EOSS Handbook Chapter 4 - Part B

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Contents

  • 4.2.5. Video system 19
  • 4.2.5.1. Camera 19
  • 4.2.5.2. Transmitter 20
  • 4.2.5.3. Signal polarization 20
  • 4.2.5.4. Antennas 22
  • 4.2.5.5. Equipment source options 25
  • 4.2.5.6. Background 26
  • 4.3. Experimenter's package / Shuttle interface 26
  • 4.3.1. Size 26
  • 4.3.2. Weight 26
  • 4.3.3. Attachment 26
  • 4.3.4. Electrical connections 27
  • 4.3.4.1. Power 27
  • 4.3.4.2. Control 27

4.2.5. Video system

4.2.5.1. Camera

<<Dave Clingerman>> We use a black and white camera that was purchased for $185 from Jactec, in La Habra, CA. It is a single multi-layer board that uses a Charge Coupled Device (CCD) array on which the images are focused. A wide angle lens of 70 degrees is their standard. The F stop of the lens is at least 16 and possibly 22 as it focuses everything from a couple of feet to infinity. A scan reversal switch is provided so that if a mirror is employed, as we do, the image transmitted is not reversed. The camera board and lens are enclosed in a plastic housing about the size of a pack of cigarettes. This lends itself very well to the modular construction of the Shuttle that we have been pursuing.

4.2.5.2. Transmitter

The heart of the video system is the video transmitter. It is a full motion, full color, NTSC standard transmitter designed and build by P.C. Electronics of Arcadia, CA. It carries a list price of $189.00. The transmitter has three inputs; microphone, line and video. The microphone is hi-Z, the line 600 ohms and the video 75 ohms, 1 volt peak-to-peak black and white or color from a baseband video source. The transmitter requires 12 volts DC at 500 ma. The output is a 1 watt peak-to-peak RF signal in the 70 cm band. The transmitter is crystal controlled and presently operating on a carrier frequency of 426.25 MHz. Three other frequencies are available at 70 cm.

4.2.5.3. Signal polarization

The Case For Horizontal

I have always advocated the use of horizontal polarization for balloon video operations due to many technical reasons and practical experience. Horizontal polarization, in the beginning (EOSS/WVN-I), was convenient due to the fact that the input to the video repeater was horizontal. All members of WVN that used the repeater were therefore, horizontally polarized on 70 cm. Elevation tracking of the balloon was actually not necessary due to the aperture of even the highest gain yagis.

The reason for elevation tracking at the launch site was the fact that we were directly under the payload. The wavefront from a horizontal omni is always parallel to the plane of the receiving antenna elements when, of course, the receiving antenna is pointed at the payload and the fact that it is a horizontally polarized yagi or collinear array. I will elaborate on that feature later. Another benefit of horizontal polarization is the fact that "noise" is vertically polarized and signal to noise ratio (S+N/N) enhancements of 20 dB or greater can be achieved depending on geological location. The pattern of a horizontal omni can be calculated from its gain as follows. We know the H-plane is 360 degrees and the surface of a sphere contains 41,253 circular degrees (or square degrees). And, we know that the gain of a dipole is 2.15 dBi. Therefore the gain (A) is:

20 Log10 of the ratio of (QE) * (QH) to 41,253. A(dB) = 20 Log10 (QE) * (QH)/41,253.

When we do a little derivation and turn the crank, we see that QE is approximately 147 degrees or � 73.5 centered on the horizon. Any null below the antenna is ~33 degrees (� 16.5 degrees) centered on the Z-axis of the antenna. In practice and in measurement it has been confirmed that the energy below the antenna is circular and will only be 3 dB down from the E-plane half power points or yielding a gain of -0.85 dBd.

Why is all this important? Because every fact of it is relative when we compare the calculations to a vertical radiator. At 100 miles out from the launch point and 20 miles above the earth (considering this a flat earth situation) the look angle is -11 degrees (using the convention that the spacecraft plane, parallel to the earth is the 0 degrees reference point or plane).

NOTE: Generally any time a distance on earth is <1/100 the circumference of the earth it may be treated as a flat earth problem.

