Hi Adam But most newer chipsets only have 8-10 correlators - these are the computationally intensive bits of the receiver that decode the GPS signal. The remaining channels are fairly dumb receivers that give signal strength, and they are constantly changing frequency looking for all available satellite signals and the associated signal strength. When one is found that is better than one currently being decoded, a correlator is assigned to it. Note that "better" does not necessarily mean a higher signal strength. Once the location of the receiver and the almanac is known then it may choose a slightly poorer signal for a satellite that is not directly overhead for better positioning. Especially if it has 4 or 5 good strong signals already - extra signals are just icing on the cake, so might as well make it tasty. I have a small quibble with this part your generally excellent post. A channel in a GPS receiver is either a correlator or it aint. The only way to measure the signal strength of a GPS signal is to use a correlator that has "the" PRN code for the channel. There is no such thing as a dumb receiver in regards to CDMA signals. You must correlate on the spreading/chirping code with a correlator. Standard detectors will only see noise(if you are close enough to the transmitter you can see the edges of the modulation in the frequency domian and the general shape/response of the transmit filter but because the codes come pseudo randomly without a correlator the average is zero. Another minor point all channnels are the same center frequency the different channels are chirped or spread with a particular PRN sequence you refer to the channels on different frequencies but the more correct term would be different spreading frequencies. (these are often confused with SAT IDs while they can be the same e.g. SAT ID 1 with PRN 1 . In many cases SAT ID x will be transmitting PRN number y because they have to recycle the PRN numbers as older satellits die off/are upgraded) "Madness takes its toll please have exact change" Phillip Things should be as simple as possible but no simpler Phillip Coiner CTO, GPS Source, Inc. Your source for quality GNSS Networking Solutions and Design Services, Now! -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of M. Adam Davis Sent: Monday, December 04, 2006 9:32 AM To: Microcontroller discussion list - Public. Subject: Re: [PIC] GPS Receivers,was Re: Why the preoccupation with bus powered programmers? On 12/4/06, Alan B. Pearce wrote: > This seems to be a benefit of having many more parallel channels in the > receiver. More channels allows it to try so many more satellite combinations > in parallel, so it finds them faster. I recently learned why some GPS receivers claim to be 16 channel, when it's useless to listen to more than 12 satellites at once (There are 24 satellites, a typical receiver cannot see more than 12 since the earth blocks at least half of them). Some extra channels are reserved for WAAS augmentation. But most newer chipsets only have 8-10 correlators - these are the computationally intensive bits of the receiver that decode the GPS signal. The remaining channels are fairly dumb receivers that give signal strength, and they are constantly changing frequency looking for all available satellite signals and the associated signal strength. When one is found that is better than one currently being decoded, a correlator is assigned to it. Note that "better" does not necessarily mean a higher signal strength. Once the location of the receiver and the almanac is known then it may choose a slightly poorer signal for a satellite that is not directly overhead for better positioning. Especially if it has 4 or 5 good strong signals already - extra signals are just icing on the cake, so might as well make it tasty. So a 16 channel may have 8 channels for searching, and 8 channels for correlating. Or it may have 8 channels for correlating, 6 channels for searching, and 2 channels for WAAS. If you have an almanac (which tells you the general orbits and frequencies of all the satellites) then on turn on you can select a set of frequencies to listen to such that you're pretty much guaranteed to get a signal from one or two satellites if you have a good view of the sky regardless of where you are on earth. This alone tells you which hemisphere of the planet you are in, and if you get two instantly then you've got a good chance of telling which continent you are on. The rest of the searching frequencies can be given another set of satellites that are very likely in your area, and giving the correlators a good set of satellites to listen to can happen very quickly on startup. The more channels it can look at on startup, the faster it will find the satellites even in an urban canyon (street level new york, for instance, where the buildings block or bounce a lot of GPS) The correlators then get to noodle on the signal for a bit until they figure out where in the pseudo random sequence they are before they're locked on. It would be easier if your GPS receiver had an atomic clock - then locking on would take only a few dozen bits of data before it figured out how much delay there was in the signal. The reason a new GPS may take several minutes to boot up is that it doesn't have a current almanac. It has to find a good signal, and listen to it for quite some time to download the latest almanac. The almanac largely contains satellite information - what satellites are active, what frequencies they transmit on, their orbit (ephemeris, etc). I wonder how difficult it would be to make a simple receiver that simply does an FFT on the GPS spectrum. That, along with a recent almanac, would give you the satellites you can hear, which might give you enough information to tell you your position to within a few thousand miles. I'm still waiting for a one channel GPS receiver made only for synchronizing clocks. -Adam - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Moving in southeast Michigan? Buy my house: http://ubasics.com/house/ Interested in electronics? Check out the projects at http://ubasics.com Building your own house? 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