Micro-Space >> Ultralight Manned Spaceflight

[rpspeck]

I mentioned navigation in my last post.  The contribution of “Coherent Radio Links” to navigation is not well known outside the professional community, but this has long been a focus of Micro-Space, ultralight system efforts.  (We have achieved a level of coherent link operation - control uplink and telemetry downlink – in a one inch diameter rocket.) 

Directional radio antennas allow tracking (and finding) rockets which fly beyond visual range.  Interferometric techniques make greatly enhanced tracking resolution available – and this has been enabled by the coherent multichannel radio receivers Micro-Space has built and used.  But among conventional radio tools  only RADAR adds the range information needed for 3 dimensional tracking from a  single instrument cluster.

High quality Doppler Shift data begins to fill in the missing data, without requiring a RADAR system (and RADAR works poorly with small – largely non conductive rocket bodies), because this effect results entirely from the changing, linear distance between the rocket and the receiver.

A modified receiver can capture and document the Doppler Shift from a stable telemetry transmitter.  In fact it is surprisingly easy to do this with a well designed, crystal controlled transmitter!  This is particularly surprising, since measurable radio frequency Doppler Shift requires motions at a “significant fraction” of The Speed of Light! 

The Speed of Light is about one million times faster than the Speed of Sound.  On the other hand, a well designed “crystal oscillator” will have a frequency stable to a small fraction of  “one part per million” for a number of seconds. 

Typical small rockets, accelerating to 500 feet per second in 2 seconds, will produce a very audible Doppler Shift in a UHF receiver with “Beat Frequency Oscillator” (used for CW – Code – and Single Sideband reception).  At 432 MHz, the tone shift will be over 200 Hertz at 500 fps, increasing to 400 Hertz for near sonic velocity, and 600 to 800 Hz for achievable supersonic flight.  Since “Sonic Boom” does not exist behind an accelerating rocket, this is a very nice way to verify predicted flight speeds. 

But, as I will discuss soon, this is only the beginning of the possibilities! 

[tnphysics]

Lunar Light Show

In the process of developing the Laser Altimeter needed in our lander, we realized that equipping our Lander (even our lightest version) with a Laser visible from the Earth is quite feasible!  A fraction of a Watt output red Diode Laser equals many “candle power”.  When this light is formed  into a one milliradian  beam, it equals millions of “Beam Candlepower”, and will be visible from the 400,000 km distance of the Moon.  This beam would illuminate a spot 400 km in diameter on the Earth.  The optimum shape will probably be elongated, 400 km north to south, and 100 km east to west.  Viewers in the right latitude range (selected by laser beam positioning commands sent to the lander) would see the red Laser light for about 3 minutes as the Earth's rotation carried them through the beam.  Flashing of the Laser, as well as its color, would aid in detecting the light visually. 

Our plans call for landing just beyond the “sunrise” line on the waxing Moon.  Only 2 to 3 days of operation will be possible before the initially cold temperature changes to excessively hot, with the sun high above the lander.  Since we do not plan to land in the dark, extremely cold environment preceding sunrise,  viewers will have to spot the Laser against a sunlit portion of the Moon.  This will be easier just after sunset on the Earth, since the Earth's dark sky will produce less veiling glare.  Viewing should be fairly easy with common sizes of amateur telescopes.

The purpose of this demonstration (beyond showing the possibility of optical communication) would be to draw thousands of people into direct participation – seeing the lander's position on the Moon for themselves - and avoiding the “Apollo Controversy” (“They didn't really land on the Moon, because ...”).  Many of those who see a “low budget” lander blinking at them from the Moon, will begin thinking about the possibility of “low budget” Human landings.

Why can't the lander be made to withstand the heat?

[tnphysics]

Most of the analysis will be done in software.  All commercial parts will be used, with shielding as required for a short stay in space. (The Radiation doses acquired by the Apollo Astronauts were all quite modest.)

Hydrazine of  all sorts is being avoided for our projects because of its hazards.  I can't envision any being allowed in a CubeSat or similar “Secondary Payload”. 

The recent computations for our spacecraft attitude control were all done  for cold gas thrusters.  We prefer “warm” CO2 for its higher storage density, and lower storage pressure, in spite of modestly lower performance.  We hope that small DOT certified containers for CO2 and N2O will be allowed in a CubeSat, given their excellent safety history and very limited  hazard potential even in the worst case.

My last posting relates to generating 6 DOF positioning forces between spacecraft using no propellant at all!  Since this is an operating system, these forces and the effects they can produce are limited by the appropriate conservation laws. Solar power is of course consumed, and at least one of the spacecraft needs to have active propulsion if modification of the interacting system's cumulative momentum is desired.

I like the idea of not using rad-hard parts.

Indeed, I believe that a commercial CPU would be overkill. Am I right on this? If so, could it also serve any other functions, such as analyzing science data?

Also, what about using the N2O as a monopropellant?

[tnphysics]

Why can't the RCS and the main engine use the same propellants and be fueled by the same tanks?

This would allow a mass savings, or would it?

