New York-based Harbinger Capital has been buying stock in wireless companies for several years, including a start-up venture called SkyTerra that planned to develop a hybrid 4G wireless network that would primarily use geostationary satellites with 40-watt transmitters augmented by very-low-power ground stations called ancillary terrestrial components, or ATCs for short. The ATCs are similar to the ground booster stations used by Sirius satellite radio to fill in dead spots in urban areas within its broadcast coverage area. The FCC granted SkyTerra the authority to use ATCs to boost signal strength in 2005.

In 2010, Harbinger bought all the stock in SkyTerra and renamed it LightSquared. The business plan called for LightSquared to use to frequency bands, 1526 to 1536 MHz and 1454.2 to 1555 MHz, for space-to-earth downlinks.

SkyTerra's downlinks would have to share spectrum with Inmarsat, also licensed to use frequencies in the 1525- to 1559-MHz band. The firm paid millions of dollars to Inmarsat to reach an agreement to shift some of its satcom frequencies to make room for LightSquared's two downlink channels.

Few eyebrows in the GPS community were raised while all this was going on because engineers saw virtually no threat from LightSquared's 40-watt satellite transmitters. Just as importantly, GPS engineers knew that the ATCs, used to fill in coverage gaps, would have to be limited to very low power so as not to jam reception from LightSquared's own satellite transmissions. A side benefit was that the low-power ATCs would not cause GPS interference.

All that changed in January 2011 when LightSquared applied to the FCC for a waiver to offer terrestrial-only service for its 4G network. Freed of the need to limit ATC ground station signal strength to a whisper to safeguard 4G reception from satellites, the terrestrial-only service would have allowed transmissions that would be millions of times stronger — a roar that could have deafened GPS receivers in the neighboring band, as well as other L-band devices with sensitive receivers.

How strong would the signal from the ATCs be? LightSquared's waiver potentially allowed it to crank up power to nearly 16 kilowatts per station. But the FCC required LightSquared to prove that there would be no significant interference with GPS. It quickly became apparent that such strong transmitters would be problematic, so LightSquared soon dialed back the peak power of the ATCs to just under 1,600 watts. It subsequently offered to use temporarily only the lower of its two L-band downlink channels in order to move farther away from the satnav band.

However, even using much lower power, the new ground station plan quickly got the attention of the GPS community, which united and created an uproar of its own. LightSquared planned to create a network of up to 40,000 ATCs, each of which could produce more than a billion times the signal strength of GPS depending upon receiver distance and elevation angle relative to the ground station antenna. Even the most-robust aviation-grade GPS shadow mask filters operating nearby would go deaf in the presence of such a roar in a neighboring sector of L-band.

American farmers joined the protest against LightSquared, expressing concern that LightSquared's L-band ground stations would interfere with agricultural GPS receivers they use to guide the application of fertilizers and pesticides. The charge was led by John Deere, a high-profile manufacturer of agricultural GPS devices.

International opposition to the LightSquared plan also mounted. The European Commission expressed opposition because it feared the high-powered ground stations would interfere with reception of its Galileo satnav system signals aboard aircraft fitted with that satnav system operating in U.S. airspace.

LightSquared responded by claiming that GPS receiver manufacturers design and build defective equipment, making it susceptible to interference outside of the protected 1559- to 1610-MHz frequency band. The firm said that “properly designed” GPS systems would suffer no such performance loss.

GPS manufacturers responded that the entire L-band sector from 1525 to 1559 MHz below the 1559- to 1610-MHz satellite navigation band, as well as the 1610 to 1660.5 MHz above the band, always was intended to be a “quiet” or low signal strength spectrum neighborhood. There was consensus among legacy L-band licensees that all users would whisper signals, not roar them.

As an aside, Inmarsat was noticeably absent from the LightSquared versus GPS battles even though it would share L-band spectrum with LightSquared if its system becomes operational. Inmarsat already had reaped sizable revenues from its agreements with LightSquared to shift frequencies to avoid interference from its ground station network. Because Inmarsat primarily serves users who operate in overwater and remote areas, LightSquared's proposed network of higher-powered ground stations in the U.S. posed no threat of interference.