GPS module wanted for timing.

Is the receiver locking only to 4 satellites at a time ?

At least with slightly higher antenna position, there should be much more than 4 satellites visible at a time. Of course, if you calculate the time solution from different combinations of 4 satellites, you then have the problem, which solution to believe. To be sure about a measuring result, measure only once :-).

In practice, most timing solutions from different combinations of satellites are quite close together, so select the median or just average the best solutions.

I do not know, what kind of foliage you have, but it shouldn't attenuate the 1.5 GHz GPS signal too much.

From memory, it has a 48MHz clock, and the output is synchronous with that. Plus the 1pps output deviation from nominal is a sawtooth wwveform with a 21ns step.

At least Lea-6T had horrible jitter at 10MHz. Average frequency was ok, though. If I remember correctly, the output was derived from 48MHz clock and anything that is a direct 48MHz/N division result was clean. I ended up using 4MHz output as reference to my project at that time.

snipped-for-privacy@downunder.com wrote in news:nfqdkdhi2gci3iu2btbk73cqvo783serpg@

It does not "lock" to ANY. It takes readings of the arrivals of timestamped pings which the satellites constantly produce.

torsdag den 12. juli 2018 kl. 16.35.25 UTC+2 skrev snipped-for-privacy@decadence.org:

which it does by "locking" onto the frequency of the satellite and the phase of the prn sequence

wrote in news:pi80tp$pr5$ snipped-for-privacy@gioia.aioe.org:

Fuck you. It does not "lock on" to a satellite. It synchronizes itself with their streams, and switches birds constantly.

It synchs to their streams and reads their arrival times with respect to each other.

Since I have yet to see tiny dishes on any GPS antenna, I feel relatively certain that a GPS device reads and synchs with the data streams, NOT "locks on to" a satellite's position.

"Lock on" would more closely match the characteristics of an earth based dish positioner and a geostationary satellite. THAT "locks on".

You might need to read up on how CDMA works.

I worked in Sorrento Valley for nearly two decades.

You know... companies like Qualcomm... the inventor of it (modern CDMA link topologies.

General Instrument... The makers of MCPC. (that one went over your head).

ViaSat, the masters of Link 16.

No, f*****ad... I do not know anything about it... Yeah, right.

Like I said before... f*ck you.

At least when correcting some of the atmospheric errors in position fixing, differential-GPS is used. In this, the DGPS station calculates a pseudo code distance correction from the DGPS station to each visible satellite individually. A distance correction of 3-30 m corresponds to a 10-100 ns time variation.

Now that the atmospheric error is similar to other nearby GPS receivers, the other stations measures the pseudo code distances then apply the DGPS correction to each measurement individually, before calculating the position fix.

Couldn't these DGPS corrections be used for accurate timekeeping ?

Of course, if the clock receiver is in a fixed well known location, it can generate its own GPS correction.

This is a bit of a cyclic analysis. The position can only be found by getting the corrected time from all the sats factoring in the path delay. So time and location are all solved together.

GPS is used for surveying where time averages are used to get very accurate locations which in turn requires much more accurate time keeping.

Rick C.

DGPS can produce postion accuracies down to the centimeter level as long as the distance between the DGPS receiver is fairly small. The local station is at Pigeon Point, about 30 miles away from Santa Cruz. I was getting about 5 meters accuracy without DGPS, and 2 or 3 meter accuracy using DGPS from Pigeon Pt. I think I could have done better.

However, the USCG is turning off their beacon band DGPS system, which has been replaced by WAAS:

That's the way it's done. Back in the stone age of DGPS, it was just a log file of positions from a terrestrial DGPS receiver. The assumption was that the errors for the users GPS receiver and the static DGPS receiver were the same at any given instant of time. So, we logged the RTCM SC-104 position records from the GPS receiver, and compared it with data from a DGPS receiver. The difference produced a much more accurate position fix. With a good view of the sky, accuracy down to the width of the antenna were fairly easy.

I don't think so. Since the differential correction system required time syncing each recorded position report, this would create a circular reference, where the uncorrected time would be used to correct itself. I don't think that will work.

Same problem as before. You can't use the current time to correct the current time.

Incidentally, before WAAS, I was looking into building a DGPS station and transmitter. For a time, I had all the necessary equipment and associated licenses. It was suppose to eventually be a free service to police and fire to improve their GPS location accuracy to the where they could quickly and reliably locate their officers and vehicles down to about 1 meter or better. The idea was to broadcast the DGPS corrections on a commercial VHF or UHF frequency, similar to what MBARI is doing to accurately locate its vessel and buoys in Monterey Bay. The stumbling block was when I discovered that the site and antenna needed to be surveyed to an accuracy of at least 1 order of magnitude better than the accuracy we expected, and that it needed to re-surveyed regularly to deal with tectonic movements. Using the GPS constellation to do the survey was not good enough. I don't recall the price, but it was more than I wanted to pay. End of project.

Today, GPS correction services are quite common and the data is easily obtained on the internet. However, that is only useful for doing post-processing of the data and not for real time.

For some application, DGPS ground stations have been replaced by pseudolytes, which are essentially GPS satellites on the ground.

