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DF Covert Datong D-903 →
The device can track any beacon between 25 and 500 MHz, 1
but in most European countries the use of law enforcement beacons was
commonly restricted to the 2 and 8 metre bands at the time.
The DF-5 is a four-channel interferometer, which uses a minimum of three
and a maximum of four
external (covert) antennas, one of which can be assigned
as a high-gain reference antenna.
The DF-5 has an external handheld remote control unit,
that shows the bearing (rose) and the relative signal strength of the
RF signal from the beacon.
It supports beacons with continuous as well as pulsed transmission.
It uses digital signal processing to suppress multi-path propagation
and to obtain an accurate and reliable bearing to a (moving) beacon.
When used in combination with an intelligent beacon,
such as the Datong D-903,
the status of the vehicle under surveillance (stationary or moving) will be
visible on the display unit.
It has a built-in electronic gyroscope
— allowing it to be used in a moving vehicle — and offers support for a
moving map display.
A number of unique features of the DF-5 are described by David A. Tong
in US Patent 4,809,012.
The price of the bare DF-5 receiver was the equivalent of EUR 28,000
to EUR 32,000 in 1993, depending on the quantity [2]. The device featured here
was manufactured in late 1994 and was updated at least once in June 1996. In The
Netherlands it was used until at least the early 2000s.
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Extendable upwards to 1500 MHz and down to Medium Frequency (MF).
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All connections of the DF-5 are at the front panel, as shown in the diagram
below. The device has no controls, but relies on the
DFC-1 remote control unit (the display)
that should be connected to CTRL1 socket at the top right.
A second (identical) DFC-1 display
can (optionally) be connected to the CTRL2 socket,
but it can only be used for monitoring purposes (not for controlling the DF-5).
Additional (optional) accessories can be connected to the AUX socket
and to the EXPANSION PORT at the
right half of the front panel.
Power – typically from the 12V or 24V car battery, or from an external power
supply unit (PSU) – is connected to the POWER socket at
the bottom centre.
The device is switched ON by means of the
rocker switch
at the bottom of the DFC-1 display unit.
At the left are 4 BNC sockets
for connection of the antennas. The largest one
– the reference antenna – should be connected to socket (4), whilst the
smaller direction finding antennas – the stubs – should be connected to
sockets (1), (2) and (3). In the device shown above, a gyroscope is fitted
below a rectangular panel at the top, marked with a big white arrow
(more about it below).
The DF-5 is fully controlled with the DFC-1 display unit shown above.
It should be connected to the CTRL1 socket of the DF-5. It comprises an
8-button keypad, a speaker, a 16-digit alphanumeric display and a bearing
display, also known as the rose. The rose consists of a series of LEDs
in a circular arrangement, pointing in the direction of the transmitter.
To the right of the rose is the signal strength indicator, which consists
of three LED bars in green, orange and red.
A small speaker is embedded in the front panel of the DFC-1. It is used
for producing the key-clicks (when operating the keypad) as well as for
reproducing the audio from the DF-5. The audio volume can be controlled
with the knob at the top.
At the bottom right is the ON/OFF switch.
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The DF-5 was used as part of a series of unmanned fixed Direction Finding
stations, known as meridians, located at strategic positions throughout
the country.
The information from these locations was sent by means of modems to the
command and control room of the police, where it was plotted on a map.
From there, it was distributed to the relevant regional investigation teams.
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Once the fixed DF station(s) had roughly determined the position of the
tracked vehicle, a special tracking vehicle could be deployed to search more
closely.
These unmarked police cars – usually a regular inconspicuous sedan – had
a DF-5 receiver installed in the trunk, whilst the
DFC-1 remote control unit/display
was mounted on the dashboard, as illustrated in the diagram below.
For proper direction finding, the DF-5 has to be used with 3 or 4 antennas,
one of which is used as the refererence antenna. The other antennas, known
as stubs were available in several forms,
so that they were not
recognised as such. In most cases, disc antennas were used for the stubs,
as they could be hidden in the sunroof of the car.
This is illustrated above as S1, S2 and S3.
