5 ms·

GPS industry rages: LightSquared 4G network would "defy" laws of physics

24 points by joelhaus 15y ago

jevinskie 15y ago
You can find the report that LightSquared submitted in the right-hand column of the page at [0]. I'm reading it now to see if their suggestion to use filters (which was very vague in the linked article) really has merit. The "Save our GPS" consortium surely doesn't think it does. [0] http://www.pnt.gov/interference/lightsquared/ http://www.pnt.gov/interference/lightsquared/
- AjithAntony 15y ago
  The proposed workaround is to start using the lower 10MHZ (1526-1536) of their allocated range immediately, while cooperating with GPS device manufacturers on strategies that will allow them to use the the remainder (1536-1559Mhz) in the future. The GPS devices operate at 1575Mhz.
- thaumaturgy 15y ago
  I'm reading the Recommendations portion of the report. It is surprisingly light on technical details. LightSquared's claim seems to be that there is only a conflict on the "edge" of one of their licensed signal bands, primarily because a small subset of GPS devices (mostly survey, mining, etc. use devices) are improperly accepting signal in adjacent bands, either accidentally or intentionally in order to carry a little additional custom data. (?) So far, after reading most of the Recommendtions report and without being knowledgeable in this specific area, I'm inclined to side with LightSquared. Groups advocating protection of wireless spectrum are frequently hysterical when it comes to anything that might affect their signal. (e.g., the hams' strong and vocal objection to various proposals to carry telecom signals over power lines.)
  - patrickyeon 15y ago
    I've glanced at the same doc, got a slightly different take from it. The higher precision devices are actually receiving data in LightSquared's licensed band (by having more receive bandwidth, they get better accuracy), on purpose. This is not the GPS devices' band to use, so IMO tough luck on them. (If you like analogies, it's like carving out a block of unassigned IPs for yourself, and then complaining when someone is actually assigned those IPs and wants to use them). The one band they tested is the one that is further away, and therefore easier for devices to reject. They claim it's not a problem for "over 99% of devices" when they are at full power there. They concede that operation the the higher frequency band (that LightSquared has licensed) would cause trouble for "a significant portion of legacy devices." The root cause of most of the problem (not the part where ultra-precision receivers use bands that aren't theirs) is that the GPS receivers expected things in neighbouring bands to remain quiet, when there was no guarantee that such would happen. This assumption made for easier and cheaper designs for them, and now all this hardware is out there that is susceptible to the interference.
    - thaumaturgy 15y ago
      Right. I might've done a bad job describing my take on the report, because I agree with all that. It's also worth noting that LightSquared is offering to provide circuitry -- which they claim to have already developed -- to any device manufacturers that want to filter LightSquared's signal. Regardless, I'm not yet seeing a single thing anywhere in this part of the report that would lead me to conclude that they're proposing something that "defies physics".
      - patrickyeon 15y ago
        The "defies physics" part would be a filter that is physically impossible to implement. As you need sharper roll-off of your filter (more rejection of out-of-bands, and sooner), you start making sacrifices and attenuating the signals in your passband as well. GPS already needs very sensitive receivers, and attenuating the signals more than they already are could make them unusable. ("Oh!" you say, "just put more amplifiers behind the filter!" Well, no, at that point the signal is inderminable from thermal noise, which you will amplify just the same. And you can't put one in front of the filter, because then the LightSquared signals don't get filtered out and they interfere...) Without seeing the specs they are being asked to hit, I can't speak to the challenges of realizing these filters. And just to be clear, there is no way for LightSquared to use their upper band and not interfere with the ultra-precision stuff that's effectively squatting there. There's no happy compromise in that situation, but as I implied earlier, I have little sympathy for the guys who weren't playing by the rules.
blhack 15y ago
If anybody was going to be outraged by this, wouldn't it be the FAA and the Us Military?
- thisrod 15y ago
  Sure they are. The enemy might send a guided missile through the window of the office next to the general's. What a disaster!
  - sbierwagen 15y ago
    Fun fact: (most) civilian GPS receivers stop working when they're moving faster than 515m/s, are more than 18,000m above sea level, or are accelerating faster than 4g. (39.2m/s^2) If you're thinking that these operational constraints sound a lot like the performance envelope of a guided missile, then gosh, you're right. IIRC, unlocked GPS recievers are classified as MTCR category II dual-use missile related components, with the usual suite of import restrictions. EDIT: Looked it up[1] for... some reason, and yep, category II, item 11, section A.3: b. Designed or modified for airborne applications and having any of the following: 1. Capable of providing navigation information at speeds in excess of 600 m/s; 2. Employing decryption, designed or modified for military or governmental services, to gain access to GNSS secure signal/data; or 3. Being specially designed to employ anti-jam features (e.g. null steering antenna or electronically steerable antenna) to function in an environment of active or passive countermeasures. So, in fact, it's just the speed restriction. The numbers I have are probably from some American arms-control law, not a MTCR restriction. 1: http://www.mtcr.info/english/MTCR-April2011-Technical-Annex.pdf http://www.mtcr.info/english/MTCR-April2011-Technical-Annex....
    - kbatten 15y ago
      It seems that the cost of fabbing your own gps chip would be peanuts compared to the cost of even one missile. Given the raw GPS data its pretty trivial to do all location processing in software.
      - 15y ago
46Bit 15y ago
Does anyone have any real worthwhile info of this entire issue? All I've seen is the few incomprehensible technical arguments and then plenty of over-popularised articles like this one. I think it's obvious that LS can't go ahead if these problems are real, but the whole debate seems to lack much real detail.
- sbierwagen 15y ago
  Lightsquared has a license to broadcast in the 1525-1559 MHz band. Since their transmitters are on the ground, the signals are powerful. GPS is 1559—1610 MHz. These signals are very weak, since the transmitters are in orbit. The pissing match is that even though on paper the two ranges don't overlap, in practice the Lightsquared transmissions tend to stomp all over the lower end of the GPS range, which make various interested parties unhappy. There's additional humor in that while it's certainly possible to get around interference like this, in practice that's dual-use technology which can be used to evade military GPS jamming.
coin 15y ago
One thing not mentioned is LightSquared misuse of the spectrum. The spectrum in question is the L1 band, which was set aside for satellite communications. These satellite transmitted signals appear weak from the ground. A loophole in the FCC rules allows the use of ground based stations to fill-in weak areas of satellite coverage. This is called Auxiliary Terrestrial Component (ATC). LightSquared's plan is to use the ATC loophole to build out a terrestrial ground-based network with very little satellite usage. They are essentially getting valuable ground-base spectrum at the low price of satellite spectrum. LightSquared blaming of GPS receiver manufactures is a bit ridicules. The GPS unit manufactures weren't expecting out-of-spec usage of the neighboring L1 band. The L1 band was set aside for satellite signals, not terrestrial. Somehow none of the press is pointing this out. Here's a good write-up http://www.directionsmag.com/articles/whats-the-deal-with-the-lightsquared-gps-clash/179780 http://www.directionsmag.com/articles/whats-the-deal-with-th...