Problems with vertical polarization

Let me now consider the vertical radiator. First, the terrestrial receiving system will have to tolerate 20 dB more noise than the horizontally polarized system. The gain of a l/4 radiator is twice that of a dipole (5.15 dBi)! Yes, gain is 20 Log10 of directivity. A ground plane antenna is essentially a dipole that has been cut in half (half the directivity, twice the gain)

The -3 dB beam width of a ground plane (using the gain equation again), is about 55 degrees (?). How did I come up with that? Let's revisit the gain equation. The gain of a ground plane is 5.15 dBi, divide it by 20 and take the antilog. Now, we have 1.8 = QH * QE / 41,253. Transpose, and we have QE = 207 degrees. Half of this is in the image and must be discounted, so the beam width is ~104 degrees or 52 degrees when observed in a rectangular coordinate system (X,Y plot). The useful lobe of a ground plane is only about 10 degrees -55 degrees above or below the horizontal (ground) plane what ever the case may be. Reference: Vertical Antenna Handbook, Capt Paul H. Lee, USN(RET.), N6PL, CQ Publishing, Inc., New York.

Beneath the antenna (inverted ground plane) is a 50 degrees cone of silence or �25 degrees which at 20 miles up dictates a hole in the center of the footprint that is ~20 miles in diameter. Further, at another 10 miles out from this hole, when the balloon is at maximum altitude the signal strength will be down by 3 dB to a yagi tracking the payload because of the lack of parallelism or the elements being displaced by 45 degrees. The cosine of 45 degrees is 0.707 and 20 times the log of 0.707 is -3 dB and at the edge of the cone of silence the attenuation is on the order of -9 dB, 'nuff said.

Taking these proven facts into consideration, we see that the center of the beam width is -36 degrees. This means that when the balloon reaches 100,000' the center of the beam is tangent with the earth only 27.5 miles out from a point directly below it and the -3 dB point is right at 100 miles or where the Brewster angle, null or point of uselessness starts. Let's examine this 100 mile reference point from the perspective of signal strength. The slant range for 100 miles out and 20 miles up is ~102 miles. The space loss for this distance is ~130 dB, however; the receiving system of the vertically polarized signal must overcome an additional 20 dB due to noise or an equivalent path loss of 150 dB at a less than marginal look angle as compared to the horizontally polarized signal that has a -3 dB point that is within the margin of acceptance.

To get an idea of the magnitude of signal we are dealing with let's look at a link; the transmitter outputs a signal of about 0.5 watt peak (27 dBm), a horizontal omni is 2.15 dBi. The signal from this system at the ground is therefore -130 + 29.15, or -100.85 dBm. A -60 dBm signal will produce the required 37 dB signal to noise ratio for a snow free or P5 picture at the TV receiver. Somewhere 43.85 dB of signal is needed to satisfy the requirement. The conversion gain of most down converters, if they are gas FET, is on the order of 30 dB with a 1-2 dB noise figure leaving 13.85 dB to come from somewhere else. How about 13.85 dB of antenna gain this could be in the form of a Boomer or a couple of long yagis and the link is closed. Experience has taught us that 20 dB fades are not uncommon and an antenna mounted preamp can solve this problem along with making up for line losses between antenna system and converter. Now in the case of the ground plane, 3 dB can be picked up in antenna gain but an additional 20 dB of noise must be overcome. The 3 dB, in this case of using the ground plane antenna, above link closure will allow a 17 dB deficit during a 20 dB fade.

Conclusions

I have presented the case for use of a horizontally polarized antenna aboard balloon-borne spacecraft. I have presented the reasons for not using a ground plane. This does not rule out ever using vertical polarization. A vertically polarized gain array is another case. I have kept the mathematics to very simple forms for clarity and understanding. I've never found much use for multiple integrals for the explanation of simple concept. What I profess here is factually evidenced in the library of E.O.S.S. videos which are available from the group's secretary.

If I can ever be of assistance to any ATV balloon group in the planning or evaluation of an antenna system please contact me.

4.2.5.4. Antennas

Types Used

The composite RF signal is radiated by a Little Wheel antenna, or a Candelabra. The Little Wheel is an omni-directional horizontal polarization radiator that has the gain of a dipole (2.15 dB over an isotropic source). The Candelabra is an omni-directional vertical polarization radiator that exhibits 3 dB gain over a dipole or 5.15 dBi. Both antennas have shown superior performance in eight of the nine launches of E.O.S.S.

A Horizontally Polarized Gain Array

Little from Big

An antenna developed in the early 1950's by W1FVY and W1IJD, the Big Wheel, was a boon to mobile two meter SSB work. During that era, an omni antenna that was horizontally polarized was desired. The Big Wheel had gain over the Turnstile and Halo. It exhibited a much more uniform omni-directional pattern and had a fairly broad frequency response, >10 MHz (1.5 : 1 VSWR).

