Batboard

Hey all...

I know this has been posted here before, but I can't seem to find it.

In a vehicle, what's the rated Top Speed for IMBE before the Dopler Shift starts to play havoc?

-Mike

Gimme a porsche and I'll find out....

APCO-25 spec (TIA-102.CAAB-A) section 3.2.2, "Operating Frequency Accuracy", specs the frequency accuracy of a mobile as 1.5 PPM for an 800 MHz mobile.

So, any Doppler shift greater than 1.5 PPM would place a perfect unit out of spec.

So, what would the speed need to be for that error? Since v will be << c, we can ignore relativistic effects and use the simple Doppler equation

Fout/Fin = sqrt( (1-w)/(1+w))

Where "w" is the speed of the radio expressed as fractions of "c".

So, for Fout/Fin = 1.0000015 and solving for w:
w = (1-n^2)/(1+n^2)

w = -1.5E-6 c, or 1005.9 miles per hour toward the base station.

In other words, if you are going fast enough for Doppler shift to put you out of spec, you are in a pressure suit and above 75000 feet, and you ain't using P-25.

Moreover, while that is the spec, a good data slicer will recover the signal with quite a bit more error than that.

Now, if you are talking a TDMA system, then you get into trouble, but not from Doppler shift - you get bit by the fact that your time of flight to the base station, and thus your time offset for when you start your burst is changing faster than the radio can compensate, and you start operating out of your time slot.

We use an 800Mhz IMBE simulcast system
When we initially put the proposal together they didnt know how a IMBE simulcast system would work in an aircraft (due to speed + altitude)
Turns out it works fine
Analog transmissions get a little "bacon frying" noise in the background due to the simucast.

Fowler

I believe you are referring to the speed in which you may want to consider using the "High Deviation TX" parameter in the Astro Trunking System Options. According to the help menu in the RSS, this feature should only be used if the unit will be moving in a vehicle at speeds in excess of 160-mph (260-kph).

There is also a warning stating that you should not enable this parameter unless system engineers have identified it as necessary, as it may result in significant degraded radio performance otherwise.

Oddly enough, there is no help available for this option in the CPS.


Todd