WO2014025375A2 - Communication between spacecraft using free-space optical telemetry - Google Patents

Description

COMMUNICATION BETWEEN SPACECRAFT

USING FREE-SPACE OPTICS FOR TELEMETRY

CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims the benefit of Provisional Patent Application Serial No.

61/687,527 filed April 26, 2012, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system and method for communicating between spacecraft. More particularly, it relates to the use of free-space optics for telemetry to communicate between spacecraft.

BACKGROUND OF THE INVENTION

The present invention relates to spacecraft, defined herein to include satellites, rockets and the payloads and sub-payloads thereof. An essential component of any spacecraft architecture is the downlink transfer of data from the spacecraft in space to a remote ground station. Typically this transfer is accomplished through a radio frequency (RF)

communications link that integrates a transmitter and antenna on the spacecraft, and a dedicated antenna and receiver on the ground. Typically these links are implemented within the microwave band with frequencies above lGHz such as the S band.

The RF communications link architecture can be complicated further for missions that incorporate multiple spacecraft (e.g., formation flying or a single rocket that has a main payload and multiple sub-payloads that each have data needing to be transmitted in real time to the ground). In cases such as these, one solution is to integrate dedicated ground stations for each of the spacecraft. This can complicate support for missions that require many payloads in space, such as science missions that integrate an array of sub-payloads to provide spatially distributed measurements throughout flight.

One goal of the present invention is to provide a new means of communication between multiple spacecraft in space using free space optics (FSO) for telemetry. It incorporates the use of super bright light emitting diodes (LEDs) as transmitters and photomultiplier tubes (PMTs) as receivers. The LED PMT-based free-space optical telemetry provides the advantage of a simpler more cost effective architecture that can be utilized for multiple spacecraft in space, over limited distances. For example, multiple sub-payloads can communicate with the main payload simultaneously, such that only a single downlink is needed to a ground station.

Another goal of the present invention is to lower the power required to implement the downlink, as both PMTs and LEDs can provide significant improvements in power dissipation versus RF systems. Additionally, the use of FSO for telemetry potentially provides a higher level of security as the known security deficiencies of wireless RF communications are removed.

SUMMARY OF THE INVENTION

The present invention includes a system and method of communication between multiple spacecraft, where the communication between the spacecraft uses free-space optical telemetry.

In one embodiment of the present invention the transmitter is a laser diode, and the laser diode may produce chained-pulses.

In another embodiment of the present invention, the receiver is a photo multiplier tube.

In another embodiment of the present invention, a filter, a baffle or a prism may be used.

These aspects of the invention are not meant to be exclusive and other features, aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Figure 1 shows a basic diagram of one embodiment of the system and method of the present invention. Figure 2 shows a block diagram of hardware used in one embodiment of the present invention.

Figure 3 represents results from signal modulation testing of one embodiment of the present invention.

Figure 4(a), Figure 4(b), Figure 4(c), Figure 4(d), and Figure 4(e), are results of testing of one embodiment of the present invention without the use of a filter or baffle.

Figure 5(a), Figure 5(b), Figure 5(c), Figure 5(d), and Figure 5(e), are results of testing of one embodiment of the present invention with a filter, but without a baffle.

Figure 6(a), Figure 6(b), Figure 6(c), Figure 6(d), and Figure 6(e), are results of testing of one embodiment of the present invention with a baffle, but without a filter.

Figure 7A and Figure 7B show plots of signal and noise values for different separation distances and different duty cycles for one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method and system of the present invention uses free-space optics (FSO) for telemetry for communicating between spacecraft in space. One preferred embodiment, described below in detail, involves a sounding rocket payload, communicating (in space) with an array of small sub-payloads.

FSO for telemetry is an optical communication technology that uses light propagating in free-space to transmit data for telecommunications or computer networking. "Free-space" means air, outer space, vacuum, or a similar medium. This contrasts with using solids such as optical fiber cable or an optical transmission line. One embodiment of the method and system of the present invention for using FSO for telemetry comprises super-bright LEDs as the transmitter. One embodiment of the method and system of the present invention for using FSO for telemetry comprises low-light photomultiplier tubes as the receiver.

The method and system of the present invention is a low cost solution for communications between spacecraft in space. The method and system of the present invention uses very low power for such communications as compared to current systems. The method and system of the present invention, which uses FSO for telemetry is more secure than RP communications. Optoelectronic and optical devices were evaluated against environmental and system level criteria. Preliminary tests used available in-house components and bench-top equipment to determine potential distances and data rates of FSO. In one embodiment of the method and system of the present invention, a light emitting diode with an optical power output of approximately six milliwatts (mW), and a photomultiplier tube, originally purposed for use on the Compton Gamma Ray Observatory, was used to transmit and receive signals over a distance of four-hundred meters in-atmosphere. In one embodiment of the method and system of the present invention, the transmission distance was increased to five-hundred meters using an optical bandpass filter. In one embodiment of the method and system of the present invention, the transmission distance was increased to nine-hundred meters using a baffle. In other embodiments, even greater distances and data rates may be achieved by using additional transmitters with greater optical power output.

The method and system of the present invention using FSO operates in a unique environment, and as such, unique factors must be taken into account with regard to the transmission wavelength. The choice of transmission wavelength must consider the presence of the sun's aurora, along with other sources of ultraviolet (UV), visible, and infrared (IR) light, both broad and narrow spectrum.

The brightest objects to consider as possible noise sources are the sun and moon, both of which present broad spectra of light. Generally, the method and system of the present invention is used at night, and though many that are used at night do not encounter direct sunlight; some may encounter direct sunlight when they reach high enough altitudes. Thus, the sounding rocket needs to keep the sun below the horizon. Direct moonlight also needs to be kept from the field of view of the receiver. Use during a new moon is preferred as moonlight reflected by the Earth can still affect the receiver.

