통신위성 정찰위성의 자료를 지상으로 중계한 SDS ( Satellite Data System )

[위종민]

그동안은 정찰위성이 찍은 자료를 위성이 저장했다가 미국이 전세계 곳곳에 중계소를 깔아두고 그 중계소를 지날 때 송신한 줄 알고 있었는데, 위성궤도상에 정찰위성의 데이터를 받아 지상으로 중계해줬던 SDS라는 위성도 있었네요. 애초 위성이 영상을 저장하지 않고, 중계위성을 통해 실시간으로 전송하는 방식이었다고 합니다. 

검은 우주에서 회전하며: 위성 데이터 시스템(SDS)과 실시간 정찰

드웨인 A. 데이(Dwayne A. Day) 2025년 5월 19일 월요일

내년은 지금까지 제작된 가장 은밀한 통신위성 중 하나가 발사된 지 50주년이 되는 해이다. 이 위성은 정찰위성에서 전송된 영상을 지상에서는 탐지할 수 없는 주파수로 수신한 뒤, 워싱턴 D.C. 외곽에 위치한 지상국으로 다시 송신했다. 이 위성 시스템에 대한 많은 세부 정보는 여전히 기밀이지만, 알려진 정보만으로도 그 설계와 개발 방식이 매우 이례적이었음을 보여준다.

지구 저궤도를 도는 ZAMAN 정찰위성의 이미지를 고궤도 중계 위성을 통해 지상으로 전송하는 시스템은 비용이 많이 들고 복잡했지만, 지상으로 직접 송신하는 방식보다 여러 가지 이점을 제공했다.

이 중계 위성은 "위성 데이터 시스템(Satellite Data System)", 약칭 SDS라는 다소 모호한 이름이 붙었고, 독특한 방식으로 개발되었다. 기밀 임무를 띤 "블랙" 페이로드를 탑재했지만, 위성 자체는 비밀이 아닌 "화이트" 상태로 공군 우주미사일시스템국(SAMSO)이 개발과 조달을 맡았다. 이 위성은 위성 정찰이라는 그림자 세계의 경계에 선 존재였고, 기밀과 비기밀 세계의 경계를 넘나들었다.

CIA와 NRO의 승인

1968년, CIA의 레슬리 더크스는 ZAMAN 프로그램을 통해 실시간 영상전송 기술을 개발 중이었으며, 영상 저장 없이 중계 위성을 통한 실시간 전송 방식을 채택하기로 했다. 당시 대부분의 정찰위성은 필름 회수 방식이었기 때문에 사진을 실제로 보는 데까지 며칠에서 몇 주가 걸렸다. 실시간 영상 전송은 새로운 접근 방식이 필요했다.

영상 중계 방식은 완전히 새로운 개념은 아니었고, 1960년대 초부터 미 공군은 SAMOS 위성에 적용을 고려했다. 직접 지상으로 송신하는 경우, 위성이 지상국 상공에 있을 때만 송신이 가능해 하루 수 분에서 많아야 수십 분밖에 전송할 수 없었다. 반면, 고궤도에 중계 위성을 띄우면 지상의 시야를 크게 늘릴 수 있었다.

CIA의 ZAMAN 프로그램(Program B)과 공군의 NRO 소속 사무국(Program A)은 여러 해 동안 갈등을 겪었고, 예산 분리 및 역할 조정을 통해 SDS의 관리 체계가 구체화되었다.

비용 분산과 책임 분산

1969년 8월, NRO 집행위원회는 SDS의 개발을 공군의 SAMSO에 위임했다. 이는 NRO에 소속되지 않은 조직으로서 매우 이례적인 조치였다. 이 결정에는 세 가지 이유가 있었다.

NRO 예산 절감: SDS를 공군 예산으로 추진함으로써 NRO 총 예산이 10억 달러를 넘지 않도록 함. 임무 보안 강화: SDS를 공군 프로그램으로 보이게 하여 소련이 정찰 목적임을 눈치채지 못하게 함. 운영 경험 활용: 위성 통신에 숙련된 공군과 Aerospace Corporation이 이미 중계 시스템을 연구 중이었음.