[mlorrey]

RCS and Maneuvering CubeSat Details

Even tiny commercial pneumatic valves provide TOO MUCH gas flow to work well for attitude control in a CubeSat, or our similar sized “Very Low Mass” lunar landers. 

I would suggest you get away from the fluid/valve/pump type design model entirely, go digital. Build an integrated chip that has a ton of tiny cavities in its face that are each filled with a high explosive chemical. Each cavity would be individually ignited via electronic ignition. This would give you zero moving parts, and very high Isp from a pulse detonation propulsion method, with very fine grained throttling of thrust of one pixel at a time up to several...

You may already be aware of this work, given that you used several of the buzzwords and laid out essentially the complete strategy, but this method of attitude and station-keeping control has been investigated.  Lewis, Janson, Cohen, and Antonsson called it Digital Propulsion in their work here:
http://design.caltech.edu/micropropulsion/99d.pdf

I worked on the project from 2000-2004 (just after this paper was published) at Caltech designing and testing bonding methods.  The basic idea is to bond an array of resistors on a silicon wafer with an array of combustion chambers in a glass layer and an array of burst diaphragms and nozzles.  The chambers are filled with fuel (lead styphnate in our case) and alligned and bonded so that each chamber is matched with a resistor and a diaphragm/nozzle.  When a "bit" of thrust is required, the correct resistor on the array is energized and about 10^-6 (theoretical) to 10^-4 (demonstrated) Newton-meter of thrust is produced.  On the order of about a million single-use bits can be placed on each side of what we would now call a ~1 kg nanosatellite (the paper linked above uses the term microsatellite before the distinctions were clear).

Thats the one. Thanks for the link. I was really impressed with the innovative ideas in a smack my forehead kind of way. I dont see in the paper what specific impulse was achieved, do you have those numers? Pluse detonation should achieve in the order of 3000 sec.

[tnphysics]

RCS and Maneuvering CubeSat Details

Even tiny commercial pneumatic valves provide TOO MUCH gas flow to work well for attitude control in a CubeSat, or our similar sized “Very Low Mass” lunar landers. 

I would suggest you get away from the fluid/valve/pump type design model entirely, go digital. Build an integrated chip that has a ton of tiny cavities in its face that are each filled with a high explosive chemical. Each cavity would be individually ignited via electronic ignition. This would give you zero moving parts, and very high Isp from a pulse detonation propulsion method, with very fine grained throttling of thrust of one pixel at a time up to several...

You may already be aware of this work, given that you used several of the buzzwords and laid out essentially the complete strategy, but this method of attitude and station-keeping control has been investigated.  Lewis, Janson, Cohen, and Antonsson called it Digital Propulsion in their work here:
http://design.caltech.edu/micropropulsion/99d.pdf

I worked on the project from 2000-2004 (just after this paper was published) at Caltech designing and testing bonding methods.  The basic idea is to bond an array of resistors on a silicon wafer with an array of combustion chambers in a glass layer and an array of burst diaphragms and nozzles.  The chambers are filled with fuel (lead styphnate in our case) and alligned and bonded so that each chamber is matched with a resistor and a diaphragm/nozzle.  When a "bit" of thrust is required, the correct resistor on the array is energized and about 10^-6 (theoretical) to 10^-4 (demonstrated) Newton-meter of thrust is produced.  On the order of about a million single-use bits can be placed on each side of what we would now call a ~1 kg nanosatellite (the paper linked above uses the term microsatellite before the distinctions were clear).

Thats the one. Thanks for the link. I was really impressed with the innovative ideas in a smack my forehead kind of way. I dont see in the paper what specific impulse was achieved, do you have those numers? Pluse detonation should achieve in the order of 3000 sec.

I am sorry, but I believe that it is nuclear pulse propulsion that has the 3000 sec Isp. The limit for chemical is 600 sec (about) @ 100% efficiency for LOX/ boron and is set by the condition that the energy released all goes into exhaust kinetic energy.

[mlorrey]

RCS and Maneuvering CubeSat Details

Even tiny commercial pneumatic valves provide TOO MUCH gas flow to work well for attitude control in a CubeSat, or our similar sized “Very Low Mass” lunar landers. 

I would suggest you get away from the fluid/valve/pump type design model entirely, go digital. Build an integrated chip that has a ton of tiny cavities in its face that are each filled with a high explosive chemical. Each cavity would be individually ignited via electronic ignition. This would give you zero moving parts, and very high Isp from a pulse detonation propulsion method, with very fine grained throttling of thrust of one pixel at a time up to several...

You may already be aware of this work, given that you used several of the buzzwords and laid out essentially the complete strategy, but this method of attitude and station-keeping control has been investigated.  Lewis, Janson, Cohen, and Antonsson called it Digital Propulsion in their work here:
http://design.caltech.edu/micropropulsion/99d.pdf

I worked on the project from 2000-2004 (just after this paper was published) at Caltech designing and testing bonding methods.  The basic idea is to bond an array of resistors on a silicon wafer with an array of combustion chambers in a glass layer and an array of burst diaphragms and nozzles.  The chambers are filled with fuel (lead styphnate in our case) and alligned and bonded so that each chamber is matched with a resistor and a diaphragm/nozzle.  When a "bit" of thrust is required, the correct resistor on the array is energized and about 10^-6 (theoretical) to 10^-4 (demonstrated) Newton-meter of thrust is produced.  On the order of about a million single-use bits can be placed on each side of what we would now call a ~1 kg nanosatellite (the paper linked above uses the term microsatellite before the distinctions were clear).

