on overall performance.  I elaborate on that in several publications – some available on this site.

No criticism is intended nor implied here; yesteryear’s designs lacked access to today’s technology, and other lifelong techies have a different set of uncommon insights (not unusual).  To fortify claims just made, I’ll do two quick things. First, for just one of many topics with potential (but unused) enormous improvement I’ll show at this site – a recognized real-world example: collision avoidance, in both two (runway incursions) and three (near miss in-air) dimensions.  Second, in addition to the 100+ book pages viewable from this site, I cite a small but representative fraction chosen from about 90 manuscripts I wrote or coauthored:

  1. “System Integration: Performance Doesn’t Measure Up,” NAECON Symposium, Dayton Ohio, 1993 —       later printed in IEEE-AES Systems Journal
  2. “Send Measurements, not Coordinates (Co-au)” IONJ, Fall 1999, pp. 203-215
  3. Unfinished Business–Glaring Absences from the Achievement List IEEE PLANS, Monterey CA 2004
  4. “ADSB (2nd-) Best Foot Forward?” (Co-au), Air Traffic Control Journal, v50  Summer 2008, pp 17-18.
  5. InsideGNSS Fall 2008, pp.29-32
  6. GPSWorld Dec 2009, pp. 8, 10, 12
  7. Robust Design for  GNSS Integration ION-GNSS, Savannah GA, Sept. 2008
  8. Aging SV’s – We Have Solutions ION-GNSS, Savannah GA, Sept. 2009

In applications across-the-board (in-air, maritime, space-related, or on land), depth of insight despite complexity is a make-or-break factor. Although that merely states the obvious, we repeatedly observe adherence to older techniques that could not capitalize on capabilities offered by recent technological advances.  In addition to the previously mentioned “slower-is-safer” constraint it is instructive to consider some further restraints:

  • Up-front needs face resistance from creatures of habit with short-term focus.
  • Younger workers, brilliantly adept with computers (operating systems, data flow, etc.) are less familiar with the functional intent of the design.
  • Many system designers have the “shoe on the other foot” (versed in theory but lacking depth of software coding or computer operations in general).
  • Emphasis on management technique produces decision-makers with insufficient technical preparedness.

These challenges must be met to avoid failure, as described in the sixth reference cited above which ends by stating “The industry can either adopt changes or continue to settle for performance levels at a minor fraction of the intrinsic capabilities available from our present and future systems.” Claims I make here can invite much skepticism. Fair enough, but those willing to explore in depth the references just cited will see potential for unprecedented benefits.