1. Background

2. Spacecraft

3. Launch and orbiting

4. Scientific results
    X-rays  Ultraviolet  Nuclear test monitoring 

5. GRAB results

6. Legacy and status

7. See also

8. References

9. External links

SOLRAD (SOLar RADiation) 1 was the public designation for SOLRAD/GRAB 1, a combination science and surveillance satellite.

It was the first satellite to successfully observe solar X-rays and also the first to conduct surveillance from orbit. Developed by the United States Navy's Naval Research Laboratory (NRL), the satellite was in many ways a direct successor to Project Vanguard, the first American satellite program. The SOLRAD scientific package aboard the satellite provided cover for the GRAB (Galactic Radiation and Background) electronic surveillance package, the mission of which was to map the Soviet air defense radar network. SOLRAD/GRAB 1 was launched into orbit via Thor DM-21 Ablestar rocket on June 22, 1960, along with the unrelated navigation satellite, Transit 2A, marking the first time two instrumented satellites had been orbited at once.

The satellite's scientific mission was a success, determining normal solar X-ray output and confirming the connection between increased solar X-ray activity and radio "fade-outs". The surveillance mission of the co-flying GRAB 1 equipment was also successful, revealing that the Soviet air defense radar network was more extensive than had been expected.

Background

The United States Navy's Naval Research Laboratory (NRL) established itself as a player early in the Space Race with the development and management of Project Vanguard (1956–1959),^[1] America's first satellite program. After Vanguard, the Navy's next major goal was to use the observational high ground of Earth's orbit to survey the locations and frequencies of the Soviet air defense radar network. This first space surveillance project was called "GRAB", later expanded into the more innocuous backronym, Galactic Radiation and Background.^[2] As American space launches were not classified until late 1961,^[3][4] a co-flying cover mission sharing satellite space was desired to conceal GRAB's electronic surveillance mission from its intended targets.^[2]

The field of solar astronomy provided such cover. Since the invention of the rocket, astronomers had wanted to fly instruments above the atmosphere to get a better look at the sun. The Earth's atmosphere blocks large sections of sunlight's electromagnetic spectrum, making it impossible to study the sun's X-ray and ultraviolet output from the ground. Without this critical information, it was difficult to model the sun's internal processes, which in turn inhibited stellar astronomy in general.^[5]{{rp|5–6}} On a more practical level, it was believed that solar flares directly affected the Earth's thermosphere, disrupting radio communications. The Navy wanted to know when its communications were going to become unreliable or compromised.^[2] Sounding rockets had shown that solar output was unpredictable and fluctuated rapidly. A long-term, real-time observation platform above the Earth's atmosphere – in other words, a satellite – was required to properly chart the sun's radiation, determine its effects on the Earth, and correlate it with ground-based observations of the sun in other wavelengths of light.^[5]{{rp|63}}

Thus, the SOLRAD project was conceived to address several NRL goals at once:

• to cheaply and efficiently produce a satellite for the GRAB surveillance mission by using a proven design.^[2]
• to obscure the GRAB mission under a scientific cover.^[2]
• to make the first long-term continuous observations of the sun in ultraviolet and X-ray light, and to correlate these measurements with ground-based observations.^[5]{{rp|64–65}}
• to evaluate the level of hazard to satellites and astronauts posed by ultraviolet and X-ray radiation.^[6]{{rp|76}}
• to better understand the effect of solar activity (including solar flares) on radio communications.^[7][8]

Spacecraft

The NRL science satellite team under Martin Votaw, composed of the Project Vanguard engineers and scientists who had not migrated to NASA upon its formation, adapted the Vanguard 3 design for SOLRAD/GRAB 1.^[8] Like Vanguard 3, the spacecraft was roughly spherical, {{convert|51|cm|ft|sp=us}} in diameter, and powered by six circular patches of solar cells.^[9]{{rp|10}} SOLRAD 1 was slightly lighter, however, massing {{convert|19.05|kg|lb|0|sp=us}}^[9]{{rp|A1-2}} (as opposed to Vanguard's {{convert|23.7|kg|lb|0|sp=us}}).^[10]

