Has anyone ever tried to build own GPS-System?

Andreas,

The point is, GPS broadcast is essential encrypted time stamps but only expensive GPS reveals this time stamp. Commercial units can only decrypt a limited amount of digits in the time and thus have less accuracy. I don't think the 1 second update is accurate to 1ns and you need more or less that accuracy for measuring distances in meters or 10 meters.

Hi,

The 1sec timeupdate is okay, you can log a time, count the cpu cycles log the next gps timestamp and you can interpolate.

Nevertheless I'm not fixed to have 1m accuracy, when it's 100m but the concept is proven it's okay.
I still want to play around with hardware and would like to get an advise what arduino -communication shield - gps shield is a good start.
I want to build at least one reception point and a beacon. First has to have com-link and gps, beacon only needs com-link. But perhaps it's wise to have 2 reception points that can also be switched to beacon mode. Shipping just one gps shield for turning the beacon into a reception point is the most expensive thing ;).

Andreas

Take a look at this

http://dexterindustries.com/Arduino-GPS_Shield.html

I think if you take this, add RF Tx/Rx as needed for comms between units, something like this
http://www.robotshop.com/433mhz-high-sensitivity-transmitter-receiver-pair-rxa30.html?utm_source=google&utm_medium=base&utm_campaign=jos

than can start writing s/w...

..not sure the gps works as you think it does.. it uses spread spectrum, not sure the time/position information is extracted by measuring "pulses with ns resolution", I think other method is used..
The "public"/"commercial"/"military" - all those are using specific separate channels in the gps band with special coding. The 1sec comming from gps is precise from long term prospective, usually there is a jitter caused by the gps receiver.p.

I think what -horn- wants to do is have the beacon send a message, the receivers all get iot and note the time it was received, talk among them selves to share those times, and then do the math to figure out where the beacon is.

hi,

that's it! :slight_smile:
And talking to themselves and doing math is a further step. Because the reception arduinos don't have to share them to eachothers. The data will be collected and analysed on a PC afterwards, because I don't need to have it in realtime. I just want to get a "when was it where" result.
Realtime, where all arduinos will communicate with eachother or with a master-node isa further step, someday :slight_smile:

Andreas

..interesting project, indeed! The only issue is to have a synchronised time across all the receiving arduinos, a big issue, I would say :cold_sweat:..
People usualy use the gps' 1 second pulse for disciplining a txco. You can hardly use it directly as the jitter is big for that kind of measurement. I would recommend you to study how navigation tools work, like LORAN or others.. The measuring time of receiving the ping from beacon is the last thing one would do :slight_smile: when having only one beacon. When having two or more beacons, each with its own synchronised atomic time, then it would be much easier as you can measure differences..
To measure with one beacon only and 100m precision required you need the time information accross all the components with a +/-150ns time alignment (all within 300ns).. To synchronise two xtal clocks with such even short-time precision would be hard for NIST as well.. :slight_smile:
PS: Maybe other approach may work - the "beacon" will play a role of a "repeater". so the arduino's will send a ping (with their own ID) and the ping will be relayed back. Knowing the roundtrip time may give you some info on the distance. When all the arduinos will repeat the pings from each other, they do not need absolute time knowledge, I guess.. But I doubt one ping will be enough as the jitter caused by the processing in arduino and the transceiver is very large, so you have to ping many times and make some averaging, or something like that..

Hi pito,

I will have a look at LORAN LORAN - Wikipedia

And this project is for http://shackspace.de/wiki/doku.php?id=project:hgg where some persons started to work on receiving signals, others are trying to find adequate hardware and I would like to build a proof of concept prototype with arduino. and IF/WHEN this works, we will change hardware to better andfaster equipment, but arduino is a nice and cheap way to check, if it is possible at all.
that's why accuracy is not so important for me, at least in this phase of the project.

Andreas

Maybe you can get together with a couple of lab accomplices and pick up some free
STM32F4-Discovery boards to plan ahead for when you need faster hardware

Hi,

for free? I don't think so! :slight_smile: But I will ask tomorrow. At least they have more MHz!

I checked http://www.sparkfun.com and would like to ask for a good combination of an Arduino mega 2560, gps module (that can also provide gps-time to the arduino) and a communication link that uses 2.4ghz and has it's own antenna AND where I can plug in an external antenna, perhaps yagi. and it would be great to have it ready so that I don't have to build and solder them on my own.

greets,

Andreas

..and the direct answer to your question: European Union is trying that (Galileo) and Russia as well (Glonass). Not sure it works today, however.. :wink:

Hi pito,

an answer to what question?

And Galileo has only 2 test satellites up there and 2 real satellites that had been launched last friday. So at LEAST they have to have 4 real satellites up there to be working in any way, AND you have to have them all covering one region, and that's very very unlikely. So it's not working unless they have 18 out of 27 satellites up there, that will be in 2015 (I think).
Glonass is not fully working, yet.

But anyway, I want to have it the other way round. That's why I don't understand your answer.

-horn-
Free, for real. Try the link, you'll see.

Free, for real.

U.S. and Canada only =(

But only £9.72 in the UK (in 12 days time) :slight_smile:
http://uk.farnell.com/stmicroelectronics/stm32f4discovery/board-eval-stm32f4-discovery/dp/2009276?Ntt=STM32F4DISCOVERY

@-horn-

Hi pito,
an answer to what question?
Has anyone ever tried to build own GPS-System?
:slight_smile:

I did once consider building a GPS, but the launch costs were simply prohibitive.

@awol: you may try harder:
http://www.kickstarter.com/projects/251588730/kicksat-your-personal-spacecraft-in-space
24x$300 :slight_smile:
p.

If you can use bursts of ultrasound instead, it would be a lot easier to do with slow hardware. Use a frequency just above 20kHz and common tweeters should quite well. Or get a whole bunch of old smoke detectors from a fire alarm company and drive the buzzers in them at the first ultrasonic harmonic.

There are two methods in use for enhancing the accuracy of GPS, Differential GPS and Real Time Kinemetrics. The simplest is differential GPS.
We were doing differential GPS back in 1986 with some pretty slow processors, so the Arduino would probably work fine for the differential calculations.
A differential system uses a fixed base station to collect psudeo-range information. The location of base station is well known, perhaps on a high precision survey point. The data collected by the base staion is transmitted to the users (WiFi would work well) (data = position + errors). The user is able to calculate his position very precisly because he can subtract out the errors in his measurements by using the measurements made at the base station. The Ardunio processors would not be doing the actual reception of GPS signals, just the data processing. Amateur Radio enthusists have been doing this for some time. The trick is to find a good GPS receiver for the base station. The old Motorola Oncore receivers could be used as I recall.