The next greatest source of light is that from stars and other distant sources. For the purposes of the method and system of the present invention this light was regarded as 'white'. This assumption was made because stars themselves emit blackbody radiation at different temperatures. While filtering this light may not be practical, other methods of limiting its impact on the receiver were studied, namely the use of a baffle. Furthermore, the mid-IR and far-IR energy (heat) given off by the surface of the Earth consists of wavelengths greater than approximately 2500 nm, thus the transmission wavelength should be at a shorter wavelength than this.

Auroral photons are generated by the collision of charged particles from the sun with the constituent particles of Earth's atmosphere. These resulting photons have a wide range of energies across the optical spectrum and are confined to specific wavelengths within the range. See, A. V. Jones, Aurora, Dordrecht- Holland/Boston-U.S. A.: D. Reidel Publishing Company, 1974. If these same wavelengths are used by a transmitting light source, then considerable noise will be present in the received signal, making the wavelengths between these emission lines much more desirable for the choice of the transmission source. Target range is nominally 400-430 nm, exclusive of auroral emissions. It is also important to have a very narrowband transmission source.

The final environmental source to be considered when determining the optimal energy of the transmission source is the geocorona. The geocorona is a result of sunlight being scattered by hydrogen in the exosphere, and does not yield photon emissions in the far- ultraviolet (Lyman Alpha) greater than 121.6 nm in wavelength. Therefore, it is best to choose a transmission source with a wavelength greater than this.

A prototype of one embodiment of the system and method of the present invention utilizes the same photomultiplier tubes (PMTs) as used for the Compton Telescope (COMPTEL) on NASA's Gamma Ray Observatory (GRO) for the receiving hardware. These 2 inch PMTs were built by THORN EMI Electron Tubes of Ruislip Middlesex England and designated as Type 9755NA (the 9755NA). The PMTs are flight ruggedized, end-on, have a transmission bi-alkali photocathode, a borosilicate plano-concave window, and ten beryl copper dynodes in a circular cage design (compact circular focus, or FAST). See, J. Macri, COMPTEL: Imaging on the COMPTON Telescope on the Gamma Ray Observatory, Critical Design Review Part III, Electrical Subsystem & Harness, Chapter 2. Presented at MBB Ottobrunn, 1984. COMPTEL's PMTs and associated electronics are purposed for scintillation detection. While they are not necessarily ideal for this application, some of their properties are advantageous. It is recognized that the opportunity exists to improve operating characteristics by using other receivers, including other PMTs or avalanche photo diodes (APDs), and the like. The back-end components originally used with the 9755N A, namely a series of output capacitors, make it well suited for AC operation. These capacitors also allow for the cathode- grounded operation of the PMT, making the connection to analog-to-digital conversion electronics less cumbersome. Another advantage was the presence of large resistors in the bleeder string (dynode chain), with 5.1 ΜΩ resistors limiting current in 'bright' environments, such as aurora or starlight.

With the 9755N A chosen for use with the prototype, an optical transmission system for the sub-payloads was then considered keeping the 420 nm peak radiant sensitivity of the PMT in mind. Radiant sensitivity is more important than quantum efficiency (QE) in the method and system of the present invention using FSO for telemetry because it does not rely on photon counting. This is because the background light is too great. Instead, it relies on the optical power incident to the PMT face. Because initial testing of the prototype system did not involve auroral emission spectra, the peak wavelength of the light transmitted was chosen to be as close as possible to the peak radiant sensitivity of the PMT, despite there being auroral emissions at 420 nm. This allowed the greatest possible reception for in-atmosphere testing, where attenuation was thought to be a larger factor. As there are gaps in auroral emissions near 420 nm, transmitters at those wavelengths should be considered as well.

The small size of sub-payloads requires that the transmission system be small and simple. Light emitting diodes (LEDs) are a good choice, as they typically have greater beam divergence than lasers. This greater beam divergence prevents connection loss of the optical pathway if the sub-payloads are not able to maintain nominal attitude, through coning for example.

High-output power units with 420 nm peak spectral outputs were used. These were Optek Technology's OUE8A series UV metal can LEDs. The OUE8A series LEDs have a 12 nm spectral half- width, with a peak wavelength of about 375 nm to about 425 nm in 5 nm wavelength increments. Their optical power output varies for each wavelength range, from 1.8 mW to 15.4 mW, when driven by 100 mA at 3.3 V. The data sheet specifies that a 1.6 A drive current is possible with a 100 pulse width and 10 % duty cycle.

The highest optical power output LED operating near 420 nm was found to be Optek's OUE8A425 UV LED. The unit used was rated at 8.8 mW to 1 1.5 mW, with a 12 nm spectral half width, and an emission angle of 18° FWHM (full-width at half-maximum), producing a 316 m full beam width at a distance of 1 km. No lenses or other optics were used in addition to the that of the can package. It is recognized that lenses and other optical devices known to those skilled in the art may be added to increase range and performance.

After the initial LED tests were concluded, greater optical power output was desired and laser diodes were studied as an option for the sending hardware. In one embodiment of the method and system of the present invention, the laser diodes used were produced by Sharp for use in Blu-ray Disc players and writers. The market for these has grown with the adoption of Blu-ray equipment, and Sharp continues to produce progressively more powerful laser diodes to keep pace with the demand for increased disc writing speeds. In one plane the Sharp lasers were found to have the same 18° FWHM divergence as the Optek LEDs, and in the orthogonal plane the FWHM divergence was 9°. This suggests that two Sharp laser diodes can provide the same coverage as a single Optek LED when they are mounted with 4.5° of separation in their mutual planes of reduced beam width.

