Tim Furniss/LONDON

The US Air Force's latest satellite is a technology demonstrator for future space systems

The US Air Force's Advanced Research and Global Observation Satellite (Argos), to be launched no earlier than 22 February, is a space version of the Swiss army knife. It carries an array of nine varied high technology experiments to demonstrate new technologies that can be applied to future military space systems.

Most of the experiments have been developed by the US Naval Research Laboratory (NRL). They are expected to show significant advances particularly in autonomous satellite navigation, superconductor device technology and in propulsion systems.

In addition, operationally, the spacecraft will be able to downlink data at a rate of 5Mb/s, more than twice the capacity of current systems, says the USAF.

The Argos is flying with two piggyback satellites: the Orsted, from Denmark; and South Africa's first satellite, the Sunsat. Codenamed P91-1, the 2,500kg Argos was built at Boeing's factory in Seal Beach, California.

It is "the largest and most sophisticated research and development satellite Boeing has built for the US Air Force", says Jim Albaugh, president of the company's Space and Communications division.

The satellite is poised for its sixteenth launch attempt from Vandenberg AFB, California, after a launch abort on 28 January. Ignition of the Boeing Delta II was stopped automatically because one of the booster's two vernier engines failed to ignite just before the main engine ignition command. It was the fifth on-pad abort for a Delta II since December 1995 and took place during the seventh attempt to launch the Argos. Six previous attempts were stopped since 15 January because of high upper level winds. Further delays have since occurred.

The Delta II will place the Argos into a 720km circular, sun-synchronous, 98.7<sup>0</sup> inclination orbit, in which it is expected to be fully operational for three years.

The Argos is part of the Air Force Space Test Programme that supports the USAF, the US Army, the Navy and the Ballistic Missile Defence Organisation. From this orbital post, the USAF hopes that it will do for future space systems what previous military technology demonstrator spacecraft have done for operational systems such as the Military Strategic and Tactical Relay (Milstar) and Global Positioning System (GPS). The spacecraft's High Temperature Super Conducting experiment, developed by the NRL will qualify superconducting digital subsystems that could provide 10 times higher speed and similar weight reduction, compared with today's silicon and gallium arsenide-based electronics.

The feasibility of autonomous satellite navigation using astronomical X-ray pulsars in place of GPS timing and navigation signals will be demonstrated by the Unconventional Stellar Aspect Payload, also supplied by the NRL.

Pulsars are small neutron stars that remain after the death of a large star in a supernova explosion. What makes them unique among neutron stars is that they are spinning rapidly, sending out regular, perfectly timed pulses of light and radio waves, seconds apart. The Argos will have a GPS receiver, but will also determine if these astronomical X-ray sources could be used as an autonomous position, attitude and timekeeping reference for military space systems. The experiment will also flight qualify three radiation-hardened, 32-bit computers.

The USAF's Phillips Laboratory has produced the Electric Propulsion Space Experiment to demonstrate reliable arc-jet thruster operation in space. An arc jet is a chemical thruster, "boosted" by electrical charges.


Electric propulsion is expected to double the payload-to-orbit capability compared with current chemical systems. The arc-jet will be used for Argos' orbital transfer, circularisation and attitude control. Military services are interested in this form of electric propulsion for the manoeuvring requirements of future global surveillance and communications systems.

The US Army's Extreme Ultraviolet Imaging Photometer will establish the behaviour of the upper atmosphere and its plasma, to improve secure radio communication systems design and the prediction of magnetic storms and characterisation of aurora, both of which interfere with communications. The experiment will also observe the earth horizon and stars to measure background radiation for future military missile defence sensors.

A US Navy Space Dust Payload will provide definitive measurements of orbital debris, resulting in a three-dimensional survey map of the dust population in low earth orbit. The payload will obtain early flight experience for the sensors and electronics that will fly on the NASA International Space Station.

The Critical Ionization Velocity experiment, from Phillips Laboratory, will help identify plumes and atmospheric wakes of missiles and orbital vehicles to improve ground monitoring and tracking sensor technology.

A number of Department of Defense systems depend on radio and microwave propagation through the upper atmosphere and ionosphere. The Argos' NRL-developed High Resolution Airglow/Aurora Spectroscopy payload will help establish an upper atmosphere "weather" prediction. This will help to predict orbital life and re-entry impact locations of satellites by forecasting the effect of the atmosphere in slowing down, or "dragging", spacecraft.

The NRL is contributing the Global Imaging Monitor of the Ionosphere, which will monitor effects of meteors, rocket exhausts and aurora on the earth's upper atmosphere.

The NRL's Coherent Radio Topography Experiment Payload consists of three-frequency radio beacon and radiating antennas to help assess the impact of the ionosphere on navigational accuracy, communications systems and radar remote sensing.

Source: Flight International