SDS는 "특별 접근 요구(Special Access Required, SAR)" 프로그램으로 분류되었으며, 이는 DSP(미사일 조기경보) 위성과 군 기상위성 외에는 없던 고급 기밀 등급이었다. NRO의 핵심 통신 페이로드는 BYEMAN 보안 시스템 내 별도 구획으로 숨겨졌고, 관련 정보를 보려면 특별한 승인이 필요했다.

기획과 충돌

SAMSO가 SDS 주관 기관으로 지정되자 내부에서 SDS를 공군 임무에도 활용하려는 목소리가 커졌다. 하지만 CIA 측은 SDS가 단일 목적의 통신 중계용 위성이어야 한다고 주장했다. 결국 1970년, 양측은 **"정보 중계 임무가 최우선이며, 부가 탑재물은 허용"**이라는 타협을 이루었다.

이후 계약 선정 단계에서 휴즈(Hughes)와 추정되는 TRW가 경쟁에 참여했고, TRW는 빠르게 탈락했다. 부가 탑재물로 고려된 DSP 미사일경보 자료 중계는 기술적 부담으로 인해 제외되었고, KENNEN 정찰위성만을 위한 중계 임무로 확정되었다.

60GHz 주파수를 사용하는 통신 페이로드는 대기권을 통과하지 않기 때문에 소련이 위성을 도청하더라도 전파를 감지하지 못하게 설계되었다.

KENNEN의 탄생

1971년, 닉슨 대통령은 ZAMAN 광학시스템 개발을 승인했고, 11월에는 이름이 **KENNEN(KH-11)**으로 바뀌었다. 이에 따라 SDS는 명확한 주요 임무를 부여받았고, KENNEN의 1976년 발사에 앞서 작동해야 했다.

1972년, SAMSO는 휴즈를 정식 계약자로 선정했고, 휴즈의 Intelsat IV 상용 위성 플랫폼을 기반으로 SDS를 설계했다. SDS는 외형상 회전형 위성 구조였고, 상단에는 회전 멈춤형(de-spun) 플랫폼에 안테나들이 장착되었다.

SDS의 운영과 진화

초기 위성들은 SIOP(통합 핵전쟁 계획) 통신 페이로드와 S-band 중계기, 1974년부터는 원자력 감지 시스템까지 탑재되었다. 하지만 부수적 임무보다 KH-11용 통신이 여전히 중심이었다.

처음 4기의 위성(F-1F-4)과 예비기(Y-1 → F-5A)는 19751980년 사이에 제작되었고, 1976년 6월과 8월에 타이탄 III-34B 로켓을 통해 발사되었다. 이후 일부 SDS 위성은 스페이스 셔틀 발사를 위해 개량되었고, Leasat(신콤 IV) 기반의 블록 2 위성들이 1980년대에 발사되었다. 이들은 높이 5.8m, 무게 최대 11톤에 달했다.

SDS의 비밀과 정치적 변화

1977년 CIA 내부 유출로 KH-11 정보가 소련에 넘어갔지만, 1978년 여름까지는 소련도 SDS-정찰위성 간 관계를 이해하지 못했다. 이는 SDS 개발 초기에 예측했던 "상호 연결성의 은폐 효과"가 유효했음을 시사한다.

SDS는 이후 공군 요구가 줄어들며 1981년 NRO의 직접 관리로 이관되었고, 프로그램 A와 B 간의 내부 권력 다툼도 이어졌다. 이후 SDS는 "백색"과 "흑색" 세계를 잇는 브리지 조직으로 유지되며 기술 진화를 수용하는 통로로 기능했다.

결론

SDS는 처음부터 철저히 은폐된 존재였지만, 실시간 위성 정찰 시대의 기반이자, 비밀과 기술 진보의 접점에 선 시스템이었다. 이제 SDS의 50주년이 다가오는 지금, 우리는 하늘 위에 떠 있는 이 수수께끼 같은 스위치보드에 대해 더 많은 것을 알게 될지도 모른다.

