This rabbit-hole began with buying a mystery MOD phone from eBay. I couldn’t find any documentation through looking up the part numbers on the phone but it bore a resembelance to equipment in a youtube video:
Army Radio Ptarmigan in BAOR A brief on SCRA 1989
After a lot of digging, it appears to be a development version of the described Static Sub-Set, a voice terminal that connects via single channel radio access (SCRA), to a wider British Army Ptarmigan trunk network. This military communications network was developed in the 1970s and operated up to the early 2000s, originally made for the British Army on the Rhine (BAOR). The system was developed by The Plessey Company, now absorbed into BAE Systems, at their Command & Control site in my home town of Christchurch, Dorset (still occupied by BAE).
{Plessey, Christchurch, Dorset}
This is the most recent photo I’ve seen of Christchurch Airfield, my Dad worked in the industrial estate at the other end but, as it was being paved over in the 70s, I must ask how much of it he remembers.
There is a fair bit of information as to the basic workings of the Ptarmigan network, I’ve included some of the materials I found useful here. The network structure, modulation scheme, channel access, are all briefly mentioned. Of course, the juicer details regarding the encryption method and actual protocols are not mentioned in the literature and are probably still secret.
Simulator Sub-Set
{Simulator Sub-Set}
{Top-view of the Simulator Subset, showing the typical sub-set keypad, 4-wire line interface and indication lamps}
{The proper static sub-set that would actually be used in the field, they keypad, connectors and lamps all look the same as my simulator version.}
Teardown
The entire unit has been taken apart, cleaned and reassembled with CAT cable wired to internal vacant 4-wire interface. There is a vacant footprint for a DE-9, for serial comms with the digital board and this agrees with ASCII contents of the EPROM. This interface wouldn’t be used on one of the deployed Static Sub-Sets but is likely here for development reasons.
Sub-Set:
- Mfr: Drumgrange
- NSN: 5820-99-126-0639
- PN: DG-799-163-600
- SN: DG 005
Handset: - NSN: 5820-99-126-1165
I originally thought that this was a digital field phone, using an interface similiar to the US 4-wire TA-1024A/U. However, when it arrived, it was a lot larger than I expected and does not appear to be for combat use. Searching the NATO Serial Numbers (NSN) returned nothing and I was stumped for a while on what it could be for. Taking it apart, there are four different cards in a DIN-41612 backplane:
- Subscriber HDB3 Interface (SHI)
- DG-799-134-230
- Subscriber Audio I/F (SAI)
- DG-799-134-220
- Subscriber Digital I/F (SDI)
- DG-799-134-200
- or DG-799-134-250
- or DG-799-163-250 <- board revisions?
- Subscriber Keyboard I/F (SKI)
- DG-799-134-210
{From top-left clockwise: SAI, SHI, SDI, SKI}
SHI
This was the first clue as to how the phone communicates. HDB3 is a line encoding scheme that was commonly used in the T1/E1 4-wire digital phone lines between telephone exchanges. This doesn’t give any information as to what data is being carried but does agree with the internal wiring of the phone. The LINE 6-pin, keyed MILSPEC connector on the top of the device, is for this differential recieve and transmit phone interface. The board has this interface circuitry and a big 15W DC/DC converter for switching the DC line voltage to the +5V,+-12V needed by the other components.
SKI
Mostly mux/demux ICs, serial-parallel converters as well as a Toshiba TMP82C55AP-2 peripharal interface chip.
SAI
{Backplane, before cleaning}
{Handset connector, MIL-DTL-5015?}
{Audio connectors, handset and for external interface}
Board full of audio circuitry to interface with the speaker/mic of the handset. This handset is connected to the unit via one of two 7-pin MILSPEC connecters. I found out later that this second connector was used to connect a fax machine or terminal via way of an external data converter (modem).
SDI
This phone is powered by an Intel A80C186XL-12, a MCU-like x86 processor. It isn’t shown in the images as the part that was there broke as I removed it to clean the boards. I’ve never seen the gold pins on one of these ceramic packages come off so easily but most of them tore off with barely a touch. I’ll need to find another 12MHz PGA-68 part, either a CMOS 80C186 or one of the NMOS 80186s. I did make sure to dump the AM27C1024 EPROM.
Another eBay find - Tactical Intercom
I found an item listed as a Ptarmigan UK/VIC-418 Station Subscriber Unit on a second-hand land rover spares website, with no photo but the NSN number looked like a match for a Static Sub-Set. Apart for a 4-wire interface, it doesn’t seem to be for Ptarmigan but I would like to get it working for use at home as an intercom.
I thought this would be something similar to the above device but it looks more like a FOB/tank intercom. The keys are a completely different layout to the sub-sets. I’m reverse engineering it, to work out it’s operation, but there isn’t a clear microphone input which is confusing as surely it’s meant to be two-way. The supply input is ~24VAC.
- NSN: 5820-99-009-0702
{Shoddy picture of the tactical intercom, taken out of it’s red metal casing and not yet cleaned}
Jane’s Military Communications 1990-91 - Janes Info. Services
I found a cheap copy of this reference book, again on eBay. It’s actually quite fascinating, with a breakdown of each country’s new military communications devices, their application, use-cases, etc. A bit concerning is the listing of each device and system as a product with a company listing (contact information), but I’m not going to critique the commercialised, military industrial complex here. I will try and scan it in at some point but I don’t have a suitable scanner at home. It does have a section on the `Ptarmigan Battlefield Communication System’, as well as similar projects such as: Deltacs/Zodiac (Dutch), CATRIN (Italian), Deltamobile (Norwegian) and Tri-Tac (American).
All these systems, bar maybe Tri-Tac, comply with the EUROCOM intercommunication standard that allows for interoperation between different European countrys’ communication networks. I haven’t been able to pin down this standard and I imagine it is still classified, but it is often referenced in the literature. It would be interesting to find this and other standards not available on the internet, such as STANAG 5040, if they are declassified for public viewing that is. There is a close relative to the intercom included in this book, the UK/VIC-408. This seems very similar, a digital intercom to be mounted on soft skinned and armoured vehicles.
resources
papers
- R. Thompson, “Ptarmigan Mobile Access - Freedom of Movement for the British Army,” MILCOM 1985 - IEEE Military Communications Conference, Boston, MA, USA, 1985, pp. 503-509, doi: 10.1109/MILCOM.1985.4795076
- L. Wentz and G. Hingorani, “NATO Communications in Transition,” in IEEE Transactions on Communications, vol. 28, no. 9, pp. 1524-1539, September 1980, doi: 10.1109/TCOM.1980.1094857
- D. K. Cheng, “Ptarmigan: A UK Secure Area-Communication System For Armed Forces,” Office of Naval Research, AD-A031, August 1976 - archived