An analysis of the design of GRAB, the first ELINT satellite

Sven Grahn


Contents


The U.S. has declassified its first space-based ELINT system, GRAB. Details of this system have been published on the web by the Naval Research Laboratory. My intention here is to try to understand better the working principles of this system.

The first GRAB satellite was launched by a Thor Able Star rocket on 22 June 1960 and was a piggyback payload with the Transit-2A navigation satellite. The GRAB satellite (60-007B, Space Command catalog number 0046) was placed in at orbit between 614-1061 km at 66.7 degrees inclination. The satellite weighed 19 kg. Its name in the open domain was Solrad-1 and the telemetry frequency for its science payload was 108 MHz. Later versions of Solrad used the then new 136-138 MHz band for telemetry.  The second GRAB satellite was called Greb-3 and was launched with Transit-4A on 29 June 1961 and the transmission frequency has been given as 136.2 MHz and 136.5 MHz. The GRAB system was later renamed DYNO(2).
 

Soviet radars targeted

The system is described as being a transponder that picked up the radar signals from two Soviet radar systems, Gage and Token, and rebroadcast these directly to the ground. First, let us determine what frequencies the Soviet radars used.

In (1) the Gage system is described as an acquisition  and the trtaadar used with the Yo-Yo missile control radar for the early  surface-to-air missile system SA-1 Guild. The radar operated in the 3 GHz range with a 2 MW power output. The Yo-Yo radar is described as a missile control radar that tracks more than two dozen targets simultaneously with flapping beams used for tracking. Six antennas which rotate cover 70 x 70 degrees. The system also operates in the 3 GHz band (E-band) and also has 2 MW peak power.

The same source describes the Token radar as an early warning radar with twin truncated wire mesh antennae similar to a widely deployed radar code-named Barlock. In the early warning mode of operation the Token radar had a range of 250-300 km with a 1 km accuracy. It operated in the 2.7-3.1 GHz range, had a pulse repetition frequency (PRF) of 375 pulses-per-second (pps). The pulse duration was 1.6-3.1 microseconds. So, interestingly both radars operated near 3 GHz. This means that the GRAB onboard receiver did not have to cover a very broad frequency range, something that simplified the design of the collection antenna.
 

RF bandwidth of the radar transmissions

The pulse width determines the RF bandwidth necessary to pick up the most of the energy spectrum of the radar. The bandwidth is 2/T, where T is the pulse width. So, for T=1.8-3.1 microseconds, we obtain a bandwidth of 650 kHz - 1.1 MHz. (The spectrum of a radar signal is shown here) However a bandwidth of a few kilohertz is sufficient to relay the basic PRF buzz and the rotation scan rate of the antenna. So, the question arises whether or not the radar signals were detected onboard with an AM detector and then only a narrow band version of the baseband signal was allowed to modulate the downlink transmitter.
 

Downlink from the GRAB satellite and its receiving antenna

First, let us look at the downlink from the satellite. The approximate frequency band can be inferred from the picture on the right (from NRL's web site) showing the receiving hut placed in friendly territory just outside the borders of the USSR. Even if the perspective distorts the proportions, it seems that the antennas array could operate somewhere in the 130-200 MHz region.

Interestingly the antennas are linearly polarized. If the satellite transmitting antennas were circularly polarized using a linear antenna on the ground would mean losing 3 dB of signal. If the satellite transmitted linearly polarized signals, the rotation of the polarization of the wave through the faraday effect would have caused regular deep fading of the signal. Most probably the satellite carried a turnstile type antenna system that generated a circularly polarized wave.

Another interesting feature of the receiving antenna is that it was mounted at a fixed elevation and rotated only in azimuth. This is a wise choice if one wishes the system to be simple. Satellites are mostly near the horizon!
 

Receiving and recording equipment - looking inside the hut

The picture of the inside of the hut is very interesting. The first thing one notices is the wheel between the two racks for hand-steering the antenna in azimuth.

The second thing that strikes you at the first glance are the two Collins R-390 shortwave receivers. These are certainly not intended for use beyond about 30 MHz, so to be used with the antenna system on top of the hut a frequency converter is needed.

Secondly, the IF bandwidth of the  R-390 is only some tens of kHz, so this means that, probably, the signal from the satellite was a detected, narrow-band version of the radar pulse train with only a few kHz bandwidth. In this way the signal could easily be recorded as an audio signal on the reel-to-reel tape recorder in the right-hand rack. Possibly the first IF at 455 kHz could be pulled out of the receiver, but its bandwidth can still not be recorded by the tape deck.

The device on top of the left R-390 looks like an old-style pulse counter. It could possibly also be some kind of display of telemetry from the GRAB satellite.
 

Antennas on the spacecraft

The figure below shows where I think the radar recption antennas are located on the GRAB spacecraft.

References

  1. The International Countermeasures Handbook, second edition 1976-77, edited by Harry F Eustace.
  2. E-mail message from space reconnaissance historian Dwayne Day, 18 March 2001.

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