Affordable Centimeter-Level GPS Accuracy  - Page 1
Forum section: electronics / general

author: RickH
author_rank: Newbie
timestamp: January 14, 2014, 08:31:56 am
content: A plug-and-play carrier phase raw measurement output GPS receiver that works with open-source RTKLIB, capable of generating centimeter-level position accuracy at $50 a pair should be interesting.Although not a complete stand-alone RTK receiver as the $900 a pair Piksi, but requires running RTKLIB software on another PC, it is still quite impressive that how quickly the centimeter accuracy RTK technology found in high-end high-price survey grade GPS receivers are becoming accessible nowadays.http://bit.ly/1dBpits

author: JBeale
author_rank: Frequent Contributor
timestamp: January 17, 2014, 05:14:11 am
content: In case you don't like hidden URLs, the NavSpark project on indiegogo is here:http://www.indiegogo.com/projects/navspark-arduino-compatible-with-gps-gnss-receiverThey are using the Skytraq Venus822 GNSS chipset and 100 MHz 32-bit LEON3 Sparc-V8 CPU+FPU.I really want this project to work and the promised performance to be realized, but I'm curious if anyone here knows how likely this goal is to be achieved? There is very little on the "affordable" end of the market with raw pseudorange outputs, as far as I know. The u-blox timing grade receivers do this but are rather difficult to source. Anyway I plunked down my $50 so my fingers are crossed.

author: Rasz
author_rank: Super Contributor
timestamp: January 17, 2014, 07:30:34 am
content: its chinese/tw = there will be no documentation or source code = good luck

author: JBeale
author_rank: Frequent Contributor
timestamp: January 17, 2014, 05:23:22 pm
content: You could be right about the documentation. But some SkyTraq Venus GPS chips are being used in the hobbyist market already, for example:https://www.sparkfun.com/products/11058

author: JBeale
author_rank: Frequent Contributor
timestamp: January 31, 2014, 05:35:26 pm
content: If you're interested, I've re-organized some info from their Indiegogo webpage into my own FAQ on the NavSpark board:http://pastebin.com/raw.php?i=1f5iXwiT

author: edavid
author_rank: Super Contributor
timestamp: February 04, 2014, 03:57:22 pm
content: Quote from: mojo-chan on February 04, 2014, 11:56:51 amThe Venus822-BD datasheet says "Tracking 15mA @ 3.3V". Is that low power for a GPS receiver?Also, the "centimetre level GPS accuracy" version (NS-RAW) doesn't allow user programming, and it won't be low power because it requires a powerful external computer to process its output with RTKLIB.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 04, 2014, 05:19:43 pm
content: Quote from: edavid on February 04, 2014, 03:57:22 pmI think that is quite low power for GPS with continuous updates, eg. not sleeping 90% of the time- but that datasheet may be talking about the baseband Venus8 CPU only, and not the full solution including RF front end.QuoteAs far as I know, true cm-level GPS solutions are currently in the $1000 (and way up) class. It remains to be proven how well the $50 NS-RAW set does... I put in for one and I'm hoping to be impressed, but my suspicion is that if cm-level happens it will only happen under ideal conditions. For example I have a ublox LEA-6T with raw output (bare module lists around $180) and after a week of tests, I find it gives me results consistent from day to day of about 20 cm in a static-PPP solution with 24 hours of averaging per data point. I have an old SiRFStar-II chipset device with raw output (DeLorme Earthmate Blue Logger) and it is about 3x worse in resolution, using the very same antenna through antenna splitter. On the other hand this is single-receiver PPP which is harder than DGPS with base and rover, but basically I'm just saying, don't expect too much.That said; low power is meaningful even for NS-RAW, if you put it on a mobile robot, plane, quadcopter... you can use a small transmitter to send the data back to your laptop etc. and still get basically real-time position without having big batteries on your remote device. Or something like a Raspberry Pi could work if you want the remote running RTK.EDIT: By the way, I see there are a few websites likehttp://rtk-pi.com/andhttp://tubbyaustin.blogspot.com/2013/06/rtklib-on-raspberry-pi.htmlwhich mention "cm-level precision" with cheap receivers but they do not show actual results, or really much detail that I could find.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: February 04, 2014, 06:58:29 pm
content: Quotevs.QuoteI smell a rat.