The Space Review: Spinning in the black: The Satellite Data System and real-time reconnaissance

The Space Review: Spinning in the black: The Satellite Data System and real-time reconnaissance

Spinning in the black: The Satellite Data System and real-time reconnaissanceby Dwayne A. Day
Monday, May 19, 2025
Next year marks the 50th anniversary of the launch of one of the most secretive communications satellites ever built, a satellite that received images from a reconnaissance satellite transmitted at a frequency that could not be detected from the ground, and then beamed them down to a ground station located outside of Washington, DC. Although many details of the satellite system remain secret to this day, enough is known about it to indicate that it was highly unusual, both in its design and the way it was developed.
This relay satellite, given the obscure name of Satellite Data System, or SDS for short, was developed under a unique management arrangement. Although it carried a highly classified mission payload—“black” in the jargon of the intelligence community—the satellite itself was developed and procured by the unclassified—“white”—Air Force Space and Missile Systems Office, thus straddling the edge of the shadowy world of satellite reconnaissance, with one foot in the light and the other in the shadows. A declassified history by Vance O. Mitchell, “The NRO, the Air Force, and the First Reconnaissance Relay Satellite System, 1969-1983,” describes how this unusual management relationship was developed—and almost fell apart—during its early years.
The CIA and NRO approve data relayIn 1968, CIA official Leslie Dirks, who was then the program manager for the ZAMAN technology program, which had been underway for several years eval‎uating technology for a real-time imaging satellite, decided to rely on relay satellites rather than onboard data storage and transmission to a ground station for a future reconnaissance satellite. Existing reconnaissance satellites returned their images to the ground on film, which meant that it could be days to weeks from when a photo was taken to when it was seen by human eyes, but near-real-time technology was on the horizon, and required new approaches to imagery transmission. By October 1969, Dirks named his assistant division chief as the manager for the relay satellites. The manager’s name is deleted in the declassified history, but he is described as conservative, detail oriented, and very methodical.
The concept of using a satellite to relay imagery from a reconnaissance satellite was not a new one and had existed since at least the early 1960s, when the Air Force considered using a relay satellite for its early SAMOS reconnaissance satellites. Transmitting high-quality imagery to the ground was difficult. Each image would contain a large amount of data, the satellite would not be able to store many of them, and if the satellite could only transmit the images while in view of a ground station, this would dramatically limit how many images could be sent each day because the satellite would only be over a ground station for a short amount of time: maybe only 15 minutes several times a day. But there was a solution: instead of transmitting signals directly down to the ground, the imagery satellite could send them upward, to a communications relay satellite in a much higher orbit, and that satellite could relay the images to the ground. This approach added complexity, but provided numerous advantages, including increasing available transmission time, and making near-real-time imaging possible.
The National Reconnaissance Office (NRO) was responsible for overseeing the development of intelligence satellites. The NRO included an Air Force component located in Los Angeles known as Program A and publicly acknowledged as the Secretary of the Air Force Office of Special Projects, or SAFSP. The NRO also included a CIA component housed in the CIA Deputy Directorate of Science and Technology's Office of Development and Engineering and known as Program B, which was then leading the ZAMAN effort. Program A and Program B had often battled each other within the NRO during its first decade of existence. In 1969, NRO officials began planning for relay satellites, and by June they became a separate line item in the NRO’s budget. The relay satellite program formally began in spring 1970 when a preliminary eval‎uation selected a small number of civilian firms for a year-long system definition phase to begin in July of that year. The plan was to downselect to a single company in October 1971.
Using an additional satellite system in high orbit to relay images from ZAMAN satellites in low Earth orbit would be both expensive and complicated. But it also offered advantages over the direct transmission to ground approach, including longer transmission times. An added advantage of the relay system was that it enabled multiple satellite constellations, not just a single satellite at a time. Another advantage was that the imaging and relay satellites would be very far from each other and the ground station and it would be difficult for the Soviets to determine that the satellites were working together, thus increasing operational security.
Many of the details of both programs remain classified, but while these early decisions about the data relay satellite were being made, ZAMAN was still primarily a technology program, not an approved satellite program. Nevertheless, it was clear to those running the National Reconnaissance Program—the formal term for the collection of top-secret intelligence satellites managed by the NRO along with their budgets—that these new systems were going to be very expensive. That created a dilemma for NRO leadership, who sought to be low-key even among those who had security clearance to know about the NRO.
Spreading the responsibility and the costsOn August 15, 1969, the NRO’s Executive Committee decided to give relay satellite development to the Space and Missile Systems Office (SAMSO). It was part of the Air Force Systems Command and not affiliated with SAFSP—the classified NRO Program A office—in Los Angeles. Unlike Program A, SAMSO was both overt and completely outside of the NRO. Giving a non-intelligence organization responsibility for a new satellite vital to national reconnaissance was extremely unusual. The only analogous situation was the transfer of a weather satellite program developed and operated by the NRO to the Air Force, which occurred around the same time. The NRO’s ExCom also gave the program an overt designation: the “Satellite Data System,” or SDS.
According to Vance Mitchell’s history of the relay satellite program, there were three reasons to give development responsibility for the Satellite Data System to SAMSO:

• Funding it through Air Force channels would hold down the National Reconnaissance Program budget. The ExCom members were concerned about the NRO budget exceeding a billion dollars, believing that this was a threshold above which their program would receive added political scrutiny from the few elected officials who were cleared to know about the NRO. Other NRO programs had already shifted money into the Air Force side to keep the total NRO budget down, and launch costs for NRO satellites were not part of the NRO’s budget. It was not until the early 1970s that the National Reconnaissance Program budget finally crossed the billion-dollar mark.
• Having SDS as an Air Force program would imply to the Soviets that it was not connected to reconnaissance and therefore enhance mission security.
• The Air Force Satellite Control Facility was responsible for communicating with satellites and had acquired considerable experience. In addition, the Aerospace Corporation had been eval‎uating data relay satellites in 1968 into 1969 and had become knowledgeable about the subject. Aerospace worked closely with the Space and Missile Systems Office.

SAMSO designated the SDS program as secret with a “special access required” (SAR) annex. There were only two other SAR programs at the time. One was the Defense Support Program (DSP) missile warning satellite, and the other was the military weather satellite program that had started as an NRO satellite system. SAR allowed the release of selected information about the NRO’s communications relay payload without divulging critical items that might compromise its mission. The CIA’s connection to SDS and details of the NRO communications payload were confined to a special compartment within the NRO’s own BYEMAN security system. Anybody requiring access to this information had to be cleared by the NRO.
Once SAMSO was designated in charge of SDS, it immediately led to questions within SAMSO and the Air Force. Air Force personnel involved in SDS development believed that since the Air Force was providing the personnel, expertise, and offices to run the SDS development, SAMSO was now more than a junior partner in somebody else’s program and should be treated as a full partner. Brigadier General Walter R. Hedrick Jr., Director of Space and Deputy Chief of Research and Development, wanted changes in SDS to make it more responsive to Air Force missions. Hedrick wanted the satellites to serve both Air Force and NRO requirements. He wanted to add secondary payloads to the spacecraft in addition to the communications relay payload.
CIA officials connected to the SDS development believed that the SDS satellites were supposed to have a single NRO communications relay payload and no other missions. They were concerned that the NRO might become a “customer” on its own relay satellite and have the satellite’s covert intelligence mission compromised in the process.
By November 1969 there was pressure to create a management agreement that both sides would accept. CIA officials agreed to allow Air Force secondary payloads on SDS but demanded a guarantee that the intelligence relay mission still had priority. In March 1970, the NRO accepted the management changes demanded by the Air Force while the Air Force guaranteed the NRO communications mission top priority.
Selecting satellites and payloads for SDSThe contract definition phase for SDS began in August 1970, a few months later than planned. Two contractors were involved: Hughes, and one other aerospace firm whose identity was deleted from the official history but was probably TRW. Both companies, like Hughes, were involved in developing communications satellites.
One of the secondary missions initially proposed for SDS was relaying data collected by Air Force DSP missile warning satellites then in development. But in summer 1970 members of the DSP program office—then operating under the deliberately obscure designation of Project 647—began to have reservations about using SDS relays for DSP satellites. Later in the year the Project 647 office withdrew from participation in the SDS in favor of relaying DSP data directly to the ground. That decision required DSP to stick with its own ground stations, including a politically sensitive ground station in Australia. It also meant that SDS again became a single payload satellite.
This change annoyed Grant Hansen, the Assistant Secretary of the Air Force for Research and Development. Hansen wanted dual or multiple users on SDS. In a January 1971 meeting with several reconnaissance officials, he discussed the options. Hansen had justified SDS in front of Congress as having multiple payloads and did not want to go back and explain why that was no longer the case. To force both SAMSO and the NRO to develop other payloads for SDS, Hansen suspended funding to SDS and placed the program on temporary hold.