Thats the one. Thanks for the link. I was really impressed with the innovative ideas in a smack my forehead kind of way. I dont see in the paper what specific impulse was achieved, do you have those numers? Pluse detonation should achieve in the order of 3000 sec.

I am sorry, but I believe that it is nuclear pulse propulsion that has the 3000 sec Isp. The limit for chemical is 600 sec (about) @ 100% efficiency for LOX/ boron and is set by the condition that the energy released all goes into exhaust kinetic energy.

Nuclear pulse propulsion is in the hundreds of thousands of sec or higher. Chemical pulse detonation can be higher than rocket engines, but yes I was thinking of pulse detonation systems that consume atmospheric O2 as oxidizer, which obviously eliminates the LOX mass from the equation.

[rpspeck]

Lunar Light Show

In the process of developing the Laser Altimeter needed in our lander, we realized that equipping our Lander (even our lightest version) with a Laser visible from the Earth is quite feasible!  A fraction of a Watt output red Diode Laser equals many “candle power”.  When this light is formed  into a one milliradian  beam, it equals millions of “Beam Candlepower”, and will be visible from the 400,000 km distance of the Moon.  This beam would illuminate a spot 400 km in diameter on the Earth.  The optimum shape will probably be elongated, 400 km north to south, and 100 km east to west.  Viewers in the right latitude range (selected by laser beam positioning commands sent to the lander) would see the red Laser light for about 3 minutes as the Earth's rotation carried them through the beam.  Flashing of the Laser, as well as its color, would aid in detecting the light visually. 

Our plans call for landing just beyond the “sunrise” line on the waxing Moon.  Only 2 to 3 days of operation will be possible before the initially cold temperature changes to excessively hot, with the sun high above the lander.  Since we do not plan to land in the dark, extremely cold environment preceding sunrise,  viewers will have to spot the Laser against a sunlit portion of the Moon.  This will be easier just after sunset on the Earth, since the Earth's dark sky will produce less veiling glare.  Viewing should be fairly easy with common sizes of amateur telescopes.

The purpose of this demonstration (beyond showing the possibility of optical communication) would be to draw thousands of people into direct participation – seeing the lander's position on the Moon for themselves - and avoiding the “Apollo Controversy” (“They didn't really land on the Moon, because ...”).  Many of those who see a “low budget” lander blinking at them from the Moon, will begin thinking about the possibility of “low budget” Human landings.

Why can't the lander be made to withstand the heat?

The Equatorial temperature on the Moon will rise to about 400K = 123 C.  This is a high, but not impossible temperature for the lander.  This creates a lot of IR emission from the lunar surface which will flood the craft, and its thermal radiators, adding to the direct solar energy input.  Admittedly, planar radiators with reflective back surfaces (facing the Moon) and with low solar absorption, high thermal emissivity surfaces facing upward will work almost as well under these conditions as they would in free space (with somewhat less than ½ the IR emissivity of two sided thermal radiators).  Solar panels, given their desired, high solar absorption, will be unable to radiate much additional heat, but additional radiator panels can work fairly well.  The additional cost to handle these conditions should be moderate.

The alternative we have actually settled on will be to land just after local dawn at a higher lunar latitude. This will reduce the “noon” thermal stress and allow 10 to 12 day operation on the moon.  It doesn't take much change in the latitude to bring the temperature peaks down significantly.

As always, it is necessary to minimize the costs of a Google Lunar Lander effort since the economics of these efforts make so little economic sense - given widespread public boredom with space efforts!

[rpspeck]

Why can't the RCS and the main engine use the same propellants and be fueled by the same tanks?

This would allow a mass savings, or would it?

As noted in my earlier analysis, the RCS needs are so much smaller than the main propulsion needs that tapping a small part of the tank pressurant is sufficient.  Very small valves and nozzles are then adequate, low in mass and very reliable.  This should be the lowest mass – as well as lowest development cost – approach.

[rpspeck]

Phase Lock to Eliminate Drift

The problem with this, very simple Doppler Detection system, is that a good crystal oscillator will drift by one part per million in a few minutes under the best of conditions and, given the air temperature drop with altitude, the drift rate for the transmitter in a small rocket will be much faster than this.  There is usually an initial delay of about 4 seconds before the adiabatically cooled, expanding air in the rocket begins to cool the crystal, and this is enough for the acceleration and “Fast Burn” velocity verification.  Long before apogee is reached, substantial frequency changes mask the Doppler effect, although the various effects can be partially resolved by linear extrapolations in captured data.

