The images below (and their sources) show the quad dishes of the Apollo Deep Space High-Gain Antenna and the Russian B529 ground station tracking antenna.

From NASA Technical Note TN D-6723 Apollo experience report: S-band system signal design and analysis which I found here

The high-gain antenna consists of an 11-inch-diagonal wide-beam horn flanked by an array of four 31-inch-diameter parabolic reflectors, as shown in figure 5. Transmitting beam widths of 40.0°, 11.3°", and 4.4° are selectable by manual switch. Reception and transmission gains corresponding to these beam widths are listed in table I. The antenna tracks by using electronic conical scan where the angle-tracking information is encoded as amplitude modulation (AM) on the phase-modulated signal received from earth. This error information is extracted within the USB equipment by a narrowband coherent amplitude detector and routed back to the antenna system, thereby providing angular displacement control.

enter image description here

My understanding of or at least theory of how this actually worked is written in this (now-deleted) comment:

At least the part of the signal from each of the four antennas is separately amplified, detected and changed to a DC signal strength level, and then the ratio of the four signal strengths is used to determine the direction and magnitude of the pointing error and used for steering and tracking.

and it's how I also think the old Soviet quad parabola tracking dishes worked, but I'd like to read further. However I am not sure. Is it possible to understand exactly how this worked?

Question: How were the signals from quad dishes of Apollo Deep Space High-Gain Antenna or Russian B-529 processed for local control of tracking?

enter image description here

Borrowed from Why have four parabolas on a ground-side array instead of just a single large one


2 Answers 2


To question about four antennas of the B-529 system: "Is it possible to understand exactly how this worked?"

In fig. 5 is a diagram of the analog formation of the B-529 antenna system of the total difference signals of linear polarization separately for vertical and horizontal polarization. There are two stages of signal addition, and in-phase signals are added in both the first and second stages. Therefore, the signal-to-noise ratio at each stage is doubled and the resulting signal-to-noise ratio becomes 4.

На рис. 5 представлена схема аналогового формирования антенной системой Б-529 суммарноразностных сигналов линейной поляризации отдельно для вертикальной и горизонтальной поляризации. Здесь имеются два этапа сложения сигналов, причем как на первой, так и на второй ступени складываются синфазные сигналы. Поэтому отношение сигнал/шум на каждой ступени удваивается и результирующее отношение сигнал/шум становится равным 4.


The system is designed according to a two-channel scheme, which provides polarized-spaced reception of horizontal and vertical polarization signals in the meter and decimeter wave ranges, followed by signal addition and automatic generation of a telemetric video signal from the channel with the best signal-to-noise ratio. Система выполнена по двухканальной схеме, обеспечивающей поляризационно-разнесенный прием сигналов горизонтальной и вертикальной поляризации в метровом и дециметровом диапазоне волн с последующим сложением сигналов и автоматической выдачей телеметрического видеосигнала из канала с лучшим отношением сигнал/шум.

The CM-178 antenna system provides reception of signals in the meter and decimeter wave ranges with arbitrary polarization of the electromagnetic wave. Антенная система CM-178 обеспечивает прием сигналов в метровом и дециметровом диапазонах волн при произвольной поляризации электромагнитной волны.

The system consists of four parabolic mirrors 6 meters in diameter with spaced phase centers (Fig. 8). Система состоит из четырех параболических зеркал диаметром 6 метров с разнесенными фазовыми центрами (рис.8). Fig.8 and Fig.9

Mirrors are placed on the CM-175 support-rotary device, which ensures their rotation in the direction of the spacecraft. To determine the direction to the object, the monopulse method is used with the formation of the total and difference signals for the azimuthal and elevation direction finding planes. The advantage of the total-difference system is the independence of the equal-signal direction from the parasitic phase incursion in the channels. Зеркала размещены на опорно-поворотном устройстве CM-175, обеспечивающем их поворот в направлении космического аппарата. Для определения направления на объект применяется моноимпульсный метод с образованием суммарного и разностного сигналов для азимутальной и угломестной плоскостей пеленгации. Достоинством суммарно-разностной системы является независимость равносигнального направления от паразитного набега фаз в каналах.

The effective area of ​​the antenna over the total channel is at least 20sq.meters in meter wave range and 25sq. meters in decimeter wave range. The width of the summary diagram is 2.5° - 3° in decimeter and 7.5° - 12° in two sections of the meter range. The maximum tracking errors at a wind speed of 20m/s do not exceed 30 arc minutes in elevation and azimuth. Used antenna emitters and feeders provide reception of signals in a wide frequency band. The frequency overlap coefficient is 7, which allows you to receive signals of all operating frequencies of the meter and decimeter ranges.

Эффективная площадь антенны по суммарному каналу составляет не менее 20 квадратных метров в метровом и 25 квадратных метров в дециметровом диапазонах волн. Ширина суммарной диаграммы составляет 2,5° - 3° в дециметровом и 7,5° - 12° в двух участках метрового диапазона. Максимальные ошибки сопровождения при скорости ветра 20 м/с не превышают по углу места и азимуту 30 угловых минут. Используемые излучатели антенн и фидеры обеспечивают прием сигналов в широкой полосе частот. Коэффициент перекрытия по частоте равен 7, что позволяет принимать сигналы всех рабочих частот метрового и дециметрового диапазонов.

