The Committee on Radio Astronomy Frequencies (CRAF) is a committee of
the European Science Foundation (ESF).
Since about 1992, radio observations done in the 322.0 - 328.6 MHz band suffer harmful interference from an unknown space station. The interference still exists at a level not different from its initial detection. In the frequency band 322.0 - 328.6 MHz the Fixed, Mobile and Radio Astronomy services enjoy a primary (shared) allocation.
Radio astronomers in the Netherlands and India were the first to report the existence of this unknown space station of which several characteristics were derived from careful analysis of the radio astronomical measurements. The radio astronomy observations were confirmed by the Leeheim Satellite Monitoring Station of the German administration. The Dutch administration filed a formal complaint at the ITU in Geneva. The ITU brought this to the attention of the USA, Russian and Chinese administrations.
CRAF was informed by the Russian administration that it could not be a Russian space station. In the meantime the US administration (NTIA) and the spectrum manager of the US National Science Foundation, Dr.T.Gergely, put much effort into this issue attempting to solve it.
At the 43rd meeting of the Committee on Radio Frequencies of the US National Research Council, CORF, in Washington (13-14 May 1998), Dr.Gergely reported that now the space station is known: the TEX satellite.
The TEX satellite was built by Defence Sciences Inc (DSI) which is now part of Orbital Sciences Corp (OSC) as part of an experiment built by Rockwell International, now part of the Boeing Company. It was launched on April 11, 1990 and its service ended in June 1991.
The satellite moves in a polar orbit (90o inclination). Its mean altitude is 372 Nmi (~689 km). Its decay is 0.005 Nmi per week or about 1 NMi each 4 years (~0.01 km and 1.85 km, respectively). The orbital period is 100 minutes, which implies about 14 revolutions around the Earth each 24 hours. The satellite visits every place on the Earth's surface at least twice each 24 hours, while the frequency of "in view" events increases with latitude (e.g. in view of the North Pole on all 14 revolutions per day).
The interference is caused by the "Beep Receive" transmissions of the satellite. TEX uses the UHF frequency of 328.25 MHz. However, given the ITU-R Radio Regulations the use of this frequency for space applications is illegal and therefore the satellite must be maintained in "quiet" mode. The satellite uses a two-way command and control link which transmits and receives on 328.25 MHz. In the communications with the ground control station, the downlink is used for satellite telemetry and the uplink for commands and schedules.
The satellite attitude control is designed to be by gravity gradient boom. This implies approximately 3 kilogram mass on a 7 meter boom. The satellite attitude is driven to maximize difference in altitudes of satellite mass and boom mass. It may become stable "right side up" or "up side down". A "Z"coil provides controlled magnetic torque to invert the satellite if necessary.
The following event occured: the TEX gravity gradient boom did not deploy correctly: it is deployed after the satellite is on orbit. During the extension the boom became bent and flexible. Therefore, it cannot hold the satellite attitude. DSI developed a control procedure to maintain the correct attitude. However, its success was moderate.
The current problem is that the UHF link was and is still functioning. The system has no "off" switch to prevent fatal unintentional shutdown and it can still receive commands and download telemetry.
The satellite was designed to be robust: if the power system bus voltage falls below a threshold, protective circuits operate to shed satellite loads, to restart the computer when the batteries have been recharged and to initiate the beep receive mode to re-establish communication with the ground station. If the flight computer "hangs up", a watchdog timer operates to reset the computer, clear the memory, restart the control program and initiate the beep receive mode.
The UHF link expects regular scheduled contact with the ground control
station, which executes normally two or more scheduled exchanges per day.
It enters the beep receive mode if no schedules are seen for 72 hour.
The schedules can become absent if the operator has neglected to uplink updated
schedules, if timely schedule uplink was not heard by the satellite, and if
the flight computer "watch-dog" timer initiates reset.
The beep receive mode is the interfering mode. The UHF link transmits a 2-second burst of status and telemetry every 72 seconds and listens for a reply. It is attemtping to re-establish contact with the ground control station. The beep receive mode continues until valid schedules are received from the ground control station. The operator must recognize the beep receive mode in order to take action. He cannot execute corrective action in absence of the beep receive mode since that would overfill the satellite command buffer, lose visibility of future schedules events in telemetry and initiate possible watch-dog timer reset. The beep receive potential is expected to continue until the solar panels cannot collect sufficient energy to sustain the system (the system cannot restart after a recharge period), the flight computer hangs up and the watchdog timer has failed, or the satellite's transmitter fails.
To keep the UHF link "quiet" requires regular operator effort and regular communication of the ground station with the satellite. An accompanying problem is that the satellite uplink appears to be degraded due to the following reasons:
Boeing is maintaining the TEX satellite and has set up a program for daily monitoring and work on the problem solution. The ground station has employed operator manpower and installed equipment to keep the satellite "quiet". OSC has a backup station for assistence when required.
Boeing began monitoring TEX on 5 September 1997. TEX had probably been in continuous beep receive mode up to that time since it has no possibility to turn it "off" at end of mission. After several Boeing activities the beep receive ended on 21 September 1997. From 21 September 1997 to 8 May 1998 (= 230 days) TEX has been in beep receive mode for 43 days only. Of these 43 days, 15 days turned out to be "avoidable" and 28 days were "spontaneous" (due to malfuntion of the satellite system).
The following ongoing actions are distinguished:
The events with the TEX satellite show that the harmful interference results
from a series of mishaps. Among those are:
CRAF assumes that deduction [a] is not an intentional error but the result of insufficient or inaccurate communications or misunderstanding somewhere between the parties involved in the frequency selection and coordination process for this space station. CRAF considers this a serious issue because of its possible consequences: The harmful interference event hurts currently the Radio Astronomy Service. Another event with a different space station might cause interference to other users of radio, e.g. the safety of life communications. And when the space station does not have the facility to kill the interference (as in this case), the consequences can be much more catastrofic.
A case as the harmful interference from the TEX satellite is therefore extremely important. CRAF trusts that the lessons which have to be learned from this case by regulatory authorities, operators, system designers and industry lead to adequate regulatory, procedural and technical improvements to prevent similar cases in future.
Knowing the details about the TEX satellite, it has to be investigated in which way and to which extent the interference problem can be alleviated.
Acknowledgements:
CRAF thanks the administrations of the Netherlands,
Germany and the USA (NTIA) for their supportive help. We also thank the ITU-R
Radiocommunications Bureau for its actions, the US Department of Defence and
the NSF Spectrum Manager and the Boeing Company to bring clarity on this
issue.