It is feasible to make both temperature compensated Crystal oscillators, and temperature stabilized units. But keep in mind that anything you fly in a small rocket has a good chance of being severely damaged, and find a way to keep the circuit costs down. Really good crystal oscillators approach 1/100 part per million drift over many minutes (or even years, with very good temperature control), but stretch the budget as “Disposable”  components.

The alternative is to give up on stabilized oscillators, and control this unit “remotely”.  By transmitting a reference signal UP to the rocket, and making the transmitter “Phase Lock” to this reference, the frequency error goes to zero (first order approximation).  The reference signal is also seen to have a Doppler Shift at the rocket receiver, and this becomes added to the Doppler Shift of the returned signal (just as it would if directly reflected as in a RADAR system), doubling the effect.

The Phase Lock process involves an electronic comparison between the weak received signal and the strong transmitter signal, with provision to adjust the transmitter so that these oscillations are synchronized.  Since it is important to keep the transmitted signal out of the receiver,  the synchronization is often implemented with a mathematical ratio between the frequencies. The transmitted signal could, for example,  be at twice the frequency of the received reference.  They would still be synchronized, but would use separate antennas and filters to keep the transmitted signal out of the receiver. 

Things become more complicated – as will be discussed next – if one wants to combine Radio Control instructions with the reference signal and telemetry data with the signal transmitted from the rocket, but this is a very desirable achievement! 

[rpspeck]

Phase Locked (Coherent) Doppler for Rocket Tracking

I will hold off a bit on describing the complications of adding telemetry and control to this “Coherent” Radio Link (and Micro-Space success in mastering them), and jump ahead  to the navigational benefits.  With the drift in measuring rocket velocity nominally eliminated by synchronizing the Down Signal to the received, Up Link Reference, the net Doppler shift (Frequency Offset) provides a very precise measurement of the rocket's “Radial Velocity”. (This is a signed speed, not a true, three dimensional velocity vector.)  The integral of this velocity gives an equally accurate measurement of the rocket's distance from the communications receiver. In a vertical launch situation this provides a very good value for the rocket's altitude with time (with a correction for the prelaunch distance of the rocket from the radio antennas). .

These measurements, and all such measurements within the atmosphere, are modified by a radio transmission velocity a bit lower than the free space “Speed of Light”.  The better known, optical correction is 0.03 percent (300 parts per million) with about a one part per million per degree temperature coefficient. The correction for radio transmission is probably a bit larger, and somewhat more variable. These corrections are for near sea level and decrease nearly exponentially with altitude, paralleling air density. For high vertical flight the total correction approaches 7 feet (2 meters).  Note that identical effects are involved in RADAR and GPS distance measurement.  Using reliable, predicted corrections, rocket altitude can be determined to a precision approaching 10 centimeters!

Even better than the absolute distance measurement are the Velocity and Acceleration measurements.  Changes in distance – beyond this relatively constant refractive correction – can be measured with a precision of as little as one millimeter. Analyzed over a modest time interval, the velocity can be determined to a fraction of a millimeter per second, and the acceleration to micro-g resolution.  All these accuracies are dramatically improved if the tracking processes occur entirely in space. For sounding rockets, the ability to document air drag in near vacuum conditions is phenomenal, and  the possibility of testing low thrust (Ion, Plasma or MHD) propulsion very real. 

“Integrating” the Doppler Shift of course involves counting the cycles of “Phase Shift” created by the motion.  Comparing the transmitted Reference signal with the returned Down Signal produces a changing phase relationship as the number of RF cycles “tied up” in transit (round trip) changes.  If the  separation distance is constant, then the time delay and number of cycles tied up in transit are also constant and the phase relationship is static.  An increasing distance increases the delay, tying up more cycles in transit.  This reduces the received Down Signal frequency, so that it falls behind the Reference  signal with a steadily increasing phase delay.  Counting “Cycles” can be done with the “Up/Down “ counter techniques used with rotary encoders. With 500 MHz radio signals, each cycle represents 1 foot (0.3 meters) of distance change (2 ft  = 0.6 meters round trip path length change).     

But “Phase Shift” need not just be counted in full cycles.  Many instruments (including those used in color TV work) measure phase shift to much better than 1/360 cycle = 1 degree of phase angle. This accuracy can even be maintained with fairly week RF signals (18 dB Signal to Noise ratio) if averaged over 100 times the communications “Data Bit” time window.  (With a barely usable communications signal, the same phase accuracy could be maintained with a longer averaging interval.)  And this “Phase Shift” resolution produces the 1 millimeter distance resolution mentioned with 500 MHz signals.

More on navigational uses of Phase Shift soon.

[rpspeck]

TRL Level 7 – 8 – 9 (the Why of Suborbital Tests)

TRL level addresses the “Technical Readiness Level” of any advanced technology, and thus the “Technical Risk” of any program which relies on that technology for success.  Given the custom development which will be required for any successful Google Lunar X PRIZE effort, this will be a very important metric to sponsors and investors. 

TRL 9 = “Actual system 'flight proven' through successful mission operations.” (In Space)
(Risk is subsequently limited by “Quality Control”, not developmental issues.)