The Sharp laser diodes have an output spectrum centered at 405 nm. Though there is a strong auroral emission at this specific wavelength, the Sharp data sheets indicate that drive current, diode case temperature, and variations in unit manufacturing can vary this wavelength enough so that it falls within the 405 nm to 420 nm auroral emission gap. The Sharp GH04P21A2GE laser diode provides a continuous optical power output of 105 mW or a pulsed optical power output of 210 mW (50 ns pulse width, 50 % duty cycle), drawing 120 mA of current at 5.4 V in continuous mode. This makes the Sharp laser diodes more efficient than the Optek LEDs by a factor of approximately five and perhaps as high as a factor of ten, depending on the power output of a particular LED, enabling the use of more devices for the same power requirements.

LEDs and lasers can both generate large amounts of heat while operating. Sounding rocket electronics are typically operated within a margin of half their maximum ratings so as to prevent damage from heat. It is advised that any optical transmitter used be tested both in- atmosphere and in-vacuum for temperature limits before failure. This also means that light output may need to be reduced according to the results of these vacuum tests, or thermoelectric coolers may be added to absorb the excess heat. The OUE8A LED data sheet indicates a maximum power dissipation of 370 mW and a maximum operating temperature of 85 °C. Sharp does not provide power dissipation values for their Blu-ray laser diodes but does indicate a maximum operating temperature of 70 °C.

Narrow bandpass optical filters were used to mitigate the problem of noise from auroral light. The filter passband must closely match the wavelength of the transmitter to allow the transmission to pass, assuming the transmitter is already matched closely to the receiver's peak radiant sensitivity. Because color filters generally allow light of a particular wavelength to transmit despite large angles of incidence, these were chosen for this application over interference filters which allow only narrow angles of incidence at a given wavelength. This is despite the fact that the transmission window of color filters is typically not as steep as those of interference filters and their transmission percentages tend to be lower as well.

In one embodiment of the method and system of the present invention, a color glass filter (model 46530) produced by Barr Associates Inc. was used as it was readily available in- house. While its 2 inch diameter matched that of the 9755NA PMT, the filter was originally ordered with the intent of detecting a specific auroral emission line of 427.8 nm, which makes it less than ideal for this application. It did, however, provide useful information about the characteristics and impact that a similar filter will have on the method and system of the present invention. A better performing (both in transmission percentage and passband wavelength) color filter is expected to be used in other embodiments.

Most, if not all, current sounding rocket's sub-payloads communicate directly with a ground station just as the sounding rocket does. In order to reduce technical challenges, one embodiment of this system and method of the present invention will utilize a unidirectional star network topology. This means that all data will move from each individual sub-payload to the sounding rocket over a unique FSO link. These data will then be downloaded to the ground station via radio link. In this case, the loss of an individual sub-payload does not equate to a loss of the entire system, unless a sub-payload begins to transmit out of turn, thus interfering with other units' communications. The downside to this topology is that the loss of the sounding rocket equates to a loss of data from the sub-payloads even if they remain operational. This situation is very similar to that of campaigns which feature a lone sounding rocket, thus there is no increased risk of failure in comparison. Given the proposed size of the sub-payloads in one embodiment of the system and method of the present invention, two-way optical communication is possible but poses a substantial challenge. This challenge is due, in part, to both the size and cost of optoelectronic devices needed for two-way communications. Bi-directional systems are also more technically complex. From a communication theory standpoint, however, the lack of feedback in a data diode can be a disadvantage. This is because an acknowledgement of the received data is not possible and therefore no resending of the information can be requested when a transmission is corrupted or not received at all. Integration of a checksum or some other form of error detection may help to counter this disadvantage.

In one embodiment of the system and method of the present invention, the use of unidirectional network topology presents a challenge from a timing standpoint as all communications will be on the same wavelength, e.g. a shared channel. This is so, unless multiple receiver/transmitter wavelength combinations are used. If any two (or more) sub- payloads transmit at the same time, their signals will interfere, necessitating the use of time- division multiplexing (TDM).

The unidirectional network topology requires that each sub-payload time its own transmissions, and this can be accomplished, for example, with a GPS receiver's one pulse per second (1-PPS) signal output. For example, in a self-polling configuration, as in any TDM design, each sub-payload will be allotted a window in which to send its accumulated data to the sounding rocket. The duration of this window is based on the number of sub-payloads on the station. This creates an inverse relation such that as the number of units is increased, there is less time for each unit to transmit. For example, with ten sub-payloads on station, and all units transmitting every second, each unit would be allotted one-tenth of a second to send its accumulated data from the previous nine-tenths of a second while the other units were transmitting their data.

With no feedback as to the efficacy of any one transmission, limiting the chance of errors in subsequent transmissions becomes a critical design consideration. As an example, if a particular burst of data is interrupted or otherwise degraded, the following data packets must not become degraded in a cascading fashion. Therefore, short duration bursts on the order of a second or less may help increase overall link fitness compared to multi-second bursts. Also, because each sub-payload will eventually go out of range, using longer bursts means that some sub-payloads could have a large backlog of accumulated data as they crossed this threshold. A limitation is met when attempting to decrease the transmission duration too far because LEDs require a non-trivial amount of time to turn on and off, and errors in timing could create overlap.

In one embodiment of the method and system of the present invention, initial considerations for calculating a reasonable dead time between sub-payload transmissions included the variance of the GPS 1-PPS signal, as well as the propagation delay of the light from sub-payloads at different distances. For example, when light travels 1 m in approximately 3.3 ns, and 1 km in 3.3 μ≤, and one sub-payload is at 100 m and another is at 1.1 km, this delay becomes non-trivial and can be used as a minimum for dead time.

