Tag: flyby

The required precision for the Phobos experiment

Thanks to Hannes DG1GGH of ESOC, we now have an idea what kind of precision is required for the Phobos flyby experiment. The answer is in the comments to this very informative and interesting post on the Mars Express blog from today. According to Hannes, they are expecting the influence of Phobos to cause a maximum deviation of 400 mHz from the nominal Doppler shift. Yes, that’s milli-Hertz, not mega-Hertz.

Obviously, this is much lower than what I had in mind when I first read about the experiment two days ago. I was thinking more along tens, maybe hundreds of Hz. Had I known this in the beginning I probably wouldn’t have considered the experiment at all. In fact, I wasn’t even aware of that measuring with such accuracy and precision was possible. Not that 400 mHz is anything special on it’s own – I can easily create a tone generator that is precise within 10 mHz. But in this case we have to keep in mind that this 400 mHz is to be considered together with an 8.4 GHz signal coming from 117.5 million kilometers away. That’s many decades in the same equation!

For the simple setup I was considering this constraint would require me to keep all oscillators in the system stable within a fraction of this limit. I could not have have achieved that with a 7.15 GHz oscillator mounted outdoors on the antenna. I don’t know the stability of the Kuhne LNC, but I am used to see “1 parts per million” in similar cases. 1 ppm @ 7 GHz is … you see my point 😉

The second error source in the chain is the USRP and the RF front end board itself. Despite all good intentions, it is still a system built of consumer grade components and millihertz precision is probably not what people have in mind when creating such devices. But I have put it on my list to measure the accuracy and precision using a good signal reference. More on that later.

Have a nice weekend!

No Mars Express experiment this time

It was a difficult decision to accept, but the Mars Express experiment with the USRP and GNU Radio is not going to happen this time!

It’s a shame because the link budget looked quite promising. An optimistic estimate gives almost 20 dB signal to noise ratio, which is more than we need, so the problem is not there. The problem is with the X-band down converter. I was looking at the KU LNC 8084 from Kuhne, which together with the KU LNA 133 BH low noise amplifier gives a total G/T of more than 30 dB/K. Unfortunately, there are two problems with this down converter:

  1. The price is slightly above what my budget can allow here and now
  2. There is a 2 week delivery time

For the sake of the experiment I could have lived with either one of the two, but not both. I know, I know… the two weeks are so close that I might even get lucky and receive it in time. But it would need to be in my hands on Tuesday, March 2, preferably on Monday to allow proper integration and testing, and that is closer to 10 days than to 14.

Using the S-band signal is not very promising either. Here, the link budget doesn’t go up due to the low power of the transmitter (5W). We can get a few dB above the noise floor if we use a filter no wider than 10 Hz – and that’s no fun when you are trying to measure Doppler shift.

Concerning the Doppler shift, I have been looking at the ephemeris generated by JPL Horizons web interface. At this time of the year, Mars and Earth are moving away from each other so the Doppler shift will be dominated by the range rate between the two planets. The orbit of Mars Express around Mars can be observed by this Doppler shift oscillating between two values. On March 3, the oscillation will be roughly between 7.3 to 12.5 km/s resulting in a Doppler shift between 200 and 350 kHz with a period of 7 hours:

