ORBIS (USAF satellite)

Background

In the ionosphere, a propagation duct can appear when a layer of lower electron density sits between two denser layers. Signals from satellites that get trapped inside these highly refractive duct walls can travel for thousands of kilometers. For example, ground stations often picked up the low-frequency signals from early Sputniks even when the satellites were on the far side of Earth. Scientists thought stable ducts for frequencies between 30 and 50 MHz might circle the entire planet, allowing communications over extreme distances.

To test this, the U.S. Air Force ran the ORBIS experiments. ORBIS satellites carried beacons that broadcast continuously on two frequencies—one above the daytime plasma frequency and one below. A dozen or more ground stations monitored these signals. Similar beacon tests, called Nora Alice, were developed by the University of Illinois and launched on Discoverer 32 and Discoverer 36 in 1961 and 1962.

Missions

ORBIS beacons flew on dedicated satellites and as experiments on other satellites, all launched in multi-satellite missions. After a launch failure on March 24, a stand-alone ORBIS satellite entered a low orbit on 18 November 1964, transmitting on 10.004 MHz for about two weeks until reentry. AFCRL observatories in Massachusetts noted extremely long-range propagation from the northwest near and after local sunrise, with signals cutting off as the satellite passed below the horizon to the south, and beyond-the-horizon propagation from the south during post-midnight hours.

Stations at Urbana, Illinois and St. Louis, Missouri observed similar effects. European stations of the Joint Satellite Studies Group reported general agreement with U.S. overhead observations but, due to high local interference, detected little long-range propagation. The program split into ORBIS Low and ORBIS High, referring to altitude. Satellite OV2-5 carried eleven experiments including an ORBIS High transmitter, launched into synchronous altitude orbit in September 1968.

OV4-3, a Manned Orbital Laboratory boilerplate launched in November 1966, carried an ORBIS Low transmitter operating at 5.005, 10.004 and 30.012 MHz, monitored by 25 ground stations worldwide; it operated for one month and reentered on 9 January 1967. ORBISCAL (OV1-17A) had two transmitters at 8.98 and 13.25 MHz, broadcasting for one week between launch and reentry in March 1969; it was a reflight of ORBISCAL 1 lost in a 12-satellite Atlas-Burner launch failure in August 1968.

Stations at Thule and Sagamore Hill Radio Observatory frequently recorded propagation ranges of 20,000 kilometers during both day and night.