GRATIFYING RESPONSES

After wide distribution of my recent InsideGNSS letter I’ve received very
encouraging responses from a number of heavy hitters. There have always been
knowledgeable individuals agreeing with the points raised therein, but current
conditions offer an increased sense of urgency. With uncertainty of support
for vital resources, a real-world precedent (five years without LORAN), and a
Defense Secretary who hates GPS, my impulse toward advocacy has grown more
determined; in fact, crystallized. Not everyone will welcome this, but it
will go down much easier if viewed as a vital opportunity, Here goes.

Among the methods awaiting basic modification for navigation and tracking, one
is especially glaring: the ubiquitous practice of sharing coordinates. Those
familiar with my work know me as a relentless advocate (in print, since 1977)
of sharing raw measurements instead of coordinates. The seemingly unremarkable
character of that step is deceptive; despite its operational simplicity, the
resulting improvements would be profound. For quick verification of that claim
recall how major errors cancel (from each satellite separately of course) in
differential GPS — and that’s only the beginning.

As important as accuracy is, additional performance traits of equal importance
are also dramatically affected. Without separate measurements, integrity
testing can’t be done. Furthermore, with partial data usage, two more main
performance criteria (availability & continuity) would be vastly improved —
in fact, calling for their redefinition to account for the immense benefit.

The list of reasons (rigorous accounting for correlations as well as different
statistics of errors in different directions, at different times, from sensors
with different tolerances; immunity of scalar measurements to an occasionally
misconstrued reference datum etc.) continues on and on. Among those not yet
mentioned here, I now choose an especially important feature for illustration:
ability to achieve precise dynamics. Flight tests by Ohio University produced
cm/sec velocity residuals for navigation (GNSS Aided Navigation & Tracking,
with results in Chapter 8 and public domain algorithms in earlier chapters),
then later for trackinga THOUSAND times better than ADSB’s 10 meter/sec.

It’s not as if we didn’t know how to accomplish these objectives. We’ve known
how to combine myriad data sources, sequentially and optimally, for well over
a half-century. Yet even now, given information from two different subsystems
(e.g., GNSS and DME), how are they processed now? Either internally (and
invisibly in costly inflexible embedded systems) or externally by averaging
coordinates. A most elementary example highlights futility of the latter:
imagine data from one sensor offering precise latitude and extremely degraded
longitude — mixed with another offering the opposite.

The fundamental nature of these reasons is matched by an equally fundamental
course of action needed to achieve the requisite goals: simply replace data
bits in standard messages. No scientific breakthroughs nor hardware redesigns
— just change what’s transmitted by UAT or Mode-S extended squitter messages.
Most of the content (preamble, error correction, etc.) can remain unchanged;
just replace information bits (latitude, longitude, etc.) by measurements.

The case is quite compelling for application of known methods, to not only
satnav but all sources of data to be used in navigation and tracking. All
benefits will become reality if we adopt, VERY belatedly, the basic step
recommended in the title of a 1999 publication

Secretary of Defense Ashton Carter’s recent statement “I hate GPS” naturally creates much concern within the navigation community. The July-August issue of InsideGNSS contains his presentation with the reaction from editor Glen Gibbons, plus my own response which delineates
* where the Secretary is badly mistaken, and
* where his concerns are legitimate.
There is a connection between the latter and our industry’s decades-long determined resistance to common-sense improvements in both performance and economy. Steps offering dramatic benefits are further described in material long available from this site. Rather than repeat those descriptions here, I now focus instead on another kind of avoidance: an urgent need to swerve away from another administrative blunder.

Recent history illustrates how the preceding expression is no exaggeration. Loss of LORAN wasn’t permanent, for reasons that were primarily capricious. Planned destruction of vital backup to a vulnerable pillar for communication and navigation wasn’t completed because the government never got around to finishing it. Hundreds of experienced professionals offered testimony in 2009 (including my “two cents’ worth” revisited ) — which failed at the time. Administrative action shut down LORAN for years, with intent to destroy it.

Poor judgment, however, is not the sole cause of unwise administrative action. Often it is prompted by poor performance; the GAO-08-467SP report provides a perfect explanation of that. Dismal as it is, it must be believed that even gross departures from responsible stewardship can be corrected. Destroying a critical resource is obviously not the answer.