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Totally Accurate Clock (TAC-2)

This product is no longer available.

Tom Clark, W3IWI, working with TAPR created an improved version of the TAC (Totally Accurate Clock). The project milestones are available.

The "Totally Accurate Clock" TAC-2 kit is intended to serve several purposes:

  • It provides a "universal" electrical and mechanical interface for a number of common OEM board-level GPS receivers including specifically:
    • Garmin GPS-20
    • Motorola Oncore
    • Trimble SK8
  • It provides interfaces for the 1 pulse-per-second (1PPS) signal generated by these receivers:
    • Low-impedance, fast rise-time 1PPS signals for "laboratory" applications.
    • RS232 level 1PPS signals for computer applications.
    • Specialized 1PPS interfaces for an add-on PCB that will stabilize a low-cost crystal oscillator to an accuracy ~ 1 part-per-billion
  • It provides several different power supply options to make use of your GPS receiver easier. The power interface is similar to (and compatible with) those used for computer disk drives:
    • A low-cost, (7805-type) regulator.
    • A high-efficiency switching power supply.
    • Direct 5 volt power.
  • It has provision for an Uninterruptable Power Supply (UPS) to buffer the GPS receiver through brief power outages.
  • It provides Battery Backup so the GPS receiver can wake up "smart."
  • It can provide isolated power for an amplified GPS antenna.

Since the TAC-2 has so much flexibility, there are several options you will have to choose from during construction. The "base" implementation will satisfy the needs of many radio amateurs with support for the Garmin GPS-20 and Motorola ONCORE Basic receivers. A few jumper changes enable support for the Motorola ONCORE VP/UT receiver. Some added parts and some more jumper changes are needed if you plan to use a Trimble SK-8.

The TAC-2 is offered as a kit which will take approxiamtely 2 or 3 hours to build.

The TAC-2 kit has been designed to be very easy to assemble. TAPR supplies detailed "Heathkit-like" instructions that only require you to know which end of the soldering iron is hot and a little knowledge on how to put things together!

One of the major interests that TAPR has in GPS timing is in the use of GPS as a way to steer ("discipline") an oscillator. By tying together a low-cost crystal oscillator and a GPS receiver, it is possible to have a Rubidium-class (better than one part per billion) frequency standard. The TAC-2 has been designed to support this function and the TAC-2 project team is working on developing the "TOC" (TAC Oscillator Controller). The plan is that the TOC will plug directly into the TAC-2 circuit board. More information on this project will made available as progress is made. No time is set for delivery of this daughterboard at this time.

Kit Availability

This kit is no longer available



View TAC-2 presentation at 1997 ARRL and TAPR DCC

What changes where made between the version 1 and version 2 designs

  • The circuitry has been improved in a number of areas:
    • better low-Z 1PPS buffers.
    • better RS232 drivers.
    • full "Plug 'n Play" support for any of the Motorola ONCORE receivers (the original PVT-6/BASIC, the 8-channel VP or the new UT-series) and several other receivers as well.

  • A number of power supply options have been provided, including
    • the TAC-2 can look like a "disk drive" when mounted in a PC
    • an optional high-efficiency switching power supply
    • an UPS (uninterruptable power supply) capability (good for a few minutes)
    • an optional regulator for external antenna bias
    • several BBRAM "keep alive" power options, etc.

  • The off-board connections for the original TAC were all soldered wires. On the TAC-2, we made extensive use of IDC ("crimp-on") connectors for ease of assembly and reliability.

  • The ONLY significant deletions from the original TAC are:
    • the on-board MMIC L-band RF amplifier.
    • The original GSFC-supplied TACs were in a metal box (we used off-the-shelf RS232 "A/B" switch boxes). The TAPR TAC-2 is a kit-form circuit board. You have to provide your own mechanical mounting (for the time being).
    • An A/B switch box would work fine as a mounting. Another possibility is that the TAC-2 circuit boards are about the size of a small computer disk drive and the power connection is the same as on a disk drive. TAPR is planning a mounting that would put the TAC-2 in a disk drive bay inside your PC.

Totally Accurate Clock (TAC)

Excerpts from TOTALLY ACCURATE CLOCK ANNOUNCEMENT, Tom Clark, NASA/GSFC (February 2, 1995)

The "TAC" name is supposed to invoke a smile on your face. Many of you remember Heathkit's "Most Accurate Clock" (a WWV receiver) and I see advertisements for VLF clocks (WWVB in USA, DCF77 in Europe) that still use the "Most Accurate Clock" name in their advertising. Since the "TAC" is 3-4 orders of magnitude better than the "Most Accurate Clock" units, the "Totally" name seems warranted (also, TAC are my initials and this was begun as a home project!).

The TAC project began when I was on sabbatical at Onsala when Bernt Ronnang got me an early prototype of a Motorola PVT-6 OEM GPS receiver. In that incarnation, the PVT-6 was pretty disappointing. When I got back home, I had Motorola update the internal firmware and found that its personality had changed completely -- it was now very precise, but it had about a 500 nsec bias. I contacted a friend at Motorola who was involved in the PVT-6 software and he told me that tests at USNO had uncovered the same error. I was added to the "beta" group, got my initial prototype updated with the latest firmware and began more detailed testing. What I then found was that the PVT-6 receiver had the best timing performance I have seen in any small GPS receiver. With a small amount of care in setting it up, it now gives 50 nsec or better RMS timing precision and biases appear to be < 20 nsec.