The satellite's SOLRAD scientific package included two Lyman-alpha photometers for the study of ultraviolet light and one X-ray photometer in the 2–8 Å wavelength range, all mounted around the equator of the satellite.^[20] Vanguard 3 (and 1959's Explorer 7) had also carried these experiments, but they had been completely saturated by the background radiation of the Van Allen belts.^[5]{{rp|63}} With that experience to draw from, SOLRAD/GRAB 1 was equipped with permanent magnets to deflect particles from the detector window.^[5]{{rp|64–65}}

The satellite's GRAB surveillance equipment detected Soviet air defense radars broadcasting on the S band (1,550–3,900 MHz).^[11]{{rp|29,32}} A receiver in the satellite was turned to the approximate frequency of the radars, and its output was used to trigger a separate VHF transmitter in the spacecraft. As it traveled over the Soviet Union, the satellite would be hit by the pulses from the missile radars and immediately re-broadcast them to ground stations below, which would record the signals and send them to the NRL for analysis. Although GRAB's receiver was omnidirectional, by looking for the same signals on multiple passes and comparing that to the known location of the satellite, the rough location of the radars could be determined, along with their exact pulse repetition frequency.^[12]{{rp|4–7}}^[13]{{rp|108}}

Telemetry was sent via four whip-style {{convert|63.5|cm|ft|sp=us}} long antennas mounted on SOLRAD's equator.^[6]{{rp|76}} Scientific telemetry was sent on 108 MHz,^[6]{{rp|78}} the International Geophysical Year standard frequency used by Vanguard.^[1]{{rp|84,185}} Commands from the ground and electronic surveillance were collected via smaller antenna on 139 MHz.^[12]{{rp|7}}

Like most early automatic spacecraft, SOLRAD/GRAB 1, though spin stabilized,^[2] lacked attitude control systems and thus scanned the whole sky without focusing on a particular source.^[5]{{rp|13}} So that scientists could properly interpret the source of the X-rays detected by SOLRAD/GRAB 1, the spacecraft carried a vacuum photocell to determine when the sun was striking its photometers and the angle at which sunlight hit them.^[5]{{rp|64}}

Launch and orbiting

The mission of SOLRAD/GRAB 1 began with its launch at 05:54 (UTC) on June 22, 1960, using the Thor DM-21 Ablestar launch system, from Cape Canaveral LC17B.^[14][15] The event was front-page news, though as much because the launch marked the first time two instrumented satellites had been carried to orbit on the same booster as for the individual satellites. Once in orbit, SOLRAD/GRAB 1 separated automatically from Transit 2A, drifting into a slightly higher orbit. SOLRAD/GRAB 1 initially circled the Earth once every 101½ minutes,^[16] varying from {{convert|611|km|mi|sp=us}} to {{convert|1046|km|mi|sp=us}} in altitude; this was a deviation from the planned {{convert|930|km|mi|sp=us}} circular orbit, caused by glitches in the second stage of the booster,^[17] but it did not affect the satellite's objectives.^[15]

Scientific results

SOLRAD/GRAB 1 transmitted more than 500 batches of scientific data between June and November 1960^[5]{{rp|64–65}} after which it became impossible to determine the angle at which the sun hit SOLRAD's experiments.^[18] Nevertheless, Solrad 1 continued to send data until April 1961 when the spacecraft was switched off from the ground, in history's first remote satellite deactivation.^[17]

The satellite communicated results in real-time, each pass providing just one to ten minutes of data,^[18] which meant that data could only be received when there was a tracking station within range – either one of Vanguard's Minitrack stations or a few other isolated receivers. Thus, only 1.2% of its active time involved actual solar observations. The magnetic deflectors proved effective, allowing SOLRAD/GRAB 1 to become the first satellite to successfully observe solar X-rays.^[5] However, they also caused the satellite to precess (wobble around its axis) like a spinning top so that its sensors were only in sunlight about half the time the sun was in the sky.^[5]{{rp|64–65}}