In August 1970 three orbital configurations for SDS were being eval‎uated. The favorite option for several intelligence officials involved putting the relay satellites in geosynchronous orbit. But this was soon rejected. Although it provided good global coverage, it had a high price tag and an unacceptable level of technical risk. The other two options offered less coverage. One of these involved placing satellites in highly inclined, highly elliptical orbits so that they would swing low and fast over the South Pole and then head high up over the northern hemisphere, putting them in line of sight with both a low-orbiting reconnaissance satellite over the Soviet Union and a ground station in the United States.
By early March 1971 Air Force and intelligence officials had identified at least six possible secondary payloads and two were considered most feasible. One of these was relatively minor: a small S-band transponder on each satellite could relay communications between the headquarters of the Air Force Satellite Control and a remote tracking station at Thule, Greenland, ending reliance upon balky land lines.
Another communications payload would support the Single Integrated Operational Plan, the Air Force’s nuclear war-fighting strategy. SIOP required communications with Strategic Air Command B-52 Stratofortress bombers and KC-135 Stratotankers. The SIOP at the time depended on ground-based high frequency broadcasts, which were vulnerable to jamming and nuclear disruption. An SDS payload in Earth orbit would be less vulnerable and could provide coverage in northern regions that were hard to cover. But according to Mitchell’s SDS history, the SIOP payload was regarded as a “heavy mother” requiring a helix antenna, transmitters, receivers, additional solar cells and cabling and structures weighing more than 136 kilograms (300 pounds). In late May 1971, the two contractor teams determined that the SIOP payload was not a good candidate and the Air Force ruled it out for SDS.
Grant Hansen was apparently displeased that once again SDS was being reduced to a satellite system with a very limited mission. A review board including representatives from Hansen’s office slashed SDS funding for Fiscal Year 1972 to force program managers to go back and find another payload for the satellites.
General Sam Phillips, who was then in charge of SAMSO but had previously played a major role in running NASA’s Apollo program, protested the funding cut. The relay program was reduced to minimum effort until they could reach an acceptable agreement, or the relay program was taken away from SAMSO and transferred back to the NRO.
Although the specific details are deleted from Mitchell’s history, Mitchell indicates that the SIOP communications payload was eventually incorporated into the SDS satellite design despite its substantial mass and power requirements.
Secrecy means bureaucracyBy spring 1971 there was increasing USAF opposition to the special access requirements (SAR) in place for SDS and the two other space programs because of the difficulties they created for management and operations. Although at least one of the SARs was eliminated around this time, Deputy Director of the NRO Robert Naka wanted to keep the Satellite Data System’s SAR in place because he believed it enabled the transmittal of important information about the relay capability without clearing personnel into the more restricted BYEMAN security system. Finally, in January 1972 Director of the NRO John McLucas removed the SAR from the SDS program and withdrew all relevant material into the NRO’s own BYEMAN security compartment.
General Phillips and one other officer did not think that an entirely covert SDS program was necessary, but they believed that SDS security should be tightened. They and NRO officials agreed that the NRO’s BYEMAN security compartment would be used to protect details on the satellite’s bandwidth, near-real-time operations, transmission, specific frequencies, and the NRO relationship. Documents about the program would be classified at the secret level and would only refer to the secondary payloads. They would also state that SDS satellites were deliberately “over-engineered” in case the Air Force wanted to add more payloads, thus explaining why such a large satellite had a relatively limited communications payload. Previously the NRO payload had been referred to as “User A” but documents would now indicate that User A had been deleted.
The birth of KENNENIn September 1971, President Richard Nixon formally approved development of the ZAMAN electro-optical imaging system. By November its name was changed to KENNEN, although it would become better known to the public by the designation of its camera system, KH-11 (see “Intersections in real time: the decision to build the KH-11 KENNEN reconnaissance satellite (part 1),” The Space Review, September 9, 2019, and part 2.) With the imaging satellite development now underway, the Satellite Data System finally had a confirmed primary mission and a deadline requiring that it become operational before the first KENNEN satellite was launched. KENNEN was initially scheduled for an early 1976 launch, although this eventually slipped to late in the year.
The communications relay payload that was developed for the KENNEN used a 60-gigahertz frequency that did not penetrate the Earth’s atmosphere. This meant that if the Soviets listened in on the KENNEN they would detect no emissions coming from it, creating the impression that it was passive even while it was sending signals up to the SDS.