*********************************************************************************
  Date__(UT)__HR:MN     Azi_(a-appr)_Elev        delta      deldot    S-O-T  /r
*********************************************************************************
2010-Mar-03 12:00 *r   44.3857   0.0718 0.78932354310229  11.1852087 137.6854 /T
2010-Mar-03 12:30 *    50.5231   3.1913 0.78946087508986  11.6142978 137.6635 /T
2010-Mar-03 13:00 *    56.5265   6.5964 0.78960254573254  11.9176387 137.6415 /T
2010-Mar-03 13:30 *    62.4270  10.2445 0.78974736351007  12.1426231 137.6193 /T
2010-Mar-03 14:00 *    68.2648  14.0936 0.78989453336189  12.3108050 137.5969 /T
2010-Mar-03 14:30 *    74.0891  18.1025 0.79004342478087  12.4288177 137.5742 /T
2010-Mar-03 15:00 *    79.9584  22.2300 0.79019340567595  12.4891937 137.5514 /T
2010-Mar-03 15:30 *    85.9418  26.4339 0.79034362769204  12.4603569 137.5283 /T
2010-Mar-03 16:00 *    92.1210  30.6697 0.79049254590969  12.2459365 137.5050 /T
2010-Mar-03 16:30 *    98.5934  34.8883 0.79063632341205  11.5023192 137.4814 /T
2010-Mar-03 17:00 C   105.4752  39.0329 0.79076193096873   8.8729735 137.4577 /T
2010-Mar-03 17:30 N   112.9049  43.0339 0.79085460119582   7.7994185 137.4351 /T
2010-Mar-03 18:00 N   121.0408  46.8078 0.79096079023597   9.7083822 137.4134 /T
2010-Mar-03 18:30 A   130.0480  50.2537 0.79108469006353  10.7829081 137.3915 /T
2010-Mar-03 19:00     140.0731  53.2460 0.79121862213031  11.4324332 137.3693 /T
2010-Mar-03 19:30     151.1869  55.6385 0.79135901132806  11.8780500 137.3470 /T
2010-Mar-03 20:00     163.3031  57.2784 0.79150401404296  12.2087075 137.3244 /T
2010-Mar-03 20:30     176.1098  58.0358 0.79165251591848  12.4639928 137.3017 /T
2010-Mar-03 21:00  t  189.0909  57.8425 0.79180373020367  12.6611863 137.2789 /T
2010-Mar-03 21:30     201.6721  56.7165 0.79195699075542  12.8035934 137.2558 /T
2010-Mar-03 22:00  m  213.4087  54.7551 0.79211158678530  12.8795690 137.2326 /T
2010-Mar-03 22:30  m  224.0838  52.1019 0.79226652008935  12.8488984 137.2092 /T
2010-Mar-03 23:00  m  233.6851  48.9097 0.79241990790189  12.5882130 137.1856 /T
2010-Mar-03 23:30  m  242.3230  45.3178 0.79256684230434  11.6337681 137.1618 /T
2010-Mar-04 00:00  m  250.1570  41.4426 0.79269056949806   8.4958690 137.1381 /T
2010-Mar-04 00:30  m  257.3492  37.3764 0.79278746204570   8.6897914 137.1158 /T
2010-Mar-04 01:00  m  264.0503  33.1954 0.79290340477852  10.4123533 137.0940 /T
2010-Mar-04 01:30  m  270.3909  28.9637 0.79303487629095  11.3531489 137.0722 /T
2010-Mar-04 02:00  m  276.4794  24.7346 0.79317518946651  11.9281843 137.0502 /T
2010-Mar-04 02:30  m  282.4067  20.5555 0.79332120377351  12.3189920 137.0281 /T
2010-Mar-04 03:00  m  288.2493  16.4696 0.79347121486610  12.6006078 137.0058 /T
2010-Mar-04 03:30  m  294.0717  12.5188 0.79362414053192  12.8071911 136.9835 /T
2010-Mar-04 04:00 Am  299.9287   8.7442 0.79377918105395  12.9534316 136.9610 /T
2010-Mar-04 04:30 Am  305.8659   5.1872 0.79393563141900  13.0406358 136.9384 /T
2010-Mar-04 05:00 Nm  311.9199   1.8902 0.79409271234490  13.0538218 136.9157 /T

Delta is the range in AU, deldot is the range rate in km/s. The S-O-T parameter is also interesting in general because it gives the angle between the Sun and the target as seen from the observer. It is important because having the Sun in the field of view of the antenna can increase the sky noise significantly.

The interesting question is still the magnitude of the perturbations caused by Phobos. 1 Hz? 10 Hz? 100 Hz? I don’t know but I am trying to convince myself that a relative Doppler shift of 10 Hz should be detectable with the USRP and GNU Radio, provided that the local oscillator of the down converter is stable. Fortunately, this is an experiment that I can do!

Aiming for planetary science with GNU Radio and the USRP

Thanks to the European Space Agency (ESA) and the Mars Express mission, we might have an opportunity just around the corner for doing big science with GNU Radio and the USRP!

On March 3, 2010, Mars Express will visit the Martian moon Phobos by performing a close flyby. According to ESA, the ESOC ops team is working with a number of possible scenarios, including one that would take the spacecraft to just 50 km above Phobos. At that distance the orbit of the spacecraft is expected to be influenced by the gravitational pull of Phobos. I knew this already for about a week when I first read it on the Mars Express Blog but it was first today that I realized the opportunities this event offers.

Continue reading “Aiming for planetary science with GNU Radio and the USRP”