Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
One needs to know the carrier frequency. Must be a high quality reference for the Cassini transmitter. On 04/03/2014 08:17 PM, Chris Albertson wrote: I just read about a discovery of a liquid water ocean on Saturn's moon Enceladus. The method used was to measure the velocity of a spacecraft as it makes a close fly-by. Gravitational anomalies will cause the spacecraft to speed up or slow down as it flies over massive objects like mountains. With three pass they now have a 3 dimensional map of density distribution. It must be very sensitive if they can tell liquid water from ice by its gravitational field. (or even rock from ice) They say they can measure the spacecraft's velocity to 90 microns per second. They do this by measuring the Doppler sift of the transmitter.I've been trying to figure out what 90 microns/sec means in terms of frequency. But I think(?) I need to know the orbital velocity of Enceladus. -- Chuck Forsberg WA7KGX c...@omen.com www.omen.com Developer of Industrial ZMODEM(Tm) for Embedded Applications Omen Technology Inc The High Reliability Software 10255 NW Old Cornelius Pass Portland OR 97231 503-614-0430 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
Hi Back when they were designing this stuff, they were very interested in getting into the parts in 10 to the 15th. They didn’t get there, but that was the desire. Bob On Apr 4, 2014, at 2:17 AM, Chuck Forsberg WA7KGX c...@omen.com wrote: One needs to know the carrier frequency. Must be a high quality reference for the Cassini transmitter. On 04/03/2014 08:17 PM, Chris Albertson wrote: I just read about a discovery of a liquid water ocean on Saturn's moon Enceladus. The method used was to measure the velocity of a spacecraft as it makes a close fly-by. Gravitational anomalies will cause the spacecraft to speed up or slow down as it flies over massive objects like mountains. With three pass they now have a 3 dimensional map of density distribution. It must be very sensitive if they can tell liquid water from ice by its gravitational field. (or even rock from ice) They say they can measure the spacecraft's velocity to 90 microns per second. They do this by measuring the Doppler sift of the transmitter.I've been trying to figure out what 90 microns/sec means in terms of frequency. But I think(?) I need to know the orbital velocity of Enceladus. -- Chuck Forsberg WA7KGX c...@omen.com www.omen.com Developer of Industrial ZMODEM(Tm) for Embedded Applications Omen Technology Inc The High Reliability Software 10255 NW Old Cornelius Pass Portland OR 97231 503-614-0430 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/3/14 8:17 PM, Chris Albertson wrote: I just read about a discovery of a liquid water ocean on Saturn's moon Enceladus. The method used was to measure the velocity of a spacecraft as it makes a close fly-by. Gravitational anomalies will cause the spacecraft to speed up or slow down as it flies over massive objects like mountains. With three pass they now have a 3 dimensional map of density distribution. It must be very sensitive if they can tell liquid water from ice by its gravitational field. (or even rock from ice) They say they can measure the spacecraft's velocity to 90 microns per second. They do this by measuring the Doppler sift of the transmitter.I've been trying to figure out what 90 microns/sec means in terms of frequency. But I think(?) I need to know the orbital velocity of Enceladus. Ranging is done by looking at the round trip time from Earth to spacecraft back to Earth. The signal on the ground is generated by a hydrogen maser. The radio on the spacecraft adds Allan Deviation on the order of 1E-15 at tau of 100-1000 seconds. The uncertainties in things like the antenna and cables on the spacecraft add similar uncertainty. The ground station antenna also flexes and moves. I'd have to go look up what the magnitude of that is, but I think it's in the same order of magnitude. For Cassini (which is what they'd be doing for Enceladus), the signals are in the deep space X-band. Transmitted from earth at 7.15 GHz, returned from Cassini at 8.4 GHz (roughly). The ratio between transmitted and received signal is 880/749 (exactly). This is called the coherent turnaround ratio and we spend a fair amount of time making sure that the turnaround is phase coherent. That is, a phase shift of 1 radian on the input signal will result in a phase shift of 880/749 radians on the output signal. The actual time delay through the telecom system is measured on the ground before launch in a temperature chamber, so any temperature variation during the measurement can be accounted for. Radio science and navigation measurements are quite impressive in their accuracy and attention to detail. measuring range to cm (out of a billion km, i.e 1 part in 1E14) and velocity to mm/s is sort of standard. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/3/14 11:17 PM, Chuck Forsberg WA7KGX wrote: One needs to know the carrier frequency. Must be a high quality reference for the Cassini transmitter. Two way measurements are most likely here (although Cassini does carry a USO). So the downlink is locked to the uplink which comes from a maser. The design of the transmit and receive system to hold that performance all the way through the chain is a challenge. Interestingly, since the light time to Cassini is hours and hours, you need a *really* good clock on the ground, because you're comparing the phase of a signal you radiated several hours ago with the phase of the signal you are receiving now. To put this in perspective.. at a range of a bit more than billion km (one way) the light time is a bit less than 5000 seconds,or 10,000 seconds round trip. So there are 4.2E13 cycles of the down link signal between Saturn and Earth. The Allan Deviation is on the order of 1E-15, so we are typically measuring the phase of that signal to a few degrees. So, things like the physical temperature of the 70 meter DSN antenna make a difference. If the optical path is, say, 100 meters, and the temperature of the dish changes 1 degree C, what's the phase change? Well, steel has a CTE of about 13ppm/degree, and 13 ppm of 100 meters is about 1mm, which is about 10 degrees phase shift at 8.4 GHz (lambda = 36mm) Solid Earth tides also feature into this. I will point out that *measuring* the performance of the radio on the ground in the lab is quite a chore. You can easily see the air conditioner cycling on and off (that bump in Adev at 1000 seconds) and diurnal cycles in temperature. most of this affects things like the cables and connectors in the test setup. And woe to the rookie engineer who thinks they can make the measurements with any old signal generator from loan pool. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/4/14 4:30 AM, Bob Camp wrote: Hi Back when they were designing this stuff, they were very interested in getting into the parts in 10 to the 15th. They didn’t get there, but that was the desire. Roughly that... http://lasp.colorado.edu/~horanyi/graduate_seminar/RSS.pdf is a good presentation with design and performance at a high level. It has plots of the USO and maser performance, etc. http://lasp.colorado.edu/~horanyi/graduate_seminar/Radio.pdf is a good paper describing Cassini Radio Science looking for Gravity waves. The Ka band system has some issues in flight. http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/11139/1/02-3097.pdf gives some numbers.. see page 19 the DSN antenna is on the order of 1E-15 at tau=1000 seconds if you're careful, 1E-14 under normal operation Tropospheric scintillation is also a factor.. I doubt we will ever see a telecom subsystem as complex and sophisticated as Cassini's ever again. The modern trend is to less redundancy, and higher level of integration in the boxes so fewer total boxes. For the Juno mission on it's way to Jupiter, we were looking for the radio's contribution to the measurement uncertainty being around 4E-16 at 1000 seconds. https://www.youtube.com/watch?v=ulzq_mlU-fA is a high level explanation of the gravity science When the Deep Space Atomic Clock (a trapped Hg ion) flies, that will change a lot of radio science, because we will be able to make more accurate one-way measurements. They are hoping for an overall improvement of 2 orders of magnitude. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
Jim, Thanks for sharing the details and preventing this subject from turning into shared ignorance. Bill Hawkins ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4 Apr 2014 08:55, Tom Knox act...@hotmail.com wrote: 90 microns is approx a freq res of about 1 x 3.66 -12 Thomas Knox Since the Doppler shift is prortional to the frequency, I can't see how one can determine the absolute frequency. But given light travels at 3e8 m/s and they can resolve 9e-5 m/s, I would have thought that the frequency resolution needed was 9e-5/3e8=3e-13. We are differing by more than a factor of 10. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/4/14 7:39 AM, Bill Hawkins wrote: Jim, Thanks for sharing the details and preventing this subject from turning into shared ignorance. It was working on this kind of thing that led me to time-nuts in the first place.. Deep Space nav is probably one of the most precise measurements made on a regular basis. How many other things are measured regularly to 1E-13 or 1E-14, as a day to day operational process (as opposed to laboratory experiments and the like). ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
