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JerBear-Lizard

Lightsquared jamming GPS signals?

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If you haven't heard about "Lightquared" you should. It poses a threat to the very existence of Geocaching!! In short, it is a company proposing to broadcast 4G cell data on a frequency that is so near GPS frequencies, that it bleeds over and screws up your GPS.

Artical from Compuworld: http://www.computerworld.com/s/article/9219021/FCC_We_won_t_let_LightSquared_hurt_GPS?taxonomyId=70

 

or google and research. This is a potentially huge issue, not only for us, but everybody that uses GPS from navigation to surveyors.

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Not that I am stirring up the pot again, but this hasn't went away and we need all 5 million geocachers to stand up and collectively yell "Hells NO!!" This topic is too important to get pushed down the list and forgotten.

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just putting it back up the list.

Then bump the old thread instead of creating yet another. As is noted, the original thread is quite long. It covers the factual details (and a ton of opinion) from about January of this year up through the Working Group's report and beyond.

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If you haven't heard about "Lightquared" you should. It poses a threat to the very existence of Geocaching!! In short, it is a company proposing to broadcast 4G cell data on a frequency that is so near GPS frequencies, that it bleeds over and screws up your GPS.

Artical from Compuworld: http://www.computerw...S?taxonomyId=70

 

or google and research. This is a potentially huge issue, not only for us, but everybody that uses GPS from navigation to surveyors.

"bleed over" makes no sense to me as an engineer and a ham operator. What do you mean?. Transmitters can make noise on multiples of the frequency called harmonics. Harmonics don't affect nearby frequencies(the first harmonic is double the frequency). A transmitter can be too strong for a receiver's front end tuner band pass filter, which affects all frequencies, not just nearby ones. That is solved by moving away or fixing the receiver. Many a ham has put better filters on their neighbors TVs, even though hams are not legally required to install filters. I think maybe the front ends of today's modern GPSs are much better than TVs IMHO and they should be fine. I have had GPS problems near high power cell or TV towers. I have pretty much ignored previous threads so this is my first reaction..

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"bleed over" makes no sense to me as an engineer and a ham operator. What do you mean?. Transmitters can make noise on multiples of the frequency called harmonics. Harmonics don't affect nearby frequencies(the first harmonic is double the frequency). A transmitter can be too strong for a receiver's front end tuner band pass filter, which affects all frequencies, not just nearby ones. That is solved by moving away or fixing the receiver. Many a ham has put better filters on their neighbors TVs, even though hams are not legally required to install filters. I think maybe the front ends of today's modern GPSs are much better than TVs IMHO and they should be fine. I have had GPS problems near high power cell or TV towers. I have pretty much ignored previous threads so this is my first reaction..

GPS is different because it's spread spectrum. There isn't a strong frequency peak you can just use a narrow filter for. The peak is spread out and barely above the noise, so you need a wide front end to get the signal at all. It's even worse for high accuracy commercial receivers and high sensitivity consumer receivers that need to go wider to get as much signal as possible. They can handle a reasonable amount of raised noise floor, but LightSquared's scheme is far from reasonable - even their "reduced impact" plan. Their spreading overlaps with GPS's spreading too much, and is a lot stronger.

Edited by GPSlug

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"bleed over" makes no sense to me as an engineer and a ham operator. What do you mean?. Transmitters can make noise on multiples of the frequency called harmonics. Harmonics don't affect nearby frequencies(the first harmonic is double the frequency). A transmitter can be too strong for a receiver's front end tuner band pass filter, which affects all frequencies, not just nearby ones. That is solved by moving away or fixing the receiver. Many a ham has put better filters on their neighbors TVs, even though hams are not legally required to install filters. I think maybe the front ends of today's modern GPSs are much better than TVs IMHO and they should be fine. I have had GPS problems near high power cell or TV towers. I have pretty much ignored previous threads so this is my first reaction..