24 points by joelhaus 15y ago

You can find the report that LightSquared submitted in the right-hand column of the page at [0]. I'm reading it now to see if their suggestion to use filters (which was very vague in the linked article) really has merit. The "Save our GPS" consortium surely doesn't think it does. [0] http://www.pnt.gov/interference/lightsquared/ http://www.pnt.gov/interference/lightsquared/

AjithAntony 15y ago
The proposed workaround is to start using the lower 10MHZ (1526-1536) of their allocated range immediately, while cooperating with GPS device manufacturers on strategies that will allow them to use the the remainder (1536-1559Mhz) in the future. The GPS devices operate at 1575Mhz.
thaumaturgy 15y ago
I'm reading the Recommendations portion of the report. It is surprisingly light on technical details. LightSquared's claim seems to be that there is only a conflict on the "edge" of one of their licensed signal bands, primarily because a small subset of GPS devices (mostly survey, mining, etc. use devices) are improperly accepting signal in adjacent bands, either accidentally or intentionally in order to carry a little additional custom data. (?) So far, after reading most of the Recommendtions report and without being knowledgeable in this specific area, I'm inclined to side with LightSquared. Groups advocating protection of wireless spectrum are frequently hysterical when it comes to anything that might affect their signal. (e.g., the hams' strong and vocal objection to various proposals to carry telecom signals over power lines.)
- patrickyeon 15y ago
  I've glanced at the same doc, got a slightly different take from it. The higher precision devices are actually receiving data in LightSquared's licensed band (by having more receive bandwidth, they get better accuracy), on purpose. This is not the GPS devices' band to use, so IMO tough luck on them. (If you like analogies, it's like carving out a block of unassigned IPs for yourself, and then complaining when someone is actually assigned those IPs and wants to use them). The one band they tested is the one that is further away, and therefore easier for devices to reject. They claim it's not a problem for "over 99% of devices" when they are at full power there. They concede that operation the the higher frequency band (that LightSquared has licensed) would cause trouble for "a significant portion of legacy devices." The root cause of most of the problem (not the part where ultra-precision receivers use bands that aren't theirs) is that the GPS receivers expected things in neighbouring bands to remain quiet, when there was no guarantee that such would happen. This assumption made for easier and cheaper designs for them, and now all this hardware is out there that is susceptible to the interference.
  - thaumaturgy 15y ago
    Right. I might've done a bad job describing my take on the report, because I agree with all that. It's also worth noting that LightSquared is offering to provide circuitry -- which they claim to have already developed -- to any device manufacturers that want to filter LightSquared's signal. Regardless, I'm not yet seeing a single thing anywhere in this part of the report that would lead me to conclude that they're proposing something that "defies physics".
    - patrickyeon 15y ago
      The "defies physics" part would be a filter that is physically impossible to implement. As you need sharper roll-off of your filter (more rejection of out-of-bands, and sooner), you start making sacrifices and attenuating the signals in your passband as well. GPS already needs very sensitive receivers, and attenuating the signals more than they already are could make them unusable. ("Oh!" you say, "just put more amplifiers behind the filter!" Well, no, at that point the signal is inderminable from thermal noise, which you will amplify just the same. And you can't put one in front of the filter, because then the LightSquared signals don't get filtered out and they interfere...) Without seeing the specs they are being asked to hit, I can't speak to the challenges of realizing these filters. And just to be clear, there is no way for LightSquared to use their upper band and not interfere with the ultra-precision stuff that's effectively squatting there. There's no happy compromise in that situation, but as I implied earlier, I have little sympathy for the guys who weren't playing by the rules.