It is recognized that the forward edge of an incoming light pulse is the most distinguishable feature of PMT output. This is in contrast to the peak signal level and the trailing edge of the output, which are less pronounced, especially during high pulse rates. Also, pulse detection is a relatively simple prospect when the signal is interpreted into the digital domain. Other encoding systems that require the output signal to maintain the same shape as the input signal, such as phase shift keying or line codes like Manchester modulation, among others, do not translate well across the transmitter-PMT bridge.

In considering these constraints, the signal modulation options deemed suitable for one embodiment of the present invention were M-ary Pulse Position Modulation (PPM), and On/Off Keying (OOK). Frequency Shift Keying (FSK) is a possible third option. If a pulse is lost in PPM or OOK, the information encoded in that single pulse is lost. Since FSK uses two (or more) pulse frequencies to represent data, it trades decreased bandwidth, compared to PPM or OOK, for the potential to mitigate errors resulting from dropped pulses.

In general, the best modulation technique uses each pulse to convey as much information as possible, while retaining a desirable level of resistance to errors. Including a degree of error prevention, such as repeating transmissions of data, using checksums, and the like, could decrease overall data rates but may ensure more complete and accurate information conveyance. While there are many types of error prevention, the ideal method for each application should be determined by testing the system to determine particular modes of failure so they can be addressed.

In one embodiment of the method and system of the present invention, the magnetometer on each sub-payload is expected to require most of the telemetry bandwidth as compared to GPS position information and housekeeping information. Additionally, the magnetometer output should be sampled at a high enough rate to avoid aliasing, but not so high that potentially unrealistic data rates of megabits or gigabits per second are required.

The magnetic field variations of interest occur at frequencies between 0 Hz and 5 Hz. If a sub-payload and its magnetometer are spinning, they should ideally do so at a rate of at least double the 5 Hz maximum, according to sampling theory. Otherwise, aliasing would occur when samples are taken at the same point in the spin of the sub-payload, perhaps to the point that a sinusoidal signal would appear to be constant. With the sub-payload spinning at 10 Hz for attitude control, the magnetometer output should be sampled at a minimum of 20 Hz.

The earth's magnetic field ranges from -50000 nT to 50000 nT in the area of interest, and a sampling resolution of between 1 nT and 2 nT is desired. In one embodiment of the method and system of the present invention, 16 bits are required for each sample (2¹⁶ = 65536), including a polarity bit. Also, each magnetometer consists of three axes, equating to a total of 960 bps. An additional 100 bps of packet header and housekeeping data are added to arrive at a data rate of 1060 bps for each sub-payload is required. In one embodiment of the method and system of the present invention, if 10 sub-payloads are ejected, and assuming the combined data accumulation rate for all units, then the transmit rate for each unit would be 10600 bps.

Following the TDM strategy above, each of these sub-payloads would accumulate data for one second then transmit it within approximately 100 ms. Even though each sub-payload is logging data at 1060 bps, it must still transmit at 10600 bps to fit within its allotted window. This data rate of 10.6 kbps should be regarded as a minimum. In one embodiment of the method and system of the present invention, the use of additional sub-payloads with greater resolution (e.g. 20 bits per magnetometer sample), and additional checksum bits is preferred. It is recognized that this may require higher sampling rates.

In one embodiment of the method and system of the present invention, the 9755NA PMT has a 2 in diameter and a detector area of 1946 mm. It is recognized that if a transmitter deviates from a 0 ° angle of incidence, the effective area of the PMT face will be reduced by the cosine of the angle of incidence. This would result in a 70 % reduction in the detector area at 45 °. This means that more than one PMT would be required for receiving the incoming light from every sub-payload's transmitter. This need for multiple PMTs is due not only to the angular response of the PMTs, but to the rotation of the sounding rocket as well. One embodiment of the method and system of the present invention features a sounding rocket equipped with four (or perhaps more) PMTs facing radially outward, with each PMT face flush with the outside of the fairing.

A basic diagram of one embodiment of the sounding rocket and sub-payload distribution is shown in Figure 1. Referring to Figure 1 , the sounding rocket 20 is shown with four photo multiplier tubes 10 which are radially distributed to receive data from several sub- payloads 30. If the sounding rocket 20 spins quickly enough, more than one PMT 10 will receive an individual sub-payload's transmission 30 as the light passes from one PMT 10 to the next. Even neglecting the rotation of the sounding rocket 20, it would still be possible for two PMTs 10 to receive a single sub-payload's 30 transmission in one embodiment method and system of the present invention comprised of four (or more) PMTs.

The potential for more than one PMT to receive a single transmission necessitates that the sub-payloads outputs be OR'ed together onboard the sounding rocket, or that all PMTs have their signals transmitted back to the ground for later comparison. A voting scheme could also be developed to compare the received transmissions of the PMTs which would help to filter out erroneous pulses. Two potential hardware options for the electronic back-end to the PMTs for analog-to-digital (A/D) conversion of the output signal include the use of a resettable threshold detector to latch the leading pulse edges of the PMT's output signal; and sampling the entirety of each PMT's output signal into the digital domain for direct transmission to the ground. The first option may require more processing in-flight and could be susceptible to erroneous triggers from noise, while the second option would likely require more rocket-to-ground bandwidth, perhaps beyond the capabilities of currently available RF links.