At an April 20, 1972, meeting of the NRO’s Executive Committee (ExCom), NRO Director John McLucas was satisfied with existing management arrangements for SDS. SAMSO would continue management, the NRO’s Program B—led by the CIA—would exercise technical oversight, and the Air Force would fund and publicly defend the program to Congress. The NRO officials also established a more streamlined chain of command from SAMSO to the Secretary of Defense level.
The NRO director also moved SDS’s BYEMAN security responsibilities from the CIA-led Program B to the Air Force-led Program A (SAFSP), which strengthened the appearance of a strictly Air Force project and enhanced Air Force authority over the program. He also ordered that there be no further mention of the other payload outside the classified BYEMAN security channel, which meant that only people with BYEMAN clearances could speak or know about SDS’s communications relay payload. Information prohibited from public release included the number of satellites, orbits, technical descriptions, launch dates, finances, and mention of ground facilities. At some point, possibly even early during the 1970s, the SDS program received the classified code name QUASAR. That name was reportedly still being used into the 21st century.
On June 5, 1972, SAMSO selected Hughes to build the satellites. According to a 2011 interview with former CIA and Hughes official Albert “Bud” Wheelon, the winning Hughes design was based upon the company’s proven Intelsat IV spin-stabilized satellite, which weighed more than 700 kilograms. The first Intelsat IV had been successfully launched into geosynchronous orbit in January 1971. Although both Intelsat IV and SDS were spinning drums covered with solar cells, SDS had a different set of antennas mounted to a de-spun platform at its top.
Hughes engineer Anthony Iorillo was one of the people assigned to the SDS program. Hughes had a problem getting sufficient numbers of its own personnel security clearances, so Air Force officers at the captain and major level with the required security clearances were detailed to work at Hughes.
Changes in payloads and operationsThe SDS had both the S-band transponder and the SIOP communications payload, but the satellite’s primary payload was always the communications relay for the KENNEN reconnaissance satellites. In August 1974, the Secretary of the Air Force approved adding a third secondary payload to the satellites, the Atomic Energy Detection System. This was introduced starting with the third satellite. Similar nuclear detection payloads—also known as “bhangmeters”—were already carried on Defense Support Program satellites. They could detect nuclear detonations in the atmosphere and space.
According to declassified Air Force documents, the Air Force started procurement with a structural test model designated X-1, followed by a qualification model designated Y-1 and equipped with most of the electronic systems to demonstrate that the satellite could perform the functions it was designed for. The initial plan was to procure four flight spacecraft (designated F-1 to F-4) and refurbish Y-1 to be a flight spare.
By the first half of 1975, testing of X-1 was completed, assembly of Y-1 was completed and it was undergoing initial testing, and fabrication of F-1 was well underway. By November 1975, the Air Force approved procurement of two additional satellites, F-5 and F-6, which were supposed to be compatible with the Space Shuttle.
The first two SDS satellites were launched into orbit atop Titan III-34B rockets in June and August 1976. The first KENNEN was launched in December that same year. Although the satellites all worked, according to several sources there were early operational problems with getting them to work together.
In 1977 a CIA employee sold a copy of the KH-11 user’s manual to the Soviet Union, giving away many of the secrets of the KENNEN satellite. However, Mitchell’s history hints that the Soviet Union did not understand the connection between the KENNEN and SDS satellites until the summer of 1978, confirm‎ing a claim that program planners had made about SDS early on, that it would be difficult for the Soviets to figure out that the satellites in highly different orbits were part of the same mission, especially since the KENNEN did not appear to be transmitting while over Soviet territory.
The fourth and fifth SDS satellites were delivered in May and October 1980, and Y-1 was refurbished, redesignated F-5A, and delivered in May 1980. In 1981 the Air Force proposed purchasing satellite F-7. It is unclear how many of these satellites were eventually launched, and one or more may have been retired to a classified storage facility at the end of the program.
