BACKGROUND
In early 1963 as a young 31-year-old engineer I was assigned the task of developing a method for mobile users to communicate through a synchronous satellite. A brief study of the problem indicated that for truly mobile communications the user should be able to use a simple dipole antenna. This led to a company funded effort to demonstrate the reception of the one-half watt Syncom 2 VHF telemetry signal on a simple dipole antenna. On 21 February, the one-half watt Syncom2 VHF telemetry signal was successfully received on a dipole antenna. Three months later, on 8 May 1964, a teletype message was repeated through Syncom 2 telemetry and command system to a ground transmitter with a power of 19 watts using 12 and 14 db. gain Yagi antennas.
When news of these tests reached Frank White of the Air Transport Association he set in motion a series of events that led NASA to fund the ATS VHF Experiment. His plan was to demonstrate two-way communication between a Pan-Am jet leaving Hong Kong with the NASA ground station at Camp Roberts California via the Syncom telemetry and command system. On Jan 27 1964, these tests were successful and within weeks NASA funded the ATS VHF experiment.
The ATS-1 is the first of a series of five spacecraft built for NASA Goddard Space Flight Center by the Hughes Aircraft Company. The objectives of the VHF repeater are as follows:
• Demonstrate feasibility of providing continuous voice communications link between a ground control station and aircraft anywhere within the area covered by the satellite
• Demonstrate feasibility of providing a network in which data from small- unmanned stations or buoys are collected via satellite and disseminated
• Evaluate feasibility of VHF navigational systems
• Evaluate airborne and ground stations required in the above mentioned networks
A fifth objective area was later added to demonstrate two-way voice and teletype communications from ships at sea anywhere in the satellite coverage area.
VHF REPEATER GENERAL DESCRIPTION
The VHF communications experiment is a frequency translation limiting (Class C) repeater receiving at a frequency of 149 Mhz and transmitting at 135 Mhz. The repeater both receives and transmits through an eight-element, phased-array antenna; Table 1 presents the repeater characteristics.
Operation of the repeater is as follows: incoming-signals at 149 mHz arereceived on each dipole element, routed through diplexers, amplified by a low-noise receiver, and shifted in phase to compensate for the relative position of each dipole antenna. The electronically controlled phase shifter in the receiver unit, driven by the waveform generators, causes the output s of each receiver to be in phase only for those signals originating from the earth. Reference sinusoids used to drive the waveform generator s are obtained from the same phased-array control electronics used to position the microwave beam toward the earth. The eight receiver outputs are summed together, filtered, down converted to an intermediate frequency (IF) of 29 megacycles, amplified, and passed through a crystal filter to limit the receiver bandwidth.
The IF is then amplified, up converted to 135 mHz, further amplified, and divided into eight equal parts. Each of the eight signals is routed to a transmitter where it is amplified, phase-shifted, and further amplified to a power level of 5 watts. Each transmitter output is routed through its respective diplexer to one of the antenna elements.
The transmitter phase shift is controlled by the waveform generator, which causes the signals from each antenna to reinforce in the direction of the earth. Provision is made to operate only odd or even sets of four transmitters, if desired, to reduce the DC power required.
It is also possible to drive the waveform generator from either the redundant Phased Array Control Electronics or to shut down this unit entirely creating a pancake antenna pattern, approximately 60 x 360 degrees that will encompass the earth during most parts of the launch trajectory and at all times after satellite reorientation.
The ATS spacecraft power supply and thermal design allow for continuous operation of the VHF experiment except during periods of eclipse. The repeater elements are supplied with -24volt and 23.4-volt regulated power. Switches are provided that allow operation of the equipment according to commands from the ground stations. Telemetered outputs are also provided which can be transmitted to earth either by the VHF or microwave telemetry systems.
The repeater is made up of nine subassemblies: eight units containing one transmitter, receiver, and diplexer; and one unit containing an up converter, down converter, waveform generator, and two voltage regulators. These units are shown in Figures 3 and 4.
TECHNICAL ADVANCES MADE BY ATS-1
1) The first VHF phased array in orbit
2) The first phased array to operate on both transmit and receive frequencies
3) The first deployable antenna on a spinning spacecraft
4) The first spacecraft repeater to use separate receivers and transmitters for each antenna element
The VHF antenna consists of eight full-wave dipoles arranged in a circle of one wavelength diameter (86 inches). Volume limitations in the Atlas-Agene shroud dictated the use of a deployable antenna. The eight-dipole elements were mounted on a radial arm attached to the forward solar panel and pivoted to place the antennas in a three-foot circle directly over the apogee motor nozzle during launch.
