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December 12, 2011

New Tests Continue to Show LightSquared Interference with GPS Devices

By Gary Kim, Contributing Editor

The latest U.S. government test of LightSquared (News - Alert) spectrum caused interference to 75 percent of global-positioning system receivers examined, a draft summary of results indicates. It is not immediately clear whether the new tests mean 75 percent of all GPS devices experience interference, or only 75 percent of the class of devices LightSquared itself says will encounter signal interference.



The results from testing conducted Oct. 31 to Nov. 4 show that “millions of fielded GPS units are not compatible” with the planned nationwide wholesale service, according to the draft seen by Bloomberg (News - Alert) News.

LightSquared has argued that its latest spectrum plan causes disruption to about 10 percent of devices. Latest test shows interference

Separately, LightSquared was notified Dec. 8, 2011 that LightSquared’s parent, Harbinger Capital Partners (News - Alert), might be investigated for possible civil securities-law violations.

Apparently, Harbinger Capital Partners, the major financier of LightSquared, has received “Wells” notices from the Securities and Exchange Commission for other reasons.  

A Wells notice is a letter sent by a securities regulator “to a prospective respondent, notifying him of the substance of charges that the regulator intends to bring against the respondent, and affording the respondent with the opportunity to submit a written statement to the ultimate decision maker.” Securities investigation?

Among the issues the SEC (News - Alert) has explored is whether Harbinger agreed to allow some investors, including Goldman Sachs Group Inc., to cash out of their holdings while barring other clients from withdrawing their money, according to people familiar with the matter. Securities law infractions

The laboratory testing was performed for the National Space-Based Positioning, Navigation, and Timing (PNT) Systems Engineering Forum, an executive branch body that helps advise policy makers on issues around GPS. It found that 69 of 92, or 75 percent, of receivers tested “experienced harmful interference” at the equivalent of 100 meters (109 yards) from a LightSquared base station.

The forum is to present its results on Dec. 14, 2011 in Washington. Additional tests are slated for 2012, specifically to test potential interference with farm equipment and scientific devices.Much of the problem appears to be that the ground-based mobile transmitters LightSquared would be using (cell towers) will be much more powerful than the weak satellite signals used by GPS receivers.

Since any radio transmitter emits signals at its primary frequency as well as many subsidiary harmonic signals at other frequencies, the relative power of those harmonic signals seems to be causing the problem.

As this illustration indicates, the primary signal (the tall center spike) also produces a range of other signals at other frequencies both higher in frequency and lower in frequency than the intended signal. It appears that what is happening is that the harmonics at higher frequency are causing interference to GPS receivers that are near tower sites.

It appears that the interference happens within 100 meters or so of towers because the signal strength, which decreases rapidly with distance, still are high enough to blot out weak GPS signals sent by the GPS satellites. 


Gary Kim (News - Alert) is a contributing editor for TMCnet. To read more of Gary’s articles, please visit his columnist page.

Edited by Rich Steeves
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