TRL 8 = “Actual system is completed and 'flight qualified' through test and demonstration (Ground or Flight)”

TRL 7 = “System prototype demonstration in a space environment.”
(The Threshold for a system “Proven in Space”)
....
....
TRL 1 = “Basic principles observed and reported”
(A concept with some “science project” to validate it.)

All funding sources – government, commercial and venture capitalists – are well tuned in to Technical Risk factors and the TRL level for space projects.  Space “Pros” justifiably dismiss enthusiasts who are not aware of these 9 levels (the middle 5 not listed above)  and unaware of the time, money and effort required to push developmental technologies from one level to the next higher level.

Simplified, a good team will need 2 to 4 attempts to work the bugs out of a system at a given TRL level and push its status up one step. For space technologies, each such effort may cost $100 Thousand to $100 Million Dollars!

The number of “attempts” are not generally a multiplicative, exponential series, but a technical “dead end” (with unresolved problems) may in fact push the TRL backward, requiring another approach.  This will introduce new problems, which require solution efforts,  before it is even possible to see if the original “dead end” has been bypassed or overcome.  These situations do multiply the “attempts” required. 

TRL 4 =  “Component and/or breadboard validation in laboratory environment.”

It is not surprising that the Space “Pros” are not impressed by even a good laboratory demonstration, when  twenty or more attempts are likely required before TRL 7 – “Successful Demonstration in Space” – (3 steps higher) can be successfully accomplished.  Some of these efforts require spaceflight and can be very expensive!

You may get the attention of the “Pros” - and funding sources – after reaching TRL 7.  But investors have no interest in a project which will take 20 experiments (each using a $10 Million Falcon 1?) before your GLXP design is even capable of  reaching the Moon!  You will need to find a much less expensive way to get through these development steps.

For that reason SUBORBITAL SPACEFLIGHT will be very important for GLXP success.  With a UP Aerospace suborbital spaceflight listing at $200,000, and modest safety constraints,  the tests can be less expensive and more frequent. Each flight can include several experiments (as can an orbital test), but two or three flights of either type are likely before you have solved the “Unexpected” problems with your system (since you didn't recognize and address them before the first flight).

I will continue to address suborbital rockets – in the GLXP context – because they are such an important, cost effective way of getting our space technologies to a credible development level.  And  until we accomplish that, “Real Money” will be out of reach.     

[blazotron]

RCS and Maneuvering CubeSat Details

Even tiny commercial pneumatic valves provide TOO MUCH gas flow to work well for attitude control in a CubeSat, or our similar sized “Very Low Mass” lunar landers. 

I would suggest you get away from the fluid/valve/pump type design model entirely, go digital. Build an integrated chip that has a ton of tiny cavities in its face that are each filled with a high explosive chemical. Each cavity would be individually ignited via electronic ignition. This would give you zero moving parts, and very high Isp from a pulse detonation propulsion method, with very fine grained throttling of thrust of one pixel at a time up to several...

You may already be aware of this work, given that you used several of the buzzwords and laid out essentially the complete strategy, but this method of attitude and station-keeping control has been investigated.  Lewis, Janson, Cohen, and Antonsson called it Digital Propulsion in their work here:
http://design.caltech.edu/micropropulsion/99d.pdf
<snip>

Thats the one. Thanks for the link. I was really impressed with the innovative ideas in a smack my forehead kind of way. I dont see in the paper what specific impulse was achieved, do you have those numers? Pluse detonation should achieve in the order of 3000 sec.

Sorry, I don't remember those numbers any more (I worked on bonding the layers together, not the performance measurements).  From a very rough estimate, though: the fuel container is 700 um in diameter and 1500 um high for a volume of 0.00058 cm^3.  The density of solid lead styphnate is 3.02 g/cm^3, and assuming an average packing density of about 0.5 (the space was only partially filled with granulated lead styphnate), take the average density as 1.5 g/cm^3.  Thus each cell holds 0.000866 g, and converting to force we have 0.00849 mN.  Each cell outputs an average of 10^-4 mNs over a time of 1 ms, for a thrust of 0.1 mN.  Thus, ISP is roughly 12.  Certainly not someting you would want to use as a second stage, but remember that the point of the system is to produce very small impulses, so bad ISP actually helps that.

[rpspeck]

More Navigational uses of RF Phase Shift

Before detailing other uses of “Phase Locked Doppler” for navigation and control of orbital, cislunar and interplanetary spacecraft, I want to discuss other uses of “Phase Detection” for tracking and navigation. 

These techniques actually have a long history.  One of the most important is used in the VOR or OMNI (VHF Omnidirectional Radio) navigation beacons long used by aircraft pilots.  These compare the Phase of an Amplitude Modulated reference signal transmitted with the radio beam, with a direction dependent Frequency Modulation. The frequency modulation is actually produced by Doppler Shift: electronically “wobbling” the transmitting antennas in a small circular path.

