Thanks for the correction! I'll have to look into this a little more. -Adam On 12/4/06, Phillip Coiner wrote: > 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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