At separation from the Agena, the spacecraft is spun up; centrifugal force causes the antennas to deploy to the 86-inch diameter at 50 rpm. Deployment takes place during the normal spin up of the spacecraft without the use of ground commands. When the spacecraft apogee motor is fired, the antennas are subjected to high Mach numbers and high heat fluxes. The rocket motor in the center of the array contains 750 pounds of propellant and burns for 40 seconds increasing the spacecraft’s apogee velocity by 6000 fps.
To withstand the severe thermal environment, the antenna elements were constructed of beryllium, flame-sprayed with aluminum oxide, and further covered with a Teflon ablative material. The high heat capacity of the ablative material and of the beryllium itself maintains the antenna temperatures below the unprotected equilibrium temperature of 3000° F.
IN-ORBIT PERFORMANCE
The following report is taken from a presentation given in May 1967 at a meeting of the RTCM in Las Vegas Nevada by Roland Boucher.
ATS-1 was launched into orbit on 6 December 1966. The VHF repeater was first operated 3 days later. Since then, it has been used to successfully communicate voice and data between NASA ground stations at Rosman, North Carolina; Mojave, California; and Kooby Creek Australia. It has sent weather facsimile pictures and has been used to determine propagation properties of the ionosphere.
Simplex air to ground communication tests have been conducted with aircraft operated by Pan American, Eastern, TWA, United, American, and Qantas airlines as well as those operated by the FAA and the U.S. Air Force. Both simplex and duplex communications were successful with a shipboard terminal constructed by Hughes. This terminal was leased to the U. S. Coast Guard and has operated successfully on the Coast Guard Cutter, Klamath at Ocean Station November in the Pacific Ocean which was described by a member of the U.S. Coast Guard. In May 1967, the VHF repeater experiment had operated successfully for over 5 months with no signs of degradation.
Prior to the launch of ATS-1, there was considerable skepticism as to the feasibility of VHF satellite communications in mobile service despite the fact that nearly every satellite to date had used the VHF band for its primary mode of telemetry and weather photo video transmission.
SCINTILLATION FADES
The uneven diffractive properties of a disturbed ionosphere can cause deep fades at VHF frequencies. These fades are normally of a very brief nature lasting typically from 8 to 30 seconds. Examination of this phenomenon by Hughes Aircraft Company under NASA contract NAS-510 174 indicated that fades of greater than 6 db. depth could be expected 0.002 percent of the time in the mid-Pacific area, Tests with ATS – 1 during the first 5 months in orbit have failed to yield any statistically significant data on scintillation fades. The rarity of their occurrence makes it almost impossible to detect them in a normal push-to-talk circuit. They do not present a serious problem to this type of communications.
MOBILE TERMINAL NOISE
Mobile terminal noise was also cited by some as a nearly insurmountable problem as late as a few months before the ATS-1 launch. Tests on aircraft and on the cutter Klamath have shown this problem can be cleared up by normal RFI practices.
SEA WATER MULTIPATH
Multi-path fades were held up as an obstacle to VHF mobile communications with fades up to 30 db predicted. Tests with the ATS-1 to date have shown multi-path propagation not to be a serious threat. In examining the records of many hours of shipboard and aircraft communication, no clear evidence of multi-path propagation fades could be found. This despite the fact that Hughes intended to use the evidence of such fades as a requirement for the development of a new type antenna for the NASA/Hughes ATS C. The fades were not found. The antenna was not funded.
EARTH-NOISE TEMPERATURE
Earth-noise temperature was cited as a possible deterrent to VHF satellite communications. The proponents of this concept reasoned that many spurious emissions from the large number of earth transmitters would form a noise blanket which would jam the satellites receivers.
Measurements taken in late 1966 by Boeing Aircraft to determine receiver noise temperatures in flight from a commercial aircraft indicated that cities could be found quite easily by the noise they created. This noise was seldom evident more than 10 or 20 miles from the city centers. Noise temperatures even at relatively low altitudes seldom were in excess of 20 db. with a 10-db-background level being more nearly an average figure.
A simple calculation involving the area of the world covered by cities indicated that this noise would not be a serious problem at synchronous altitude. Corroborating evidence was the fact that most satellite command systems operate in the VHF band. Any serious problems in the uplink would certainly have been discovered before 1966.
The launch of ATS-1 proved this point. Up-link receiver sensitivity of the ATS-1 spacecraft is essentially that measured in the laboratories.
