[MRCA] [MMRCG] The backpack GPS project

[Ray Fantini]

That system has the external pre amplifier, or maybe its an external preamplifier down converter? Don't know yet. Its located right next to the battery box. Somehow assume that the brochure infers that the box is a L1 C/A receiver and that's still is a protocol for GPS and would assume that it should work.  Thinking of separating the antenna and preamplifier (downconverter?) and setting up the whole mess with that stuff outside and the receiver on the bench externally powered and experimenting with all the weird base settings and see if I get any love that way.
Just pure speculation based on looking at the innards of the receiver that the two receivers look to be built for VHF/UHF like the  old NAVSTAR system and maybe to make this system compatible they stuffed a external L band downconverter and that what's between the antenna and receiver and is more than a preamp? Just don't know if the old NAVSTAR system used C/A protocol in a BPSK format. Maybe that's why the receiver has so many profiles upon start up and unless you select the appropriate format it's not happy?
Problem right now id that I have a bench full of PRC-74 that need to be aligned and have to get all that stuff out of the way.

Ray F/KA3EKH



From: MMRCG at groups.io <MMRCG at groups.io> On Behalf Of Chance Wolf via groups.io
Sent: Monday, October 6, 2025 11:40 AM
To: mrca at mailman.qth.net; MMRCG at groups.io
Subject: Re: [MMRCG] [MRCA] The backpack GPS project


One thing I noticed just on a brief glance through that brochure is that they reference field configurations as needing an antenna and preamp both, so perhaps it's looking for some specific antenna that's just been replaced in your setup with a generic L1 passive puck type.


From: mrca-bounces at mailman.qth.net<mailto:mrca-bounces at mailman.qth.net> <mrca-bounces at mailman.qth.net<mailto:mrca-bounces at mailman.qth.net>> On Behalf Of Ray Fantini via MRCA
Sent: Monday, October 6, 2025 7:15 AM
To: mrca at mailman.qth.net<mailto:mrca at mailman.qth.net>; MMRCG at groups.io<mailto:MMRCG at groups.io>
Subject: [MRCA] The backpack GPS project

Everyone agrees that it's a fun box to have something that weighs thirty pounds that's strapped to a allice frame to do what the smallest I phone will do only better, but if you don't understand the attraction to the use of obsolete and arcane technology you should not be reading this in the first place.
So far I have determined that the backpack system is a Magnavox MX 4400 and may be one of the earliest systems out there. Attached is a copy of the sales brochure from back in 1987, the backpack can be seen on page 5
Test with the system have provided some information, first when the system powers up it wants to know what "Plan" you want to run. There is a list of about twenty different navigation plans for locations throughout the world along with several cryptic modes that I do not understand. Loading the CONUS US plan results in just a message that the antenna appears to be obstructed after a while when it cannot find any satellites. One of the other plans, I cant remember the name right now has somewhat the same results but did notice that after ten or fifteen minutes the clock function sets itself correctly so assume something's getting thru.
Going to assume that this is not just the old NAVSTAR VHF/UHF system but do not know if its got the smarts to decode modern L1 band data or not? The antenna definitely looks like a L1 antenna but not certain about the software. Unfortunately for some weird reason on this version they eliminated the SATS and STAT buttons. The buttons are there but not labeled. Assume this set had a limited version of firmware?  All other buttons and otherwise its identical to the commercial 4400 or 5400 receivers. Will go thru when I have time and write down all the different navigation plans that are offered and see if that has any useful information.

Ray F/KA3EKH


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