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
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.
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.
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!
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.