The Denver Area group of ATVers elected to use horizontal polarization for their ATV simplex operation in the 70 cm band. Horizontal would provide some isolation from 450 MHz FM operation which is vertical. The video repeater was not yet a reality and our simplex operation entailed all points of the compass. Yagi antennas would have worked and did for some. However, in a "roundtable" on our activity night you could work a rotator to death and stations off the sides and back of the beam would be left out. We needed a fairly efficient radiator that was omni-directional and horizontally polarized. A scaled down Big Wheel�or Little Wheel�seemed to be a natural to fit the bill.

Little Wheel Description

Many of you may have seen a Little Wheel and not recognized it. To describe this array takes longer than building one. It looks like the skeleton of a Three Leaf Clover. Each skeleton leaf is made of a wavelength of material (rod or tubing). The element is formed by bending the material into a Clover Leaf looking loop by making two 90 degree bends a 1/4 wavelength in from the ends.

The elements are mounted to hub that insulates the ends from on another with element phase preserved. The three elements paralleled will have an impedance of 12 ohms. A 50 ohm match is acquired through the use of a capacitor or a capacitive stub. A coax is then attached to the hub to complete the array assembly. The array is tuned by bending the quarter wavelength sections toward or away from each other, vertically, whatever is required to achieve resonance in the desired part of the band.

Prior to the launch of the Western Vision Network/Edge Of Space Sciences (WVN/EOSS) ATV Balloon Experiment, Mr. Bill Brown�WB8ELK, Technical Editor of 73 Today magazine�was consulted as he had flown some semblance of a down scaled Big Wheel on his early ATV balloon experiments. So for us antennas were one of the main topics for consideration. It was thought that in order to increase ATV participation the balloon ATV transmitter could be on the ATV repeater input frequency (426.25 MHz). That way the flight of the balloon could be followed by watching the repeater output, 1253.25 MHz. And so, a Big Wheel scaled to 70 cm was placed on the spacecraft. It performed remarkably well and continues to perform to all expectations not only for EOSS but all across the country.

Today, newcomers to the facet of HAM Radio that flies ATV on helium filled weather balloons may wish to experiment with the Little Wheel, a proven high altitude ATV radiator. By using horizontal polarization on the balloon flights for ATV transmission may attract greater participation than vertically polarized radiators. This participation could be brought about by the fact that there is a lot of "small signal" work done at 432.1 MHz. These very serious VHFers have their arrays horizontally polarized. Sometimes the arrays are made switchable to vertical for long haul FM or even circular for Amateur Satellites. At any rate there are an abundance of operators out there that may not be intimately interested in ATV ballooning but could track and report if asked.

We see that an antenna development for one facet on amateur radio led to its scaling and use in a second facet which has ultimately placed it at the "edge of space."

A Vertically Polarized Gain Array

Background

According to my ledger a considerable number of groups are flying the "Little Wheel" antenna on their balloon payloads for the ATV radiator. I am, of course, am very happy that the "Little Wheel" has enjoyed such success and enhanced the enthusiasm of ATV Ballooning (kiting, rocketry, RCing and not quite yet Frisbiing). However; not everyone in the nation or the world has chosen to propagate their video (ATV) signal in the horizontal.

Recently, in Denver, the local ATV repeater group Colorado Amateur Television League (CATL) has decided to try a vertical input on the video repeater, (426.25 MHz input/1253.25 MHz output). All interested ATVers then had to put a 90 degree twist on the tails of their horizontally polarized 70 cm antennas so the repeater could once again be accessed. This change in philosophy and ultimately polarization caused a bit of a problem. With a horizontal input to the ATV repeater and with the balloon borne ATV transmitter on 426.25 MHz we used to repeat our balloon launch video throughout our service area. Now, in order to capitalize on the maximum amount of ATV participation during the local balloon launches, "Edge of Space Sciences, Inc." (EOSS) had to request their technical committee come up with a spacecraft antenna that would be vertically polarized, light weight and perform considerably better than a "rubber duck".