author: ve7xen
author_rank: Super Contributor
timestamp: February 04, 2014, 07:51:06 pm
content: Hoping they'll be useful for timing, good timing modules are pretty expensive or old and power hungry / large. Backed. Might almost be enough peripherals onboard to implement a basic GPSDO without too much else.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 04, 2014, 07:59:20 pm
content: If you read through all the comments on the Indiegogo site, you will note that RAW output was not originally part of the offering, but they added it by customer request. At first try they could not get RAW output working with RTKLIB from VENUS 822 so they were going to use an older VENUS 6 chipset as the backup plan, which they stated upfront. Later you will note that they did report success with RTKLIB from the Venus 8 family, so after that there was no hardware limitation in place for Arduino programmability, so what happened then?I don't know but it's easy to imagine there was some company meeting and the marketing-strategy people ruled out programmability for the RAW output version. Logically enough, because if it's just provided as library, you could buy the less expensive basic unit and simply reprogram it to be the (slightly) more expensive RAW one. The fact that Oliver posted the point about market segmentation on page himself, suggests to me he's not trying to hide what's going on. I can hardly criticize this strategy when it still ends up the cheapest raw-output GPS I'm aware of.Compare that with the other vendors in the GPS space who do the same thing, but without actually saying it. In days gone by, the SiRF-Star II GPS chipset had an undocumented feature providing raw pseudorange and carrier phase. This was removed in SiRF-Star III, allegedly for this reason.With the 10-nsec resolution I was also thinking about GPSDO. I note they have a 24-bit PWM which when filtered might be useful as a DC control signal as well; so it's possible you might get it to work without needing a separate controller.

author: edavid
author_rank: Super Contributor
timestamp: February 04, 2014, 08:44:36 pm
content: Quote from: ve7xen on February 04, 2014, 07:51:06 pmI don't think so... Oliver said:QuoteMaybe John can translate that, but it doesn't sound promising for GPSDO use.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 04, 2014, 08:52:50 pm
content: Quote from: edavid on February 04, 2014, 08:44:36 pmI'd say Navspark could be used in a "basic GPSDO" (maybe not a "fancy GPSDO"). Oliver is saying that it's not as good or feature-ful for precision timing as a $200 solution (LEA-6T). However position-hold does not necessarily improve your timing over the long term, it just helps with short-term fluctuations. If your local oscillator is stable enough and you're only using the GPS for long-term drift correction, it may not affect accuracy much, particularly if your antenna has a good view of the sky.He also said Navspark does not have TRAIM, which is an integrity monitor that can warn you if the calculated timing error exceeds some limit, but that is just an information feature, it does not by itself change the accuracy. You could get the same information by measuring instantaneous deviation between the 1PPS from the GPS and from your local disciplined oscillator.It's just a question of what your goal is, and your budget. It's not a high-end GPS, rather the opposite, but for the price of one LEA-6T you can get quite a few Navspark units and compare their outputs.

author: ve7xen
author_rank: Super Contributor
timestamp: February 04, 2014, 11:11:11 pm
content: It's also programmable. Not sure exactly what that means, but it may be fairly easy to implement TRAIM and position hold in a custom firmware. Affecting how the time solution is calculated is probably not possible... but it'll be fun to play with anyway, I think. Seems a lot more practical than 20 year old Motorola Oncore units.

author: edavid
author_rank: Super Contributor
timestamp: February 04, 2014, 11:13:52 pm
content: Quote from: ve7xen on February 04, 2014, 11:11:11 pmProgrammable means you can run your own code, but the GPS code will just be a blob, so it can't be modified.

author: amyk
author_rank: Super Contributor
timestamp: February 05, 2014, 12:12:52 pm
content: Quote from: JBeale on February 04, 2014, 05:19:43 pmFor comparison that is the same power consumption asthis MT3339 module, but thisQuotesuggests it consumes rather a lot more.Quote from: edavid on February 04, 2014, 11:13:52 pmModifiedeasily, that is... "when there is a will, there is a way."

author: JBeale
author_rank: Frequent Contributor
timestamp: February 06, 2014, 09:20:28 pm
content: By the way, I thought I'd never heard of the embedded LEON3 SPARC-V8 cpu that NavSpark is using. But maybe I've been using one without realizing it.Today I read the description for the (rather expensive) SDK for developing embedded applications on the Roving Networks WiFly (standalone wifi module- now owned by Microchip). It says: "Based on GCC, GNU Make and the LEON BCC Compiler System" making me think they've got a Sparc-LEON2 or LEON3 in that module, running their wifi and TCP/IP stack. Just for what it's worth.http://www.microchip.com/stellent/groups/sitecomm_sg/documents/devicedoc/jp557986.pdf