GPS message on request - Page 1
Forum section: electronics / beginners

author: rakeshm55
author_rank: Regular Contributor
timestamp: September 06, 2017, 12:02:13 pm
content: Hi,Currenlty i am working with a GPS module based on MT3339http://www.alphamicrowireless.com/franchises/globaltop-technology/smart-gnss-evb-patch-modules.aspx....On power up the device continuously sends the GPS data as expected....Is there any means to command the device to send GPS data on request??.... I have tries sleep by sending sleep command and later collect data but does this force the GPS module to reacquire lock information...How to implement command response for standard GPS modules....

author: Jeroen3
author_rank: Super Contributor
timestamp: September 06, 2017, 12:10:11 pm
content: GPS needs to be continuously receiving and calculation in order to keep track of the satellites that are expected to move in range.Phones have assisted GPS, they receive the list of satellites to look for via internet. If they can guess where they are the receiver is adjusted to listen directly for satellites in range.

author: sasa
author_rank: Regular Contributor
timestamp: September 06, 2017, 01:23:36 pm
content: I have different GPS module, but briefly looking in datasheet for your module, it is not quite different...The most of cheap modules are based on ARM and have quite amount of internal SRAM in order to hold and calculate position based on received data from satellites. It is usually enough to predict GPS satellites position for a 3 days.If put in sleep, on hot start/wake-up it only have to read internal RTC date and time, last position before put in sleep and from table get predicted satellites orbit which cross the sky, then listen these satellites in order to calculate new position. All that may last less than 1s, I do not see actual info in the datasheet.I do not claim that is how is implemented hot start, however that is how it is the fastest to do. Time needed for GPS module I have to return new position from hot start is less than 1s.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 06, 2017, 01:35:19 pm
content: $PMTK314 followed by 19 zeros?Seehttp://www.microchip.ua/simcom/GPS/Application%20Notes/MT3339%20Platform%20NMEA%20Message%20Specification_V1.00.pdfpage 25.

author: FreddyVictor
author_rank: Regular Contributor
timestamp: September 06, 2017, 01:37:01 pm
content: Quote from: rakeshm55 on September 06, 2017, 12:02:13 pmYou can turn all messages off, and then enable a message type when you want to receive it (and then disable it again)not sure how long you have to wait for the message tho'have a look at the $PMTK314 command for MTK3339

author: rakeshm55
author_rank: Regular Contributor
timestamp: September 07, 2017, 05:06:08 am
content: Thank youthis worked!!!

author: cdev
author_rank: Super Contributor
timestamp: September 07, 2017, 05:18:10 am
content: Newer GPSs have power saving modes that let them wake from sleep to capture and save their location at regular intervals. This takes almost no power if they already have the ephemeris in memory, or if they download from the network. So no more warm start or anything like that is necessary. Also, soon they will be accurate to just one or two centimeters.However, all bets are off if they are off AND out of network coverage (for example, in the middle of the ocean or desert) so cant download an ephemeris off the net. Unless somebody turns it on, then they will have the broadcast satellite ephemeris in around 14 minutes.

author: rs20
author_rank: Super Contributor
timestamp: September 07, 2017, 05:34:37 am
content: Quote from: cdev on September 07, 2017, 05:18:10 amHeh yeah, to the same extent that ADSL2+ runs at 24 Mbps orthis transistorcan carry 600 Amps

author: sasa
author_rank: Regular Contributor
timestamp: September 07, 2017, 05:47:19 am
content: Quote from: cdev on September 07, 2017, 05:18:10 amYes, as soon as automotive vehicles is fully tested and certified by law regulative, military GPS band resolution will probably be available for civil usage.

author: hamster_nz
author_rank: Super Contributor
timestamp: September 07, 2017, 07:37:31 am
content: Quote from: sasa on September 07, 2017, 05:47:19 amThe high accuracy GPS can be obtained from the regular C/A code by following the carrier directly, rather than down converting to an IF. A technique called "Carrier phase recovery". The GPS signal has a wavelength of around 20 cm, so if you can follow the carrier phase you get get very precise timing (and there for positioning) information.However, with those levels of accuracy the changes in the Ionosphere (like the current solar storm) can causing positioning errors....(I am still amazed that you can know where the space vehicles are, in a 20,000+ km orbit, to within a few mm).

author: cdev
author_rank: Super Contributor
timestamp: September 07, 2017, 03:27:20 pm
content: GNSS has very rapidly been getting better, and the hardware smaller and cheaper. Even without corrections of any kind being applied besides the broadcast ones, the shift to multi-constellation GNSS (one of my GPS's can do GS+Glonass and another can do GPS+Beidou plus both have SBAS, which has been there for awhileAdding additional "bird"(s) of another kind - when even just one beidou sat is there, on my cheap Navspark or using GPS+Glonass seems to have made a huge improvement in the quality of the solution. Without any additional computation going on in my computer.Its really quite amazing.When I use Rtklib, even with my cheap antennas, it gets even better, although there still are some bugs I have yet to work out. Performance is still not as good as I was getting way back in 2011, 2012 or so with just GPS and SBAS. But thats my fault because he better corrections you just were describing which I used to apply are missing.(I stopped using it for a couple of years and during that time correction products in SP3 format which used to live on a NASA web site - which still are free and exist on the network - have moved and I have to re-locate them and enter them in to RTKlib's file grabbing tools and also set up all the paths for everything again in at least a dozen different places.)