Selective Availability still in use?

[+The A-Team]

Yesterday on Twitter, I saw a story about new GPS technology that would provide accuracy down to a few centimetres. Someone commented on the story that GPS had always had that capability (which I believe is true), but that consumer GPS has random noise added to limit resolution. I replied to them pointing out that Selective Availability was turned off 15 years ago, but they replied and asserted that SA is still enabled many times a day and that the military wouldn't give their reasons for doing so.

 

It came off as a bit tin-foil-hat-ish, but I'm wondering if anyone knows if there's a grain of truth to their assertion? I fully expect that there are methods used in certain areas (ie. military conflict areas) to limit GPS accuracy, like jamming, but is SA still used daily? Supposing it was, wouldn't it affect the entire satellite constellation and all signals would be degraded worldwide? If so, I'd think we cachers, if anyone, would notice.

 

Come on, all you conspiracy theorists, show me some evidence that what this person is asserting is true!

[+wmpastor]

 

Come on, all you conspiracy theorists, show me some evidence that what this person is asserting is true!

 

I can't find some caches. There's your proof!

[+larryc43230]

Come on, all you conspiracy theorists, show me some evidence that what this person is asserting is true!

 

I can't find some caches. There's your proof!

^ ^ ^

That's my story, and I'm sticking to it!

 

--Larry

[+MartyBartfast]

If so, I'd think we cachers, if anyone, would notice.

 

Come on, all you conspiracy theorists, show me some evidence that what this person is asserting is true!

 

I think that pilots of large ships would notice when they started crashing into harbour walls, unless of course the introduced error is only a metre or two as opposed to the ~50m of the original SA errors.

[+AustinMN]

Yesterday on Twitter, I saw a story about new GPS technology that would provide accuracy down to a few centimetres. Someone commented on the story that GPS had always had that capability (which I believe is true), but that consumer GPS has random noise added to limit resolution. I replied to them pointing out that Selective Availability was turned off 15 years ago, but they replied and asserted that SA is still enabled many times a day and that the military wouldn't give their reasons for doing so.

 

It came off as a bit tin-foil-hat-ish, but I'm wondering if anyone knows if there's a grain of truth to their assertion? I fully expect that there are methods used in certain areas (ie. military conflict areas) to limit GPS accuracy, like jamming, but is SA still used daily? Supposing it was, wouldn't it affect the entire satellite constellation and all signals would be degraded worldwide? If so, I'd think we cachers, if anyone, would notice.

 

Come on, all you conspiracy theorists, show me some evidence that what this person is asserting is true!

 

Every day thousands of aircraft rely on SA being turned off 24/7/365.

 

GPS (even the encoded military channels) have never had centemeter-level accuracy. There are systems that use GPS signals that can get centemeter-level accuracy, but they rely on a locally generated correction signal, and the correction is still only as accurate as the placement of the equipment generating the local correction signal. In the systems I have read about, proximity to the correction device has an impact on accuracy, to the point where a couple of kilometers away (under ideal conditions), the correction is not much better than the uncorrected signal.

 

I believe the FAA has investigated mechanisms that would provide local correction for an entire metro area, but concluded that with SA turned off and the implementation of WAAS it was unnecessary. Every consumer GPS receiver manufactured in about the last 10 years or so is equipped to receive the WAAS signal. If someone is using an older GPS, they could think that Selecive Availability is on because the ionosphere (you know, the ionosphere that the WAAS signal is supposed to correct for) will introduce it's own error. WAAS is only usable in North America, but many other parts of the world have their own correction system.

 

Something that can introduce huge errors (I discovered this by accident) is carrying a cheap digital camera. Until I figured out what was going on, my old GPS would sometimes head out across a lake at 45 MPH, and after being gone for a few minutes, would then come back again from some other direction, generally after I put the camera away. A couple times, the excursion was several miles. This happened with or without WAAS. I'd bet all kinds of other devices (early iPods, old laptops, old or cheap cell phones, cheap calculators, etc.) could do the same if given an opportunity.

 

There are lots of other ways to fool oneself while using a GPS (such as expecting accurate signals while in a skyscraper canyon, or in a jungle during a rainstorm). There are so many of these anomolies that I would bet that anyone claiming SA is active has at least one, and probably several, going on at a time...and also has no idea that they are doing it to themselves.

 

Austin

Edited May 6, 2015 by AustinMN

[TahoeJoe]

I'm sure search and rescue would have something to chime in about this if SA was to be turned back on.

[+The A-Team]

Yep, I'm pretty sure the guy I referenced is just a nut-job who doesn't know what he's talking about. As you've all pointed out, it would very quickly become apparent if SA was turned on for even a short period of time with so many entities relying on GPS these days. Planes would start crashing, drivers would get lost, geocachers' heads would explode...

 

Thanks for the background, AustinMN. I guess I always knew there was some kind of additional level of technology required to attain the centimeter-level accuracy, but I'd never looked into it very much. I guess that would be DGPS, which I assume is what professional surveyors would be using.

