I was watching a really interesting show on the Discover Channel, or maybe it was the learning channel, I dont quite recall. At any rate, they were talking about GPS, and explained that there were originally 2 signals sent down to earth. one was reserved for military use, and was accurate to a few inches, the other, was a degraded signal that was for public use. This was for done as a matter of security. As of the year 2000, only one signal is boradcast, and that is the more accurate one. They further explained, that in order for GPS to work, 3 total satelites has to send a signal to the GPS receiver, and used simple triangulation to pinpoint the location within a few inches. Pretty logical, the FCC has been able to do this with people broadcasting personal hacked equipent for a long time, ie: rogue broadcasting on a military channel, police channel, or other “off limits to the public” frequencies.
Then they went into a remote beacon locating device that is used in boats, planes and the like. Specifically http://www.vitmarine.com/acr-2790.html the 406Mhz type, the link shows the specific one they were speaking of. Of course, the ones in planes and boats are a little more sophisticated, but operate on the same principals, The part I do not get. the 406 need not talk to 3 satelittes, only one, and it is able to send a precise location to a company that looks up the registered number it broadcasts, determine if it is a boat, plane etc from a registration database, and also pinpoint the coordinates of the distressed vessel.
So the question remain, how is it that GPS takes 3 satellites, and this simple 408 device only needs one. Without triangukation mathemtaics, how can a point to point figure anything out? Or perhaps, it only boradcasts the distress beacon signal, and along with that works in conjunction with GPS to also send coordinates as well, anyone know?
There are still at least 2 transmissions - one that is available to cilvilans and one reserved for the military. The milatary can degrade the accuracy of the civ. models. Some milatary models use the civ frequency with a correction for the degradation.
gps can use 3 sats to find out it’s location but normally use 4. If it does use 3 it uses the earth surface as a fourth location. The reason for the 4 is that there is an error in triangulation in practice (in theory it works) - the 4th is used to correct the error.
If I understand the OP correctly the 406 is the emergency beacon device that was just approved for land use. If so it uses gps (IIRC) to locate itself and just uses one sat. to broadcast it’s location. gps units don’t broadcast the sats do.
The COSPAS-SARSAT home page has information, especially this PDF document. There are a few differences between this and GPS. First, GPS gives you the longitude, latitude and altitude. The COSPAS-SARSAT assumes that you are on the ground, so it doesn’t give you the altitude. Which means you need one fewer satellite. Second, the COSPAS-SARSAT assumes you are not moving (probably a valid assumption since it’s an emergency beacon), and takes multiple measurements as it flies by above you. So it takes some time to acquire your position. You need to wait up to an hour for a satellite to come into view, and a few more minutes for it to acquire your position. On the other hand, GPS gives you instantaneous measurement anywhere, any time and can be used on a moving vehicle.
Properly speaking, GPS receivers do not “triangulate.” They measure the time difference between the arrival of signals from various satellite pairs, which places them on a hyperbolic surface with respect to the satellites’ positions. They then calculate the intersection of several of these hyperbolic surfaces to determine a fairly precise position.
The change that took place in the GPS signal in the year 2000 was removal of the artificial degradation in the civilian (non-encoded) part of the signal. Prior to that, the signal was “dithered” to give a typical accuracy of 120’ or so. Without this, typical accuracy is 40’ or so (it will depend on antenna quality and location, received signal strength, etc.).
You can get accuracy in inches, but only by leaving a receiver in one location for a long time and averaging the positions it calculates every couple of seconds.
Xema, do you have any sites on your info, this is quite contrary to about 1/2 hour worth of information and diagrams and satellite 3d demos they showed on television. Again, I can not remember what the show was, but it was either Learning Channel, History, etc, and I consider them pretty reputable, but I would like to know and understand correctly.
I specifically remember them speaking in great detail about triangulation math, and 3 satellites. I also specifically remember them talking about any off the shelf GPS receiver now able to get accuracy to a few inches, and in real time. However, I remember thinking to myself, this does take all the fun out of geocaching.
Xema is just talking about semantics. Strictly speaking, triangulation means measuring the bearing (angle) of the target from two known points to calculate the location of the target. GPS measures difference in distance to three known points so it’s not triangulation in the traditional sense.
But nowadays, I think any method of calculating position based on multiple reference points is called triangulation.
IIRC, when GPS was being introduced to general aviation (small planes), the story was:
non-precision signal (civilian): +/- 50’
precision signal (US military only): +/- 3’
and, theoretically, differential GPS (addition of a ground-based signal, and math I will never understand) could produce position accurate to within 6". (real handy when landing in 0 visibility)
Also, there are so many satellites up, you can have more than 4 a lot of times. Many aviation GPS routinely use 8-10 satellites at once. More accuracy that way too. The $500 hand held jobs are really impressive to anything 10-15 years ago. What can and is being done in the surveying worlds and the aerial mapping world is really astounding.
Can you say 1 part in a 2-3 million? I knew you could.
Big difference as explained above in GPS and their different uses and costs and emergency beacons. Apples and oranges sorta…
GusNSpot I don’t think aviation medels use more then 4 at a time but monitor more then 4 and use the best signal of the group.
Also there are 2 WAAS sat’s that were launched to allow corrections to civ. gps’s. THis allows civ’s to get accuracy to about 6 ft. Without it the best you can expect would be 12 ft if you left it in one spot for a looooooooog time (a day) You will never get down to inches with a standard civ gps.
Just in plain language, from the first link to the beacon, this device a GPS dataport in it. A GPS receives the transmitted signals from a few of the 27 geosynchonous GPS satellites, finds it’s location in 3D, then sends that data to the beacon, which TRANSMITS that location data on a UHF band radio frequency to another satellite, which then goes down into a control center. The military has a system called the PLRS, Position Location and Reporting System, which does essentially the same thing using military allocated UHF radio frequencies, which stop at 399.99 MHz.
The GPS feature of the beacon is just a backup. The COSPAS-SARSAT satellites have their own method of determining position. It’s done by measuring the Doppler shift of the signal as the satellite passes overhead. As the satellite approaches the transmitter location the signal is blue-shifted, and as it flies away the signal is red-shifted. The satellite is in polar orbit, so the latitude of the beacon can be determined by where the Doppler shift becomes zero. The amount of shift depends on line-of-sight speed between the satellite and beacon, and this is depends on how far the beacon is from the path of the satellite; if it’s directly beneath the path then the shifts (both red and blue) are greatest. So you can calculate how far the beacon is from the path. This means that a single pass with a single satellite narrows down the beacon position to two positions - its latitude is known, and you know it’s X miles away (could be east or west) from a certain longitude. A second pass will remove the ambiguity.