You are correct. I did most in my head late last night and kind of lost my focus as I was finishing. I was attempting to see roughly what timing accuracy was needed. I meant to end the sentence with a question mark. Thomas Knox Date: Fri, 4 Apr 2014 13:01:12 +0100 From: drkir...@gmail.com To: time-nuts@febo.com Subject: Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency? On 4 Apr 2014 08:55, Tom Knox act...@hotmail.com wrote: 90 microns is approx a freq res of about 1 x 3.66 -12 Thomas Knox Since the Doppler shift is prortional to the frequency, I can't see how one can determine the absolute frequency. But given light travels at 3e8 m/s and they can resolve 9e-5 m/s, I would have thought that the frequency resolution needed was 9e-5/3e8=3e-13. We are differing by more than a factor of 10. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/4/14 5:01 AM, Dr. David Kirkby wrote: On 4 Apr 2014 08:55, Tom Knox act...@hotmail.com wrote: 90 microns is approx a freq res of about 1 x 3.66 -12 Thomas Knox Since the Doppler shift is prortional to the frequency, I can't see how one can determine the absolute frequency. But given light travels at 3e8 m/s and they can resolve 9e-5 m/s, I would have thought that the frequency resolution needed was 9e-5/3e8=3e-13. We are differing by more than a factor of 10. It's actually even more tricky, if you think about it, because what you are really doing is making the measurement over some time period, and the path length of signal is continuously varying during that time. Not only is Cassini doing it's flyby of Enceladus (and you're looking for small deviations in trajectory from those due to an idealized point source masses), but you've also got your ground stations on Earth moving due to planetary motion, daily rotation, as well as things like solid earth tides moving the DSN station up and down by tens of cm during the measurements. Gravity science in deep space is a very time-nutty activity.. it's basically finding all the various sources of change, modeling them, and driving the uncertainties as low as possible. They use a collocated radiometer to compensate for the extra delay of the atmosphere of earth. JPL has all those folks computing earth rotation models, and that figures in (hey, you need to know the rotational velocity of earth pretty accurately, to take that out of the equation). The folks who do this spend a lot of time looking at residuals plots and trying to make them look like a flat line of zero width. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On Fri, Apr 4, 2014 at 6:19 AM, Jim Lux jim...@earthlink.net wrote: Radio science and navigation measurements are quite impressive in their accuracy and attention to detail. measuring range to cm (out of a billion km, i.e 1 part in 1E14) and velocity to mm/s is sort of standard. Looks to be about one order of magnitude better than standard. They claimed 90 microns/sec velocity in this case. I was looking for a document that shows the design of the radio science system in the spacecraft but did not find it. Best I can tell is they use a phase locked receiver transmitter as a kind of transponder so the high precision clock is on Earth. They say this is the first time they are able to detect mechanical movement in the ground station antenna in the Doppler data. I guess 90 uM/sec sensitivity just about everything is a noise source. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/4/14 9:34 AM, Chris Albertson wrote: On Fri, Apr 4, 2014 at 6:19 AM, Jim Lux jim...@earthlink.net wrote: Radio science and navigation measurements are quite impressive in their accuracy and attention to detail. measuring range to cm (out of a billion km, i.e 1 part in 1E14) and velocity to mm/s is sort of standard. Looks to be about one order of magnitude better than standard. They claimed 90 microns/sec velocity in this case. I was looking for a document that shows the design of the radio science system in the spacecraft but did not find it. One of the links in the other message I posted has some of the links Best I can tell is they use a phase locked receiver transmitter as a kind of transponder so the high precision clock is on Earth. Exactly. Cassini carries a deep space transponder or DST, which is the predecessor of the Small Deep Space Transponder(SDST) which has been flying on most missions from JPL. APL has their version on Messenger (Mercury) and New Horizons (Pluto). The basic technique is to have a phase locked receiver with very narrow loop bandwidth (a few Hz) that locks to the uplink carrier. Traditionally that would be at around 9 MHz (called f0 in the coherent transponder world), and the receiver LO would be at 748 * f0, so the IF is at f0. That same oscillator is then multiplied up by 880 to generate the X band transmit signal. (hence the 880/749 ratio). S-band transponders use a similar scheme with 240/221 as the ratio. Early X-band radios used S-band designs with an added x4, just as many modern Ka-band radios use a x4 on the output of a Xband transmitter (e.g. Cassini is a X up/X down and X up/Ka-down system. the DST and SDST use DROs as the microwave oscillator. The SDST uses a VCXO that's around 80 MHz (8f0), but other than that, it's pretty much the same design approach. In the SDST, the carrier tracking loop and data demodulator is implemented in a digital ASIC which drives a DAC to control the VCXO. Newer coherent transponders do things a bit differently. They use the same stable XO, but then use a pair of NCO/DDSs to generate the reference oscillator for the multiplier/PLL for Rx and Tx. The tracking loop (and its filters) is implemented in digital hardware (FPGA). There's some cleverness in setting things up so spurs don't bite you, and that changes in the crystal frequency don't propagate through. By the way, for the best