blhack 15y ago

If anybody was going to be outraged by this, wouldn't it be the FAA and the Us Military?

thisrod 15y ago
Sure they are. The enemy might send a guided missile through the window of the office next to the general's. What a disaster!
- sbierwagen 15y ago
  Fun fact: (most) civilian GPS receivers stop working when they're moving faster than 515m/s, are more than 18,000m above sea level, or are accelerating faster than 4g. (39.2m/s^2) If you're thinking that these operational constraints sound a lot like the performance envelope of a guided missile, then gosh, you're right. IIRC, unlocked GPS recievers are classified as MTCR category II dual-use missile related components, with the usual suite of import restrictions. EDIT: Looked it up[1] for... some reason, and yep, category II, item 11, section A.3: b. Designed or modified for airborne applications and having any of the following: 1. Capable of providing navigation information at speeds in excess of 600 m/s; 2. Employing decryption, designed or modified for military or governmental services, to gain access to GNSS secure signal/data; or 3. Being specially designed to employ anti-jam features (e.g. null steering antenna or electronically steerable antenna) to function in an environment of active or passive countermeasures. So, in fact, it's just the speed restriction. The numbers I have are probably from some American arms-control law, not a MTCR restriction. 1: http://www.mtcr.info/english/MTCR-April2011-Technical-Annex.pdf http://www.mtcr.info/english/MTCR-April2011-Technical-Annex....
  - kbatten 15y ago
    It seems that the cost of fabbing your own gps chip would be peanuts compared to the cost of even one missile. Given the raw GPS data its pretty trivial to do all location processing in software.
    - 15y ago

46Bit 15y ago

Does anyone have any real worthwhile info of this entire issue? All I've seen is the few incomprehensible technical arguments and then plenty of over-popularised articles like this one. I think it's obvious that LS can't go ahead if these problems are real, but the whole debate seems to lack much real detail.

sbierwagen 15y ago
Lightsquared has a license to broadcast in the 1525-1559 MHz band. Since their transmitters are on the ground, the signals are powerful. GPS is 1559—1610 MHz. These signals are very weak, since the transmitters are in orbit. The pissing match is that even though on paper the two ranges don't overlap, in practice the Lightsquared transmissions tend to stomp all over the lower end of the GPS range, which make various interested parties unhappy. There's additional humor in that while it's certainly possible to get around interference like this, in practice that's dual-use technology which can be used to evade military GPS jamming.

coin 15y ago

One thing not mentioned is LightSquared misuse of the spectrum. The spectrum in question is the L1 band, which was set aside for satellite communications. These satellite transmitted signals appear weak from the ground. A loophole in the FCC rules allows the use of ground based stations to fill-in weak areas of satellite coverage. This is called Auxiliary Terrestrial Component (ATC). LightSquared's plan is to use the ATC loophole to build out a terrestrial ground-based network with very little satellite usage. They are essentially getting valuable ground-base spectrum at the low price of satellite spectrum. LightSquared blaming of GPS receiver manufactures is a bit ridicules. The GPS unit manufactures weren't expecting out-of-spec usage of the neighboring L1 band. The L1 band was set aside for satellite signals, not terrestrial. Somehow none of the press is pointing this out. Here's a good write-up http://www.directionsmag.com/articles/whats-the-deal-with-the-lightsquared-gps-clash/179780 http://www.directionsmag.com/articles/whats-the-deal-with-th...