The Time and Frequency Division of NIST is now offering a network time service to deliver UT1 time. The service will consist initially of two parts:
1. A time server that transmits UT1 time in standard NTP format. (For example, see RFC 1305 for an explanation of the NTP format.) This server should respond to time requests in NTP versions 3 or 4 from most standard NTP clients. The server will treat all requests in client-server mode independent of the actual mode of the request. Broadcast modes are not supported. The reply will have no special indication that the transmitted time is UT1 and not UTC.
The server will be synchronized to UTC(NIST) with an offset of approximately 1 microsecond RMS. The difference between UTC(NIST) and UTC is typically a few nanoseconds, and is tabulated in Circular T of the BIPM, which is published monthly. (www.bipm.org). Rapid UTC (UTCr) gives daily values of the difference between UTC and UTC(NIST) and is computed by the BIPM at more frequent intervals. The server will compute UT1 time by adding the DUT1 value for any MJD to UTC(NIST). The value used for DUT1 will be the value determined by the IERS (http://www.iers.org) and published in IERS Schedule A for the MJD of the request when it is received by the server with no adjustments or interpolation.
The accuracy of a time message received by a user will be limited by the stability and reciprocity of the network delay between the user's system and the server. It will generally be no better than a few milliseconds, but may be much poorer and might be better than this value in some particularly favorable circumstances.
Access to this service will be limited to users who register a static network address with NIST, and understand that the time is UT1 and not UTC. The registration requirement is to prevent confusion between this server and other NTP services operated by NIST. The server will not reply to a request from a network address that has not been previously registered.
2. A digital service that will provide DUT1 and possibly Earth orientation parameters as a simple text string. The details of the service are still being developed, but the general method will be for a user to transmit a time to the server as a Modified Julian day number (and possibly in some other formats) and receive the parameters in return. The parameters will be derived from IERS Schedule A, and will initially return the values as of 0 hours on the MJD of the query. Some form of interpolation may be added at a later time if that proves to be useful to the community.
The digital service is intended to provide only contemporary data, and will not store data older than about 30 days. The accuracy of the data will be determined by the accuracy of the extrapolation inherent in the Schedule A, and will be on the order of a few milliseconds in DUT1 for a 30 day extrapolation.
We expect that these data will be available with no restriction and without prior registration with NIST.
We are soliciting comments about the best way to implement this service.
Here are the server names, locations, and IP addresses for NIST Internet Time Service - the UT1 server is at the bottom of the page.
To register for the UT1 service or to send comments or questions, please address:
Time and Frequency Division NIST Mail Stop 847
325 Broadway Boulder, Colorado 80305
1 June 2015