signal strength on a GPS

Can I guess? 100% is full scale on some manner of RSSI (relative signal strength indication) buried inside whatever chipset is used in the TomTom 2535. I run into the same thing in cell phones and wi-fi chipsets. The relationship between receive signal level in -dBm and whatever the chipset produces is not linear. The output is usually in the form of an 8 bit number from 0 to 255. In order to convert that into signal level in -dBm, a lookup table would be needed. For cell phones, they just coarsely quantize it into 4 or 5 "bars" for the user, but produce the real signal level in -dBm in the "service mode" page. Looks like TomTom took the easy way out and just produced a percentage of full scale. I suspect that if you dive deep into the well hidden GPS diagnostic pages in your TomTom, you might find the signal levels in -dBm. Wi-Fi is more of the same. Some GPS and Wi-Fi chipsets now have an internal lookup table for the conversion. Either way, the number produced is still bogus if you have an external GPS antenna amplifier. The indicated signal level will be what appears at the receiver input, not what is at the antenna.

Disclaimer: I couldn't find the chipset used and therefore could not RTFM to see if my guess is correct.

You might get a more specific answer in sci.geo.satellite-nav on TomTom support site:

or on multiple TomTom user forums.

sci.geo.satellite-nav

Well, the way a GPS receiver works, there is no way to actually measure the signal. It is below the noise of the receiver! What they do is autocorrelate the signal with a deciphering key and if the data passes a checksum, it is good. So, they must be measuring the ratio of good to bad decoded data blocks.

Jon

A software engineer probably doesn't know what a dB is.

dataBase?

Best regards, Spehro Pefhany

Thanks will chk there.

Not much help there, from what I've seen. Can't get to anyone at Tomtom that might know what the answer is ( I suspect (sh)he is locked up in a Faraday cage).

Agin thanks for lead to sci.geo.satellite-nav

Jim

dBm as in "don't bother me".

Using Occam's Razor, I suspect your simple answer may also be the correct answer to the question. It may also have been something equally simple, such as running out of program space or running out time to code the output in dBm.

You're an idiot.

They measure timing flag packet arrival times from three satellites.

It's worse than just the signal being below the noise: the way a GPS receiver works, the received signal is always relative to the received noise: all but the most expensive GPS receivers pass the signal through a comparator (basically, or a "1 bit ADC", or a "splitter", but it's all the same thing). It works because unless there's a jammer in-band, the satellite signal is buried in noise, so the noise works to linearize the "ADC".

So there is no way to measure the signal strength as such, which means there is no way to compare it against any absolute reference.

And, that means that the question "100% of what?" is a good one, and (I'll bet) one that you'll not find an answer to easily.

On a sunny day (Sun, 16 Sep 2012 00:03:50 +0000 (UTC)) it happened Jim Whitby wrote in :

Probably signal to noise ratio, in fact bit error rate. In GPS (the US system) all sats transmit at the same frequency, their data xored with some pseudo random code that is different for each sat. So they likely actually use the bit error rate for each satellite. That is how 'clean' the detection of each sat signal is in the total signal after comparing it its code.

Google for example: IS-GPS-800B.pdf for a lot of technical details.

Percent of 1-bit correlator output vs ideal full scale output. This is a measure of SNR.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

OK, explain to us what a dB is.

If you use a 4-quadrant (Gilbert cell) multiplier (mixer) instead of a XOR gate, the despread signal will definitely have a positive SNR in the 1000/50 Hz bandwidth) and the absolute signal power should be easily measurable.

On a sunny day (Sun, 16 Sep 2012 19:48:12 +0300) it happened snipped-for-privacy@downunder.com wrote in :

No, you misunderstand the GPS system. The satellites all transmit at the same frequency, their digital signals are xored before modulation with a pseudo random code, a so called 'Gold code'. This code is different for each satellite. Basically this is done with a shift register. In the receiver AFTER demodulation the digital signal is run through a similar shift register, and one by one tested for the correct Gold code (each sat has its own code). In case of noise, you do not get an exact match against the know codes, and the number of wrong bits is then a measure for the signal to noise ratio.

If it's Trimble, the measurement is relative to AMU (Arbitrary Mystery Units).

its own code).

number of wrong

The GPS signal is an ordinary direct sequence spread spectrum (DSSS) signal. It can be received with a mixer/multiplier/xor gate by multiplying the received signal with the same chip clock (in this case

The shift register and bit error detection you are talking about has to do with signal acquisition _before_ the receiver chip clock generator has been synchronized with the transmitter chip clock.

I didn't understand a damn thing in that document!

its own code).

number of wrong

Do they x-mit a different freq. for military use?