GPS selective availability

[Two_dogs]

With the threat of a war looming in Iraq, will my trusty GPS suddenly have a degradation in performance due to selective availability and/or signal degradation.

:} I forsee a position error in the order of the great circle route distance between Washington D.C., and Baghdad, Iraq. :}

[Hillbilly]

Selective avalability was removed from the system a couple of years ago. The latest generation satellites have the ability to degrade the accuracy within a small region, while leaving the rest of the world alone. So unless your company requires you to overfly Iraq you GPS should not be effected.

[OzExpat]

I have an idea that SA is still hard-coded "ON" in the programming instructions within TSO-129 Receivers. One of the specifications for TSO-145/6 Receivers is that SA is turned off. In any event, there are more GPS satellites in orbt now than there were in 1991.

Having said that, however, it might be a good idea to keep getting RAIM forecasts. That might save a bit of embarrassment.

[criticalmass]

OzEx speaks wisely. Always integrate all navigational data from multipel sources (including the Mark 1 calibrated eyeball) before making critical decisions.

I would not be too surprised if older GPS receivers give some erroneous readings if and when hostilities break out.

The US DoD giveth - the US DoD taketh away!

[exrba]

Its a bit hard to cross reference with other sources, including the Mk1 eyeball, when your in the middle of an NPA.
Not to worry though, SA was switched off years ago.

[*Lancer*]

Hillbilly, that's all well and good, but let's just hope the US DoD know exactly how close airliners fly to Iraq! (I've even seen Air France cut the corner).

[Ang737]

THE WHITE HOUSE
Office of the Press Secretary
For Immediate Release May 1, 2000
STATEMENT BY THE PRESIDENT REGARDING THE UNITED STATES' DECISION TO STOP DEGRADING GLOBAL POSITIONING SYSTEM ACCURACY

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 midnight tonight. We call this degradation feature Selective Availability (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 commercial users worldwide. Last year, Vice President Gore announced our plans to modernize GPS by adding two new civilian signals to enhance the civil and commercial service. This initiative is on-track and the budget further advances modernization 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 capabilities to worldwide users free of charge.

My decision to discontinue SA was based upon a recommendation by the Secretary of Defense in coordination with the Departments of State, Transportation, Commerce, the 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 consistent 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 military 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, emergency response, oil exploration, mining, and many more. Civilian 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 applications to emerge and continue to enhance the lives of people around the world.


---------

Now that the looming war is upon us you will find more and more RAIM predictions indication that RAIN wont be available down south due to the sats being directed into the northern hemisphere


Ang
;)

[AMRAAM]

Thanks ANG

I have been looking for that PR for some time.

But !

There is NO chance of the satelites bieng moved north.

The GPS system relies on the satelites bieng in the correct position for the system to work. They are moved, when required, after passing over thier appropriate ground station and an assesment on condition and location are done. This usually happens once or twice a day. I am not exactly sure how many set orbits there are, but there are at least 4 with a few satelites in each ring.

GPS uses a time signal vs known location from the almanac which is part of every GPS. This is why your GPS takes such a long time to lock on, if at all, if the system date/time is wrong. Was very common with early units.

I do however think the USDOD will be doing an assessment on this decision given current world events, maybe a SECURITY measure to be taken very soon.

I am interested to see the SA over small areas as mentioned earlier, can anybody expand on this.

[8 8th's Blue]

I can't see the US giving GPS to the world if it could be used effectively against them.ie they won't turn it off. As mentioned earlier they now have the ability to "localise" the error. I think the biggest threat to aviation will be possibilty of a steep increase in fuel prices!

88b

As far as controlling SA I could only speculate as to how. Maybe the alter the clocks in the sattelites with in certain regions? I don't think the US government will give that secret up in a hurry!

[OzExpat]

I can't see the US giving GPS to the world if it could be used effectively against them
If you ever have the chance to fly faster than 999 knots (? I've forgotten the actual speed, but it's something like that, as a groundspeed), you might find that your civil (ie non-military) GPS receiver will turn itself off. This probably won't prevent use of a civil GPS in a hostile military aircraft, but it will prevent them being used in ICBMs, etc.

[Jenny Talia]

the 999kt limitation is a receiver characteristic, as opposed to a system characteristic. Receivers do not transmit, so there is no way for the greater system to know what an individual unit is doing. An organisation with appropriate knowledge could quite easily 'build out' the 999kt limitation in individual receivers. Besides, methinks ICBMS arent the ones that they're actually worried about as they can pretty well navigate autonomously.

[OzExpat]

Well, I DID say...

ICBMs, etc.

Underlining added for effect... and neatness... :D

[compressor stall]

is the 30m SA "error" really going to make a difference with a dirty bomb in the front? :confused: