Expert Advice: From the Front Lines of Interoperability

Expert Advice: From the Front Lines of Interoperability
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3734


Jan 1, 2009
By: James C. Litton
GPS World

For all the talk about and good will towards interoperability of GNSS sys=
tems, one might=20
assume that such hands-across-various-waters agreements are nearly or tac=
itly=20
accomplished. While this is not out of the question, neither can it be te=
rmed as within=20
sight. Detailed discussions at the December 8=E2=80=9312 meeting of the I=
nternational Committee on=20
Global Navigation Satellite Systems (ICG) in Pasadena revealed how far we=
have to go. I=20
attended the meeting and witnessed a few of the devils at work in these d=
etails.

The ICG was established on a voluntary basis as an informal body to promo=
te cooperation on=20
matters of mutual interest related to civilian satellite-based positionin=
g, navigation,=20
and timing. The committee=E2=80=99s mandate is to encourage compatibility=
and interoperability of=20
global navigation satellite systems by the countries that launch and main=
tain them, and=20
promote the use of terrestrial GNSS technologies to support sustainable d=
evelopment,=20
particularly in developing countries.

From the perspective of a civil user or that user=E2=80=99s equipment su=
pplier, the progress made=20
toward agreement on the very definition of compatibility and interoperabi=
lity by the work=20
group established at ICG-2 in Bangalore for ICG-3 in Pasadena (in this ca=
se Working Group=20
- A or WG-A), does not encourage optimism about the potential outcome fro=
m ICG-4 in St.=20
Petersburg, scheduled in 2009. Working Group - D (WG-D), the group concer=
ned with=20
international geodetic reference systems and time steerage, had the great=
er momentum=20
toward agreement. Working Group - B (WG-B), established to agree upon the=
enhancement of=20
performance of GNSS services, has a complex task that involves establishi=
ng waveforms and=20
other multipath mitigation and refraction compensation methods. WG-B firs=
t met in=20
Bangalore for this purpose in September 2007.

As an aside, one might also note that the military user and equipment sup=
plier cannot be=20
wholly satisfied that adequate firewalls or moats will exist, either. But=
in all=20
likelihood, that user=E2=80=99s provider will be able to negotiate a modu=
s vivendi bilaterally,=20
rather than to reveal its requirements in detail to all providers and the=
ir beneficiaries.=20
There were repeated references to the standards and regulations imposed b=
y other=20
international regulatory groups, such as the International Telecommunicat=
ions Union (ITU);=20
however, it was clear that in the critical areas of compatibility and int=
eroperability=20
(especially the latter), the broader and less detailed such standards and=
regulations are,=20
the better.

The concentration on English syntax and semantics would have graced a mas=
ter=E2=80=99s class in=20
that subject, including contributions expressed in Russian to be translat=
ed by the Russian=20
co-chair of the WG-A, Sergey Revnivykh. (The other co-chair is David Turn=
er of the U.S.=20
National Executive Committee for Space-Based Positioning, Navigation, and=
Timing). One key=20
factor is that the satellite systems are defined and in various stages of=
operation and=20
are currently compatible (receivers already exist for joint operation), a=
nd that ideal=20
interoperability will be given much lower priority than the requirements =
of national=20
security and national or regional leadership.

Interoperability. WG-A has determined that negotiations toward interopera=
bility will be=20
bilateral, not multilateral, giving each pair of competitors the opportun=
ity to forge=20
agreements which can become precedents for all. Given the momentum of the=
separate=20
systems, this approach seems most likely to result in compromise than one=
in which all=20
interests are presented simultaneously.

Ideal interoperability was perhaps most eloquently expressed by Professor=
Brad Parkinson=20
during his keynote presentation at the plenary session, as being =E2=80=9C=
=2E . . a single receiver=20
which could obtain an accurate four-satellite solution from one satellite=
from each of the=20
four global constellations: GPS, Galileo, GLONASS, and Compass.=E2=80=9D =
The concept that such a=20
receiver would be minimally different from one that would obtain such a s=
olution from only=20
a single constellation was not defined, but to pursue the notion of ideal=
ity to, perhaps,=20
an impractical level, such a difference would seem to be a self-actuated =
software function=20
on the best available signal. That computer processor technology and soft=
ware technology=20
are rapidly evolving towards a software GNSS receiver should enable concu=
rrent development=20
or technology insertion in future generations of user equipment, which ca=
n adapt to a=20
greater level of interoperability as market and political conditions allo=
w.

Jamming. The elephant in the room about which nobody was talking is the c=
apacity to jam=20
the systems of enemies, and to make the jamming of one=E2=80=99s own syst=
em much harder. The=20
greater the reliance on smart weapons, instantaneous targeting, continuou=
s surveillance,=20
and electronic warfare, the greater the value of these measures and their=
countermeasures.

At the same time, the increasing reliance on accuracy and availability fo=
r the civil user,=20
some of which is driven by civil defense requirements, compels adequate a=
llocation of=20
spectrum and power and the use of sophisticated coding methods. Both regi=
mes are supported=20
by the same technologies, the cost of which must be recouped by applicati=
ons that generate=20
value in times other than those of conflict. Nevertheless, there was a cl=
ear resolve to do=20
the fact-finding to determine the extent to which interferences generated=
by systems that=20
were nominally or fully compliant with ITU regulations. It was officially=
noted at the=20
initiative of the European Union members of WG-A that ITU regulations did=
not provide=20
adequately precise standards for complete avoidance of interference.

It would be tendentious of me to report the specific debates on wording, =
all of which were=20
carried out in a very civil manner. David Turner, with no disagreement, e=
xpressed in WG-A=20
a need to have =E2=80=9C. . . at least two more [WG-A] meetings before St=
=2E Petersburg. . . =E2=80=9D The=20
need in many applications to have both spectral diversity and common cent=
er frequencies=20
was noted, again (as in IGC-2), with the caveat that it may not be feasib=
le to achieve=20
both. It seems that while there is skepticism regarding the processes and=
their speed,=20
there is little cynicism about the objectives of the ICG or whether the U=
nited Nations is=20
the best institution to try to achieve them.

The upshot of the meeting is that the creation of a system of GNSS system=
s will not be=20
seamless and will not evolve continuously; however, the agreement by Work=
ing Group-C on=20
reference frames and time steerage toward International Terrestrial Refer=
ence Frame and=20
Coordinated Universal Time without leap seconds may well be a pattern tha=
t can be followed=20
to achieve as much compatibility and interoperability as national and reg=
ional interests=20
will allow.

From a commercial perspective, being able to encourage and potentially i=
nfluence the=20
adoption of such ubiquitous methods of measurement, enabling manufacturer=
s to more=20
effectively utilize multiple signal sources in a single platform, is extr=
emely important.=20
This is especially true for companies like mine that focus on providing t=
he ability to=20
utilize any and all signals as they become available, in order to maximiz=
e the benefits to=20
civil users around the world =E2=80=94 and after all, they are the ones p=
aying for these systems.

JAMES C. LITTON is business development manager for Septentrio Satellite =
Navigation and is=20
based in Los Angeles, California. Previously, he worked at NavCom Technol=
ogy.