Advanced Communications Technology Satellite (ACTS)

About ACTS

Operations

Overview

The Advanced Communications Technology Satellite Ka-band payload became operational on September 23, 1993. Although NASA's original experiment program has concluded, ACTS is still operational 24 hours a day, 365 days a year. The spacecraft is located at 105.2° W longitude where it will remain, even after it is shut down. The ACTS spacecraft is controlled by the Master Ground Station and the Satellite Operations Center.

The spacecraft now operates in an inclined orbit mode. This means that north/south station-keeping is no longer maintained on the satellite and earth stations must now track the satellite (see Ephemeris Data).

There are certain times when the satellite is not available to experimenters. During two periods a year, the ACTS solar panels are eclipsed and the satellite's communications payload is shut down (see Eclipse Operations) making it unavailable for use.

Antenna Coverage

This graphic depicts the spot beam locations for ACTS.

You may also view a larger graphic.

Ephemeris Data

Ephemeris data is satellite position data with respect to some co-ordinate or reference system. ACTS ephemeris data is an earth-centered reference system that provides predictions of satellite position in latitude, longitude and altitude on an hourly basis. Latitude is in milli-degrees. Additionally, the range, azimuth and elevation from the NASA Ground Station are listed.

The data is posted as a plain text file (text file, 225 Kb). Last updated June 18, 2002.

Eclipse Operations

At times the spacecraft communications payload is not available for users. A constraint on the operation of the satellite occurs during the spring and fall equinoxes. During the equinoxes the spacecraft's solar panels are eclipsed from receiving full sunlight. This effect is common to all geosynchronous satellites.

Fortunately, an eclipse's impact is typically low because it occurs during the first hours of the day, local Eastern time, when not many users are active. To conserve weight, the spacecraft’s batteries were not sized to provide power to the communications payload. Therefore, for 22 days before and after the equinox (44-day period eclipse season), the communications payload is shut down so that only essential bus functions remain operating.

The hours of shutdown are typically between 0600-0830 GMT which allows sufficient time before the eclipse period to gracefully shut down the system, and then afterwards, to re-energize critical spacecraft components before bringing the system up again. The peak shadow time is about 70 minutes in duration for a week before and after the equinox. The duration of the outages symmetrically tapers around the equinox.

Inclined Orbit Operations

Hydrazine fuel is consumed by the ACTS spacecraft to maintain its desired position in space (stationkeeping). North-south stationkeeping maneuvers used the bulk of the annual hydrazine fuel consumption of ACTS. East-west stationkeeping control required far less fuel usage.

In 1997, with the continued interest in using ACTS by its various users, NASA investigated extending operations by discontinuing north-south stationkeeping maneuvers. It was determined that operations could be extended by approximately 31 months before exceeding spacecraft attitude control limitations.

Beginning in July 1998, north-south stationkeeping maneuvers were discontinued to conserve hydrazine fuel. Orbital inclination has been increasing at a rate of 0.76 degrees per year reaching an inclination of 2.5 degrees by August 2001. In this inclined orbit operations mode, ACTS operations continued to support a NASA-organized experiments program through May 31, 2000. Inclination information can be found in the ephemeris data table (text file, 225 Kb).

In the absence of any corrective maneuvers, the inclination plane for any object at a geostationary orbit height will increase to a maximum of 15 degrees and return to its original inclination over a period of 54 years.

Retirement Information

Since launch in 1993 the ACTS spacecraft was actively maintained at its assigned orbital station of 100° West longitude. When spacecraft retirement planning began in early 1999, updated fuel estimates indicated a very low amount of remaining hydrazine. The safest option to minimize orbital debris issues was to move the satellite to the orbital gravity well at 105.2° west longitude on the geostationary arc.

In April 2000 as ACTS neared its planned end of mission, the routine east/west station keeping maneuvers that maintained this 100° position were discontinued. ACTS began a slow natural drift towards the gravity well at 105.2°. On June 18, 2000, a 5-minute burn of the West facing hydrazine thrusters on the spacecraft accelerated the drift rate to 0.075 degrees per day. To move a spacecraft in orbit, the orbit height is either raised (moves West), or lowered (moves East) from the geostationary altitude.

The activity of moving ACTS past the several active commercial satellites between 100° and 105° was an involved process since ACTS is very low on fuel. It could not be maneuvered far above its neighbors for the relocation. As a result, a daily conjunction analysis was done by NORAD/Cheyenne Mountain Operations Center (CMOC) to predict the minimum spacing between ACTS and the satellites it was about to pass. The commercial satellite operators were kept informed of ACTS current orbital elements and the ACTS project was advised of planned station keeping maneuvers by the satellites it was approaching.

The westward drift of ACTS was stopped by a 4-minute burn on August 11, 2000 followed by a 30-second trim burn on August 16, 2000. After monitoring the satellite orbital stability for nearly two months, a final 3-second trim burn was done on October 12, 2000. Long term predictions for the natural gravitational effects on this parking orbit indicate less than 0.2° variation from 105.2°. This will assure that the retired ACTS spacecraft does not impinge on nearby active geostationary satellites.