Eventually, the first series of satellites was replaced by an updated version designed to be compatible with the shuttle from the start. The second block of SDS satellites was based on Hughes’ Leasat (Syncom IV) design, which was a squat cylinder made to take advantage of the shuttle’s wide payload bay. Five Leasats were built and launched from the shuttle, although the antenna configuration would have been significantly different for an SDS version. It appears that four of these block two SDS satellites were launched. They had the same 4.3-meter diameter as the Leasats, but whereas the Leasats were 4.3 meters tall, the block two SDS satellites had a bigger “antenna farm” on top and were 5.8 meters tall. The Leasats had a deployed mass of 6,894 kilograms, but the block two SDS satellites were anywhere from 2,268 to 4,536 kilograms heavier, depending on the source.
In the late 1990s, the NRO surprisingly unveiled a new communications relay satellite that appeared to be based upon Hughes’ Intelsat VI design that may have been a block three version when later satellites launched on Atlas II rockets. In early 2017, NASA revealed that it had been offered a spare satellite from an unnamed government agency. That satellite was clearly the one seen in the late 1990s. (See “Spinning out of the shadows,” The Space Review, March 13, 2017.) It is likely that any block four SDS satellites were three-axis stabilized and based upon a commercial comsat bus like the earlier satellites.
The more things change…In October 1976, the Air Force announced long-range plans that did not include SIOP payloads on future SDS satellites. Instead, the SIOP payloads would be mounted on the planned Air Force Milstar communications satellites. Milstar was a highly ambitious and complex communications satellite system that would support multiple Air Force requirements. When first conceived, the Air Force planned to have Milstar satellites in geosynchronous orbit as well as a constellation of satellites in medium-altitude polar orbits. The satellites in their different orbits would be able to communicate with each other, creating a complex interlocking communications network around the Earth. They were also supposed to be protected against enemy jamming and hardened to survive the effects of nuclear weapons. If it worked as planned, Milstar would provide a tremendous leap in communications capability for multiple Air Force and other users.
The CIA’s Leslie Dirks asked members of his staff to eval‎uate including the SDS relay capability on the Air Force’s Milstar. The initial concept was for three Milstar satellites in polar orbits to perform the relay capability for future KENNEN satellites. But CIA officials quickly grew skeptical about this proposal. Milstar was going to be very complex and face technical risks and problems in development resulting in delays that could affect the KENNEN relay mission. In addition, the NRO’s communications relay payloads would then become secondary payloads for satellites that had many other Air Force missions. CIA officials questioned what would happen if one of the NRO’s payloads failed on a Milstar satellite: would the Air Force launch an expensive replacement satellite simply to fulfill the NRO’s requirement? Two of Dirks’ aides recommended against putting the KENNEN communications relay payload on Milstar and Dirks agreed.
Dirks’ decision proved to be a good one. The early Air Force plan was for Milstar to begin operations in 1982, but Milstar soon ran into major development problems. Ultimately, the first Milstar did not launch into space until 1994. The Air Force had to postpone plans to transfer the SIOP communications payload from SDS to Milstar, and SDS continued carrying SIOP payloads into the 1990s.
Shuffling responsibilitiesDirks’ decision to not transfer the KENNEN communications relay payload to Milstar meant that the SDS program would have to continue, and since the Air Force no longer had a requirement for SDS, the program would have to be transferred to the National Reconnaissance Office, with NRO funding and BYEMAN security measures. In November 1981, NRO Director Pete Aldridge approved the transfer of responsibility for SDS.
Aldridge’s decision created controversy. Brigadier General Jack Kulpa, who headed SAFSP and was therefore the NRO’s Program A director, lobbied to transfer the SDS from SAMSO into Program A, arguing that this would provide continuity and Program A had sufficient experience to run the program, although KENNEN was run by the CIA’s Program B and there was still an ongoing rivalry between personnel in Programs A and B. Yet another suggestion was to create an NRO Program D office solely to manage the relay satellite program.
SDS Director Colonel Clyde McGill and his supervisor, SAMSO commander Lieutenant General Richard Henry, lobbied to leave SAMSO responsible for SDS. They argued that withdrawing SDS into the National Reconnaissance Program benefitted nobody. Both the NRO and the Air Force needed SDS to serve as a “bridge organization” that could work in both the white and black worlds and provide access to evolving technologies for both sides. Although Mitchell’s history of SDS is unclear on this point, apparently Henry and McGill were successful at convincing Aldridge to maintain SDS as a SAMSO-led program, at least for a few more years.
Now that the 50th anniversary of the dawn of the SDS is near, perhaps we will soon learn more about this enigmatic switchboard in the sky.