The 4th antenna – the reference antenna – was placed further towards
the rear of the roof (R4), and looked like a typical cellphone antenna of the
era. In The Netherlands such antenna's were commonly used with the analogue
(1G) mobile telephone network ATF-3.
In most cases, dual-band disc antennas were used for S1 and S2, so that the
vehicle could be used for tracking beacons in the 2m and 8m band.
For the 8m band, only three antennas were used (S1, S2, and R4).
For tracking beacons in the 2m band, all four antennas were used
(S1, S2, S3 and R4).
When the DF-5 is used in a driving vehicle, it is important that
the arrow of the gyroscope
points in the forward (driving) direction.
If the device will be placed horizontally, the gyroscope has to be
mounted into the top cover. If the device will be
used vertically, the gyroscope has to be fitted into the
centre compartment at the rear of the device.
The electronic gyroscope can be
fitted in four different orientations,
so that the DF-5 can be placed in all possible positions (N, E, S, W).
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Portable direction finding
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When getting close to the tracked vehicle, the tracking vehicle often had to
be abandoned, as otherwise it might attract the target's attention.
In such situations, the observation teams (OT) used a portable miniature
receiver, such as the single-channel
SRX-1, to do the last bit on foot.
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The DF-5 has a number of improvements over its predecessor,
the DF-4:
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- Suitable for all frequencies between 25 and 500 MHz (not restricted to one band)
- Suitable for tracking beacons outside the police bands
- Ability to scan an area for alien beacons
- Support beacons from foreign law-enforcement observation teams
- Double sensitivity (+6dB)
- Built-in PET stabiliser
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Although a correlative interferometer gives a much more accurate bearing
to the transmitter than conventional direction finding techniques, such as
Watson-Watt or even Doppler, the
reading can be erratic in urban environment, especially in earies with large
concrete or steel structures.
For this reason, the DF-5 is equipped with a so-called PET 1 stabiliser –
a device that intelligently filters the bearing data, before presenting it
on the display. This is done by using an additional computer platform
that applies low-pass filtering, and that takes the movements of the
tracking vehicle itself into account by using data from a motion sensor and
an electronic gyroscope.
For the older DF-4, the PET stabiliser was available as
an external add-on, that was connected between the CTRL1 port of the DF-4
and the DFC-1 display. In the DF-5 it is an integral part of
the unit, that is fitted internally on top of the main computer board.
The front panel connectors (CTRL1, CTRL2, AUX and EXPANSION) are all provided
by the PET board. The PET was developed in 1992 and is described
by David A. Tong in US Patent 5,280,293
of 11 Sep 1992.
➤ Leaflet
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PET = Performance Enhancer for Tracking.
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The actual DF-5 receiver is housed in the milled aluminium enclosure
shown in the image on the right. It has large removable lids at the top and
bottom, plus three smaller ones at the rear.
All connections are at the front panel.
The device is usually supplied in the leather pilot case
shown above, but can also be used outside the case.
It is imortant that it is installed in the tracking vehicle, with the large
white arrow pointing in the forward (driving) direction. The receiver is controlled from the DFC-1 display unit.
➤ Look inside the DF-5
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All controls and indicators of the DF-5 are on the external DFC-1 unit shown in the image on the right. It should be connected to the CTRL1 socket of the DF-5,
and shows the bearing to the transmitter.
All user interaction is via this unit.
If necessary, a second DFC-1 unit can be connected to the CTRL2 socket of the
DF-5, but this extra unit cannot be used for controlling the receiver.
It is for displaying only.
➤ Look inside the display
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The DF-5 is entended for use in combination with intelligent second-generation
tracking beacons, such as the Datong D-902 and (better) the
D-903 shown in the image on the right.
The beacon is housed in a milled aluminium enclosure and is powered by an
internal lithium battery pack. It has a series of twelve strong neodymium
magnets at the rear, allowing it to be covertly attached to (or under) a
vehicle.
➤ More information
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Intelligent beacons fitted with a receiver,
like the D-903/TRSI shown above,
can be configured and controlled remotely, by means of the
CMD-3 telecommander shown in the image on the right.
