Winter 2007 Issue

Homing In

RDF in the Headlines

By Joe Moell, KŘOV

This Cingular Wireless tower has three sets of directional
antennas to increase channel capacity by dividing coverage into multiple sectors. The antennas do not have sufficient directivity to provide accurate angle-of-arrival information on incoming signals. (Photo by Joe Moell, KŘOV)

Whether for sport or for public service, stories about RDF (radio direction finding) are of interest to the general public as well as to ham operators. Although they rarely explain the technical details, the mainstream media often present news items that include electronic signal tracking. Let’s dig into some recent ones.

As I write this, there is much in the news about James Kim, a Senior Editor for CNET, who perished while seeking help for his stranded family in the mountains of Oregon after a severe snowstorm. CNET is reporting that the successful search for Kim’s wife and children was greatly aided by a brief signal from his cell phone.

The very remote spot where the Kim family vehicle became snowbound was blocked from all the towers of his provider, Cingular Wireless. They could not call 911. However, two clever engineers from Edge Wireless, the regional operator, scoured the call data records and found a single “ping” from James Kim’s phone at 1:30 AM to a tower near Glendale, Oregon. This brief transmission was acknowledgement of a waiting text message.

Kim drove out of that “sweet spot” before picking up the message, but the automatic transmission exchange was enough for the two Edge engineers to determine that his phone was less than 26 miles southwest of Wolf Peak in the little-used “Z” coverage sector. They then employed computer models to determine areas of mountain shadowing and combined that data with their own knowledge of the region to direct searchers toward Bear Camp Road, which is where Kim’s family was found in the vehicle. Kim had driven down that road by mistake because a vandal had broken a lock and opened the gate to it.

The Kims’ planned route for their Thanksgiving holiday was from Interstate 5 to Gold Beach via NF-23, but they took a wrong turn. Instead of backtracking, they continued in hopes of finding another road back to NF-23. That would have been tricky even in good weather. I remember a mobile hidden transmitter hunt in the Oregon coastal mountains back in June 1991. VHF signals constantly fluctuated in amplitude and direction due to the steep forested terrain. The poorly maintained logging roads were a confusing maze. Eventually, April and I found the hidden transmitter, and then we faced the challenge of finding the right way out.

E911 and RDF

The Kim tragedy points out how far this country is from the goal of 100% cellular coverage and accurate location of persons who call for help via cell phones. For many years, dispatchers at the approximately 10,000 PSAPs (Public Safety Answering Points) around the USA have had near-instant indication on their computers of the street addresses of landline callers to 911. In 1996, the FCC demanded in Docket 94-102 that cellular and PCS providers send the exact location of their 911 callers to dispatchers and gave them until October 1997 to start doing it.

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