NASA's Atomic Clocks.

The Trimble FTS8400 is Obsolete

Users of the Trimble FTS-8400 GPS timing receivers will soon need to replace them because they will stop working on August 21, 1999. NASA and their partners around the world operate the Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) networks. These networks use the FTS8400 GPS timing receivers. The current status indicates that:

• The Trimble/FTS8400's originally cost about $30,000 some 15 years ago and are due for a replacement with more modern hardware.
• The FTS8400s will die on August 21, 1999 because of the GPS equivalent of a Y2K problem – the GPS week 1024 roll-over.
• The CNS Clock™ performs a factor ~10 better than the FTS8400s, and cost under $1000.
• The CNS Clock™ needs to be used with a counter, and an excellent commercial time interval counter, the HP53131A, costs only $1725.
• The present logging software that supports the FTS8400 needs to be replaced. A cost-effective solution is to plan to acquire new Pentium PCs (running Windows NT Server) for each station and using an adaptation of the Tac32 support software.

Why Do We Need A GPS Clock?

Each SLR and VLBI station is equipped with a precision frequency standard – a Hydrogen Maser at the VLBI stations, or a Cesium standard at the current SLR stations, or a GPS-disciplined Rubidium or Crystal standard in the new SLR2000 stations. This working "flywheel" can be called the "house clock" and serves as the reference for picosecond-level precision time intervals (including microwave carrier phase reference). This long-term performance of this frequency standard must be calibrated by a source of accurate epoch time directly traceable to the international time services (i.e. the USNO Master Clock). There is a need for preserving/improving the ability to compare the "house clock" with the USNO Master Clock because:

• The scientific applications of VLBI and SLR data requires accurate (sub-microsecond) epoch time tagging in order to be of any use.
• Efficient analysis/processing of the VLBI/SLR data requires the guarantee of long-term consistency of these time tags.
• Monitoring of the accuracy of the frequency generated by the "house clock" (the Maser/Cesium/Rubidium standard mentioned above), requires comparison with an independent, outside time standard.

In past years, NASA solved these problems by carrying (not too portable) travelling clocks to all their sites. This was later replaced/augmented by the use of LORAN timing. Then in the early 80's they made the transition to GPS, using the (now outmoded, antique) Trimble FTS8400 units (costing ~$30,000); the FTS8400 's have done yeoman's service, but it is now time for their retirement.

It is asserted that the SLR and VLBI networks will continue to have a mandatory requirement for GPS-based timing forever! Since the VLBI and SLR networks have similar timing requirements, and since their activities can be made much more cost-effective if they have a common solution, and because the present FTS8400-based capabilities will die in about a year a solution to this problem is needed now.
Some side benefits to the early adoption of this scheme are:

• FTS8400 users will save money (by improving operational efficiency and by not attempting to maintain the old FTS8400s);
• users will achieve an order-of-magnitude improvement in the accuracy of the epoch time available at their stations; and
• this solution will solve some station communication and coordination problems (by better integration of the information flow using the Internet).

All this can be accomplished for a one-time cost around $5000 per station!

Some Specific Implementation Issues

The HP53131A Time Interval Counter (TIC) is an impressive device. The HP53131A has 10-digit resolution, and makes time interval measurements with 500 psec one-shot resolution. [An alternative is the HP53132A , with 12-digit and 150 psec, costing $2595. There is no compelling need for spending the extra $870. In the following comments, all HP53131A features apply to the more expensive HP53132A]. The HP53131A is a half-width, 3.5" high module. Rack mounting adapters cost an additional $52. It might be desirable to purchase the $100 option that puts the input connectors on the rear panel. Under any circumstances, a properly equipped HP53131A counter is under $2000.

Additional options: Some higher stability time-base options ($600, $900 or $1500) options are available, but are unnecessary since the "house clock" is already available. 3 GHz ($800) or 5 GHz ($2350) frequency counter options are available but would be unnecessary since the '131 is to be devoted to TIA measurements between the "house clock" and GPS.

The HP53131A has two independent data outputs. It comes equipped with full HPIB (IEEE-488) control. In addition, it also has a simple-to-use RS232 output (normally intended for a serial printer) on a standard 9-pin connector (a 9-pin female-to-female null modem cable is all that is required to plug it into a PC). When running in TIA mode, this port delivers data that looks like:

10.311,8  u     < Raw time-interval samples,
10.155,8  u     < output once/second with
10.285,8  u     < 500 psec resolution
10.251,8  u
10.094,8  u
10.226,3  u
10.194,8  u
N      : 100     < Shows 100 second TIA interval,
STD DEV: 0.057,107  u  < followed by Std Dev (57.1 ns),
MEAN   : 10.183,871 u   < Mean offset (10183.871 nsec),
MAX    : 10.311,8   u  < and extrema seen during this
MIN    : 10.038,3   u  < (100 sec) TIA sample interval.
10.223,3  u     < Then the '131 resumes the
10.236,8  u     < once/second output until the
10.206,8  u     < next TIA (100 sec) interval.

This data can be captured and then passed it through a simple filtering program which reformats the data into two files – the TIA (typically 100 sec) samples, and (optionally) the "raw" 1 second data samples. This allows for easy import into EXCEL or MATLAB.

The Glue Required

The CNS Clock™ hardware and Tac32 software were developed to provide high accuracy timing capabilities to support R&D activities of the air transport industry. It solved the particular problem of needing to synchronize several Windows NT computers being used for data acquisition with aircraft. The CNS Clock™ hardware was developed incorporating Dr. Clark's TAC-2 design.

Discussions with Dr. Clark (NASA) led to the design of this software module to integrate HP53131A counter support into Tac32 to meet the needs of the timing community that formerly used the FTS8400. Thus, the Tac32Plus TIC module is designed to log either time-tagged "raw" (1/sec) TIC measurement and/or the TIA statistical measurements from a HP53131A in a format suitable for use by other programs (like EXCEL, MATLAB, etc).

Tac32Plus runs on any 32-bit Windows platform (W95/98/NT) and the only computer requirement is two serial ports (one for the TAC-2, one for the HP53131A counter).

Networking of Data files

1. The Windows computer can "put" the files via its standard internet connection; or
2. If the computer is running Windows NT, it can be available as a file server on the Internet; or
3. If the station is running Microsoft Networking (MSN), then the data files are directly accessible by other computers at the station; or
4. In the case of the VLBI stations, where the Field System is running LINUX, the SAMBA software supplied with LINUX also supports MSN.