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Dwayne Day can be reached at zirconic1@cox.net.
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[정명기]

미국처럼 전 세계를 상대로 통신위성을 깔아두었다면 그냥 통신위성으로 자료를 보낸 뒤 통신위성에서 지상으로 암호화해서 보내주면 되는 것 아닐까 싶네요.
우리나라라면 북한 상공에서는 우리나라 소유의 통신위성과 교신이 가능할테니 대북자료는 실시간 수집이 가능할지도요?

[위종민]

북한 상대로는 정찰위성에서 바로 자료를 쏴 줄 수도 있을 것 같은데, 중국이나 일본, 미국, 러시아 같은 곳의 자료를 수집해야 한다면 별도의 통신중계위성을 운용해야 할 필요가 있긴 하겠네요.

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즉응성이 필요 없고 기밀성이 필요 없는 경우에는 세계 10여곳의 중계소와 계약해서 지상으로 자료를 주고 받고 하는 것 같습니다. 예전에는 무궁화 위성의 통신 채널 중 일부를 군용으로 썼는데, 이제는 군 전용 통신위성도 있군요.

무궁화 위성을 기준으로 하면, 남한의 중심부인 금산읍을 초점으로 하고 있고, 반경 약 500+km 정도의 원이 통신 가능 범위라고 나옵니다. 이정도면 신의주-김책시를 연결하는 정도까지가 통신 가능 범위로 보입니다. 중국은 산둥반도의 끝자락, 일본은 후쿠오카-히로시마 선 정도까지가 통신 가능 범위일 듯 합니다.

군 전용 통신위성이라면 전시에는 북한 전역을 통신가능하게 초점을 조정할 수 있겠지만, 통신위성 전부를 군용으로 돌리는게 아닌 이상 중국, 일본, 러시아 등의 영역은 쉽지 않겠네요.