In one embodiment of the system and method of the present invention, an alternative receiver configuration comprises one PMT located in the center of the sounding rocket facing upwards or downwards. A system of mirrors, prisms, or the like, could be used to collect the transmitted light and direct it to the single PMT. This embodiment could also minimize the complexity of the receiver hardware.

It is recognized that PMTs should be operated within an envelope of input flux levels bounded by dark current and general noise at the low end, and saturation at the high end. Therefore, it may be necessary to increase the output of the sub-payloads' transmitters as they move away from the sounding rocket, since the incident flux at a PMT's face will decrease by 1/r² where r is the separation distance of a sub-payload from the sounding rocket. This creates a potential mode of failure and an unneeded design complication if the transmitters can operate at full power for the duration of flight without overwhelming the PMTs. If the saturation recovery time of the PMT is short, it may be possible to operate the transmitters at full power even if the PMTs are peaked at first, but then operate nominally soon thereafter. During testing of a prototype system no saturation of the PMT by the LEDs was seen, but depending on the choice of transmitting hardware, it may occur. In addition, minimizing the accumulation of heat generated by the transmitter may also warrant the use of this strategy.

In one embodiment of the method and system of the present invention, a method of compensating for increasing separation distance would be to use multiple light sources on each sub-payload. There, a staged increase in the number of functioning sources over time, as opposed to an increase in any single unit's drive current could be utilized. Multiple light sources may also be required to obtain the necessary optical power from the transmitter.

In one embodiment of the system and method of the present invention, each light source could be driven by its own amplifier from a common signal. One source would be used initially, with additional ones being activated as the sub-payload moved away from the sounding rocket. For example, one source would begin broadcasting immediately after ejection, and as the sub-payload moved away from the sounding rocket, it would quadruple the number of broadcasting sources for every doubling of distance until all sources were activated.

Whatever the compensation strategy, it should be exclusive of signal modulation as the same variables (such as pulse width or height) should not be modified. The compensation strategy requires that the approximate separation velocity of the sub-payload and the sounding rocket be known. This velocity data could be acquired by on-board GPS calculations, or by pre- testing the rocket motors for thrust values beforehand. In Figure 2, a block diagram of the hardware used for testing one embodiment of the system and method for the present invention is shown. Exemplary equipment operating considerations and aberrations are in parentheses. In Figure 2, the diagram shows a Function Generator 1 (Tektronix AFG 302 IB); a Violet LED in series with a resistor 2 (OPTEK OUE8A425G, 47 Ω, 49.3 Ω measured, 1 W Current Limiting Resistor, 5% tolerance, 25 ppm/K Temperature Coefficient); a High Voltage Supply 3 (ORTEC 456 0 - 3 kV model set to 1.32 kV); a PMT 4 (THORN EMI Electron Tubes 9755NA); a DC Supply for preamp 5 (TENMA 72-6610 set to 24 V); a Preamplifier connected to PMT dynode output 6 (HP 5554A); and an Oscilloscope 7 (Tektronix THS720A) with 50 Ω impedance coax cables with BNC terminations.

There were two types of tests conducted on certain embodiments of the method and system of the present invention. First, tests of the PMT and the LED inside a dark chamber were performed to demonstrate OOK signal modulation. Second, outdoor tests were conducted at several measured distances to demonstrate the operation of the system in a more realistic setting.

For the signal modulation tests, a custom machined dark chamber was used as an operating enclosure for the PMT and LED to provide a benchmark. The chamber measured 10 inches in height and 6 inches in diameter and could be separated into two halves at the middle, with the PMT mounted in the bottom section facing up and the LED in the top section facing down. The separation distance from the LED lens to the PMT face was measured at 10 cm. The PMT was built up with a COMPTEL Dl bleeder string with flight grade components, while the LED was connected through a BNC jack and external resistor. The enclosure was coupled together for the 10 cm tests and separated by the appropriate distance for the long distance tests.

In general, good PMT handling practices, such as leaving the tube in the dark and allowing it to 'warm up' before use, were followed as given in the Hamamatsu PMT Handbook. See, Hamamatsu Photonics K.K. Editorial Committee, Photomultiplier Tubes: Basics and Applications, 3rd ed. Shimokanzo, Iwata City, Shizuoka Pref, Japan: Hamamatsu Photonics K.K. Electron Tube Division, 2006. Also, the dark chamber required the use of cables approximately 1 m in length for the high voltage input and signal outputs. These cables were brought into the chamber through a rubber sleeve of similar length to ensure that no light entered the unit. Shorter cables are recommended for future tests to help reduce output signal noise.

In one embodiment of the system and method of the present invention, the HP 5554A (5554A) was used as the trans-impedance amplifier for the PMT. Trans-impedance amplifiers are unique in that they 'translate' small currents into more easily measured voltages. Product documentation for the 5554 A recommends that its signal input have a 50 Ω impedance. The 9755NA bleeder string was equipped with two signal outputs, one from the anode and one from the final dynode. Because the final dynode had a 50 Ω output impedance, this was used instead of the anode output for connection to the 5554A. The only disadvantage to this setup was the resulting inverted signal output. Most importantly, it was the amplifier's output of the PMT signal and not the PMT signal itself that was viewed with the oscilloscope in these tests. Any aberrations in the amplifier output could therefore be attributed to the amplifier and not the PMT.

An example of this was the 5554A's output of a persistent, undesirable 49 kHz sinusoid, in addition to the desired output signal. The equipment was exchanged one item at a time to find that the preamp was the source. Adjusting the preamp's physical location and orientation with respect to the other equipment helped to reduce this signal to below 50 mV_p-p. This is suggestive of signal coupling as a potential source of the sinusoid.