An approximate view of the total and difference diagrams is shown in Fig. 9. In the direction of the antenna axis (α = β = 0), the difference signal is zero, and the total signal is maximum. When the object deviates from the equal-signal direction, the signal amplitude in the difference channel characterizes the value, and the phase with respect to the phase of the total channel indicates the side of the deviation. The difference channel signal is used in the CM-175 servodrive to rotate the antenna axis along the corresponding angular coordinate to the object.

Примерный вид суммарной и разностной диаграмм приведен на рис.9. На направлении оси антенны (α=β=0) разностный сигнал равен нулю, а суммарный максимален. При отклонении объекта от равносигнального направления амплитуда сигнала в разностном канале характеризует величину, а фаза по отношению к фазе суммарного канала - сторону отклонения. Сигнал разностного канала используется в сервоприводе CM-175 для доворота оси антенны по соответствующей угловой координате на объект.



This phrase right here:

The antenna tracks by using electronic conical scan

Isn't entirely helpful. Electronic conical scan was a thing in radar, a sort of intermediate step between lobe switching (also known as sequential lobing) and monopulse techniques. As far as I can tell, the CSM USB radio used a sort of half-way house that one source referred to as "single channel monopulse". Interestingly, at least one NASA document referred to the CSM antenna tracking system as "sequential lobing" (Lunar Far Side Communication Satellites) but this seems to be a one-off and therefore probably a mistake.

For true monopulse tracking you can have four receivers, the outputs of which are munged together to provide a sum signal and two difference signals, typically one for azimuth (taken from horizontally separated receivers) and one for elevation (taken from vertically separated receivers). This provides you with continuous elevation and azimuth error values. I won't go into much more detail of monopulse tracking systems as there's a lot of stuff freely available out there already which will do a better job than I could. The "single channel monopulse" system switches back and forth between providing azimuth and elevation error signals at some rate. I'm not entire sure what the cycle rate was in the CSM, but I see references to 50Hz.

From Advanced S-Band Transponder Study Program, 1968:

This system has nominally been termed a single channel monopulse tracker, however, it retains the properties of a sequential lobing system because of the sampling technique. In succession, the receiver input ($\Sigma + \Delta$) contains azimuth information and then elevation information in much the same manner that a sequential lobing technique samples information in each quadrant. The only difference is that $\Delta Az$ and $\Delta El$ signals are formed by the antenna sum-and-difference network instead of by processing the pulse amplitude received from each lobe

Single channel monopulse antenna tracking system

The "CAD" is the coherent amplitude detector (which I believe is the same thing as a synchronous envelope detector, if you were interested), and "AGC" would be automatic gain control, a signal derived from some other CAD that's not shown in this particular block diagram. The error values would have been fed back into a servo control system to direct the antenna.

The USB ground stations, like the Russian antenna described in A. Rumlin's answer, used true monopulse techniques to help point their antennae, with a cluster of four horns driving a single big parabolic antenna. The S-Band study program above suggested that the CSM system be upgraded to a monopulse tracking system, using the existing wideband phase-difference detector (WBD):

Recommended monopulse antenna track system

The antenna was also apparently pointed at Earth using an IR sensor, though I only found one reference to that: Proceedings of the Apollo Unified S-band technical conference under "CSM Antenna Characteristics".

Also of possible interest is the fact that the high-gain antenna is in fact two high gain antennae: the 11-inch wide-angle horn in the middle, with the widest beam-angle and lowest gain, which would presumably have been used closest to Earth. That bit is itself composed of four horns, and so could probably be switched into the exact same tracking hardware that the parabolic antenna array would have used.

  • $\begingroup$ Oh this is really interesting, thanks! I'm going to have to dig in and think about this. I'm stuck at a very basic point though. Using some form of traditional conical scanning, I can determine the angular distance that a signal's source is from the boresite (or nominal) direction that my quad array is pointing because I have each of the four pointing in a slightly different direction. It could be a one-way measurement; the source of the signal could be a beacon that you are tracking, or a radar reflection. $\endgroup$
    – uhoh
    Commented Dec 11, 2019 at 14:29
  • $\begingroup$ But in monopulse mode, using polarization for example, it seems you either have to encode your own transmitted pulse (e.g. vertical polarization to the left, horizontal to the right), or if you are receiving a beacon then it will have angular encoding; they transmit vertical to the left, horizontal to the right, and when we measure the ratio, we know our position relative to their boresight. Maybe my problem is that I learned about conical scanning in the context of tracking; pointing a receiver antenna, and not in radar... $\endgroup$
    – uhoh
    Commented Dec 11, 2019 at 14:32
  • $\begingroup$ See Why is the reflector on this millimeter-wave antenna spinning? $\endgroup$
    – uhoh
    Commented Dec 11, 2019 at 14:32
  • 1
    $\begingroup$ @uhoh there's such a thing as receive-only conical scan, too (COSRO), developed to hide the fact that you're using a conical-scanning radar from your target by using a simple emitter and a conical-scanning receiver. In any case, because the monopulse lobes are not aligned, returned signals will have slight phase and amplitude differences, from which you can extract pointing errors continuously, rather than at the rate determined by your conical scan rotation rate. $\endgroup$ Commented Dec 11, 2019 at 14:37
  • $\begingroup$ Thanks again, I'll start reading further in the morning post-coffee. $\endgroup$
    – uhoh
    Commented Dec 11, 2019 at 14:45

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