These two types of modulation (AM and FM) – which have little interaction in good receivers – are separated from the RF signal and compared with a phase detector or phase comparator. The standard aircraft form uses a dial to set the desired aircraft path from the VOR transmitter as a “Radial” compass  direction, and a single needle shows if the plane is to the right or left of that path.  (A TO or FROM flag indicates whether the received signals have the relative phase for outbound flight in that direction, or if they have the reversed – 180 degree shifted – phase relationship indicating inbound flight along the same compass track from the opposite side of the transmitter.)

Other application have also been in widespread use.  But for our, customized use, these techniques also have great value. Closely related to the VOR application, a phase comparison of the RF signals from two antennas can give extreme accuracy in identifying the direction to the transmitter.  Two antennas, separated by a one wavelength distance (66 centimeter at 500 MHz) will see a 180 degree phase shift for a 30 degree change in the line of sight direction to the transmitter.  If these two signals are electrically combined in a single “antenna array”, a 30 degree FWHP “Beam Width” results. But if a good phase detector is used to compare the signals, the resolution becomes 1/6 degree = 10 minutes of arc. If the array of antenna elements is increased to a 20 meter width, the beam width can be reduced to about one degree.  But two antennas spaced 20 meters apart will increase the Phase Detector based resolution to 1/180 degree = 20 seconds or arc!   It is true that RF signal reflections can seriously distort this analysis, but additional antenna units can detect and correct for some of these errors, and antennas to monitor a rocket nearly overhead can have high immunity to such reflected signals.

Thus, by applying Phase Detection techniques to the output from a small group of antennas, the “Line of Sight” direction to a small rocket can be determined to very high resolution (even with a low power transmitter), and when combined with information from the “Coherent” (Phase Locked) Doppler system, the distance to the rocket along that path is obtained to even higher accuracy.  Simple tools thus yield extreme navigational precision in three dimensions.

For vertical flight into “Space” (at 100 km altitude) using the described system, a one milliwatt transmitter would be sufficient to provide second by second tracking of the flight with 1 to 5 meter lateral resolution (usable for guidance corrections) and better than 1 cm altitude resolution.

(Careful design, construction and calibration are necessary to consistently achieve the best performance mentioned.) 

Next: Space Tracking for Lunar Transfer, Lunar Descent and Orbital Rendezvous for Sample Return.

[rpspeck]

Micro-Space response to a forum comment:

Thank you!  I really hope that by sharing what I have learned in decades of rocket work – and related development – that I will be able to help others push “entrepreneurial” efforts into space.  I worry that the technical details may not interest many readers.  On the other hand, these details are often necessary for success!  Some are things known to “Old Rocket Pros”  who would prefer to discourage “Newbies” who think they can catch up, rather than help by sharing their wisdom.  Other details  have actually been forgotten by most “Space Pros” who just use the systems built a half century ago.  But modern electronics, electro-optics and microcontrollers can greatly simplify older techniques.  Adequate performance in launch and control can be obtained without duplicating historic systems.  Radically lower infrastructure and operating costs are possible.

For example: in 1959 a huge amount of money was being spent on global communications systems to link satellite tracking stations. Today, this can be accomplished by renting a few Iridium or Globalstar phones for remote areas,  plus dial up and web links to volunteers listening to your satellite in cities!

[rpspeck]

Control and Telemetry in a Phase Locked RF System (1)

Modulation forces a detectable modification on the transmitted Radio Frequency (RF) energy.  Usable modifications retain the primary characteristics of the electromagnetic radiation (so that the signal can be coupled through available cables, handled by practical antennas,  aimed, amplified and detected without unreasonable difficulty).  With such modifications, the RF signal can carry information.  The simplest – and still widely used – approach is to turn the RF signal off and on to communicate using an agreed upon code.  This is called CW (Continuous Wave) communication because when the RF is on, its waveform is steady and continuous.

To integrate any modulation scheme into a “Doppler” (interferometric) navigation system, the phase of the RF signal must be preserved in spite of the uplink “Control” modulation or the downlink “Telemetry” modulation. 

This becomes iffy with the on off approach, because the phase is unknown when the signal is off.  This may be tolerable for short off intervals, but still is risky if unexpected acceleration could change the velocity during such an interval.  It is better if the Amplitude is changed, but not reduced to zero, so that phase detection is still practical. 

This brings back the still common “Amplitude Modulation” (AM) modulation mode.  Since it can be implemented in a linear format, it can be used to transmit Voice, Music and Video.  But these modes have lost favor for a number of reasons, the best known being their gradual degradation form “noise” when the signal is weak.

Digital format modulation avoids this gradual effect (Graceful Degradation) and substitutes a VIOLENT systems failure, with weakening signal strength, including Worst Case Artifacts unless robust “error correction protocols” are incorporated in the data processing.  Only the availability of such protocols make CDs, DVD and Digital TV possible!  Since we are discussing Telemetry and Control which will be interfaced to digital systems, our modulation can be assumed to be digital with some level of error identification and correction included.

