I was sailing in Boston Harbor this past week and when I was within the 1/2 or so of the Logan airport glide path, my GPS (Garmin 230) started to repeatedly loose satellite fix. When I finally got far enough away from the airport everything returned to normal and has worked fine since. So I guess I'm trying to figure out whether I have a GPS problem, or whether some the airport electronics where screwing up the GPS. Any one have any thought or similiar experience? Bryce S/V Spellbinder H410
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Same problem w/Garmin 48
Bryce: We made a trip to SF bay a couple of weeks ago. Got to the confluences of the Sacramento and San Joaquin rivers and the same thing happened. My charting software said we were going over 200mph, we lost the signal and then everthing was back to normal. This only lasted for maybe 5-10 mins. Came back through the same general area on the way back and it worked fine. Just figured that the Presidents plane was flying so they were screwing with the system again?
Bryce: We made a trip to SF bay a couple of weeks ago. Got to the confluences of the Sacramento and San Joaquin rivers and the same thing happened. My charting software said we were going over 200mph, we lost the signal and then everthing was back to normal. This only lasted for maybe 5-10 mins. Came back through the same general area on the way back and it worked fine. Just figured that the Presidents plane was flying so they were screwing with the system again?
Gremlins
As a matter of general interest, near the Federal building in LA near the 405/Wilshire, my cell phone has dumped for 15 years. My Loran often went south when off LAX, but nearly always when dark and in pea soup fog. Same near Point Loma in San Diego. The GPS seems to be more reliable, although sometimes losing a fix for no apparent reason. Good luck; RD
As a matter of general interest, near the Federal building in LA near the 405/Wilshire, my cell phone has dumped for 15 years. My Loran often went south when off LAX, but nearly always when dark and in pea soup fog. Same near Point Loma in San Diego. The GPS seems to be more reliable, although sometimes losing a fix for no apparent reason. Good luck; RD
Lost Signal
GPS must receive at least two satilite signals to work. Some times since everything is moving it takes a few minutes to acquire another signal after one is lost. The airport should have no effect on your signal.
GPS must receive at least two satilite signals to work. Some times since everything is moving it takes a few minutes to acquire another signal after one is lost. The airport should have no effect on your signal.
Selective Availability
The Pres. signed the edict to remove dithering from the signal which makes regular GPS more equivalent to dGPS. SA(selective availability) is still alive and well.
The Pres. signed the edict to remove dithering from the signal which makes regular GPS more equivalent to dGPS. SA(selective availability) is still alive and well.
GPS information
Actually, GPS needs a minimum of four sats to correctly identify your position, though some units can deduce it from three. The sat signal allows the unit to figure out its distance only from a given sat. This creates a sphere of possible points in space. The second sat creates a second sphere. This is where most people get confused. The intersection of these two spheres is NOT only two points. Its a circle of possible points. The third satellite reduces the possibilities to two points. One of these is the correct answer. One of these is 11,000 miles off (the radius of the sat orbit) Some units are smart enought to recognize the wrong one as way off, and eliminate it. Others require the fourth satellite to limit the nexus of possible positions to one. As to why you might lose your signal near the airports, or any other modern comunications facility: Remember in high school when you tied a string to a doorknob and if you shook it just right you got a standing wave? Remember how if two standing waves align you get a really BIG wave? This is the premise behind the actual signal the sat is sending. In order for the gps unit to know where it is, it has to know how far away the sats are. In order for it to do that, it has to know how long the signals are taking to get to it. The problem is there is no baseline. So what they do is beam a time signal to the sats. Each then beams it earthward and it is received by the GPS unit. The signal looks like a square wave with sets of pulses longer or shorter to carry data. The unit uses a fifth sat (or, in earlier units one of the four) as a baseline. It then looks to see how far out of phase the signal from the baseline is as compared with any given sat signal, and uses that to determine the time difference between the sats. This gives the difference in distance between the two sats as compared with the GPS unit. Do this with all the sats and you wind up with a nasty algebra problem but one with a limited number of solutions. You are at one of the solutions. Add more sats, you get fewer possible solutions, and therefore greater accuracy. Hence 12 channel units. So here's the problem with the airports. To the GPS, the when the signals from each sat is overlaid with the signal from the reference, an interference pattern emerges. This is similar to the old turntables where a strobe was used to allow you see how fast the turntable was spinning. Imagine then putting another strobe in the room. You'd lose the ability to read the table. This is what happens when you start slinging digital patterns around near a gps receiver. Even though the freqs are way different, you get harmonic interference, and you get drop outs. Still with me? - wow, and I bored even myself. Justin - O'day Owner's Web
Actually, GPS needs a minimum of four sats to correctly identify your position, though some units can deduce it from three. The sat signal allows the unit to figure out its distance only from a given sat. This creates a sphere of possible points in space. The second sat creates a second sphere. This is where most people get confused. The intersection of these two spheres is NOT only two points. Its a circle of possible points. The third satellite reduces the possibilities to two points. One of these is the correct answer. One of these is 11,000 miles off (the radius of the sat orbit) Some units are smart enought to recognize the wrong one as way off, and eliminate it. Others require the fourth satellite to limit the nexus of possible positions to one. As to why you might lose your signal near the airports, or any other modern comunications facility: Remember in high school when you tied a string to a doorknob and if you shook it just right you got a standing wave? Remember how if two standing waves align you get a really BIG wave? This is the premise behind the actual signal the sat is sending. In order for the gps unit to know where it is, it has to know how far away the sats are. In order for it to do that, it has to know how long the signals are taking to get to it. The problem is there is no baseline. So what they do is beam a time signal to the sats. Each then beams it earthward and it is received by the