SAMOS ELINT Satellites
(Program 102 / 698BK / 770)

When the SAMOS program was established, there was also a subsystem/component letter F assigned, for ELINT (Electronic Intelligence) payloads. Their planned primary purpose was to detect electromagnetic emissions from Soviet radar systems in a frequency range of about 30 MHz to 40 GHz. The USAF's initial plan envisioned two kinds of ELINT payload: F-1, which was to fly as a secondary payload on SAMOS photoreconnaissance missions, and F-2, which was to be the only payload of its Agena control module. The SAMOS F-type systems were also referred to as "Ferrets", a common name at the time for airborne ELINT systems. This article covers all Agena-based ELINT satellite systems directly related to the SAMOS F program.

SAMOS F-1/F-2/F-3

F-1 was an experimental payload to verify the basic concept of satellite-based radar ELINT. It used vacuum tube technology, and could detect radar emissions with frequencies in the S- and X-band. It recorded the pulse width, pulse repetition interval and frequency of each signal it found, and transmitted these data as a 10 kilobit/sec digital stream to the ground tracking station (either in real-time, or after storing the data on a magnetic tape). The first three F-1 systems were launched as secondary payload on SAMOS E-1/E-2 missions, but only one of them actually reached orbit, on flight SAMOS 2 on 31 January 1961. Even though the F-1 didn't last long before the power supply failed, it could record the emission of an American tracking radar and validated the basic viability of the concept.

By the time of the first orbital F-1 mission, the scope of the follow-up program had been expanded. The F-2 payload was to transmit digital data like the F-1, but scan three frequency bands instead of F-1's two. The F-3 system was to provide wide-band (6 MHz) analog output, to get more fine-grained characteristics of radars detected by the F-2. While F-1 used vacuum-tube electronics, the successors would use solid-state technology, resulting in significant weight savings. Therefore the Atlas first-stage of the photo-reconnaissance SAMOS program could be replaced by Thor-based rockets.

In 1962, when a directive was issued that all military-related space programs should use numerical designators instead of names, the SAMOS ELINT program was designated as Program 102. By that time, the generic F-2 and F-3 payload designations had also been dropped, and replaced by more specific ones, describing the frequency ranges covered by the payload, and whether it returned a digital data stream or wide-band analog data. The two-band F-1 system was relabeled Group 0, while three different three-band F-2/F-3 configurations became Group 1, 2 and 3. Digital and analog data were designated with -D and -A suffixes, respectively. Therefore the full new designation of the F-1 payload became Group 0-D. Groups 1, 2 and 3 together covered frequencies between 59 MHz and 12.5 GHz. As for all reconnaissance-related air and space programs, the newly-formed National Reconnaissance Office (NRO) took over the management of Program 102. The payloads of the program received individual Mission numbers in a 71xx-series, starting with 7151.

The first actual launch of Program 102, in February 1962, used an existing F-1 payload, which had been placed in storage after the SAMOS E-1/E-2 program had been cancelled. This Group 0-D system operated for 6 days until the batteries ran out. It was the first satellite ELINT mission to return data of Soviet radar systems. The next flight, launched on 18 June 1962, carried a Group 2-D payload. It failed after less than two days, when an erroneous command sequence broke the tape recorder, ending the mission. From the this flight onwards, all SAMOS ELINT satellites orbited at altitudes of approximately 500 km.

Before the next mission, the program designator was changed to Program 698BK. The third flight was successful for two days, when a battery failure ended the mission. The fourth flight, Mission 7154, was the first one to use a Thrust-Augmented Thor (TAT) first stage, and was fully successful with a new record duration for the program of 10 days. Flight #5, Mission 7156, was the first one to carry an "analog" payload, and also the first one to feature an Agena with solar arrays for significantly longer mission durations. Until February 1966, a total of nine of 9 SAMOS ELINT missions had been launched, of which the final one reached a duration of 210 days. In 1965, the numerical designator was changed to Program 770. Program 770 covered not only the ELINT systems of SAMOS F lineage, but also the NRO/Navy POPPY family of small ELINT satellites.