performance, you want to actually move the receiver LO to keep the signal at the same place in the IF, as opposed to doing some sort of block conversion and tracking entirely in software. That way, you don't worry about the phase vs frequency characteristic of the IF filters: you're always at the same place. All you have to worry about is phase vs temperature at one frequency. That said, the Electra UHF proximity radios use a LO that goes in big steps, so their coherent turnaround performance isn't as good (although, conceivably, one could characterize the IF group delay characteristics and build an equalizer in software) We're also going to GaAs VCOs because they have wider turning ranges. Historically, transponders are made in extremely limited quantities (3-4 units every few years) and they have a lot of touch labor for tuning (e.g. you get your frequency assignment years in advance, and you order crystals at the right frequency, etc.). There's a fair amount of reuse of spare transponders (e.g. a mission which is flying 1 or 2 will buy an extra, and then when they successfully launch, will hand off that spare to the next mission) which leads to all sorts of channel assignment issues (Opportunity and MRO have the same DSN channel, for instance), so there's been interest in designs which can have their channel selected after manufacturing. DROs don't have the tuning range to cover the whole 50MHz X-band, and certainly not the 500 MHz Ka-band. We spent a couple years trying to make a dual control input DRO with a coarse and fine inputs, but it didn't work out so well. As readers of this list will appreciate, a quiet oscillator has high Q resonators, and that is the opposite of what you want for wide tuning range. We developed some prototypes using GaAs VCOs whcih seem to work quite well, and that's the direction we'll probably go in the future. Personally, I will be happy if I never have to fool with making DROs again. High performance DROs are the epitome of touch labor, and being basically a mechanical resonator, have microphonics, temperature coefficients, picky alignment during assembly, etc.. A monolithic solution which uses lithography is FAR better, if you can get it to work. They say this is the first time they are able to detect mechanical movement in the ground station antenna in the Doppler data. Yes. they've made a big effort to do this in preparation for Juno (and Bepi-Colombo) because they're trying to push the radio/gravity science
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
gravitation measurement, particularly gravitation measurement in space is based on the Eotvos -effect see here: http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_effect and here: http://en.wikipedia.org/wiki/Lor%C3%A1nd_E%C3%B6tv%C3%B6sand from the begin of the space exploration many space crafts using accelerometer based on that Eotvos pendulum, invented by Eotvos in the eighteen-hundreds [the richest oilfields in the United States were discovered by Eötvös' Pendulum. The Eötvös pendulum was used to prove the equivalence of the inertial mass http://en.wikipedia.org/wiki/Inertial_mass and the gravitational mass http://en.wikipedia.org/wiki/Gravitational_mass accurately] so no speed no time measurement is necessary... ~ a former co-worker of space projects. A. Pummer On 4/4/2014 9:01 AM, Jim Lux wrote: On 4/4/14 5:01 AM, Dr. David Kirkby wrote: On 4 Apr 2014 08:55, Tom Knox act...@hotmail.com wrote: 90 microns is approx a freq res of about 1 x 3.66 -12 Thomas Knox Since the Doppler shift is prortional to the frequency, I can't see how one can determine the absolute frequency. But given light travels at 3e8 m/s and they can resolve 9e-5 m/s, I would have thought that the frequency resolution needed was 9e-5/3e8=3e-13. We are differing by more than a factor of 10. It's actually even more tricky, if you think about it, because what you are really doing is making the measurement over some time period, and the path length of signal is continuously varying during that time. Not only is Cassini doing it's flyby of Enceladus (and you're looking for small deviations in trajectory from those due to an idealized point source masses), but you've also got your ground stations on Earth moving due to planetary motion, daily rotation, as well as things like solid earth tides moving the DSN station up and down by tens of cm during the measurements. Gravity science in deep space is a very time-nutty activity.. it's basically finding all the various sources of change, modeling them, and driving the uncertainties as low as possible. They use a collocated radiometer to compensate for the extra delay of the atmosphere of earth. JPL has all those folks computing earth rotation models, and that figures in (hey, you need to know the rotational velocity of earth pretty accurately, to take that out of the equation). The folks who do this spend a lot of time looking at residuals plots and trying to make them look like a flat line of zero width. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Water on Enceladus - What does this imply about NASA'a ability to measure frequency?
