Gemini Radio Systems 

Sven Grahn



The Gemini spacecraft was a huge step forward over the Mercury spacecraft. The level of sophistication is well reflected in the unusual reliability of this spacecraft despite its complex mission and rapid launch sequence. However, its radio systems were a compromise between sophistication and speed of development. There was not enough time to develop something like the Unified S-Band system used on Apollo and the Space Shuttle. Geminiís radio systems therefore included separate equipment for each function.

 

The table below is mostly based on (1) but also on information from COSPAR Information Bulletins. The Gemini-6 and Gemini-7 frequencies come from an old issue of the COSPAR bulletin. These frequencies were not mentioned for any other Gemini spacecraft. In (1) there is no command frequency given for the Gemini itself but only for the Augmented Target Docking Adapter. I have assumed that the same frequency band is used for the manned Gemini spacecraft itself.
 

Purpose

Modulation

Power, Tx

Data rate kbps

Frequency

Acquisition aid

CW

200 mW

246.3 MHz

Recovery beacon

50 W pulses

243.0 MHz

UHF Voice Tx/Rx

AM

3 W

296.8 MHz

HF Voice Tx/Rx

AM

5 W

15.016 MHz

Real-time telemetry

PCM (NRZ)-FM

2 W 

51.2

230.4 MHz

Delayed time telemetry

PCM (NRZ)-FM

2 W 

112.6

246.3 MHz

Back-up telemetry

PCM (NRZ)-FM

2 W 

51.2/112.6

259.7 MHz

Gemini-6 TM

PCM (NRZ)-FM

?

?

249.2 MHz

Gemini-7 TM

PCM (NRZ)-FM

?

?

249.3 MHz

Rendezvous radar Tx

Pulse

1150 W

250 Hz PRF

1528 MHz

Rendezvous radar Rx

Pulse

1150 W

250 Hz PRF

1428 MHz

C-band radar Tx

Pulse

0.5 or 1 kW 

-

5765 MHz

C-band radar Rx

Pulse 

5690 MHz

S-band radar Tx 

Pulse 

 

2910 MHz

S-band radar Rx

Pulse 

 

2840 MHz

Digital Command System Rx

PSK

-

 

406-450 MHz

 
The orbital C-band transponder had a 1 kW output, while orbital transponder transmitted at 500 W. The command receiver could handle Real Time Commands (RTC) or Stored Program Commands. The RTCs triggered nine relays.

The Agena target vehicle had two redundant telemetry transmitters operating on 246.3 MHz with a +/-150 kHz deviation and 2 watts RF ouput. The command receiver of the target used a frequency in the 406-450 MHz range. The C-band transponder of the target operated on the same frequencies as the Gemini spacecraft itself.

Telemetry from the ATDA was picked up in Florida on 246.3 MHz by my friend Richard S Flagg when he was a member of the University of Florida Student Tracking Station in Gainesville. (Click here to listen). The Student Tracking Station also managed to receive voice from the Gemini spacecraft. Listen here for a short transmission from Gemini-9 on 296.8 MHz where the range and range-rate to the ATDA target are called out..

 

The first space based radar

The radar receiving antenna system consisted of three dual spiral antennas 16.5 cm in diameter.  The receiving antennas, along with the transmitting antenna, form a square array 0.82 wave lengths apart. There were three receiving antennas: the azimuth antenna, the elevation antenna, and the reference antenna. The elevation and azimuth antennas were rotatable. The transmitting antenna and the reference antenna do not rotate. The spiral antennas were raised a quarter wavelength above the radar faceplate and they had a circumference of 2.4 wavelengths, i.e. 52 cm. The reference antenna was used alternatingly with the azimuth an elevation antennas to determine the bearing to the target. The terms azimuth and elevation may seem confusing, but they are used in the Gemini Familiarization Manual. When the spacecraft was oriented along the local vertical and with its longitudinal axis along the orbital velocity vector they were equal to yaw and pitch, of course.

There is a reason why the transponder transmits a 6 microsecond pulse in reply to a 1 microsecond pulse. The signal on the two horizontal and subsequently the two vertical receiving antennas must be present for an interval long enough to compare their phase relationships.

The method that was used to measure bearing was based on the fact that the transmission lines from the three receiving antennas were wired so that the RF voltage induced in the azimuth and elevation antennas were 180 degrees out of phase with the RF voltage induced in the reference antenna, if the target transponder was on the radar boresight axis. The sum of the compared voltages was zero. If, however, the target was off-boresight, e.g. in azimuth, the path lengths to the reference antenna and to the azimuth antenna will be different. Therefore, the phase difference between the RF voltage induced in the two antennas will not be 180 degrees. As a result, there was not complete cancellation as before. An error  voltage proportional to the displacement from the boresight axis will result. 

The method used to null out the error voltage constituted the interferometer method of angular measurement. The method depended on a peculiarity of a spiral antenna. The spiral antenna shifts the phase of the RF voltage induced in it as it is rotated about its center. Therefore the 180-degree phase difference between the azimuth and reference antennas could be restored by rotating the azimuth antenna. The amount of azimuth antenna rotation required to restore the null is proportional to the target displacement in azimuth. The error voltage was used to drive a motor which rotated the antenna until the error voltage was again zero.

The antennas on the Agena target are both dipole and helices and the generate an antenna pattern almost without nulls. See figure here.
 

Communications antennas

The HF antenna was 3.9 meters long when extended in orbit. The C-band transponder antenna  system for ascent and re-entry used three helical antennas on the forward end of the cabin. A phase shifter was used to make the antenna pattern vary and fill out all voids. The orbital C-band transponder used an annular slot antenna mounted on the equipment section of the adapter. It is flush-mounted with the skin as can be seen in the figure above.

Detail of UHF stub antenna

 




Location and purpose of UHF whip antennas


References

  1. The Project Gemini Familiarization Manual, SEDR 300, August 1966


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