SAMOS F Launch List

• No.: Sequential flight number for the SAMOS-based satellites of Programs 102, 698BK and 770.
• Name: Unclassified name of the satellite. The OPS number ("OPS" standing for "Operations") is a random number given to all military-related satellite launches between 1963 and 1983 (when "OPS" was replaced by a sequential "USA" number). Before the introduction of the OPS numbering system, the only unclassified reference to the satellite was an FTV (Flight Test Vehicle) number. For SAMOS-related satellites, the FTV number was identical to the Agena serial number.
• Mission: Each mission had a unique NRO Mission number in the 71xx range, starting from 7151. One mission was cancelled.
• COSPAR ID: International designation of the satellite

Program 102/698BK/770 launches, SAMOS F-1/F-2

MULTIGROUP

As early as mid-1962, it had become clear that the separate frequency coverage by Group 1/2/3 payloads had some significant drawbacks. The frequency bands had to be preselected many months before the actual mission, because designing, setting up and testing the antenna configuration was a complicated and time-consuming process. Because of this, the so-called MULTIGROUP payload was designed. It covered 8 frequency bands, each with its dedicated antenna, and 4 of these bands could be selected for each mission with a supposedly minimal turn-around time. The expected much shorter mission preparation times didn't work out as planned, though, leading to the development of the STRAWMAN payload system.

Program 770 flew three MULTIGROUP missions between December 1966 and January 1968. Improved solar arrays and batteries enabled still longer mission durations, reaching almost 15 months for Mission 7163. All MULTIGROUP satellites carried one or more secondary SIGINT payloads.

Image: NSA

MULTIGROUP (OPS 1879)

Program 770 "MULTIGROUP" Launch List

• No.: Sequential flight number for the SAMOS-based satellites of Programs 102, 698BK and 770, continued from the preceding "single group" missions.
• Name: Unclassified name of the satellite. The OPS number ("OPS" standing for "Operations") is a random number given to all military-related satellite launches between 1963 and 1983 (when "OPS" was replaced by a sequential "USA" number).
• Mission: Each mission had a unique NRO Mission number in the 71xx range, continuing from the last number of the preceding series.
• COSPAR ID: International designation of the satellite

Program 770 launches, MULTIGROUP series

STRAWMAN

The MULTIGROUP payload was superseded in Program 770 by a system named STRAWMAN. It had new electronics, a solid-state core memory for the digital data, and an improved tape recorder for the wide-band analog data. A new S-band space-to-ground link allowed for a download rate of 128 kbit/sec, and both the digital downlink and the uplink were encrypted for the first time. Four STRAWMAN missions were launched between October 1968 and July 1971, and they had durations between 12 and more than 20 months. The launch vehicle was the more powerful Long-Tank Thrust-Augmented Thor (LTTAT, a.k.a. Thorad) to handle the increased payload weight.

All STRAWMAN missions carried THRESHER and REAPER payloads, which provided digital data about radar sites with a location accuracy of 24 km (15 miles) and 8 km (5 miles), respectively. The first two missions carried a payload named CONVOY, to record detailed data about Soviet ABM (Anti-Ballistic Missile) radars of the SA-5 missile system. The final STRAWMAN payload was named HARVESTER, flew on the fourth mission, and was also designed to intercept SA-5 radar signals. However, an antenna connector of HARVESTER failed, leading to the loss of this part of the overall mission.

Image: NRO

STRAWMAN (OPS 8373)

After STRAWMAN, there were apparently no further dedicated large ELINT satellites launched into low-earth orbits. The operational requirements were presumably covered by smaller subsatellites and secondary payloads.

Program 770 "STRAWMAN" Launch List

• No.: Sequential flight number for the SAMOS-based satellites of Programs 102, 698BK and 770, continued from the preceding MULTIGROUP missions.
• Name: Unclassified name of the satellite. The OPS number ("OPS" standing for "Operations") is a random number given to all military-related satellite launches between 1963 and 1983 (when "OPS" was replaced by a sequential "USA" number).
• Mission: Separate NRO Mission numbers were assigned to each ELINT payload of a STRAWMAN satellite. THRESHER was apparently considered the main payload, as it continued the 71xx sequence of the SAMOS ELINT series. The other payloads had mission numbers in the 72xx range, continuing a series established by earlier SIGINT subsatellites and payloads.
• COSPAR ID: International designation of the satellite

Program 770 launches, STRAWMAN series

Main Sources

[1] Dwayne A. Day: Ferrets of the high frontier: US Air Force ferret and heavy ferret satellites of the Cold War, The Space Review, April 2009
[2] David D. Bradburn et.al.: The SIGINT Satellite Story, NRO, 1994
[3] Gunter Krebs: Gunter's Space Page (for launch lists)

Back to Directory of U.S. Military Rockets and Missiles, Appendix 3

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Last Updated: 8 July 2025