GOES satellite time code dissemination :

Abstract

The N a t i o n a l Bureau o f Standards, i n cooperation w i t h t h e N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n (NOAA), has been d i s s e m i n a t i n g a t i m e code referenced t o UTC(NBS) v i a two o f NOAA's g e o s t a t i o n a r y GOES s a t e l l i t e s s i n c e 1975. A review o f t h e GOES t i m e code system, t h e performance achieved t o date, and some p o t e n t i a l improvements i n t h e f u t u r e w i l l be discussed.The disseminated t i m e code i s o r i g i n a t e d from a t r i p l y redundant s e t o f atomic standards, t i m e code generators and r e l a t e d equipment maintained by NBS a t NOAA's Wallops I s l a n d , VA s a t e l l i t e c o n t r o l f a c i l i t y . I t i s r e l a y e d by two GOES s a t e l l i t e s l o c a t e d a t 7 5 O W and 135"W l o n g i t u d e on a continuous b a s i s t o users w i t h i n North and South America ( w i t h over1 apping coverage) and well o u t i n t o t h e A t l a n t i c and P a c i f i c ocean areas. Downlink frequencies a r e near 468 MHz. The s i g n a l s from b o t h sate1 1 i t e s a r e monitored and c o n t r o l l e d from t h e NBS l a b s a t Boulder, CO w i t h a d d i t i o n a l m o n i t o r i n gi n p u t from geographical l y separated r e c e i v e r s i n Washington, DC and Hawaii. Received t i m e code accuracies a r e t y p i c a l l y b e t t e r than 1 ms i f t h e u s e r o n l y a p p l i e s a c o n s t a n t c o r r e c t i o n t o compensate approximately f o r h i s geographical l o c a t i o n o r b e t t e r than 100 p s i f manual o r automatic c o r r e c t i o n s are a p p l i e d f o r p a t h delay u s i n g s a t e l l i t e p o s i t i o n d a t a encoded i n t o t h e GOES t i m e code s i g n a l s .Performance experience w i t h t h e r e c e i v e d t i m e codes f o r p e r i o d s r a n g i n g from several y e a r s t o one day i s discussed. Results a r e a l s o presented f o r s i m u l t aneous, common-view r e c e p t i o n by co-1 ocated r e c e i v e r s and by r e c e i v e r s separated by several thousand k i 1 ometers.Based on t h e general acceptance o f t h e GOES t i m e code, NBS and NOAA have r e c e n t l y extended t h e i r formal Memorandum-of-Agreement t o c o n t i n u e t h e GOES t i m e code o p e r a t i o n s f o r a t l e a s t an a d d i t i o n a l f i v e -y e a r p e r i o d ,

The GOES time code service, 1974-2004: A retrospective

“…The time code frame contained a 40 bit synchronization message, a 32 bit time-of-year message including the day of year, and the UTC hour and minute; an 8 bit message containing the current offset between UTC and the astronomical time scale UT1, and a 52 bit ephemeris message containing the satellite’s latitude, longitude, and height above the Earth’s surface minus a bias of 119 300 µs [ 31 ]. The remaining 108 bits were originally unused, but by 1984 were used to store the year, expected-accuracy indicators, system status information, and indicators for daylight saving time and leap seconds [ 32 ] ( Fig. 6 ).…”

“…The original TCG configuration updated the position data to the satellites once every 30 min [ 28 ]. This was later reduced to 4 min [ 33 ], and eventually to 1 min [ 32 ]. The more frequent position updates smoothed the received data and improved the quality of the service.…”

“…These oscillators were originally owned and maintained by NIST, but in the later years they were provided by NOAA. The TCGs were referenced to the atomic oscillator frequency and set to be 260 000 µs ahead of UTC(NIST), with an uncertainty of ±10 µs throughout the lifetime of the service [ 32 ], and ±1 µs during the last nine years (1996–2004). The TCGs were adjusted from Boulder in time increments of 0.2 µs, the period of the 5 MHz time base.…”

“…GOES time at the CDA was synchronized by NIST personnel using a number of techniques throughout the years, including the use of television signals from nearby Norfolk, Virginia, LORAN-C transmissions from Cape Fear, North Carolina [ 29 , 30 ], portable clock carries from Boulder to Wallops Island [ 39 ], and eventually through the use of GPS signals [ 32 ]. The performance of the station clock was continuously monitored from Boulder using a remote controlled data logger [ 40 ], which was replaced by more modern equipment in 1999.…”

GOES satellite time code dissemination :

“…Periodically, these results are compiled and published to document the performance of the GOES time code system. One example can be found in Reference [4] . Figure 4 is a plot of the observed daily averages of UTC(NBS) -GOES/East over a period of about 4 years where no corrections have been made for satellite position variations.…”