X-rays

Only 20% of the data indicated X-ray output in the ranges the SOLRAD package could detect. Nevertheless, the data supported several important findings. It established the sun's normal X-ray radiation levels during times of inactivity: less than 6x10⁻¹¹ Joules/cm²/sec in the wavelengths the satellite could see. When X-ray output was observed strongly in excess of this baseline, it was usually correlated with solar activity visible from the ground. The data also showed that the X-ray output could change significantly in as little as one minute, underscoring the need for constant observation.^[5]{{rp|64–65}}

When detectable X-ray output exceeded three times the normal rate,^[5]{{rp|64–65}} radio 'fade-outs' occurred, confirming the link between solar X-ray variability and the strength of the Earth's ionized thermospheric layers.^[8] These thermospheric disturbances were not just caused by solar flares, but also by active solar prominence regions as well as bright surges and subflares at the edge (or limb) of the sun.^[5]{{rp|64–65}}

Ultraviolet

SOLRAD/GRAB 1 did not find a correlation between solar ultraviolet output and thermospheric disturbance,^[5]{{rp|53}} and the Lyman-Alpha detectors were deleted from the later SOLRAD 3/GRAB 2 mission.^[19]{{rp|28}}

Nuclear test monitoring

It had been hoped during design and development that SOLRAD/GRAB 1 would be able to identify above-ground atomic tests, which produced strong emissions of X-rays in the bands that the satellite could detect. If a nuclear test ban treaty between the United States and the Soviet Union were to go into effect, SOLRAD/GRAB 1 or its successors might then be able to detect unauthorized tests by the Soviets. However, no spikes corresponding to known Soviet atomic tests were conclusively found in SOLRAD/GRAB 1's data. The Vela-Hotel satellites were later purpose-built for the task after the ratification of the Partial Nuclear Test Ban Treaty in 1963.^[20]

GRAB results

SOLRAD/GRAB 1 was the world's first operational surveillance satellite. While the SOLRAD package searched for X-rays and ultraviolet sources, the satellite's GRAB sensors listened for the output of Soviet air defense radar pulses in the microwave "S band" over a circular area {{convert|6500|km|mi|sp=us}} in diameter beneath it.^[13]{{rp|108}}

Though the satellite's surveillance equipment functioned from launch until their failure on September 22, 1960, GRAB 1 only returned 22 batches of data, its first delivered on July 5, 1960.^[17] This was due to President Eisenhower's insistence that every GRAB transmission be personally approved by him for fear that the Soviets would discover the satellite's espionage mission.^[11]{{rp|32}} Nevertheless, even this first limited surveillance endeavor yielded valuable information, including the revelation that Soviet air defense activity was more extensive than expected.^[17]

Legacy and status

The SOLRAD/GRAB series flew four more times (once successfully), finishing with the SOLRAD 4B mission launched April 26, 1962. The next three SOLRAD missions flew with GRAB's signals intelligence successor, Poppy, 1963–1965. The final five SOLRAD satellites were stand-alone scientific satellites, three of which were also given NASA Explorer program numbers. These flew from 1965–1976. In all, there were thirteen operational satellites given the SOLRAD designation.^[2] The GRAB program was declassified in 1998.^[17]

See also

• General information on the SOLRAD project
• General information on the GRAB project

References

External links

• [https://www.n2yo.com/satellite/?s=46 SOLRAD 1's current orbital information]

• Assyriska Föreningen
• Assyriska Föreningen in Norrköping
• Assyriska IF Norrköping
• Assyriska KF Norrköping
• Assyro
• Assyro-Babylonian literature
• Assyro-Chaldean Rite
• Assystem
• Asszonyfalva
• Asszonyfalvahavas
• Asszonyvasara
• Asszonyvására
• Assétou Kolga
• Assú
• AS Săgeata Stejaru
• As-Sūdān al-Inglīzī al-Maṣrī
• (AST)
• Asta (air base)
• Asta B 518
• A stabilization fund
• Asta Binnie
• A Stab In The Dark
• A Stab in the Dark
• A Stab in the Dark (disambiguation)
• Asta Birna Gunnarsdottir