There were two amplification setting adjustments for the 5554 A. According to the unit's manual, the 2x setting on the second amplification adjuster yields the highest dynamic range of the several possible settings, so this was used throughout the testing. The first adjustor, the pulse type, was set to provide maximum amplification, 1 OOOx, with a sharp leading edge and an exponentially decaying tail pulse with a 100 μβ time constant. The bandwidth of the preamp was not known.

In FSO On/Off Keying (OOK), long chains of identical logical values can develop as a series of either Is or 0s. If the Is are coded as pulses and 0s as dead time, for example, a long series of Is could have the potential to 'pile up' and become indistinguishable. Conversely, a long chain of 0s could cause the receiver to loose synchronization with the transmitter, since synchronization may not be nominal as a result of propagation delays. Thus, testing for each problem became important. To replicate the sustained pulse chain, the function generator's burst mode was used to produce a series of square pulses in rapid succession, followed by a dead period. While these tests were carried out in the dark chamber, different results may occur at greater distances.

Figure 3 shows the results of this test using a 4-pulse chain at 100 kHz frequency and

10 % duty cycle. This demonstrates that the PMT was able to return to a rest state after receiving each pulse, thus preventing any 'pile up' or overlap. The oscilloscope's frequency indicator (in the lower right of the figure) indicates the number of received pulses per second. The constant value of this indicator implies that there were few, if any, dropped pulses. Both 500 kHz and 1 MHz pulse frequencies achieved similar results, as did chains up to 30 pulses in length.

Difficulty arose, however, when pulse chains with higher duty cycles were transmitted. For example, a 6-pulse chain at 500 kHz and a 50 % duty cycle exhibited individual pulses almost too close together to be distinguishable, potentially complicating its decoding. This would seem to indicate that shorter duty cycles would be best in OOK.

Without a dedicated M-ary PPM modulator and demodulator, it is difficult to determine the accuracy of this modulation technique over an FSO link. As shown in the preceding figures, the number of pulses per second can be very accurately received. While this implies that the average pulse-to-pulse duration is relatively constant, it does not definitively demonstrate PPM capability since pulse-to-pulse duration could vary over much shorter time periods.

In order to test the system's reaction to increasing distance, an outdoor location with a long line of sight that allowed for easy movement between test locations was found. The moon had set less than an hour before the beginning of the test, with the sky just beginning to brighten from the sun as the test concluded. Preparation of the site began with marking out a receiver location. Marks were then placed along the roadside every 100 m, extending to 900 m. Once dark, with the Milky Way partially visible and no ground based light in the PMT field of view, the receiver hardware was put in location. Power for the equipment was provided by a 750 W inverter connected to a battery in an idling vehicle. The transmitter was powered by a 350 W inverter also connected to an idling vehicle. The transmitter was moved to each 100 m mark where measurements were made and screen captures of the oscilloscope output taken. Several issues arose from using automobiles. First, the transmitting vehicle was equipped with day time running lights that illuminated the ground in front of the vehicle, creating a source of background optical noise for the PMT. Also, the oscilloscope was unable to obtain a lock on the LED pulse frequency, despite it doing so while connected to grid power in previous 90 m distance tests. It is assumed that this was due to the use of an inverter. The inverter powered function generator may also have contributed to this by not being precisely timed. EMI could also have been a factor.

All tests were conducted with a 0 V - 5 V square wave at a 10 kHz pulse frequency, as indicated by the function generator. Through prior calculation and testing, this 5 V maximum was estimated to put out 6 mW of optical power, though this value was not measured directly. Occasionally the frequency was increased to 20 kHz, which merely doubled the number of pulses per unit time, with no ill effects on the detection of the pulses. Referring to the Figures, the distance comparison figures do not use the same vertical scale for all panels so that each pulse shape could fit within the given space. The horizontal scales are all 50 μ8. Also, as the dynode signal is inverted and was not righted for the captures, each pulse is effectively upside down, with the pulse of interest being negative.

Figures 4(a)-(e) are a compilation of oscilloscope output screen captures for the range of distances for results without the use of a filter or baffle. In Figure 4(a) the PMT output of LED pulses was at a 100 m distance with a 500 mV scale. In Figure 4(b) the PMT output of LED pulses was at a 300 m distance with a 200 mV scale. In Figure 4(c) the PMT output of LED pulses was at a 400 m distance with a 100 mV scale. In Figure 4(d) the PMT output of LED pulses was at a 500 m distance with a 50 mV scale. In Figure 4(e) the PMT output of LED pulses was at a 900 m distance with 50 mV scale. Figures 4(a)-(e) were at a 10 % duty cycle.

In one embodiment of the present invention, 400 m was the extent to which the signal could be detected above the noise floor. At both 500 m and 900 m the light from the LED was not discernible in the oscilloscope output, but it was still visible to the human eye, suggesting high atmospheric attenuation and/or noise (general signal or background optical). At 900 m a rough sinusoid was seen in the preamp output signal that was most likely the 49 kHz preamp sinusoid mentioned previously. Raising the duty cycle of the LED output to 50 % did not improve signal reception enough to allow transmission from 500 m. The noise floor was similar to that of the 10 % duty cycle pulse tests with the only significant difference being that the after pulses were spaced evenly between the actual pulses, as opposed to occurring directly after the actual pulses. The initial pulse and after-pulse could be the result of AC coupling in the output of the PMT bleeder string, the preamplifier, the oscilloscope, or even the patch cables. Unfortunately, whether the oscilloscope was AC or DC coupled during the test was not explicitly recorded, but it was most likely DC coupled. The double-pulse was not observed prior to the distance tests.

