2x old GPS receivers: How do they work? - Page 1
Forum section: electronics / rf-microwave

author: wootwoot
author_rank: Contributor
timestamp: June 02, 2024, 02:03:50 pm
content: HiI enjoy hiking and after falling back to using phones recently to navigate a difficult route, I remembered how much fun I had using older GPS receivers as a kid, like an actualeTrex- Trust worthy so that you were not going run off after the wrong bearing but just basic enough that you still needed the map and there was a bit of challenge there, some technical finesse required. So I bought 2 of the oldest units I could find with the full intention of hiking with them and I dunno, going and finding a few geocahesA Garmin GPS 75 from 1994 and aGPS 50 from around 199275manual(mirror), 50manualimage: retro_gpsHehe I can't believe one evenmade it onto GetawayAU$2,000 to AU$4,000 in 1992 apparently. RBA reckons that's AU$4,500 to AU$9,000 in 2023 dollars!Got the 75 first and went outside where it began tracking satellites. Being so old, these units don't contain any EEPROM or flash and they back up volatile memory with a 3V lithium cell. Without that any almanac or user data (waypoints ect) is lost at power down. So once it looked like it was going to fix I went inside and ripped it apart, found the dead cell and a bunch of leaking electrolytics. Corrosion wasn't too bad (it seems to attack the solder more than the copper?) so replaced those washed and dried the board, and began poking around. Interestingly it runs on an x86 intel processor (16 bit 186), has I think 256k of ROM, 64k RAM, and the Hitachi display drivers are huge and seem to take up a disproportionate amount of space on the PCB. The board looks like it has many layers and comparing it to the electronics in my 1991 Subaru (a project that I have made a few repairs) and old discmans ect it looks quite dense. The case is seriously water tight. Any wiring or copper contacts that pass outside the case are sealed with epoxy and there is a moulded rubber seal between the case halves where they join. There is even a sachet of silica gel inside. The rubber is still supple. The GPS 50 has the same construction but its digital board is a little simpler and runs on a 188.The unit is constructed in 3 'layers', the first being a two layer plastic membrane/flat flex that sits under the rubber button sheet. This passes through a slit in the case that is sealed with epoxy where it passes in to join the digital board at a ZIF connector. The digital board contains the screen, processor and other digital stuff. Then, connecting with an array of 8 0.1 inch header pins, tipped upside down, is a board that is entirely covered with a massive RF can. Folding it out gives access to the top that is tack soldered down. I had to take a look. Lots of passives, a number of trimmers I dare not touch, a couple of interesting PCB RF filters (I think) and a few ICs, look like little opamps and one that through a search looks like an IF mixer. There is a lovely big silver can there, hopefully you can read the markings. I think its an oscillator. There are specific errors codes in the manual about a high precision oscillator.The thing that strikes me, and the reason I'm making this post:I don't think I can see any sort of GPS specific ASICin this device!! It all seems to be implemented using a relatively small number of off the shelf ICs and discreets! (Unless its the Mitsubishi chip I couldn't identify, next to the ROM). So, I was wondering if any RF or GPS pros here could take a look at my board shots and describe what various parts of the circuit are doing. Is early, vanilla GPS actually reasonably simple to implement? Would there be a whole bunch of clever signal processing going on in what is a fairly basic low power CPU?The things work great! You can see on my test trace, where I attached a remote antenna to the roof and went on a short tour-de-drivethru around eastern Melbourne, how well it works (thanks Clinton). Its not as accurate as a modern unit, but they are much better than I expected them to be! They were tracking 7 to 8 (maximum) satellites the whole time.I began by logging the GPS 50 in QGIS. Where the dots begin, that's where I swapped over to the 75. I downloaded the track log and plotted the points when I got home. I am experimenting with the 'Resolution' algorithm, as the memory size is only 1,024 track points that roll over in a FIFO scheme. With the limited lat long precision, you can see the quantisation on the mapThe GPS 75 is probably the device I'll use the most. it has the ability to sync Routes and Waypoints via RS232 (andGPSBabel) and has track memory. The old GPS 50 seems to only capable of outputting NEMA sentences on its RS232 port. No syncing possible.With the success I've had with these, I wonder if Dave should run out and sit with thatSony Pyxisfor a while. Untill they can recover almanac data, fixing takes a long time. The 50 took about 25 minutes but for some reason, I don't know if it was the particular arrangement of the constellation when I tried it, the 75 took over an hour. Now, with the orbital data in memory both receivers take like 30 seconds, sometimes less.Battery life on 4x 2,500mAh NiMH is about 10 hrs

author: wootwoot
author_rank: Contributor
timestamp: June 02, 2024, 02:56:03 pm
content: Board shots of the GPS 50. RF section I am pretty sure is the same

author: wootwoot
author_rank: Contributor
timestamp: June 02, 2024, 03:03:27 pm
content: And now look, a serious question: Is this a legitimate RF Distributed-element filter?Because jokes aside, this looks more like a peenus than the Blue Origin rocket.