GPS System Failure Imminent

[Jay Honeck]

I spent a fascinating few hours with an avionics engineer yesterday. He showed me an FAA-generated map of the US where GPS approaches are no longer authorized, because our GPS constellation of satellites has become unreliable.

Specifically, the satellites were designed to last ten years. Many are now 30 years old, their orbits have become wobbily and their electronics buggy. Even though the FAA is talking about NextGen being entirely GPS dependent, there is no current plan to replace these aging satellites.

The short term solution is for avionics manufacturers to make their instruments use the Russian version of GPS, which is newer, more reliable, and accurate to 1.5 meters. This, of course, leaves us totally dependent on a foreign country for our instrument approaches -- a bad idea, IMHO.

Anyone else heard about this?

 

[Jim Logajan]

Speaking of GPS failures, I wonder how many people are aware of the NOTAMs on tests that can affect usage in large areas, like the possible ones in the following document that cover two dozen potential test periods covering several states over two weeks (ends tomorrow):

http://www.alpa.org/portals/alpa/fastread/2013/docs/NSAWC13-02_GPSFlightAdvisory.pdf

 

[Jim Logajan]

Let's just say this: because of the low power nature of the GPS signal it really doesn't take a lot of power to jam it ("GPS denied"). GPS spoofing is a different matter.

By way of confirmation, and although they don't say what power was being used, and their radius of possible impact is no doubt conservatively large, there is nothing localized about this testing when the 50 ft AGL radius is 246 nm (testing ended today and described here: http://www.alpa.org/portals/alpa/fastread/2013/docs/NSAWC13-02_GPSFlightAdvisory.pdf)

 

[Jim Logajan]

WOW, Just WOW. The fact that he asserts the orbits of the GPS constellation are getting "wobbly" should have been a clue. An orbit is a never ending ballistic curve. To make them "wobble" would require constant application of energy in various vectors to create the oscillation. I think the straps on his straight jacket are a little loose.

On one hand, the gravity of the earth varies with location (i.e. J[sub]2[/sub] flattening), and the gravity of the sun and the moon also cause perturbations - as does the solar wind, among other causes. One could say these cause "wobbles" in a simple central-force ballistic orbit.

But on the other hand, since they are known and can be numerically modeled to desired accuracy and can (in any case) be compensated for, none will cause system failure.

 

[Jim Logajan]

It's been a long time since I had celestial mechanics, (40+ years), please define a "simple central-force ballistic orbit"

My use of the word "ballistic" was admittedly redundant. My use of "central-force orbit" was intended to mean a computation of the orbit using only the force from a single point. That can yield a "simple" equation for the motion for all of time. Hence the origin of my convoluted sentence.

Why were you studying celestial mechanics? Not too many degrees or professions have use for that knowledge.