Figures 5(a)-(e) are a compilation of oscilloscope output screen captures for the range of distances with the use of a colored glass filter. The results are from one embodiment of the present invention with a filter, but without a baffle. With the filter, transmitted pulses at 500 m were detected, despite significant noise in the signal. This effectively increased transmission distance by 25 % compared to the results without the filter. The signal was not detected beyond 500 m in this configuration but there may be a larger difference when the system and method of the present invention is operated in outer space.

In Figure 5(a) the PMT output of LED pulses was at a 100 m distance with a 500 mV scale. In Figure 5(b) the PMT output of LED pulses was at a 200 m distance with a 500 mV scale. In Figure 5(c) the PMT output of LED pulses was at a 300 m distance with a 200 mV scale. In Figure 5(d) the PMT output of LED pulses was at a 400 m distance with a 100 mV scale. In Figure 5(e) the PMT output of LED pulses was at a 500 m distance with a 100 mV scale. Figures 5(a)-(e) were at a 10 % duty cycle using a colored glass filter.

The LED pulses at 400 m, both with the filter and without the filter (Figures 4(c) and 5(d)), are recognizable but the pulses are much less distinct when viewed through the filter. This could possibly be a result of condensation that formed on the glass filter during the tests. However, the overall noise still appears to be less with the use of a colored filter.

Increasing the duty cycle of the pulses to 50 % did not make them more distinct and in fact made them less so, especially at 500 m. This, along with the lack of improvement in the filterless 50 % duty cycle test compared to the filterless 10 % duty cycle test, puts into question the use of high duty cycle pulses to improve detectability and range. In addition, short duty cycles may help improve reception as drive currents can typically be increased in comparison to long duty cycle pulses.

Figures 6(a)-(e) are a compilation of oscilloscope output screen captures for the range of distances with the use of a baffle, but without a filter. The results from using the optical filter supported the idea that background optical noise was present. To better understand the effect of this noise, a baffle was used to limit it in a different manner than the colored filter. The baffle itself was a 4 inch diameter schedule 20 PVC pipe, 43 inches in length, with a semi-gloss black interior. While such a large baffle would not be used on a sounding rocket, a smaller baffle that reduced the field of view of each PMT would require the use of additional PMTs to compensate for the reduction in field of view depending on the shape of the baffles.

In Figure 6(a) the PMT output of LED pulses was at a 500 m distance with a 200 mV scale. In Figure 6(b) the PMT output of LED pulses was at a 600 m distance with a 100 mV scale. In Figure 6(c) the PMT output of LED pulses was at a 700 m distance with a 100 mV scale. In Figure 6(d) the PMT output of LED pulses was at a 800 m distance with a 100 mV scale. In Figure 6(e) the PMT output of LED pulses was at a 900 m distance with a 100 mV scale. Figures 6(a)-(e) were at 10 % duty cycle using a baffle.

With the baffle placed in front of the PMT, the viewing angle was reduced to approximately 5.5°. The baffle results in Figure 6 suggest that background light did hamper the PMT in the previous two tests. For example, in the tests without the baffle there was no signal recovery at 900 m (See Figure 4(e)), but in the test with the baffle there was a distinct pulse display (See Figure 6(e)).

Increasing the LED pulse duty cycle to 50 % with a baffle yielded a PMT signal that was larger than that for the 10 % duty cycle. While the pulse signal was greater in magnitude, the leading edge of this signal was no less distinct, which demonstrates that increasing the duty cycle has mixed results.

In comparing Figure 5(e) and Figure 6(a), it is clear that the signal strength was increased more by the baffle than the filter at the same 500 m distance. This could be a result of starlight or scattered light from terrestrial sources within the filter passband acting as noise. Also, combining the filter and the baffle decreased the signal strength. This suggests that the filter used did not operate as expected because of condensation, or perhaps another less obvious factor. For example, dawn was approaching when the longest distance tests were conducted and the additional light could have contributed to the background optical noise.

Figure 7(a) and 7(b) show plots of signal and noise values for different separation distances at different duty cycles for one embodiment of the system and method of the present invention. Pulses were visible with the oscilloscope when the signal level was above the noise level on the graph (SNR > 1), but once the signal level dropped below the noise level on the graph, no pulses were visible (SNR < 1). Figure 7(a) represents data for the 10 % duty cycles, and Figure 7(b) represents data for the 50 % duty cycles. In each plot, the top line is the signal and the bottom line is the noise for that particular distance and configuration (filter or no filter, baffle or no baffle, and the like).

The evaluation of environmental and system level criteria, along with the testing of optoelectronic devices and bench-top equipment, yielded promising results. In one embodiment of the present invention, light pulses at 10 kHz from an LED were detected using a repurposed in-house PMT at distances of 400 m during in-atmosphere tests. In one embodiment of the present invention, distances of 500 m were achieved with a colored glass optical filter. In one embodiment of the present invention, distances of 900 m were achieved with the use of a baffle. In one embodiment of the present invention, short duty cycle pulses worked best.

In one embodiment of the present invention, more optical power could be used, such as through the use of multiple laser diodes. This may help to increase the effective transmission distance, and data rates could also be improved if testing with a pulse position modulator proves that PPM is viable.

In one embodiment of the present invention, the 9755N A PMT used shows merit as a receiver, but improvements to its electronic interface are recommended along with finding an adequate analog-to-digital solution, including error correction, if needed. It would also be prudent to quantify atmospheric attenuation in a vacuum chamber to better compare in- atmosphere results to those expected in space. Vacuum chamber testing should also be done to determine heat dissipation needs for the optoelectronic devices.

