March 14, 2015

Old GPSes just keep on truckin'

Trimble Transpak II (c) 1993 P/N 19437-60 
I picked up a Trimble Transpak II and a Trimble Pathfinder a while back on eBay, and never did get around to sorting them out and getting them mounted in the Suburban.  I think it's time I took a stab at that again.  Maybe I should back up and explain how I got here.

First, the specifications:

Trimble Transpak II
Copyright 1993
v4.20 Firmware
6 L1 band (1575.42 MHz) receiver 
External Serial Data Port
Internal Antenna / External Antenna Port
9-32VDC 5W Power
P/N: 19437-60
Trimble Pathfinder Basic
Copyright 1990
v3.14 Firmware
6 L1 band (1575.42 MHz) receiver
External Serial Data Port
Internal Antenna / External Antenna Port
9-32VDC 5W Power
P/N: 14992-60

How many GPSes does a guy need?

It seems like all electronic communication or internet-ready devices are coming with built in GPSes these days.  On my last trip I had an iPad, a Playbook, a Blackberry, a Holux M-1000 data logger, and a Garmin dash-mounted GPS... all with built in GPS functionality. So why do I need another?  Well, maybe I don't, but I discovered most of the devices quickly lost functionality when I was out of cell/data range, and these old GPSes never had such a crutch.  They also dont have maps in them to get out of date, which is great - but navigating the roads with them isn't very handy.  What they are really good at is telling you when you're getting close to a pre-designated waypoint, OR tell you where you are within 10 feet.  Yes, these devices from the early 1990s, when the Seattle Grunge scene was at the top of the charts, and the first Iraq war, perform better now than when they initially hit the streets.  Why?  Well, at the time the US military, who control the GPS satellite constellation, were "fuzzing" the signal with a technique called Selective Availability; so it would wander and not be very accurate. They stopped doing so on May 2, 2000 - supposedly permanently.  These units are ruggedized, and look like they could be thrown out of a moving tank in the desert and keep on working. I'm not sure how waterproof they are, but they are at least water resistant.

For power, they take a lead acid rechargeable battery pack, 8 AA batteries (AA battery pack shown in the pictures above), or car-cigarette adapter for 12V power; but interestingly will take anywhere from 9-32V.  So, you can power these things off a 24V NATO power source, a 12V civilian vehicle, or... I don't know what would put out 32VDC..?

Another funny quirk came up; Y2K and planned obsolescence.  Both of these Trimble receivers are on the list on non-Y2K compliant devices, so why didn't they explode on January 1st 2000?  Evidently the GPS signal has at least three parts to it (I'm sure there are more than 3 but I haven't looked up the standard, if it's even published in sufficient detail) The coordinates are independent of time or date; they continue to be accurate down to 0.1 arc seconds (10 feet according to Brooke Clarke's web site - Thank you Brooke!) and I haven't noticed any issue with coordinate precision.  Then there's the time, which is accurate to the second, as far as I can tell.  However, the date is way off - I'm not sure when Trimble expected these units to last for in the field, but 2015 wasn't planned for.  On March 12th 2015 the date read July 27th 1995.  If anyone can explain this I'd really like to hear it.  I presume there's some math involved that can explain it and the date chosen isn't random.

So, why again?

Well, provided I can hard wire the power to it in the Suburban, and mount it in a tidy fashion on the dash, it will function as an interesting talking point, an excellent timepiece, and a backup GPS that will surely never fail.  It's lasted this long, it'll last through the apocalypse.


  1. I have one of these, too. The date does not worry me and the rest work really well. I bought it to put in an ex-army Land Rover Series 2A FFR but have sold that and now have an M151 MUTT which is 24V so it will now be the proud recipient of the Trimble. It is not all that fancy like modern units but it does give position, use waypoints, plot track and is accurate so it will be useful in the bush where nearly all modern GPS units do nothing much more...

  2. Just wish I could find a vehicle mount...