But keep in mind: There Are No Digital Radio Systems.  Everything about Electromagnetic Radiation, its generation, transmission, reception, and detection is inherently a linear process. Modifications can be induced  by voltage or current step transitions, and these in turn can be controlled by computer “bit line” outputs.  The magnitude of electronic voltages can be detected by a “comparator” circuit and “shaped” to become a computer “bit  line” input.  This process is inexact, adding noise through its uncertainty, but a decision is made about the present voltage and a subsequently “noiseless” bit of data is received by the computer.  It “seems to be” digital communications.  Done well, it works.


Returning from that side note,  AM modulation has other drawbacks:  it requires linear control of the Radio Frequency amplitude, which is fairly inefficient and modestly difficult.  Worse, the amplifiers which one might like to add to a transmitter, and those one must add along a very long cable connection, are not linear and significantly degrade the accuracy of the AM modulation. And worse yet, in mobile applications the extreme amplitude change resulting from signal absorbers, reflectors  and “multipath” interference are difficult to separate from the more modest amplitude changes deliberately added to the signal.  Motorola mastered this thorny problem with their practical “Car Radio” almost 80 years ago, and profited greatly as a result.

Many of the drawbacks of AM are less important, and easier to avoid, when the information transmitted is digital – on/off  bits (with a nonzero “off signal” to maintain phase analysis for our use).  But it remains a bit harder to use and has not been implemented in the convenient and attractive Integrated Circuits available for FM operation.  Thus AM  remains a possible, but unattractive option.

END 1

[Priest546]

even it is a theory there is a possibility that this thing may come true we never now what happens in the next day.,.
_________________
Humidifier Filters

[rpspeck]

4/18/09 Comment

Interesting reading, as always. However, I'm wondering how you arrive at that 1 mW RF would be sufficient...

At 500 MHz and 100 km, the free space loss would be 126 dB so, since 1 mW is 0 dBm, your signal strength will be less than -126 dBm. The best sensitivity I've seen in professional communication receivers for VHF/UHF is around -130 dBm for very narrow bandwidth that might be too narrow for this use. Galactic noise and polarization losses (Faraday effect) will also have significant influence on VHF/UHF when you are so close to the limit.

RPS
I have personally observed 10 nanovolt 50 Ohm RF signals with a 0.5 dB noise figure preamp in "single sideband" CW mode (0.01 microvolt).  This is not unreasonable with <1 nanovolt/sqrt(Hz)  noise level for the front end (300K source temperature), but implies about 100 Hz bandwidth for my ear at the optimum tone frequency.   The 50 Ohm 0.01 Microvolt RF would be -147 dBm.

Few people are talking about 100 Hz bandwidth, but I have achieved signaling with much lower bandwidth using synchronous detection - which is inherent in the phase locked modes. Data rates would be limited, but 100 Baud actually requires less bandwidth than this.  SMAD (Wiley J. Larson) lists 300K, 100 baud, 0.01 uV RF with an OK (10^-3) Bit Error Rate, even without error correction.
(Eb/No = 6.8 dB)

I did not suggest that this would be accomplished  with a dipole receiving antenna. I was just looking at a 14.4 dBd, 70 cm Ham antenna. Four of these would provide a very nice tracking array.  Using dynamic mixing to combine these signals, a net gain of  >20dBd would be available.  If each was modified to Cross Element configuration, and the resulting 8 RF signals properly amplified and processed, dynamic mixing of the cross components would accommodate Faraday and mechanical rotation with little loss.  These antennas might also approach 30K “Galactic Noise”, and see an additional  20dB S/N improvement as a result.

This is 20 dB better than the signal  you computed (which I agree with, except for <100% antenna efficiency – possible 50%), and 40 to 60 dB actual Signal/Noise. (With a 10 milliwatt transmitter, we could be talking about communication from the Moon.)  My Phase Accuracy assumes a  good S/N, but averaged to 1 Hz bandwidth. One degree phase accuracy (1/60 radian) needs S/N Voltage = 60x  in its effective bandwidth, =  36 dB with 1 Hz, or 16 dB S/N with 100Hz bw.   

No, this is not a simple receiver system.  Yes, this is exactly the kind of hardware I have been building,  and to an extent using for our rocket flights. 

[rpspeck]

Control and Telemetry in a Phase Locked RF System 2    4/24/09

Many communications systems now use narrow band FM  =  Frequency Modulation to add information to an RF signal.  (This is somewhat different from “wideband”, Broadcast FM.  The latter is much less efficient, requiring far more transmitter power but, at best,  it offers better linearity and lower noise than AM.  Broadcast FM  shares some of the advantages and weaknesses of Digital Broadcast, although it is much less energy efficient than the latter).  FM neatly avoids the amplitude linearity problems in amplifiers, transmitters and receivers (a problem noted for AM). One of the best things, for small rocket systems, is that tiny Integrated Circuits offer high gain (extremely nonlinear) “limiting amplifiers” which boost the received signals up to a  constant voltage level before analysis.