Preliminary Design

As Chief Scientist / Technical Committee Chairperson of EOSS, without hesitation or reservation, I elected to take up the challenge and launched into a rather extensive research program to design, create and test a vertically polarized, gain array. The research went something like this; KISS Method - what's first and available? The stinger, the spike, the quarter wave monopole, the coaxial collinear, the "J", the old sewer pipe type monopole was even considered. The monopoles around a quarter wave length were known to be semi-radiating dummy loads, to one degree or another. Even though, mathematically proven they do exhibit 3 dB over a dipole, the Brewster angle (or Pseudo Brewster Angle) is too great to place the main lobe maxima on the horizon when the edge of space is encountered. The monopoles exhibit narrow bandwidths due to a high Q.

Physically, if affixed rigid to a near spaceborne vehicle, they are subject to and influenced by spacecraft dynamics, then you have the polarity characteristics that resemble the tail of a horse, swatting flies. How about the 5/8 wave or a coaxial collinear to solve the problem. Not quite, similar problems to those previously presented will be experienced. Given; however, the Brewster angle will be lessened and the energy will be more on the horizon. They are inefficient as they burn up power; first in the matching scheme secondly in its dielectric. Considerable consideration was given to a configuration that would shift the high current node inherent at the base of radiators such as a quarter wave monopole or 5/8 wave radiator which burns up a considerable amount of power in heating the ground plane.

Any light weight "spike" type of radiator will pendulum. Sure there are techniques for reducing the penduluming but they involve added weight which isn't acceptable. The accounts I've read of ATV Balloon operations that have used stingers and such for their ATV signal radiators indicate it's definitely not the way to go.

How about a solitary folded dipole, in an effort to gain the bandwidth required to properly propagate an ATV signal? Same problems with stability as the monopole, plus the added problems of mounting vs. feeding. For example; mount it on the side of the spacecraft and you shadow the opposite side seriously hampering reception on the two adjacent sides. Dangle it below the spacecraft and the pattern will be distorted by the feed line. How about employing a pair of folded dipoles for balance and reducing the feedline effects by moving the feedline far enough out of the pattern so as not to effect either radiator? Then a nice bi-directional array has been created that will twist and turn on the feed line and drive operators nuts that are trying to view and/or record the video, as the signal strength goes from P~ to P(N) and back again.

Detail Design

The folded dipole, to me, still had some interesting features I thought could be pressed into service if I could come up with the right combination / configuration and win the approval of the EOSS technical committee or for that matter the EOSS Membership-At-Large. With calculator in hand an investigation was undertaken to determine what sort of design could be conjured up that would allow the basic folded dipole to produce a pattern that is omnidirectional, in the horizontal plane and exhibit a predominately vertical E-field. The folded dipole was examined from the very basics such as optimum material diameter of the top and bottom radiating elements. Similar diameters allow a 4:1 impedance transformation and a terminal impedance of 288 ohms. Why not three radiators around a center hub and stand them off by 0.25 wavelength? That way the pattern would became more than bi-directional and the feed impedance would be a third of a the folded dipole by itself. That feed impedance of approximately 90 ohms would be easy enough to match to 50 ohms with a 0.25 wavelength of RG-62 (93 ohms) or a balun of sorts like a bazooka (coaxial Q-bar). The geometric mean of 50 and 90 is 67. A quarter wave length of 75 ohm coax would probably suffice. However, the bazooka would provide some structural integrity to the array and therefore could not be flexible. The bazooka would also serve as a partial reflector to this three element array.

Build

Creating a 50 ohm air line that is a quarter wave at 70 cm was only a matter of turning the crank on the coaxial transmission line equation. Brass tubing 7/16" O.D. and 0.375" I.D. of approximately 0.035 wall provided the larger diameter (D). It also fits very nicely over the threaded end of a BNC connector when the threads are removed. The other variable, the smaller diameter, (d) is then 0.1226". A piece of standard 1/8" (0.125) brass brazing rod or hobby tubing would work very nicely as the center conductor for this length of air line. If made of rod, one end needs to be tapered to a point -More- from about one inch back so as to fit the BNC center conductor solder cup. Once soldered in place this center conductor/connector assembly could be inserted in the 0.375" brass tube and the connector soldered to the tube. The opposite end of this tube is to be fitted with a 3/8" brass nut that has had a 3/8" drill bit run through it to remove the threads and allow it to be slipped over the brass tube. Three of the six flats of the nut, every other one, require a 0.093" diameter hole in the center of the flat. These will later receive the very ends of the folded dipole elements. The tube can now be slipped through the nut to the point where it would be flush with the edge of the nut and then soldered in place. A teflon shoulder washer is needed to serve as both "center conductor spacer" and "element end insulator". This can be created from 1/4" thick teflon sheet. The shoulder part would be 0.625" diameter, 1/8" thick while the center portion is 0.375" diameter and 1/8" thick. A hole 0.125" diameter is to be drilled in the center of this shoulder washer in order to pass the center conductor. The washer could then be placed over the center conductor and slid down till the 0.375" portion would fit inside the brass tubing and bottom out against the 0.625" diameter shoulder. A 0.750" diameter brass disk with a 0.125" hole in its center could be fitted on top of the shoulder washer and soldered to the center conductor.