The telecommander can be used to alter the operating mode of the beacon
(continuous or pulsed), and can be used to enter sleep, deep sleep or coma
mode, in order to reduce the chance of detection by the targeted party.
➤ More information
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The reference antenna was usually disguised as a regular car phone antenna of
the era. The actual direction finding antennas – the stubs – were
available in several forms, such as a roof rack, or as plate antennas with a
magnetic base.
The most popular solution however, was to use three disc antennas that were
installed below the blinded window of a sunroof. The image on the right shows
a typical car phone antenna of the era. We are unable to show the original
covert disc antennas at this time.
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Each DF-5 came with a full set of manuals, one for the DF-5 itself and one
for each of the other parts, such as the tracking beacons
and the telecommander.
We do have these manuals in our collection, but as they contain written
– potentially confidential – information, we are unable to disclose them
at this time.
➤ Overview of documentation
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Data from DF-5 units in fixed stations
could be sent by MODEM to a central facility, where it was plotted on a map.
In addition, when the DF-5 was used in a moving vehicle, the current location
and the measured bearing could be projected onto a local moving map display.
Datong produced a PC-based environment for this, known as NAVIGATOR,
but police services also used in-house developed software. The DF-5 is also
compatible with the Becker Avioscout — a map display for airplanes
and helicopters.
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The simplified block diagram below shows how the DF-5 works. At the top left
are four antennas: three small stubs – S1, S2 and S3 – and a larger one –
the reference antenna (R4). The antennas are connected to four highly
identical receivers (1-4). The four receivers have a common Local Oscillator
(LO) — in fact a Beat Frequency Oscillator (BFO) — so that they are tuned in
tandem.
The outputs of the four receivers are fed to four lock-in amplifiers that are
driven by a common I/Q signal that is derived from the reference receiver (4).
Each lock-in amplifier provides a pair of DC voltages that represent the phase
of the associated receiver, relative to the reference channel.
The DC voltages are low-pass filtered – to remove noise – and
converted to the digital domain (i.e. digitised) after which they are
processed by an internal computer. The system is calibrated by a built-in
calibration oscillator of which the signal can be fed directly to the antenna
inputs. This way, the software can cancel out any differences between the
receivers. The distances between the antennas can be entered into the
computer's non-volatile memory, allowing the software to calculate the correct
bearing from the measured phase differences. As a final step, the system is
calibrated with a real beacon that is placed at some distance in front of the
tracking vehicle.
The computer communicates via a serial bus with a built-in
PET stabiliser, which in turn provides the interfaces to the outside
world. Display(s)
and optional expansions are connected to the PET.
The PET also has a built-in motion sensor and an electronic gyroscope, so
that it can compensate for the movements of the tracking vehicle itself when
presenting the bearing on the display.
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The DF-5 is housed in a milled aluminium enclosure that has compartments at
the top, bottom, front and rear. The compartment at the top is covered by a
large panel that is held in place by 12 recessed screws at the top. After
removing the screws, the panel can be taken off, as shown here:
Two board are fitted in this compartment:
the computer board – which fills the entire space –
and a smaller PET board that is fitted on top.
The computer board is connected to the receivers at the bottom. It comprises an
Intel TN80C196 microprocessor with RAM, an EPROM with version 4.3 of the
firmware, battery backed CMOS RAM (settings), and lots of
4th and 6th order low-pass filters.
The PET board
takes about 2/3rd of the space and is fitted on top of the
computer board. It is a fully autonomous unit that communicates with the
computer board via a serial RS232 interface, and to the outside world
(e.g. the DFC-1 display) via
several further RS232 interfaces.
It has its own Intel 80C196 microprocessor with RAM, EPROM and
battery backed CMOS RAM to hold the user's settings. The board contains
a motion sensor, and is connected to an
electronic gyroscope that is fitted
either in the top panel
or the rear panel.
These motion sensors are used to take
the movements of the tracking vehicle into account when presenting the
bearing data to the display.
The large compartment at the bottom of the DF-5 holds the receivers, the
local oscillator (LO), the I/Q modulator and the lock-in amplifiers. The
latter provide the signals for the analogue filters on the computer board.
