Inverted Selective Availability (ISA)

4/01/96

A Pentagon Report Issued Last Friday Ordered Operational Late Sunday

(3/31/96 at 19:00:00 EST) a new Inverted Selective Availability (ISA) implementation that promises to revolutionize the Global Positioning System (GPS). ISA uses the control scheme already in place for Selective Availability (SA) but in place of the intentional degradation of the civil, C/A code GPS signal, ISA introduces slowly varying corrections that remove not only SA errors, but also remove the ionospheric, tropospheric, and broadcast ephemeris errors that are the largest sources of bias errors in the C/A code stand-alone GPS receiver. ISA not only marks the end of Selective Availability but represents a major improvement in the "un-degraded" signal that was available before the implementation of SA.

ISA corrections are formed from the hundreds of Differential GPS (DGPS) monitors used by the US Coast Guard and other federal agencies within the United States in conjunction with similar DGPS monitor systems implemented in other nations world-wide. A new ISA branch of the GPS Control Segment, located at Falcon AFB in Colorado, models Global Area System (GAS)corrections from a world-wide array of monitors using techniques perfected in Wide Area Systems (WAS).

According to the report, initial tests indicate a horizontal positioning accuracy of 2-3 meters (95%)and a vertical error of 3-4 meters (95%). Users are cautioned that noise errors and local bias errors (such as multipath) will not be removed by ISA.

While the advantages of ISA are many, there is a "down-side." It is expected that receiver prices for low-end recreational receivers will begin to increase for a time, as manufacturers rush to improve the poor designs previously masked by SA biases.

Many thousands of recreational users will now be able to accurately quote positions with precisions of tenths of seconds of latitude and longitude. Early reports suggested that Internet resources previously devoted to "newsgroup" discussions of the merits of SA and the failure of the Department of Defense (DoD) to realize that DGPS and "averaging for a long time" had already made it possible for terrorists to locate buildings and fly airplanes might be freed by the ISA implementation. These resources have already been "re-captured" by these newsgroups and used to begin a series of inquiries into ways of removing multipath so as to be able to obtain new ISA-aided position "readouts" to accuracies of hundredths of seconds, apparently now

A Precision Required In Location Fixes Or On Letterheads.