GPS is different because it's spread spectrum. There isn't a strong frequency peak you can just use a narrow filter for. The peak is spread out and barely above the noise, so you need a wide front end to get the signal at all. It's even worse for high accuracy commercial receivers and high sensitivity consumer receivers that need to go wider to get as much signal as possible. They can handle a reasonable amount of raised noise floor, but LightSquared's scheme is far from reasonable - even their "reduced impact" plan. Their spreading overlaps with GPS's spreading too much, and is a lot stronger.

OK, I may be getting it ... or not. In my world the FCC assigns a range of frequencies called a band. You can use any frequency for a transmission as long as any type of modulation of that frequency does not generate frequencies outside the band. The modulation type can be AM, FM, SSB, spread spectrum and on and on. Are you saying the FCC is proposing overlapping bands or sharing of a band?

 

EDIT: I may be missing a key point about spread spectrum. My thinking it is just multiple frequencies for one info stream.

Edited by John E Cache

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Problem with Lightsquared's project is that it is licenced to use a weak signal satellite band adjacent to the GPS band we listen to.

Lightsquared applied for and got a qualified waiver to use their band for strong terrestrial signal stations that are very close to the GPS band. Most modern GPS are designed to get the weak satellite signals, not to reject adjacent frequencies. Ever stand close to a commercial repeater stack and try to listen to 2m? Front end overload.

 

I asked a while back about what the uplink frequencies would be for the terrestrial stations (two way of course there). There are phones now operating in the band close by the GPS. I fear that the cell phones/mobile devices would have to be fairly strong themselves to make it to the NEW terrestrial stations from any distance... Question is what will that do to the GPS user standing close by... or even the mobile device user himself while trying to use the GPS option?

 

I don't see it flying well... in the other thread I stated an opinion (only) on how it might be made to work with some cooperation from other 'partners' chipping in their available frequency resources... and using the Lightsquared sat allocation as it is licenced now for download/upload to the NEW sites only. Since they are a wholesaler and not an end user provider, that works for my thinking.

They have been partnering up lately to boot.

 

Hope that helps a bit...

 

Doug 7rxc (VE7RXC)

Edited by 7rxc

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Disclaimer: I am not a signal processing guy. What I know about it is what I've picked up being around GPS over the years. So please bear with me when I don't know where to draw the line between what's obvious to any EE/RF/DSP person and what's specific to the GPS flavor of spread spectrum. I apologize in advance for anything stupid I might say. I'll also limit the discussion to pre-modernized L1.

 

OK, I may be getting it ... or not. In my world the FCC assigns a range of frequencies called a band. You can use any frequency for a transmission as long as any type of modulation of that frequency does not generate frequencies outside the band. The modulation type can be AM, FM, SSB, spread spectrum and on and on. Are you saying the FCC is proposing overlapping bands or sharing of a band?

 

EDIT: I may be missing a key point about spread spectrum. My thinking it is just multiple frequencies for one info stream.

At least as applied to GPS, spread spectrum does the opposite. It allows multiple info streams on one frequency.

 

The band that GPS uses is centered on L1, and all GPS satellites transmit on L1. The PRN codes modulated on the signal are so high rate (~1 MHz C/A code, ~10 MHz P code), that they really are a lot like modulating noise on the signal, spreading it across a wide bandwidth. But the receiver knows at least the C/A code and can remove it from the signal and use it for accurate ranging. It's also pretty resistant to some kinds of jamming - an in-band spike isn't as bad as it could be because it only takes out a narrow part of the signal.

 

So, GPS is using this chunk of spectrum with more power in the middle and tailing off at the ends. A basic receiver might only use the middle part, and more advanced receivers will also go out farther into the tails. Compared to other kinds of modulation, the power drop-off for spread spectrum is very gradual by nature. A limited amount of overlap between bands is expected. As long as the other guy's modulated noise isn't too high compared to your modulated noise, it's acceptable.

 

The adjacent bands to L1 are pretty much limited to satellite services, so you'd never be close to a strong transmitter. Lightsquared's spectrum was originally approved to be mainly on satellites, with the option to use some low power transponders to extend the service to places where you couldn't get the satellites. The waiver that has caused the kerfuffle would allow them to use higher power transponders out in the open.

Edited by GPSlug

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This is a rehash of previous threads cited earlier in this thread. Closing in favor of existing topics.

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