As we prepare for the upcoming publication of the 2008 GPS Receiver Survey in the January issue, I would like to direct everyone’s attention back to the article by Frank van Diggelen in the January 2007 GPS World titled “Lies, Damn Lies, and Statistics.” The author cites the relationships between the various accuracy measures used in the community and the need to include the metric, as well as the numerical value when defining accuracy. Ironically, the latest receiver survey

at that time, appearing in the same issue, demonstrated that many manufacturers ignore Frank’s admonition to include that metric when specifying an accuracy value.

In my consulting experience, weapons folk usually use circular error probable (CEP) as the implied measure of accuracy. This leads them to infer that a number given to them — lacking a stated metric — is a CEP.

I recall when, in the late 1990s, we would discuss the pros and cons of retaining Selective Availability (SA). Several colleagues in the Pentagon thought that the 100-meter number agreed to by the Department of Defense (DoD) and the Federal Aviation Administration (FAA) was a CEP number — which is the median or 50 percentile miss — when in actuality it was the 95th percentile value of the measurement scatter, that is, the distance from the aimpoint (or for surveying, from the survey antenna) containing roughly 95 percent of the measurements.

So the metric inferred/implied does make a difference. In the late 1990s, while SA was still activated, we took two days of measurements at a northern Virginia facility. The 95th percentile scatter we observed was within a meter or so of the 100-meter value. And the CEP of the demonstrated scatter equated to about 45 meters, generally consistent with van Diggelen’s chart, when you consider the north and south 1−sigma values of the tested sample were not the same. The point here is that while SA may have deterred some unauthorized users from exploiting GPS for landing aircraft, the weaponeers would probably have killed to get a 45-meter CEP on their weapons or weapons platforms from GPS.

And that CEP figure was obtained by operating the receiver in a four-satellite solution mode. As an aside: we continued our tests to include a receiver operating in an all-in-view mode, similar to the mode used by non-embargoed civil GPS receivers at the time. When we ran our receiver in that all-in-view mode the actual median miss — that is, the CEP — of a two-day sample was about 25 meters. So much for SA . . . and the effect of the number of receiver channels on accuracy. 

Before I leave the discussion of van Diggelen’s article, I’ll take slight issue with Frank on his assumption that “If you see an accuracy specified without a metric . . . it is usually CEP.” Maybe that’s the case with the military. But, based in part on his own cell-phone accuracy experience, and my experience in working with the FAA, that is a very “iffy” assumption.

So, I’m suggesting that we accompany accuracy numbers with the appropriate metric. But what about presentations to the layman, be he a congressman who might be trying to determine the relevance of GPS in cell phones, or the local town board member who’s trying to determine whether it’s worth allocating funds to surveying fire hydrants, or whether all emergency vehicles should be GPS-equipped? Or a presentation to a defense official who asks whether it’s necessary to have a weapon platform equipped with GPS when the weapon itself is GPS-equipped?

This audience might well ask the questions in terms of “accuracy.” But what they should be asking is “How does this address my real problem?” To the question of funding GPS for emergency vehicles, the proper answer might be: “Sir, what you really want to know is how fast we will be able to isolate a heart-attack victim in a crowded parking lot.” To the DoD official, it might be: “Sir, given GPS, I’ll tell you the number of weapons we will have to deliver to the target to ensure a 95 percent probability of destruction.”So; help me out here, Chicago. Does anyone really care about accuracy?

We in the industry sure should, and be precise as to how we define it — the first point made here. But I would suggest that we do our customers a favor and translate that definition into a measure of effectiveness that the customer is more concerned about, but may not even know how to articulate. And in going through the process, we might gain a better appreciation of the effect of accuracy on the services we’re trying to provide.I could go on. Even if the receiver manufacturers define their metric in GPS World’s annual receiver survey, so many other environmental conditions affect the numbers. Most importantly, the dynamics.

So, let’s be precise amongst ourselves. Shouldn’t we educate the audience to ask the relevant questions, and shouldn’t we do our homework and provide answers to those questions?

Ahh. For the perfect world.

A consultant to U.S. corporations that support the Department of Defense and the intelligence community, Terry McGurn previously served for more than 30 years with the Central Intelligence Agency, most recently as senior analyst for assessing foreign navigation and guidance technologies. He participated in the drafting of the 1996 Presidential Decision Directive on a national GPS policy, and is currently a member of the National Space-Based PNT Advisory Board.