17 June 2015

Harris RF-5800 'Selective Call', MSK 2000Bd/1000

('Systeme3000-ALE' equivocation)

We recently had the opportunity to get and study the Harris Selective Call waveform (we name it as Harris-ALE but it's not its official name!): it's an MSK modulation at 2000 Baud speed, followed by short MFSK-8 125 Baud in non-standard MS-188-141A (ALE), acf is 50 ms (100 bit). We focused on the initial 3 seconds segment in order to define modulation mode and other features in order to help to clarify the above misunderstanding (see later).

At first glance it looks like a serial-tone signal: carrier frequency 1600 Hz and OQPSK modulation at 2000 Bd speed (pic. 1). As reported in this article by Sergey Makarov (SergUA6) http://signals.radioscanner.ru/info/item281/ "it is not easy to tell apart between QPSK and MSK since they are tightly connected".

Pic. 1- Harris Selective Call and its phase-plane
By measuring the main parameters of the signal we had (pic.2):

baudrate (Br) = 2000 Bd

bandwidth (Bw) ~ 3000 Hz

shift (Sh) = 1000 Hz

Pic. 2
That said, and according to what reported here http://signals.radioscanner.ru/info/item68/ we are pretty sure that the Harris Selective Call is not an OQPSK but rather an MSK 2000/1000 waveform, 2000Baud speed and 1000Hz shifted. As it is clearly visible in pic.2, are verified those relationships that are specific to MKS signals, i.e.: Bw = 1.5 Br and Sh = Br/2 as shown in the cited article (pic.3):

Pic. 3 - MSK relationships (from radioscanner.ru)

Moreover, into the Harris signal, there is another clue that points to MSK. As you can see in the picture 4, there is a long state staying in one frequency, if should it be PSK, the frequency should come back to carrier and stay there till a new phase change. 
Pic. 4

 The shape of the eye diagram (pic. 5) also points in this direction (MSK with BT = 0.3):

Pic. 5
The Harris signal has an ACF value of 50 mSec (100 bit) and it is characterized by the presence of what looks like a signature, something like a 'business card':

As said, this is a safe-origin signal since it was recordered some years ago during Harris RF-5800 acceptance tests: at that time it was supposed to be standard in Falcon-II series radios. Harris equipment is wide spread in NATO countries, but this mode, although available, seems to be not much used.

With these points in mind, we asked ourselves the question that we indicated at the beginning: the so-called Thales “Systeme3000-ALE” (reported also in a post on this blog) has a waveform that is very similar to the Harris-ALE' one, also aurally they just sound in the same way. Browsing the web, there are logs by listeners reporting both the signals: altought the Thales waveform be almost 99.99% OQPSK, may be the case that these two signals could be confused each other ?

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