FM is not a natural for a Phase Locked system, because “Phase Lock” requires a frequency matching a stable reference. But “Frequency” is actually the “Rate of Phase Change”, and  “Phase Modulation” transients will be detected by FM systems.  If the modulation is contrived as a “Return to Zero” phase deviation, it will be detected in the FM system as a bipolar (+/-)  frequency cycle and have no net effect on the Phase used for Doppler analysis.  “Narrow Band” FM by definition involves phase deviations of less than ½ RF cycle with maximum level  modulation.  A fraction of that is sufficient for detection with a good signal, and the impact of this on the Doppler Phase analyzer can be predicted, and compensated.  FM actually works quite well producing a fixed AVERAGE frequency with audio inputs, since these are usually symmetric AC signals, and are normally forced to have no DC frequency offset component. This is much more difficult to arrange with digital signals, but not impossible. 

A quick way to produce a “Net Zero” effect with digital Frequency Modulation (called “Frequency Shift Keying” = FSK) is to transmit each bit followed by its compliment (adding powerful error detection capability).  A variety of  techniques can encode a block of data with both a “Net 50%” sum constraint and error correction capability. This may require sending a  sub-block as compliments plus adding “Parity” type compensation bits.  Another technique is to use compensating “Bipolar” +/- Frequency Shift Keying so that the average frequency is fixed.  Many of these techniques also improve standard FSK, by allowing “Automatic Frequency Control” to keep the receiver optimally tuned in spite of Doppler shift and transmitter drift. Most of the techniques I just mentioned send fewer real data bits with a given link bandwidth then would simple FSK, but other error detection and correction  techniques also add extra bits, and reduce the real data rate.

Micro-Space is working with some of these techniques to allow the addition of “Phase Locked”  capability to the FSK hardware we have been using for nearly two decades. Results are promising.

[rpspeck]

Control and Telemetry in a Phase Locked RF System (3)

The best modulation method, for these systems, produces the greatest energy efficiency and maintains Phase Lock with a single “quirk”.  It resembles AM, but also captures all the benefits of FM.  It is BPSK = Binary Phase Shift Keying.  The way to split all possible Phase conditions evenly into two categories is to “bisect” the polar phase plot, and establish the reference phase states 180 degrees apart.   These two states have the greatest separation, for a fixed RF voltage, and will remain reliably above the noise level with the weakest signal.  Analyzed for Amplitude (using a phase sensitive detector), the states become not X and 0, for a given RF power, but X and -X.  From this standpoint, the detector voltage is doubled  with no increase in detector noise.  Although the RF is on continuously (not off for part of the time as with CW), the voltage difference between the two states is what would be obtained with Four Times the transmitter power!  This is the communication mode used for the most challenging interplanetary links.  Since both the frequency and phase are stable with any modulation data, it is well suited for Phase Locked use. 

The “quirk” mentioned is that the inevitable, small phase errors that occur are analyzed incorrectly (backward) when the RF phase is reversed. The detected errors must be used in a control loop to maintain the “Lock”, and incorrect analysis will magnify the errors instead of reducing them.  Avoiding this problem is awkward, but not particularly difficult. Correction of the small errors does not have to be done immediately, but can be implemented with a modest time delay. The error detected in a particular interval can be saved until the phase state for that bit has been determined , and then applied with a corrected polarity if necessary. 

The “Limiting Amplifier” in FM receiver circuits works perfectly with these signals, since the phase reversal is preserved even if the actual amplitude is hidden. The FM circuit's Frequency Detection function can even be used to guide the receiver tuning very close to the Lock condition, allowing that state to be established.  These are desirable features, and worth pursuing.  It is true that phase lock is harder to achieve that FM tuning, and can be very challenging with a weak signal.  In addition, it is easy to lose track of which phase represents 0 data, and which represents a 1 (a brief signal interruption, producing one nanosecond of timing uncertainty, can do this).  Some predictable structure must be embedded in the signal to allow recognition of the correct polarity.  With the bidirectional command/telemetry loop used for Doppler navigation, this confusion can be easily detected by programming the rocket system to echo a predetermined selection of the command bits (probably with a fixed bit as well). A failure could trigger a brief reference transmission, which would reset both systems to the correct phase states.

Modest complications bring profound benefits.  The very high resolution tracking data mentioned earlier becomes available: the signal to noise level is enhanced: the RF link bandwidth is minimized, because of  exact tuning, to further reduce the noise, and a wide range of efficient amplifiers can be used.  Micro-Space is modifying our rocket Radio Systems to work in this mode. 

At present, we are using this mode in our “ElectroMagnetic Tractor Beam” demonstrators, which also use phase detection to determine unit positions.  The BPSK data controls operation of the mobile unit, and synchronizes magnetic drive forces.  We used related technology a number of years ago in a patented, optical detection and ranging system. We have also used “Phase Analysis” for precision sensing with a number of mechanical position sensors, and in automated, optical film measuring interferometers. Related to this is synchronous detection, which we have applied in many systems dating back to an old “Moon Bounce” communications effort. 

We have been “toying” with parts of this system for a number of years, without forcing the subsystems into workable form or shrinking them to a size consistent with our FM (FSK)  production assemblies.  But we are now “biting that bullet” and will soon have units in rockets ready to fly.