GPS module does not latch to any satelltite - Page 1
Forum section: electronics / projects

author: ppTRN
author_rank: Regular Contributor
timestamp: February 06, 2025, 10:13:42 am
content: Hi everyone,I am trying to use a SIM68ML GPS module in my design, but unfortunately it appears to be unable to latch to any satellites. I tried it in direct sky view of course. Let me provide some more informations:- The module communicates via a serial TX & RX, TX outputs NMEA messages, RX can receive PMTK inputs- The TX line output correctly the messages, although it clearely says that no satellites are in viewCode:$GNGGA,002909.799,,,,,0,0,,,M,,M,,*53$GPGSA,A,1,,,,,,,,,,,,,,,*1E$GLGSA,A,1,,,,,,,,,,,,,,,*02$GPGSV,1,1,00*79$GLGSV,1,1,00*65$GNRMC,002909.799,V,,,,,0.00,0.00,060180,,,N*59$GNVTG,0.00,T,,M,0.00,N,0.00,K,N*2C- This message is outputed without any software configuration, it starts when it is supplied with voltage.- The antenna I choose is a patch passive one made specifically for GPS (1176MHz - 1606MHz). I tried also other ones with no results- You can find attached both schematic and pcb.- As you can see there is the possibility to add an active antenna. If I use this passive antenna I simply do not populate the 10ohms resistor. This leaves a stub on the antenna line. I tried and phisically cut that stub withouth any resoults.- The antenna track is 50ohms matched (calculations done with kicad tool)- I tried several libraries (tinyGPS just to mention one) but it is my understanding that all they do is decode the messages sent by the module- I monitored the RX line of the GPS and the microcontroller (ESP32S3), regardless of the library used, does not send any commandAny clues on why it is not properly working?

author: inse
author_rank: Super Contributor
timestamp: February 06, 2025, 10:24:35 am
content: Did you try an active antenna as well?

author: ppTRN
author_rank: Regular Contributor
timestamp: February 06, 2025, 10:29:05 am
content: Not yet but it's on its way. I hope I will be able to try it tomorrow

author: ftg
author_rank: Frequent Contributor
timestamp: February 06, 2025, 03:15:02 pm
content: You say SIM68ML, but your schematic shows SIM28ML.SIM68ML has a FORCE_ON pin for turning it on it seems, which SIM28ML lacks and has just "NC" on that pin.But if it is outputting stuff on it's TX pin, then it is clearly powered.With the patch antenna in use, does it really not get a lock in 15minutes while outdoors?How is the patch soldered in and how much groundplane does it have?Btw what voltage is VBAT? As the symbol on the right in the posted schematic seems to be for a two cell battery.I have linked the SIM28ML and SIM68ML hardware design guides below, so that others can also follow along and don't need to google them.SIM28ML Hardware Design guide:https://simcom.ee/documents/SIM28ML/SIM28ML_Hardware%20Design_V1.01.pdfSIM68ML Hardware Design guide:https://simcomturkiye.com/pdf/GNSS/SIM68ML/SIM68ML_Hardware_Design_V1.00.pdf

author: ppTRN
author_rank: Regular Contributor
timestamp: February 06, 2025, 03:54:13 pm
content: Thank you for providing extra material for all other users. Let me answare all your points- both 68 and 28 are pin to pin compatible and the force_on pin on my device is not used, as permitted from the datasheet- Yes it is powered since it outputs some reasonable data- I did not actually waited for such a long time, maybe two minutes. Do you think I should wait more? I did not expect that and I am going to try immediately.- Everything was soldered using a stencil and an oven, all the soldered pads looks very good. Surrounded by many cm in every direction by ground, ground plane also on bottom layer- Vbat is straight forward from a LiPo cell. It is permitted, in terms of voltages, by the manual. I did not add any bypass capacitor tho... my bad; but I am sure that inside the module there are plenty, and it is only 4 cm away from the battery itself.- There are two cells in parallel (actually just one is soldered right now)Thanks again

author: ftg
author_rank: Frequent Contributor
timestamp: February 06, 2025, 03:59:46 pm
content: Yes, you should wait longer.Cold start with an unassisted GPS at least used to take 11-12minutes.As it needs to get the almanac and that takes some time.So in 15 minutes a working GPS receiver, with a working antenna, with a clear view of the sky while outdoors should get a GPS lock.Modern GPS chipsets are remarkably sensitive and often work even with less than ideal antennas.

author: ppTRN
author_rank: Regular Contributor
timestamp: February 06, 2025, 04:54:30 pm
content: Unfortunately even after 30 minutes in my balcony it did not latch. I will try tomorrow on a more open area and with an active antenna

author: ppTRN
author_rank: Regular Contributor
timestamp: February 07, 2025, 09:04:24 am
content: UPDATE: with an active antenna it worked immediately. In just a few seconds the device latched with 12 satellites and provided me with all data.So what is going on?Tried again the passive one desoldering the inductor, but again with no results. I am now suspecting that the antenna might have something wrong but I mean, what are the odds? The next suspect might be the stub that is left when the inductor feeding the antenna with DC is desoldered...What should my next move be?In the meantime I am leaving the device outside hoping that it will somehow latch to some satellites.This is the patch antenna:https://www.tme.eu/it/en/details/f0201p010mc137bufl/gps-antennas/2j-antennas/2jf0201p-010mc137b-ufl/And this is the active one:https://www.tme.eu/it/en/details/atgg46015-bp/gps-antennas/yic/

author: Gyro
author_rank: Super Contributor
timestamp: February 07, 2025, 09:47:52 am
content: To me, that patch antenna seems a little too good to be true. 'Flexible, ground plane independent, self adhesive, very very thin, and coax cable terminated'. When I compare that with the much thicker ceramic patch antennas which have specific groundplane requirements and are positioned as close as possible to the GPS module with a controlled impedance track, it makes me wonder if GPS patch antenna technology has made amassiveleap forward (to be fair, there have clearly been advances), or there is something else I don't understand.The fact that it acquiressoquickly with the active antenna, leads me to think that there is either an issue with quality or signal level from the passive one.P.S. My own limited experience - especially regarding the loss of the on-board LNA (Reply #33)...https://www.eevblog.com/forum/projects/ebay-u-blox-lea-6t-gps-module-teardown-and-initial-test/

author: ftg
author_rank: Frequent Contributor
timestamp: February 07, 2025, 11:35:39 am
content: I find it annoying that the 2JF0201P does not seem to have any application notes or mounting instructions for it.Or in general how to achieve optimum performance while using it.Like is it meant ot be mounted on the inside of a plastic case, or is it ok being glued down on a PCB ground plane.The datasheet radiation pattern images make it look like it's measured in free space, not on a ground plane.It is also linearly polarized, according to the manufacturerhttps://www.2j-antennas.com/antennas/single-internal-antennas/2jjf0201p-high-precision-gps-gnss-l1-l2-l5-l6-flexible-polymer-adhesive-antenna/456, so it will take a 3dB hit on that as well.2J's site also has no application notes, no guides on mounting and no contact details for support.But the linearly polarized GNSS antenna is rated for 25W.I would expect it to work outdoors. but who know's how it is supposed to be mounted.

author: ppTRN
author_rank: Regular Contributor
timestamp: February 07, 2025, 01:44:10 pm
content: Well, I think at this point I am discarding these flexible patch antennas and I am going for a ceramic one.I will buy a couple and see what I get