On 4/4/14 9:58 AM, Alex Pummer wrote: gravitation measurement, particularly gravitation measurement in space is based on the Eotvos -effect see here: http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_effect and here: http://en.wikipedia.org/wiki/Lor%C3%A1nd_E%C3%B6tv%C3%B6sand from the begin of the space exploration many space crafts using accelerometer based on that Eotvos pendulum, invented by Eotvos in the eighteen-hundreds [the richest oilfields in the United States were discovered by Eötvös' Pendulum. The Eötvös pendulum was used to prove the equivalence of the inertial mass http://en.wikipedia.org/wiki/Inertial_mass and the gravitational mass http://en.wikipedia.org/wiki/Gravitational_mass accurately] so no speed no time measurement is necessary... ~ a former co-worker of space projects. A. Pummer I don't know many spacecraft that carry a gravity sensor these days. Apollo 17 carried a gravimeter based on a vibrating string accelerometer. There's a proposed experiment for 2017 or something to do the equivalence test. There's also a proposal to put a spring gravimeter (ISA) on an ESA lunar lander, and to include it in the proposed MAGIA mission, which uses the two satellite GRAIL/GRACE approach as the primary measurement. I think Bepi-Colombo is carrying the Italian Spring Accelerometer to Mercury. GRAIL and GRACE measure the distance between two spacecraft very precisely (using carrier phase measurements on board) to infer the gravitational field of the body they are orbiting around. Gravity science by measuring range and Doppler is popular because it's cheap. You already have to have a radio on the spacecraft, so the incremental cost for the science is the labor of the toilers on the ground, who can be inexpensive graduate students working on their dissertation in non-real time. There *is* a cost to specifying and measuring the performance of the spacecraft radio to support this kind of precision, but that again, is just a cost penalty and it's small. It doesn't add risk, mass, or power. Adding an instrument adds mass, power, data bandwidth and programmatic risk (what if the instrument isn't delivered on time, or interferes with some other instrument or the spacecraft). Radio science adds no mass, no power is required and the data is entirely gathered on the ground, where it's cheap. There is a slight operational cost: doing an 8 hour gravity science pass with the data rate turned down to minimum so you can have maximum power in the carrier, as opposed to in the data sidebands. And, this is changing. As we move towards faster telecom radios, keeping good radio science performance is challenging. When we sent bits at 10 bits/second, you needed really good close in phase noise, and so, the radio science performance came for free. But when you send 10 Mbps, the phase noise inside 10 Hz isn't as big a deal, and people are starting to ask why are we spending extra time, money, mass, risk to achieve insanely high radio science performance... That's why Bill Folkner led a development effort to develop a purpose specific low mass, low power radio science instrument (RSTI). A 1kg, few Watt, 1 liter box that you could put on anything to do precision radio science (and nothing else). The prototype is (hopefully) going to fly as part of LMRST, which is a 2 or 3U cubesat. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