UNANTICIPATED PROBLEMS
The ATS uplink is in the land mobile band. In the early phases of the in-orbit test program, strong signals were heard in the satellite s passband. Many of these were conversations in English from what appeared to be military personnel. The conversations contained description of maintenance operations on jet aircraft indicating that a ground terminal used to communicate with mobile airport vehicles was transmitting to the spacecraft. Other signals in the spacecraft ‘s pass band that have been annoying at times have contained considerable 60 cycle modulation, indicating they originate from an earth borne transmitter.
None of these emissions proved detrimental to the test program after December 12. On that day, the severity of the jamming indicated that up link ERPs in excess of 2 kilowatts were present
A number of interested parties, through the cooperation of NASA and Aeronautical Radio listened for a one-week period in an effort to determine the origin of these strong signals. They were not present during this one-week period and have not returned. In the first 5 months as equipment and operating procedures have improved both in the aircraft and shipborne tests, this problem, which seemed so serious on 12 December, has been nearly forgotten. Today VHF communications via satellite have been shown to be feasible for aircraft and maritime mobile application.
THE ATS-3 VHF EXPERIMENT
Spacecraft operating at microwave frequencies operate their transmitters well below peak power (transmitter Back-Off). The VHF Experiment on ATS-3 Replaced the Class C RF amplifiers used on ATS-1 with linear RF amplifiers. This was important because it greatly reduced the inter-modulation distortion inherent in multi-channel transmitters. These transmitters were solid state and used a class A/B final stage; The DC power required was reduced 1/2 db. for every 1 db. of back off. This was a very important discovery since power is a very expensive commodity on any Spacecraft.
At low elevation angles multipath can cause a significant loss in signal for short periods of time as the reflected signal alternately cancels and adds to the direct signal. Circular polarization can eliminate this problem when used by receiver and transmitter that was later verified in field tests with TACSAT in 1969.
Hughes designed and tested circular polarized replacements for the dipole antenna elements on ATS-3. Unfortunately, NASA did not approve their use. Meanwhile Boeing designed a circular polarized flush mounted VHF antenna for the 747 aircraft. C.A. Petry at ARINC worked with the airlines and FAA to produce a spacecraft compatible aircraft radio set in ARINC Specification 546.
When the first Boeing 747 was delivered to Pan Am, it was equipped with and ARINC 546 communication transceiver and a circular polarized antenna. This aircraft was equipped for satellite to aircraft communications.
ATS-3 was launched successfully on November 5, 1967, and positioned over the Pacific Ocean. Together with ATS-1 nearly global communications were possible at VHF frequencies.
Hughes designed a small inexpensive VHF terminal for the US Coast Guard that was installed on the USS Glacier, the ship used to resupply the Antarctic Base. Sun spot activity was heavy during the 1967-68 winter, and HF radio was unusable for long periods of time. The $4000 Hughes satellite terminal got through every time.
In the fall of 1969 I was selected as a representative of the State Department to the CCIR Conference on satellite communications. Captain Charles Dorian and I were able to persuade the Russian delegate to support the US position to authorize VHF aircraft communications by satellite. France led the opposition. The Russians brought along the eastern bloc, even Havana supported us. The French opposition was defeated – WE WON
Unfortunately, France played politics better than we did. As I understand it, they got NASA to oppose Aerosat in exchange for France support of the Space Shuttle. In any case, I received a phone call in Geneva from Hughes saying NASA pulled the plug – ITS ALL OVER. I had spent nearly almost 10 years in the pursuit of a VHF Aeronautical Satellite to no avail.
At least the military did not have to play these politics. Both Russia and the US adopted VHF communications (TACSAT). The Syncom and ATS experiments produced at least two winners.
Completely independent of my employment with Hughes, I had developed the concept of an electrical powered battlefield surveillance drone and a Solar Powered high altitude spy plane. Dr. Bob Roney told me Hughes was not interested.
I left Hughes Aircraft in January 1973 and successfully proposed both aircraft to DARPA. The prototype electric powered battlefield drone flew that year and was shown on Los Angeles television. The 32-foot span proof of concept model of the spy plane flew on solar power alone in 1974. A patent for the electric powered aircraft was granted on May 18, 1976.
POSTSCRIPT
On September 29, 1995, Ben McLeod and Bob Bohanon (Both from Pan American) organized a 30th anniversary celebration in Washington DC. Personnel from ATA, ARINC, Bendix, Comsat, FAA, FCC, Hughes, NASA and or course Pan-AM were in attendance. We all were all thrilled that the aging Frank White was able to attend and were sad that other important contributors from Comsat, Collins Radio, and the US Coast Guard were unavailable or deceased.
Our Space Heritage 1960-2000 