Technical Interference Questions
April 28, 2011 By: Alan CameronQ&A from the April 21 webinar 'LightSquared and GPS: Our Story So Far'
Herein follow all the questions posed, broken into several topic groups. Some answers have been given. For many questions, we haven’t marshaled the time or energy to address – or maybe we plain don’t know. But the process is open for you to contribute. If you would like to suggest an answer to a question, scroll down to the bottom of this webpage and click on the “Add Comment” link. Be sure to give the number of the question you are answering! You must also include your name and job title (affiliation or employer is not necessary) for your answer — or further comment — to be considered for posting to this webpage. Discuss away!
This page contains questions, and some answers, under the general topic of Technical Interference Questions. See also:
50. What will happen if Lightsquared reduces its power and can prove to FCC it has filered its signal to prevent interference with GPS signals?
A. Lightsquared would probably need to deploy more base stations.
— Logan Scott, consultant (LS hereafter)
51. Explain the difficulty in designing/building an adequate filter for L/S transmitters and/or GPS receivers.
A. Main problem for LightSquared filter is dealing with high power levels. For GPS, insertion loss and temperature sensitivity.
— LS
52. Are passive high-Q resonators preceeding the GPS LNA useful? Or will the LNA NF detoriarate too much?
A. Insertion loss, temperature sensitiveity, and phase linearity/gain flatness are the main issues.
— LS
53. If the LightSquared transmitter has a filter, what happens when the filter fails?
A: This is part of the Lightsquared monitoring reliability function I alluded to. Lightsquared needs to monitor and shut down immediately if this happens.
— LS
54. What impact will the LightSquared product have on power grid synchrophasors?
A: This is primarily a timing issue. If GPS is lost, then phase & frequency discipline is lost. Need to determine if LightSquared is the problem and take corrective action.
— LS
55. The output power of an Iridium handset is 1.5-6W in the 1616-1626 band. At a range of a few metres this can have incident power on a GNSS antenna equivalent to several kW at a kilometre. Given the general similarities, why do you expect LightSquared to be a serious problem?
A: Iridium handsets are not commonly used in urban settings because of high airtime charges. Also, sat coverage in urban areas is not great since building penetration is not all that good (high elevation angle).
— LS
56. What front end limits do you put on your receiver (band limits, attenuation). How much would it cost to make your receiver attenuate the out of band emissions?
A: Absent power consumption issues; my first line of defense would be high IP3/1-dB compression. Then I don't have to muck about with the antenna and filters which introduce all sorts of performance issues vis. Accuracy.
— LS
57. Are the LS ground transmitters REALLY 15kW?? Is that ERP, or something else?
A: They are authorized for 15 kW but LightSquared indicates they will use 1500 Watt max EIRP.
— LS
58. Are Adaptive antennas such as GAS-1 and ADAP liable to react to the Lightsquared signal (either inband or outband) and provide nulls at those directions?
A: That is a very good question.
— LS
59. Logan mentioned a receiver possibly losing lock. If they have trouble keeping lock, will they be able to lock to begin with in these cases?
A: Generally, the signal acquisition threshold is about 6 dB higher than track thresholds in older receivers. This distinction is less in high correlator count receivers.
— LS
60. Can military GPS technology designed to provide anti-jam capability help in the commercial space to address this interference?
A: Maybe in a few select stationary applications but mostly No.
— LS
61. 1400 Watts at the surface ?
A: Max 1500 Watts EIRP is specified by LightSquared, 15000 Watts is authorized by the FCC.
— LS
62. Are there alternative technologies such as the Alcatel-Lucent LightRadio which would cause less interference than the 40,000 towers?
63. What would be considered a safe "buffer" between Lightsquared and the GPS frequencies?
64. Assuming the interference is based on antennae location, is there any information on the distribution of transmitting towers and the potential areas of interference?
That’s all for now, folks! There are plenty more questions and some answers to post, but we’re running out of time and this newsletter has to go to press. Check back later for more material – and post your own answers for consideration, at the “Add Comment” link below.
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