It is recognized that there are other instruments that may be used to operate over smaller spatial scales. One example is the Electron Retarding Potential Analyzer, or ERPA, which is already carried on sounding rocket missions. See, MacDonald, et al., Comparisons of Thermal Electron Measurements on two Sounding Rocket Experiments. AGU Spring Meeting Abstracts, 2005. The ERPA measures electron temperatures in the ionosphere and is small enough to fit within a sub-payload. While understanding the spatial gradients of the magnetosphere is important to scientists, studying gradients of electron temperatures in a similar manner could also yield interesting findings. Such gradients could be studied over distances of one or two kilometers which may be attainable given the results discussed herein. Based on the qualitative analysis and research conducted, along with preliminary testing, the results indicate that FSO is a promising telemetry system for sounding rocket sub-payloads, and the like.

In certain embodiments of the method and system of the present invention, the transmitter is a light emitting diode, a laser diode, or any light source that emits light within the receiving range of the receiver (PMT, avalanche photo diode or regular photo diode).

In certain embodiments of the method and system of the present invention, the receiver is a PMT, an avalanche photo diode, or regular photo diode.

In one embodiment of the method and system of the present invention, the transmitter transmits at a wavelength from about 122 nm to about 2500 nm. In one embodiment of the method and system of the present invention, the transmitter transmits at a wavelength from about 375 nm to about 425 nm. In one embodiment of the method and system of the present invention, the transmitter transmits at a wavelength from about 405 nm to about 420 nm. The transmitters will transmit at as narrow a wavelength as practicable given the receiver and any color filtering devices. One specific wavelength may be chosen of the many notches within the auroral emission spectra and the other light sources.

In one embodiment of the method and system of the present invention, the transmitter has an output power from about 2 mW to about 210 mW. In one embodiment of the method and system of the present invention, the transmitter has an output power from about 10 mW to about 105 mW. The power output of the transmitter is limited only by present technology. The power output is higher when short pulse widths are used and the duty cycle is lower.

In one embodiment of the method and system of the present invention, the transmitter produces chained-pulses. In one embodiment the number of chained-pulses is from about 2 to about 50. In one embodiment the number of chained-pulses is from about 4 to about 20. A theoretically unlimited number of pulses may be sent as the peaks do not compound. Single pulses are also possible.

In one embodiment of the method and system of the present invention, the transmitter chained-pulses have pulse frequencies from about 100 kHz to about 1 MHz. In another embodiment of the method and system of the present invention, the pulse frequencies range from about 10 kHz to 50 MHz.

In one embodiment of the method and system of the present invention, the transmitter duty cycle of the chained-pulses is from about 10 % to about 50%. In another embodiment, the transmitter duty cycle is from about 5% to 90%.

In one embodiment of the method and system of the present invention, the transmitter chained-pulses have pulse widths from about 1 ns to about 1 ms.

In one embodiment of the method and system of the present invention, there is a plurality of receivers on a spacecraft. The number of receivers may be up to 100. In another embodiment, the receivers are PMTs arranged in a circle around the spacecraft facing outward. The number of PMTs is determined by the requirement that the angle of incidence of light from another spacecraft is not too great. In yet another embodiment there is only one PMT inside the spacecraft, facing up (or down) with holes in the outer skin of the spacecraft fairing, or if not holes per se at least windows, through which light from the transmitters can travel. In a five window configuration, the light would travel through a single window and hit the other side of the fairing where a mirror is located to direct the light downward (or upward) to a single PMT. This removes the need for multiple PMTs and the associated circuits.

In one embodiment of the method and system of the present invention, there is a plurality of transmitters on a spacecraft. The number of transmitters may be 2 to 100.

In one embodiment of the method and system of the present invention, the transmitter's signal is focused on the receiver with the use of mirrors, prisms, or other optical devices known to those skilled in the art.

In one embodiment of the method and system of the present invention, the signal to noise ratio is improved with the use of filters, baffles, electric filters and other circuits to better interpret the signal coming off the PMT. While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention.

Claims

CLAIMS What is claimed:

1. A system for communicating between spacecraft using free-space optical telemetry comprising:

a first spacecraft with a transmitter for transmitting free-space optical telemetry, and;

a second spacecraft with a receiver for receiving the transmitted free-space optical telemetry.

2. The system for communicating between spacecraft using free-space optical telemetry of claim 1 , wherein the transmitter is a diode.

3. The system for communicating between spacecraft suing free-space optical telemetry of claim 1, wherein the transmitter produces chained-pulses.

4. The system for communicating between spacecraft using free-space optical telemetry of claim 1 , further comprising a filter.

5. The system for communicating between spacecraft using free-space optical telemetry of claim 1 , further comprising a baffle.

6. The system for communicating between spacecraft using free-space optical telemetry of claim 1, further comprising a prism.

7. The system for communicating between spacecraft using free-space optical telemetry of claim 1 , wherein the receiver is a photomultiplier tube.

8. A method for communicating between spacecraft using free-space optical telemetry, comprising:

providing a first spacecraft with a transmitter for transmitting free-space optical telemetry; and

providing a second spacecraft with a receiver for receiving the free-space optical telemetry.

9. The method for communicating between spacecraft using free-space optical telemetry of claim 8, wherein the transmitter is a diode.

10. The method for communicating between spacecraft using free-space optical telemetry of claim 8, wherein the transmitter produces chained-pulses.

11. The method for communicating between spacecraft using free-space optical telemetry of claim 8, further comprising the step of providing a filter.

12. The method for communicating between spacecraft using free-space optical telemetry of claim 8, further comprising the step of providing a baffle.

13. The method for communicating between spacecraft using free-space optical telemetry of claim 8, further comprising the step of providing a prism.

14. The method for communicating between spacecraft using free-space optical telemetry of claim 8, wherein the receiver is a photomultiplier tube.