GPS - working or broken.......
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From: Berkshire, UK
GPS - working or broken.......
My understanding of GPS equipment is that the GPS module in ones sat-nav receiver does the bulk of the calculation on the received data and squirts out position, speed, direction and time. The display and map related calculation stuff is done in software in the unit.
Is this correct?
I have a cheap GPS based speed display unit which reads a bit odd. Used in the car I expected it to display a proper accurate reading of speed, more accurate than the car built in speedo. It doesn't. Below 10mph it reads less than the car speedo does, over 10mph is starts to read more than the car speedo which, when compared with other GPS equipment, over reads itself. The comparison GPS equipments I have used are a Garmin Nuvi 2599 car satnav, a Garmin Aera 500 flying satnav and a Garmin 12xl walkers GPS. It is not a Garmin issue! The comparison GPS units all agree with each other and, in previous tests, they agreed with flying GPS systems from Honeywell and Skyforce.
What is going on?
The cheap display I have is the second one I have had. The first unit was supposed to display MPH but actually showed speed in KMH. There are no controls on the items, you just plug them in to a USB for power and wait for lock onto the satellites and you are good to go. nearly.
Is this correct?
I have a cheap GPS based speed display unit which reads a bit odd. Used in the car I expected it to display a proper accurate reading of speed, more accurate than the car built in speedo. It doesn't. Below 10mph it reads less than the car speedo does, over 10mph is starts to read more than the car speedo which, when compared with other GPS equipment, over reads itself. The comparison GPS equipments I have used are a Garmin Nuvi 2599 car satnav, a Garmin Aera 500 flying satnav and a Garmin 12xl walkers GPS. It is not a Garmin issue! The comparison GPS units all agree with each other and, in previous tests, they agreed with flying GPS systems from Honeywell and Skyforce.
What is going on?
The cheap display I have is the second one I have had. The first unit was supposed to display MPH but actually showed speed in KMH. There are no controls on the items, you just plug them in to a USB for power and wait for lock onto the satellites and you are good to go. nearly.
Joined: Oct 2019
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From: USA
That is correct.
The amazing thing is that a GPS can read 0 mph (or kph). It is getting time/distance information from satellites that are traveling at 8700 mph (14,000 kph) and a continuously changing component of that to the receiver. They have no idea if they are moving relative to the surface of the earth other than by calculation; they can't look out the window of a car, for example, and see if the scenery is passing by.
I find it amazing that after gathering all that data it would screw up the result. My supposition is that the display is taking the output of a GPS receiver which is supplying correct information and is doing some other averaging or conversion incorrectly. It is so difficult to do the initial processing of the GPS satellite cluster transmission that it would surprise me they would make a stupid error at that point. However, someone doing a fancy display and just using the output of a GPS receiver could easily mess that part of the pipeline up.
Getting to the bottom of this would involve opening up the unit and tapping in to the output of the GPS receiver to read the messages it outputs to the rest of the display electronics. Typically this is a 10 Hz rate over a serial connection, but there may be other possibilities.
Does the display indicate the number of satellites are in view?
The amazing thing is that a GPS can read 0 mph (or kph). It is getting time/distance information from satellites that are traveling at 8700 mph (14,000 kph) and a continuously changing component of that to the receiver. They have no idea if they are moving relative to the surface of the earth other than by calculation; they can't look out the window of a car, for example, and see if the scenery is passing by.
I find it amazing that after gathering all that data it would screw up the result. My supposition is that the display is taking the output of a GPS receiver which is supplying correct information and is doing some other averaging or conversion incorrectly. It is so difficult to do the initial processing of the GPS satellite cluster transmission that it would surprise me they would make a stupid error at that point. However, someone doing a fancy display and just using the output of a GPS receiver could easily mess that part of the pipeline up.
Getting to the bottom of this would involve opening up the unit and tapping in to the output of the GPS receiver to read the messages it outputs to the rest of the display electronics. Typically this is a 10 Hz rate over a serial connection, but there may be other possibilities.
Does the display indicate the number of satellites are in view?
Last edited by MechEngr; 24th August 2025 at 18:11. Reason: fixed the rate. 1 -> 10
Thread Starter
Joined: Sep 2004
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From: Berkshire, UK
erm, it really isn't that clever!
See https://www.ebay.co.uk/itm/405333360...Bk9SR8jVkcybZg
I think. Seems to be a monster link I cut and pasted from Ebay, sorry.
See https://www.ebay.co.uk/itm/405333360...Bk9SR8jVkcybZg
I think. Seems to be a monster link I cut and pasted from Ebay, sorry.
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From: Berkshire, UK
After the post office screwed** up the delivery I didn't get around to taking the unit out in the car until after the 31 day returns deadline.....
So I might open it up and have a play. I did look at NMEA message formats in relation to using a GPS Rx module in another project. I can feel an Arduino project coming on. I might be able to use the display parts with a new case to make it work as required.
** the dozy postman tried to deliver the unit to the wrong address. Upon finding the householder out he threw my parcel over the rear fence where it sat in torrential rain for the day and night. We know, we stayed in that day due weather! A week or so later, when the recipient got back from a holiday away, he tried to deliver it to us but we were away for 10 days. The Post Office have still not worked out what happened to my parcel but agree that the proof of delivery photo does not show my property.
So I might open it up and have a play. I did look at NMEA message formats in relation to using a GPS Rx module in another project. I can feel an Arduino project coming on. I might be able to use the display parts with a new case to make it work as required.
** the dozy postman tried to deliver the unit to the wrong address. Upon finding the householder out he threw my parcel over the rear fence where it sat in torrential rain for the day and night. We know, we stayed in that day due weather! A week or so later, when the recipient got back from a holiday away, he tried to deliver it to us but we were away for 10 days. The Post Office have still not worked out what happened to my parcel but agree that the proof of delivery photo does not show my property.
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What the heck on that pricing? I am looking at the chip price and it is $5 in buys of 1000. How ever do they get the entire RF front end, the display driver, the display, and the housing for the next couple of dollars? Amazing. A working board for hobby use is $20-25, not including a microprocessor to extract the data.
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For what it cost me it isn't worth the cost of sending it back and trying to prove it is reading in error. I have a couple of GPS Rx decode and RF front end modules which only cost 3 or 4 pounds during coving lockdown but didn't get used. I will have a contrast and compare session with these and also the Garmin GPS12xl which I think can be set to output the NMEA messages for reading on a tty terminal.
Testing is on the move might be the tricky bit.
Testing is on the move might be the tricky bit.
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From: Scotland
Thread Starter
Joined: Sep 2004
Aviation Qualifications: PPL
Posts: 883
Likes: 34
From: Berkshire, UK
I just spent a bit of time refreshing my understanding of NMEA sentences and realise that I didn't see any sentences that have MPH in them, only apparently KPH and Knots. Maybe the conversion from one of them to MPH is the erroneous part?
Still going to open the unit up and see what makes it tick.
Still going to open the unit up and see what makes it tick.
