VS stoppen met opzettelijke verslechtering van GPS
GEODESIA 2000-6
Per 1 mei 2000 heeft president Clin
ton besloten dat de VS stopt met de
opzettelijke verslechtering van het
GPS-signaal (zie kader). Deze zoge
naamde Selective Availability (SA)
hield in dat baan- en klokfouten wer
den aangebracht, zodat autonome
plaatsbepaling met GPS voor de civie
le gebruiker minder nauwkeurig was
dan mogelijk. Deze verstoring kon
weliswaar grotendeels teniet worden
gedaan door gebruik te maken van dif
ferentieel GPS (DGPS), maar dat was
alleen weggelegd voor professionele
gebruikers, en dat bracht extra kosten
en ongemakken met zich mee.
Dat SA zou worden uitgezet, was al
enige tijd duidelijk: in 1996 kondigde
Clinton al aan dat SA vóór 2006 zou
worden uitgezet. Op 2 mei 2000, om
4.05 uur UTC, oftewel 6.05 uur Ne
derlandse tijd, was het zover. In fig. 1
dr. ir. H. van
der Marei,
afdeling
Geodesie
TU Delft.
Tabel 1.
De tekst van het
door het Witte
Huis uitgegeven
persbericht.
is te zien dat om 4.05 uur de fout in de gemeten afstand
naar de satellieten dramatisch afneemt. Duidelijk zichtbaar
in de grafiek is dat SA een periodieke fout in de afstand
naar de GPS-satelliet geeft, die ook nog per satelliet ver
schilt. Dit is vooral het gevolg de zogenaamde 'clock-dithe-
ring', het opzettelijk verslechteren van de satellietklok. De
fout in de door de GPS-satellieten uitgezonden banen was
altijd al veel minder.
Staandaardafwijking
95% percentiel
2D-positie
Hoogte
2D-positie
Hoogte
0.00-4.05 uur
32 m
56 m
66 m
109 m
4.05 - 8.10 uur
3,1 m
5,4 m
5,2 m
12 m
Het effect van SA op de berekende posities is te zien in
fig. 2. In het bovenste deel van de grafiek staan de 3D-posi-
ties van 2 mei 2000 0.00 - 4.05 uur UTC, toen SA nog aan
stond, en in het onderste deel van 4.05 - 8.10 uur, toen SA
net uit stond. De nauwkeurigheid van (autonome) plaats
bepaling is met een factor 10 verbeterd. De verbetering in
Statement by the president regarding the US dec
Today, I am pleased to announce that the United States will
stop the intentional degradation of the Global Positioning
System (GPS) signals available to the public beginning at mid
night tonight. We call this degradation feature Selective Avail
ability (SA). This will mean that civilian users of GPS will be
able to pinpoint locations up to ten times more accurately than
they do now. GPS is a dual-use, satellite-based system that
provides accurate location and timing data to users worldwide.
My March 1996 Presidential Decision Directive included in
the goals for GPS to: encourage acceptance and integration of
GPS into peaceful civil, commercial and scientific applications
worldwide; and to encourage private sector investment in and
use of U.S. GPS technologies and services. To meet these goals,
I committed the U.S. to discontinuing the use of SA by 2006
with an annual assessment of its continued use beginning this
year.
The decision to discontinue SA is the latest measure in an on
going effort to make GPS more responsive to civil and com
mercial users worldwide. Last year, Vice President Gore an
nounced our plans to modernize GPS by adding two new civi
lian signals to enhance the civil and commercial service. This
initiative is on-track and the budget further advances moderni
zation by incorporating some of the new features on up to 18
additional satellites that are already awaiting launch or are in
production. We will continue to provide all of these capabili
ties to worldwide users free of charge.
My decision to discontinue SA was based upon a recommen
dation by the Secretary of Defense in coordination with the
Departments of State, Transportation and Commerce, the
:ion to stop degrading GPS accuracy
Director of Central Intelligence, and other Executive Branch
Departments and Agencies. They realized that worldwide
transportation safety, scientific, and commercial interests
could best be served by discontinuation of SA. Along with our
commitment to enhance GPS for peaceful applications, my
administration is committed to preserving fully the military
utility of GPS. The decision to discontinue SA is coupled with
our continuing efforts to upgrade the military utility of our
systems that use GPS, and is supported by threat assessments,
which conclude that setting SA to zero at this time would have
minimal impact on national security. Additionally, we have
demonstrated the capability to selectively deny GPS signals
on a regional basis when our national security is threatened.
This regional approach to denying navigation services is con
sistent with the 1996 plan to discontinue the degradation of
civil and commercial GPS service globally through the SA
technique.
Originally developed by the Department of Defense as a mili
tary system, GPS has become a global utility. It benefits users
around the world in many different applications, including air,
road, marine, and rail navigation, telecommunications, emer
gency response, oil exploration, mining, and many more. Civi
lian users will realize a dramatic improvement in GPS accuracy
with the discontinuation of SA. For example, emergency teams
responding to a cry for help can now determine what side of
the highway they must respond to, thereby saving precious
minutes. This increase in accuracy will allow new GPS applica
tions to emerge and continue to enhance the lives of people
around the world.