The TAC project now involves both hardware and software. Let me briefly describe both to you.


The core of the TAC consists of a GPS. Several are going to be supported in the TAC-2 design (Motorola ONCORE, Garmin GPS-20, Trimble SK-8) The circuit board allows these various GPS to be mounted to the board.

The TAC-2 adds a number of desirable features:

  • The 1 PPS output signals are buffered through a 74AC04 gate to improve the drive capabilities and to act as a "fuse" to prevent damage to the receiver in case of an operator goof. Up to three independent buffered outputs are provided, and the buffers will drive about +2 to +2.5 volts into a 50 ohm termination. The normal logic polarity is positive going at the epoch time, but this can be inverted if desired by some simple jumpers.
  • Up to three open collector 1PPS signals are also available, negative going. Normally these would be used to drive display LEDs, but they can be used for other purposes.
  • The add-on board includes an RS232 driver that provides 1 PPS time synchronization to an attached computer. The 1 PPS signal is normally connected to the computer's DCD input.
  • The RS232 I/O to the computer is buffered and isolated from the receiver to act as a "fuse" to prevent damage to the expensive receiver in case of an operator goof. An RS232 OR-gate is supplied for the receiver input signal to allow RTCM SC104 Differential GPS signals to be fed to the GPS receiver.

  • Other Sites and Pages

  • Tom Clark, W3IWI Site

  • Photos of putting together a TAC-2 by Henk Uijterwaal

    Internal GPS units that will work with the TAC-2

    Related GPS Files and Pages:

    GPS System Standard Positioning Service Signal Spec (pdf 1.2Meg)

    Annex A, B, & C of GPS Standard Positioning Service Signal Specification (pdf 1.3Meg)

    NAVSTAR GPS User Equipment Introduction (pdf 2.4Meg)

    Various Motorola Oncore Information

    Time Service Department U.S. Naval Observatory

    Operating the Motorola GPS Engines In NMEA-0183 Mode McKinney Technology, Doug McKinney (rtf, 20K)

      For those of you interested in how GPS rcvrs are made, and more particularly want more details on the custom chips that are in the Motorola ONCORE receivers.

    Project History

    July, 1997

    The kit is finished and shipping!
    June 1st, 1997
    Due to a problem with board production, the Dayton HamVention introduction was delayed. The board is back in the shop and we expect to have kits available the end of June. Keep an eye on TAPR-BB for the offical announcement and pricing.

    March 15th, 1997

    Testing continues and a final production board is nearly completed. Documentation is being completed and will be reviewed by between 3 and 5 people. More information will be presented when the final production kit cost and availability is know. The kit is looking very good. The pictures to the right are (top - component side of the TAC-2) and (underneath - Side view of the TAC-2 with Motorola Oncore-VP mounted underneath).

    January 1997

    The project team has the rev-b boards in hand and are building. When docs are written and testing is completed with this round, we will be asking for Beta testers. Details on how to particpate in the beta-testing will be announced in the Winter TAPR PSR (Packet Status Register) and on the TAPR-BB e-mail list. It is expected that a small group of 5-10 people will be asked to particpate in the beta-testing.

    November 1996

    The project team is reviewing the design of the second alpha board. The second alpha-board will be put into the board shop (3-4 week turn) once everyone feels that it is ready. It is not expected that a beta run will be done until the Spring.

    October 1996

    The first alpha boards arrive. Alpha testers installed components and found that they work! Documentation begins. Project teams decides to add functionality and additional components to board. This requires a change in the board layout and a second alpha board run.

    September 1996

    Purchase of the alpha PCB authorized by the TAPR BoD.

    Assembly of the TAC-2 with GPS receivers

    Content created by Henk Uijterwaal

    One of the candidates for the GPS receiver is the TAPR Total Accurate Clock (TAC). This unit uses the Motorola Oncore VP GPS receiver. The unit comes as a kit, on this page you can see the various steps in assembling the kit.

    The unassembled kit ready to be soldered together

    Our soldering iron.

    The assembled kit. The empty positions on the PCB are for other GPS receivers, the red-black cable is the power cable.

    The Motorola Oncore GPS receiver (right), ready to be mounted on the TAC unit (left).

    The TAC unit (top PCB) with the Motorola (bottom PCB) mounted on it.

    The finished unit with the backup battery and all cables, ready to be hooked up to a computer and antenna.

    The software that comes with the unit displays the time and position, here is a screen dump made after the unit had been running for a couple of hours. The display shows:

    • The local time (14:07:44).
    • The location of our office (latitude 52o22.37934 and longitude 4o53.37072), on the bottom left hand side of the screen, and
    • The number of satellites from which it receives signals on the right hand side of the screen: 7 satellites are visible, 5 give reliable signals

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