My apologies for my fumbled around. I haven't done anything with GPS for about 15 years.

afaik every cell phone basestation has a gps reciever for timing and phones already download info from them to get a quicker GPS fix (A-GPS)

I don't know if they also use it for D-GPS, but it seems like all the bits and pieces are there

DGPS is sometimes used, but not in all systems. Someone had the bright idea of time synchronizing all the GSM and CDMA2000 cell sites to make handoffs between sites much easier and seamless. That's a good idea, but requires that all the sites in the system synchronize their clocks to a common source such as GPS. I don't recall the timing accuracy required, but I'm fairly sure it doesn't require DGPS.

AGPS (assisted GPS) is a different nightmare. Basically, the GPS receiver in the typical handset does not provide a sufficiently accurate location fix to satisfy the FCC and the alphabet soup of served agencies. So, some providers use TDOA (time difference of arrival) information from cell sites near the user to improve the location accuracy, and to shorten lock time.

The data from the cell site is sent over the internet to a location services server. The data from the handset it sent to the cellular service provider and then forwarded to the location services provider. The two (or more) data sets are conglomerated into lat-long-altitude fix, which is then forwarded to the PSAP (public safety answering point). Note that the data sent by the handset is NOT it's lat-long-altitude, but rather the raw time delays from each satellite. That allows for applying DGPS corrections at the location services provider. The only parts of the system that requires a GPS receiver is the handset, and the location services server for applying DGPS corrections.

Again, I'm far behind on the location technology in use today because I haven't been involved in GPS for a long time.

ts

I'd would have thought it was mostly a matter of not wasting time and batte ry on searching for satellites and downloading almanac at 50bps when you have a megabit data connection to a cell tower that has all the info

ts

I'm trying to understand this. Are you saying the normal operation of a ce ll handset does not require DGPS, but the location requirements for emergen cy services does. So rather than add DGPS to cell phones for this purpose they punt the heavy lifting to the cell towers by providing them with the r aw timing data from the handsets rather than processed location data?

Rick C.

bits

cell handset does not require DGPS, but the location requirements for emerg ency services does. So rather than add DGPS to cell phones for this purpos e they punt the heavy lifting to the cell towers by providing them with the raw timing data from the handsets rather than processed location data?

I don't know, but it wouldn't surprise me if it was a leftover from phones not having enough horse power to but do call a and calculate position

Correct. However, I'm mostly talking about a basic bottom of the line "feature phone", not a smartphone. The smartphone has a better chipset capable of computing the location directly from the satellite delays and feeding the location to a mapping program. The "feature phone" does not. However, E911 has to work with both types of phones, as well as phablets and maybe some data only IoT (internet of things) devices. In other words, the lowest form of cell phone that you can find, which could easily be 15 years old. I use an LG VX8300 which qualifies.

One might think that having a smartphone makes things easier by simply sending the lat-long-altitude to the PSAP directly. Nope. In order to do DGPS and AGPS corrections, one needs the individual satellite delays. That would mean that the smartphone would need to process the WAAS, DGPS, AGPS, etc data instead of the location services server. That's totally impractical and much easier to send the raw delays to the location server instead.

Exactly. In the distant future, there may be nuclear powered smartphones with the battery capacity, horsepower, and bandwidth to do it all in the handset, but not at this time.

Battery life and cell phone horsepower were big parts of the decision to do the location processing offsite. If you scribble the various possible topologies on paper, where the processing can be done in the handset, at the cell site, at the wireless provider, on a central server, or in the cloud, I think you'll find the simplest, cheapest, fastest, and most versatile implementation is the current method.

The accuracy problem is rather interesting. When the FCC held qualification trials for the various proposed methods of improving location accuracy, AT&T decided to use a short cut. Since a huge percentage of 911 calls came from callers in vehicles, AT&T simply snapped (rounded off) the indicated location to the nearest roadway on the map database. That was sufficient to meet the original specs. The only problem is when someone calls sufficiently far away from a road, or from a location that doesn't have an address.

I think these are the current specs:

A good question to ask is why anyone needs such extreme levels of accuracy? If there's a fire, or incident, it's easy enough for the calling party to describe the location. I can contrive some unusual situations where that's not possible, but that's unusual and hardly justifies burdening the entire population of cell phone users with the cost of the hardware, software, and services. So, why the extreme accuracy? My guess(tm) is that only the various police and spy agencies can benefit from that level of detail as they track someone or something around the country.

I was with you until now. I have a 15 year old hand held GPS which does WA AS processing on an ancient Motorola processor running at something like 33 MHz I believe. I don't think the processing is so hard really. The full data for WAAS is sent by the WAAS sats. I'm not so familiar with DGPS or A GPS. The one real advantage of using the location services server is that the server has all the info all the time while the cell phone may have been turned off until the moment the emergency call is made and so not have a f ull lock or data update.

I don't know the accuracy rating for WAAS functionality, but my experience is the GPS claims roughly 10 feet. But that may be with averaging over man y seconds. I know the unit will do that, but I don't remember if the 10 fo ot number was after considerable averaging or not.

I'm a bit surprised at this. GPS processing in general is relatively light duty. I suppose the DGPS and AGPS might be more math intensive.

People are very bad at explaining where they are or how to get there. A fr iend has several times castigated me for not having my house number at the end of the 900 foot driveway. Emergency services would not have a way to t ell which driveway to come down to find me as well as the USP and FedEx guy s. Even a single minute can make a huge difference in reaching a fire. I really need to do that tomorrow. DGPS won't help this issue at all. The h ouse might be well located, but the driveway isn't even shown on Google map s very well.

Rick C.