All circuits are shielded by tightly soldered metal cans, as a result of
which we are unable to see their contents. The circuits are interconnected
with high quality coaxial cables with SMB connectors.
At the rear are the control lines from the computer board.
The diagram above also shows the three milled-out compartments at the rear
of the enclosure. At the right is the
power converter. At the left is the
ribbon cable that feeds the control signals from the computer board to the
receivers. The gyroscope
must be fitted into the compartment at the centre
when the device will be used in the upright position. In this case it is
fitted at the top.
The DFC-1 display unit is housed in a customised standard die-cast aluminium
enclosure that consists of two shells, with flat inserts at the top and at
the bottom. The insert at the bottom holds the sockets and the ON/OFF switch, whilst the
one at the top holds the volume control.
The bottom case shell holds the processor board with two daughter cards:
a small one with the 16-character alphanumeric LED display, and a larger
one with the bearing indicator, or rose, that consists of a circular array
of LEDs. To the right of the rose are three rows of green, yellow and red
LEDs that form the signal-strength indicator (S-meter}.
The circuit comprises
a 87C51 microcontroller and two MM5450N LED and Display Controllers.
The main board also holds two audio amplifiers – one for the receiver and one
for the key-clicks – and some logic for the serial port.
The upper case shell holds the keypad, a small speaker and two windows:
a large grey one for the rose, and a narrow red one for the 16-character LED
display. Adhesive foam pads are present at several positions to keep the
windows in place. The two case shells are held together with four large
bolts in the corners of the rear side. The DF-5 is enabled with the ON/OFF
rocker switch.
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When we received our DF-5, it was incomplete and in unknown condition.
Luckily it came with the original operating instructions, which greatly
helped in determining the working principle of the device. As the receiver
had previously been used in a tracking vehicle, most of the cables and the
stub antennas had been lost, but these could easily be replaced by
non-resonant alternatives.
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Furthermore, the device had been in storage for
nearly 20 years after it was decommissioned in the early 2000s, as a result
of which the backup batteries were likely to be exchausted by now.
This means that channel settings, calibrations and user settings were
likely to have been lost.
One of the first things that needed our attention was the power socket, which
is located at the centre of the front panel. For reasons that are beyond our
comprehension, the manufacturer has choosen to use a
3-pin XLR socket for this
and (worse) to use an
unconventional wiring.
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On the DF-5, the (+) terminal is connected to pin (1) of the XLR3/P, whilst in most countries this pin is used for the (-) terminal. With lack of a proper
wiring standard, and to avoid damage as a result of accidentally plugging in
the wrong cable, we decided to swap it for an XLR4/P part.
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Next, the interconnection cable between the DF-5
and the dispay unit
was restored. The orginal 9-pin D-sub connector had been swapped for a diferent one
by a previous owner, for easier installation in a vehicle.
The correct wiring of the 9-pin connector
was deduced from the internal wiring of the PET board, and was later
verified against the wiring specified in the user manual of another Datong
direction finder — the DF-6.
Together with the DCF-1 display unit, this cable is a minimum
requirement to bring the DF-5 to life.
It should be connected to the CTRL-1 port.
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With all cables in place, the newly made power cable was used to connect
the DF-5 to a 12V DC power source. An old
carphone antenna with magnet mount
was used as the reference antenna, and short improvised helical antenna's were
used as stubs. We also used a signal generator to simulate a beacon.
After switching the system ON (with the rocker switch at the
bottom of the display unit), the device came alive and immediately produced
a bearing on the display (rose).
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So far, the following restoration work has been carried out:
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- Leather case cleaned
- DF-5 exterior and front panel cleaned
- Cable between DF-5 and DFC-1 reconstructed
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- Disc antennas
- Technical documentation
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Below is the pinout of the 3-pin male XLR
receptacle at the centre of the front
panel, which is used for the power input. Note that this connector is not
wired as expected, and has the (+) terminal on pins 1 and 2, and the (-)
terminal on pin (3). This is different from other equipment.
