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Javad Ashjaee’s LightSquared Solution Presentation at the PNT Meeting Print E-mail
Written by Marc Cheves, PS   
Saturday, 12 November 2011

 

Due to time and space constraints, my editorial in the next magazine does not tell the complete story about what happened at the recent PNT meeting. Because of the significance of the LightSquared issue, the PNT changed its schedule to allow Javad to present his solution the day before the public meeting began. Assembled were scientists and top-level experts from all over the industry, both in the public and private sectors. They were given the opportunity to grill Javad, and according to reports, Javad answered every question to the satisfaction of all present.

Probably the most significant comments at the public meeting were made by Tom Stansell, who moderated the LightSquared panel. Stansell, one of the pioneers in GPS and a fifty-year veteran in the industry, opened his remarks by saying that Javad is “One of the single most innovative people in his field, bringing out products that make his competitors unhappy and uncomfortable at times because he is a leader.”

Prior to the Survey Summit last summer, Javad spoke of guarding against “aggression” toward GPS. But just before the Summit, as soon as he understood that an engineering solution was possible, he quickly began to develop a fix which he announced at the Summit. In the LightSquared press conference on October 13, Javad frankly admitted that the GPS industry has not done a proper job of designing receivers: the industry knew that a threat existed from future L-band transmissions. More about this can be found HERE and HERE.

Javad began his presentation at the PNT meeting with a slide showing how communications are the most expensive and difficult part of receiver design. (Note: Javad’s 76-page PowerPoint PNT presentation can be found HERE.) Whether by UHF or cell phone, the connection is often tenuous and difficult to maintain. With LightSquared, Javad sees a much more robust and cheaper means of relaying the correction signals.

Next, Javad displayed a graphic showing the spectrum, and where all the players lie in that spectrum. Unknown to me was the fact that LightSquared’s proposed use lies on both sides of the GPS spectrum. The two LightSquared bands lying left, or below the GPS band, are referred to as 10L and 10H (what we’ve been referring to as the lower and upper band), and the band lying to the right, or above GPS and GLONASS, is referred to as 10R.

Something I’ve always admired about Javad is his ability to put highly technical subjects in terms that are easy to understand. And so, for the next part, Javad gave a clear explanation of power measured in decibels. Counter-intuitively, the larger the decibel value—measured in milliwatts, dBm—the weaker the signal. And the scale is logarithmic: for example, 10dB is 10 milliwatts, but 60dB is one million milliwatts. Non-military GPS signals are -133 dBm, but LightSquared is -10dBm. Stated more simply, LightSquared is a tenth of a milliwatt, but GPS is a twenty trillionth of a milliwatt. As you can see, the GPS signal is far weaker than the LightSquared signal.

The problem then becomes, How to eliminate the interference from the powerful LightSquared signal while still receiving the weak GPS signal? To evaluate his solution, Javad devised four separate tests. The first test is component simulation and analysis, and the second test is sine wave in-circuit measurements (see PowerPoint). Both of these tests clearly showed that the Javad solution works to exclude the LightSquared signals, both above and below the GPS spectrum.

Javad next showed the costs for the “fix” in the low noise amplifier (LNA) circuitry: a $30 ceramic filter, and four tiny 24-cent surface acoustic wave (SAW) filters. (Note: an LNA acts as a cascade and is how a receiver takes the extremely weak GPS signal and makes it useable.)

The third test was to place a LightSquared signal generator and a modified receiver in an anechoic chamber. These chambers completely block all outside signals, thereby allowing the scientists to isolate the equipment being tested and evaluate it without any extraneous “noise.” Using the built-in spectrum analyzer capability of the Javad-VS receiver, this test also clearly showed that above- and below-band LightSquared signals were blocked from the receiver.

The fourth test, a zero baseline configuration—what Javad called the ultimate test—placed un-modified and modified receivers in a very high-multipath environment. (Zero baseline = one antenna feeding two receivers.) The results of this 15-hour test also clearly showed that the receiver was able to maintain a ±0.2mm accuracy on carrier phase signals for GPS L1, GPS L2 and GLN L2. Due to the fact that GLONASS uses FDMA and not CDMA (like GPS), GLN L1 was slightly higher, but was still corrected by almost two thirds (we’re still talking about only a millimeter on GLN L1).

Stansell closed the panel discussion by saying, “I think we learned, thanks to Javad, about a very clever solution to a particular problem for a particular range of products—the products he is most familiar with,” he said. “It may or may not fit in some of the other applications. What we have not addressed is sort of the elephant in the living room. That is the cost, and time delay, and changeover process if LightSquared is allowed to go forward. Will it be the lower 10, upper 10? That has to be resolved. There are very large questions remaining to be discussed, and [they] may or may not be fully solved in a short period of time."

In a subsequent conversation I had with Javad, he said he is working on a solution for the upper of the two lower bands, so even though LightSquared itself says that the 10H solution might take years, given the speediness of Javad’s lower band solution, I wouldn’t be surprised if the years turn into months. In his opening remarks, Stansell alluded to the fact that Javad has always stood alone with his innovations and approach. But whether it’s having a hand in getting GLONASS adopted by the precise community around the world, or coming up with a solution for the LightSquared issue, his presentation at the PNT meeting was one of his finest hours. To put his money where his mouth is, he brought 40 modified TRIUMPH-LS “LightSquared-Protected” receivers to the meeting, offering them to any and all who wanted to test for themselves.

Looking to the future, Javad said his company will begin manufacturing “LightSquared-Integrated” receivers in May 2012, for RTK positioning using the proposed LightSquared broadband network for high-speed communication. More information can be found HERE

 
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