Three, 36" lengths of 0.093" (3/32") diameter brazing rod were marked in the center and from that mark measured 6.0" both ways from it and marked again. From each of these marks measure 5.0" toward the ends on the rod and mark again. This leaves about 7.0" on each end of the rod. Now, it's just a matter of folding it up like a coat hanger. The elements (top & bottom) need to be a half inch between centers which allows the terminal impedance to be approximately 288 ohms. The two, 7.0" end pieces when bent parallel to each other will be approximately 2.0" apart. The very ends are then bent in a very mild "S" curve to bring the ends within a half inch of each other. One of the ends of a single element are to be inserted in one of the holes in the brass nut that has previously been soldered to the brass balun tube. The other end of the element is to be soldered to the brass cap that was previously soldered to the center conductor of the balun. All three elements will be soldered in place in like manner.

Test

Testing of the array was accomplished by first sweeping the device with a Texscan UHF Sweep Oscillator, Wiltron SWR Bridge and presenting a visual indication of bandpass on an HP-1743 Oscilloscope, using it in the X vs. Y mode. Some slight bending had to be done to bring the 1:1.0 portion of the curve to the portion of the spectrum where I wanted it, (425-431 MHz). Resonance at desired bandwidth accomplished, next the E & H plane pattern measurements and last but not least the gain determination.

The National Bureau of Standards (Boulder Labs) where I work has several antenna ranges, all usually tightly scheduled. I patterned my outdoor range after theirs. My test methods may be a little crude but very repeatable. I made my rotational platform sufficiently far (30 meters) from the source as I am only interested in "far field" measurements. After calibration the array was rotated in the "H" plane to ascertain any discrepancies such as deep nulls anyplace in the 360 degrees of coverage. The smoothed pattern was so smooth that the transition from element to element was barely discernible (<+/-1.5 dB). Next the array was turned in the E-Plane and what was anticipated and hoped for of the pattern became reality. In the design stage I had anticipated the balun/bazooka might act as a reflector to the upper portion of the elements, it did. The pattern observed was a skewed main lobe center of the E- plane, by thirty five degrees. Some minor lobes were of course evident at the top of the pattern but for all intents and purposes were insignificant. The envelope appeared to very much resemble a classic "cosecant squared" relation that is very desirable of search radars on airborne platforms and quite relative to our balloon missions. A hard copy of both E & H planes were plotted. Here was a case where an antenna was created out of necessity to fulfill the requirements of a previously good array but of the wrong polarization. The design phase related and the outcome of the project. I offer the dimensions and construction details in case this type of antenna will serve your purposes as well as it has ours (EOSS). If you'd rather just purchase one of these please let me know. It is available through Olde Antenna Lab as the Candelabra.

4.2.5.5. Equipment source options

An optional resource of the video system is a Digital Video Board produced by Elktronics of Findley, OH and retailing for $99.00. This board uses a Read Only Memory (ROM) can be programmed with custom video frames. These frames can be graphics (logos), call signs, messages, or whatever the imagination dictates. The frames are "on-board" timer controlled and can be interspersed with live video from the camera. The board operates on 12 volts DC and draws about 160 ma. We have used it on some missions where the over all current budget is minimal. Another option available in the video system is the Digital Audio Board from Ming, Inc. of Sacramento, CA, for $125.00 . This board can be configured to output a prerecorded message under "on-board" timer control, or store and forward a message on command. It is a 12 volt device that draws 200 ma. The memory is 4 megabytes long using Digital Random Access Memory chips (DRAMS). The audio is routed to one of the video transmitter subcarrier inputs and becomes the audio on the video down link. We noticed, early on, that the DRAMS are very sensitive to RF fields and therefore hasn't been used much on the Shuttle. An option available to the video system that is actually part of another system that will be described by another author is the microprocessor control. One of the many control features is the ability to position the camera in elevation, +/- 90 degrees. This is done through a voltage reversing relay that positions the mirror or, as in past cases, the camera itself. There are times when it is advantageous to be able to look other than just at the horizon. The Earth beneath the Shuttle can then be observed. This has been a distinct advantage to the Fox Hunters. The terrain of the shuttles intended landing area can be observed and identified. Positioning of the mirror for the camera to image skyward affords witnessing and confirmation of balloon release, when so commanded.