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- +8 to 30V DC (typically +13.5V)
- +8 to 30V DC (typically +13.5V)
- 0V (ground)
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As there is no proper standard for wiring an XLR3 (audio) connector for
supplying DC power, its use it strongly discouraged. In order to avoid
confusion and potential damage to the equipment by plugging in the wrong
cable, we have decided to replace the socket by a
4-pin XLR4 and wire it
according to international conventions. Below is the pinout when looking
into the socket.
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- 0V (ground)
- not connected
- not connected
- 12V (ground)
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WARNING —
Note that an unconventional wiring scheme is used for the power socket.
Check the power cord and ensure which pin is connected to the chassis,
before connecting the DF-5 to a 12V DC power source.
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The CTRL1 port is used for connection of the
primary DFC-1 display,
which is also used to control the receiver.
It communicates with the DF-5 via a 9600 baud 8N1 RS232 interface.
If necessary, a second DFC-1 display can be connected to the CTRL2 socket,
but this cannot be used for controlling
the DF-5 receiver. Below is the pinout when looking into the socket:
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+12V brown when DF-5 is switched on TDX pink to RXD of DFC-1 n.c. - not connected ON yellow when connected to ground Audio green to DFC-1 0V black Ground RXD grey from TXD of DFC-1 n.c. - not connected GND blue Audio ground
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If necessary, a second DFC-1 display can be connected to the CTRL2
socket, and can be placed for example near the driver seat.
Note that the second display cannot be used
for controlling the DF-5 receiver. This can only be done from the primary
display that is connected to CTRL1.
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+12V brown when DF-5 is switched on TDX pink to RXD of DFC-1 n.c. - not connected ON yellow when connected to ground Audio green to DFC-1 0V black Ground RXD grey from TXD of DFC-1 n.c. - not connected GND blue Audio ground
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This port is intended for the connection of optional expansion
units, such as an antenna switcher and a tracker command transmitter
(telecommander). The wiring of this connector is currently unknown,
but it is assumed that it provides a 9600 baud 8N1 RS232 interface as
well as power.
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This port was intended for future expansions.
The wiring of this connector is currently unknown, but it is
assumed that it provides a 9600 baud RS232 interface with modem lines
as well as some digital lines, and probably also a 12V power supply.
Pinout when looking into the socket:
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- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
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- ?
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- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
- ?
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Below is the wiring of the 7-pin DIN socket
at the bottom of the
DFC-1 display/controller, when looking into the socket.
Note the strange numbering of the pins. This socket should be connected
to the CTRL1 socket on the DF-5 (see above).
Pinout when looking into the socket:
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RXD pink Data from DF-5 Audio green Audio from DF-5 TXD grey Data to DF-5 ON yellow Switched to ground GND blue Audio return +V brown Power supply (from DF-5) 0V black Power GND (chassis)
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Type Correlative interferometer radio direction finder Purpose Beacon tracking Users Law enforcement agencies Receivers 4 Frequency 25 - 500 MHz Steps 10 Hz Channels 100 Sensitivity -152 dBm Bandwidth 100 Hz - 8 kHz in 8 steps Stabiliser PET (standard) Power 8 - 30V DC Dimensions 365 x 333 x 107 mm Case 485 x 385 x 210 mm DFC-1 200 x 90 x 28 mm Weight 10 kg
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- Reference antenna (cell phone antenna)
- Ski-rack antenna
- Antenna pattern
- Disc antenna
- Plate antenna
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50-GPSPSU-0010 GPS receiver with RS232 interface (AUX) ? Antenna selector DIRECTOR PC with moving map display software ? Telephone modem
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- DF-5 Leaflet
Datong, October 1993. CM303606/R.
- PET Stabiliser leaflet
Datong. PET.PM3 2990. Date unknown.
- User Guide for DF-5
Datong, 20 August 1993 (based on 1st draft of 19 May 1993).
- User Guide for Beacon System model D-903
Datong, 19 February 1992. Issue 1.1.
- User Guide for Second Generation Command Transmitter, Model CMD-3
Datong, Issue 1.3, 5 February 1992
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Confidential document. Not for public release.
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