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

Journal of Research of the National Institute of Standards and Technology, March-April, 2005 by Michael A. Lombardi, D. Wayne Hanson

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Two GOES satellites were continuously tracked. The western satellite, or GOES-West, was located at 135[degrees] west longitude. It was tracked from the CDA and sites in the states of Washington and Hawaii. The eastern satellite, or GOES-East, was located at 75[degrees] west longitude. It was tracked from the CDA, and stations in Santiago, Chile and Ascension Island in the South Atlantic. The ground stations other than the CDA were unmanned and known as Turn-Around Ranging Stations (TARS) [28]. This system of three ground stations provided three slant ranges to the satellite that allowed the location of the satellite to be determined geometrically (Fig. 4).

Data collected through trilateration was used to generate the six Keplerian orbital elements (a, e, i, [OMEGA], [omega], M) that described the shape and orientation of the satellite's orbit at a given epoch. The semi-major axis, a, is the average distance in kilometers from the center of the mass of the Earth to the satellite. For the geostationary GOES satellites, a is nominally 42164 km. The eccentricity, e, is a measure of how close the orbit is to being circular. A perfect circle would have an eccentricity of 0. The orbit inclination angle, i, is the angle between the orbital plane of the satellite and the equatorial plane of the Earth, and is always near 0[degrees] for GOES. The right ascension of the ascending node, [OMEGA], is the angle from the vernal equinox to the ascending node of the satellite. The ascending node is the place where the northbound satellite crosses the equator. The argument of perigee, [omega], is the angle from the ascending node to the perigee, or the point on the orbit nearest to Earth. The mean anomaly, M. is an angle describing where on the ellipse the satellite is at the epoch. At perigee, the mean anomaly is 0[degrees]. At the highest point of the orbit (apogee), it equals 180[degrees]. Once these six orbital elements were obtained, it was straightforward to calculate the satellite's position at a given epoch, and to estimate the position (with increasing uncertainty) for a number of days after the epoch. This allowed the position of GOES-East and GOES-West to be precisely known, making the satellites a logical platform for a time code service.

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The GOES system also offered a convenient way to send time information to and from the satellites (Fig. 5). In addition to their continuous photography of the Earth's surface and the collection of space data related to Earth/Sun interaction, the GOES satellites also collected data from remote sensors located on Earth. These remote sensors, called data collection platforms (DCPs), are used to monitor flood, rain, snow, tsunami, earthquake, and air/water pollution conditions [28]. Some DCPs are equipped with both a receiver and a transmitter. When an interrogation message is received from the satellite, they transmit their stored data using frequencies near 401 MHz through the satellites and back to the CDA at Wallops Island. The CDA continuously relays interrogation messages to both GOES-East and GOES-West via two 18.3 m diameter parabolic antennas at an S-band frequency of approximately 2034 MHz. The interrogation messages are sent back to Earth at downlink frequencies of 468.825 MHz from GOES-West and 468.8375 MHz from GOES-East at a data rate of 100 bits per second, using just 400 Hz of bandwidth [29, 30].