[Moun10Bike]

There is an encrypted channel in the GPS signal that is separate from the civilian signal. It takes special hardware (and I assume security clearance) to access it, and it provides superior accuracy. I'm betting that's what he's referring to.

[jholly]

There is an encrypted channel in the GPS signal that is separate from the civilian signal. It takes special hardware (and I assume security clearance) to access it, and it provides superior accuracy. I'm betting that's what he's referring to.

Yeah that channel accuracy is so good it can guide a bomb right through your front door, and if you have two it will do the right or left.

[+tomfuller & Quill]

Even if you wanted to get a DAGR http://en.wikipedia.org/wiki/Defense_Advanced_GPS_Receiver You would not want to pay $1832 for one.

The first GPS I ever used (before 2000) cost the state of PA $5000. It had a full rubberized keyboard and weighed 3.5 lb. I took waypoints for 3 3 minute sessions and then used correction software on the computer (differential correction) to get the spot within 2 feet (most of the time). The Corvallis Microtechnology MC-GPS had a rechargeable battery and we always carried a spare charged battery.

[knowschad]

I saw a similar article, but without that claim about SA. It did say that surveyor grade GPSrs have had that ability for years, but it implied, and I assumed, that it was simply a matter of cost and technology.

[+BBWolf+3Pigs]

Even if you wanted to get a DAGR http://en.wikipedia.org/wiki/Defense_Advanced_GPS_Receiver You would not want to pay $1832 for one.

The first GPS I ever used (before 2000) cost the state of PA $5000. It had a full rubberized keyboard and weighed 3.5 lb. I took waypoints for 3 3 minute sessions and then used correction software on the computer (differential correction) to get the spot within 2 feet (most of the time). The Corvallis Microtechnology MC-GPS had a rechargeable battery and we always carried a spare charged battery.

 

And even if you had a DAGR, you'll still need crypto keys to get the added accuracy.

 

(Edit: typo)

Edited May 7, 2015 by BBWolf+3Pigs

[+AustinMN]

I believe the FAA has investigated mechanisms that would provide local correction for an entire metro area, but concluded that with SA turned off and the implementation of WAAS it was unnecessary.

 

I made this statement above but additional research has proven it to be incorrect. LAAS (known internationally as GBAS) has been implemented at a handful of airports globally. There are several ground-based systems that have been developed.

 

Austin

[+jwmoe1973]

Even if you wanted to get a DAGR http://en.wikipedia.org/wiki/Defense_Advanced_GPS_Receiver You would not want to pay $1832 for one.

The first GPS I ever used (before 2000) cost the state of PA $5000. It had a full rubberized keyboard and weighed 3.5 lb. I took waypoints for 3 3 minute sessions and then used correction software on the computer (differential correction) to get the spot within 2 feet (most of the time). The Corvallis Microtechnology MC-GPS had a rechargeable battery and we always carried a spare charged battery.

 

My Oregon 450 has proven itself more accurate than the DAGRs we used. They were typically unreliable and more often than not glitched...

[knowschad]

Not only is SA not turned on, but the NOAA is in the process of setting up a completely new datum to provide even more accuracy. (link )

 

Replacing NAVD 88 and NAD 83

NAD 83 and NAVD 88 will be replaced in 2022, and there are many related projects to make sure the transition goes smoothly. Read the NGS Ten-Year Plan to learn more and continue to visit this web-page for more information.

 

 

 

Why is NGS replacing NAD 83 and NAVD 88?

 

NAD 83 and NAVD 88, although still the official horizontal and vertical datums of the National Spatial Reference System (NSRS), have been identified as having shortcomings that are best addressed through defining new horizontal and vertical datums.

 

Specifically, NAD 83 is non-geocentric by about 1.5 meters. Secondly, NAVD 88 is both biased (by about one-half meter) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today. Both of these issues derive from the fact that both datums were defined primary using terrestrial surveying techniques at passive geodetic survey marks. This network of survey marks deteriorate over time (both through unchecked physical movement and simple removal), and resources are not available to maintain them.

 

The new reference frames (geometric and geopotential) will rely primarily Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) as well as an updated and time-tracked geoid model. This paradigm will be easier and more cost-effective to maintain. Read our white paper for more information.

 

Edited May 13, 2015 by knowschad

[+DukeOfURL01]

Then how do Surveyors get centimeter accuracy (or better)?

[+UMainah]

Then how do Surveyors get centimeter accuracy (or better)?

 

http://en.wikipedia.org/wiki/Real_Time_Kinematic

[+gpsblake]

I thought they could turn on SA in certain areas of the globe or block GPS altogther by regions, for example they could turn it off or skew for the country of Iran or whatever country. That might be my faded memories or old information.

[+AustinMN]

I thought they could turn on SA in certain areas of the globe or block GPS altogther by regions, for example they could turn it off or skew for the country of Iran or whatever country. That might be my faded memories or old information.

 

The satellites broadcast a signal. Anyone on the same side of the globe as the satellite can listen to it. While it might be possible to jam the signal in a local area, it would not be by Selective Availability.