4.2.5.6. Background

The video system has been through many iterations to get where it is today. However, it is very versatile due to the stress we have placed on modular construction, reconfiguration of this and other systems to meet mission requirements, is not labor intensive. The six portions or components that make up our present video system have been covered. It is by no means a cast in concrete system or sacred philosophy. We have created a working system that has been the product of much trial, suggestion and change. We will continue to modify both hardware and philosophy in an effort to better the performance and meet the requirements of our student project engineers and managers. This is the mode in which we wish to function and the intent of our charter. I am very much of the belief that we are only limited only by our imaginations. Don't be afraid to dream, visualize a new concept, let others know what you're thinking and together we will make things happen. I have been associated with Amateur Television (ATV) since the mid 60's and it is my belief that Scientific and Educational Ballooning has been the biggest boost to this facet of HAM Radio since its inception. "It's Amateur Radio that you can see".

4.3. Experimenter's package / Shuttle interface

<<Mike Manes>> As of November 1992, the EOSS Shuttle has rather limited support for experiments. We are engaged in weight and power reductions and development of a more versatile command and telemetry interface with a goal of simple adaptability to a wide variety of potential experiments. The current plan is to provide for four 0.0 to +5.0 V 8-bit analog inputs reported via telemetry in Vdc, several TTL level digital inputs and outputs, and a 1200 baud bi-directional serial bus for more complex telemetry and commanding. This specification will be updated when that project is completed early 1993. Experimenters are warned not to use the following data as definitive. The EOSS Tech Committee should be consulted for the latest information prior to experiment construction.

4.3.1. Size

20 cm (8") cube max

4.3.2. Weight

170 g (6 oz) max

4.3.3. Attachment

Flush to bottom of Shuttle, 4 points on 12.5 cm (5") grid using tie wraps & dowels.

Foamcore board 5 mm or more thick with hot-melt, or RTV glued and reinforced joints has proven itself strong, light and thermally insulative. Additional thermal insulation in the form of building-grade Styrofoam sheet may be added if required. The package should be completely covered in aluminum foil for EMI protection. Elmer's glue is effective for foil attachment. Foil seams at access covers should lap at least 2 cm.

A typical EOSS flight system profile may appear as follows:

4.3.4. Electrical connections

All electrical interface to the experiment will be via a 12-pin female Molex connector mounted on the rear vertical surface of the Shuttle located on vertical center and 10 cm (4") above bottom surface. Mating connector is a Radio Shack RS 274-232 or equivalent. Pin-outs are <TBD> at this time.

4.3.4.1. Power

The Shuttle can supply up to 100 mA of +10 to +15 Vdc unregulated battery power for a 3-hour flight duration. This source is commandable on and off via ground command, and is interrupted by the Controller automatically under low battery and touchdown conditions. EOSS reserves the right to kill experiment power to preserve flight safety. This source is protected by a 125 mA slow-blow fuse. Experimenters should supply RC or LC filtering to avoid conducted EMI. This source can present up to 100 mV PP of 16 KHz noise from the ATV transmitter. Additional power requirements must be supplied as part of the Experiment Package.

4.3.4.2. Control

1. Telemetry

At present there are 4 spare 0 to 3.32v analog telemetry inputs available. Each is sampled at 30-sec intervals and reported as Lens Temperature. The only other available means of returning real-time information to the ground is via the television camera, which can be aimed 10 degrees past the nadir. Additional data logging must be provided by the Experiment.

2. Digital Commands

a. Three 200 mA max open-collector NPN (2N2222) outputs referenced to power ground are commandable on and off by ground command issued manually using a DTMF (telephone touchtone) keypad. These are called Functions A, B and C.

b. A resistively current-limited (250 mA) source of polarity-selectable battery power pulsed at 3 selectable msec pulse widths via DTMF keypad from the ground. This source was originally designed to drive a 6V R/C servo motor in angular steps in either direction. Function B is common to the drive for this source.

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