GPS unit. The signal looks like a square wave with sets of pulses longer or shorter to carry data. The unit uses a fifth sat (or, in earlier units one of the four) as a baseline. It then looks to see how far out of phase the signal from the baseline is as compared with any given sat signal, and uses that to determine the time difference between the sats. This gives the difference in distance between the two sats as compared with the GPS unit. Do this with all the sats and you wind up with a nasty algebra problem but one with a limited number of solutions. You are at one of the solutions. Add more sats, you get fewer possible solutions, and therefore greater accuracy. Hence 12 channel units. So here's the problem with the airports. To the GPS, the when the signals from each sat is overlaid with the signal from the reference, an interference pattern emerges. This is similar to the old turntables where a strobe was used to allow you see how fast the turntable was spinning. Imagine then putting another strobe in the room. You'd lose the ability to read the table. This is what happens when you start slinging digital patterns around near a gps receiver. Even though the freqs are way different, you get harmonic interference, and you get drop outs. Still with me? - wow, and I bored even myself. Justin - O'day Owner's Web
thanks
I'll just try not to be near an airport or other major communications system when I can't use visual navigation. I suspected some sort of interference, since I had the same result going both into and out of the inner harbor. thanks for the feedback. Bryce S/V Spellbinder
I'll just try not to be near an airport or other major communications system when I can't use visual navigation. I suspected some sort of interference, since I had the same result going both into and out of the inner harbor. thanks for the feedback. Bryce S/V Spellbinder
Superb Cap'n Justin
Excellent treatise on the vagaries of the GPS, Justin. Gonna keep this in my archives. Don
Excellent treatise on the vagaries of the GPS, Justin. Gonna keep this in my archives. Don
That confusing explanation from Justin
Justin's words about interference are somewhat tangled and I'm not sure where he is trying to go with it. It is true that multiple fixed transmitters will set up standing waves. But the GPS satellites orbit the earth twice per day using 3 orbital planes and any standing waves from them won't last but a moment before the geometry changes. Another potential GPS reception problem is in "urban canyons" such as downtown in major cities where there can be tremendous multipath (where the receiver sees the same signal multiple times from multiple reflections off the buildings, just like "ghosts" on non-cable TV). And there is the blocking of signals except from sats nearly overhead. But neither of these should be a problem on a boat in a harbor. When Justin says "the signals from each sat is overlaid with the signal from the reference" I don't know what reference he means. Is there a pseudolite at Boston Logan? Or do you mean a DGPS transmitter? The answer is actually much simpler. In the area of the airport their are many transmitters, some of them very high power. The airport's scanning radar alone puts out a signal that will be a million times stronger than the signal from a GPS satellite 11,000 miles away. And you will find transmitters for all manner of guidance and communications. They go by the names VOR, ILS, possibly MLS (what a beast), NDB, then there's controller voice and data communications, corporate voice and data communications, that already mentioned scanning radar, possibly ground control radar, all the transponder replies from local aircraft, and others will all contribute to set up very high level local RF fields. Your GPS is a consumer electronic device and is very poorly shielded compared to avionics or military electronics. The local RF levels near an airport are so great that it just overwhelms the internals of your GPS, probably saturating both the RF and IF sections. Like trying to talk to the cute girl at the rock concert while standing in front of the speakers as the band plays. "WHAT?" "WHAT?" "WHAT?" Maybe that is all Justin meant.
Justin's words about interference are somewhat tangled and I'm not sure where he is trying to go with it. It is true that multiple fixed transmitters will set up standing waves. But the GPS satellites orbit the earth twice per day using 3 orbital planes and any standing waves from them won't last but a moment before the geometry changes. Another potential GPS reception problem is in "urban canyons" such as downtown in major cities where there can be tremendous multipath (where the receiver sees the same signal multiple times from multiple reflections off the buildings, just like "ghosts" on non-cable TV). And there is the blocking of signals except from sats nearly overhead. But neither of these should be a problem on a boat in a harbor. When Justin says "the signals from each sat is overlaid with the signal from the reference" I don't know what reference he means. Is there a pseudolite at Boston Logan? Or do you mean a DGPS transmitter? The answer is actually much simpler. In the area of the airport their are many transmitters, some of them very high power. The airport's scanning radar alone puts out a signal that will be a million times stronger than the signal from a GPS satellite 11,000 miles away. And you will find transmitters for all manner of guidance and communications. They go by the names VOR, ILS, possibly MLS (what a beast), NDB, then there's controller voice and data communications, corporate voice and data communications, that already mentioned scanning radar, possibly ground control radar, all the transponder replies from local aircraft, and others will all contribute to set up very high level local RF fields. Your GPS is a consumer electronic device and is very poorly shielded compared to avionics or military electronics. The local RF levels near an airport are so great that it just overwhelms the internals of your GPS, probably saturating both the RF and IF sections. Like trying to talk to the cute girl at the rock concert while standing in front of the speakers as the band plays. "WHAT?" "WHAT?" "WHAT?" Maybe that is all Justin meant.
Clarification -
The standing wave effect with the GPS is in the encoded signal, not in the actual broadcast wave. The pulse is what NOAA calls 'pseudorandom' and is infact an encoding of a time signal. Lay the code over itself and it will line up sometimes and not at others. Tron is quite correct that if the unit were actually looking for interfence in the beam itself, sat movement would make it change very quickly. Justin - O'day Owner's Web
The standing wave effect with the GPS is in the encoded signal, not in the actual broadcast wave. The pulse is what NOAA calls 'pseudorandom' and is infact an encoding of a time signal. Lay the code over itself and it will line up sometimes and not at others. Tron is quite correct that if the unit were actually looking for interfence in the beam itself, sat movement would make it change very quickly. Justin - O'day Owner's Web
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