Activity Feed › Discussion Forums › GNSS & Geodesy › Antenna height problem?
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John Hamilton, post: 412162, member: 640 wrote: they reprocessed all of the CORS several times with newer antenna models, etc
So it is important that my ellip hts be compared to those published on the current adjustment, and the fact that they had different numbers in the past is not relevant.
John Hamilton, post: 412162, member: 640 wrote: there are some electronics in the antenna,
I was under the impression that the electronics in the antenna (when the whole receiver was not there) was just a preamplifier to boost the signal so cable loss didn’t drop the signal strength into the noise. Under that assumption the differential delay could be a factor. But I haven’t examined those circuits to confirm the idea, so you could be right.
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I was told many years ago that cable length effect is immaterial for positioning purposes, at least for the lengths normally encountered in field equipment. (Timing purposes may be a different matter.) A quick search didn’t turn up anything positive, though I note that the CORS guidelines document limits its comments about cable length to signal loss characteristics, suggesting that lengths greater than 30 m should use low-loss cable.
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I assume your nearest CORS may be Marion. The time series plot makes it look like it may be broadcasting a height a cm or two more than it is. You could process a rinex file for Marion for the day of your observations and see what height it was that day and make the adjustment at the rover (just for fun)
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I hope the dimension you use are those in this diagram from the NGS site. In the early years the field observers were always blamed for poor processing results under the assumption that they had mis-measured their antenna heights. Once fixed-height poles were adopted that excuse was no longer possible.
On the issue of the impact of antenna cable length:
Expecting an OPUS ellipsoid height (the mean of three solutions) to agree with the result of an adjustment with respect to fixed control is optimistic.
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linebender, post: 412177, member: 449 wrote: You could process a rinex file for Marion for the day of your observations and see what height it was that day and make the adjustment at the rover (just for fun)
That idea has crossed my mind, but a quick look discouraged me from carrying through to statistical significance. Is there information there that would let one improve on the OPUS report? To what extent was it a real move and how much was statistical variation?
GeeOddMike, post: 412188, member: 677 wrote: I hope the dimension you use are those in this diagram
Yes. I got them from the diagram on the antenna but they match that drawing.
GeeOddMike, post: 412188, member: 677 wrote: On the issue of the impact of antenna cable length:
That article deals only with the gross delay, which my post acknowledged had no effect on position. I was considering differential delays between L1 and L2 and would like to find any discussion of that aspect.
GeeOddMike, post: 412188, member: 677 wrote: Expecting an OPUS ellipsoid height …to agree with the result of an adjustment with respect to fixed control is optimistic.
As I said, the size of the mismatches are quite good. What bothers me is having several offsets all with one sign and wondering if that indicates a bias.
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Bill93, post: 412206, member: 87 wrote: I was considering differential delays between L1 and L2 and would like to find any discussion of that aspect.
The coaxial cable will not cause differential delay between L1 and L2. It is a TEM transmission line, which is not dispersive and has equal delay at all frequencies. (Some transmission lines, for example waveguides and printed-circuit-board microstriplines, are somewhat dispersive.) However, filters are another matter—there are usually ceramic preselector filters in both the antenna and receiver for L1 and L2 separately (or perhaps a selection of wider bands so as to include L5, GLONASS, and so on). These filters will have different group delays and will delay the signals slightly differently (on the order of a few nanoseconds). For that matter, the satellites also have similar filters, and so it’s the grand combination of all these filters in cascade that determines the L1L2 bias for a given satellite and receiver. Since (excepting GLONASS for now) the GNSS systems are CDMA, they have nominally the same spectrum across codes, so group delays in the receiver are exactly the same across codes. There are small manufacturing variations across SVNs; the IGS keeps track of these.
Try the Johnson and Graham “black magic” books for a discussion of transmission-line nonidealities.
Cheers,
Peterps: to be really pedantic, practical coaxial cables have frequency-dependent skin-effect and dielectric losses, and this makes them very slightly dispersive. But the effect is negligible at these frequencies.
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Just going from memory on some stuff from last summer, I would look at the relative height compared to the absolute height. All of my Trimble stuff still uses the relative height. My opinion is that Trimble needs to update EVERYTHING to the absolute height. The number you are looking for may be 74.83mm
For the record, what I know about this may fill a thimble, probably not. It’s just that I was chasing something similar last year with the help of a few on this board.
James
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JaRo, post: 412296, member: 292 wrote: All of my Trimble stuff still uses the relative height.
If by “stuff” means their software, that is not relevant because OPUS uses an absolute model.
pmonta, post: 412211, member: 10428 wrote: The coaxial cable will not cause differential delay between L1 and L2. It is a TEM transmission line, which is not dispersive and has equal delay at all frequencies.
This statement usually follows the point in a tutorial where they have assumed losses to be negligible. If you use a coax at frequencies where the losses are significant, it is not strictly true.
See for example this university lab discussion which pushes an RG-58 coax to frequencies that you would not ordinarily use it for, and notes a rather extreme dependence of phase velocity on the frequency. Skip to the conclusion at the end if you don’t want to wade through it.
http://alignment.hep.brandeis.edu/Lab/XLine/XLine.htmlI’m a little suspicious of the experiment design and controls on this one, but it does indicate a similar effect.
https://www.comsol.com/paper/download/213171/hegde_abstract.pdfL1 and L2 frequencies are in the range where nearly any coax is becoming significantly lossy, although not necessarily seriously so. That leads me to think there may indeed be some effect of cable length on GPS accuracy. Unfortunately, the Trimble cable does not have an RG-xxx number stamped on it to look up its characteristics.
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Bill93, post: 412457, member: 87 wrote: If you use a coax at frequencies where the losses are significant, it is not strictly true.
Yes; I did mention it in my postscript. If my arithmetic is correct, for 10 meters of RG-58, the difference in group delay between L1 and L2 is about 4 ps, or 1.2 mm in free space. For a lower-loss feedline such as LMR-400, it’s a little smaller, about 2 ps. (Cable delay decreases with increasing frequency.) This is swamped by the group-delay differences in the satellite filters, antenna filters, and receiver filters. For example, if the group delay of the receiver’s L1 filter is 4 ns and the L2 filter 3 ns, then that’s 1000 ps of L1/L2 differential delay, lumped in with the much smaller amount from the coax.
I should have read more carefully upthread—I see now what you were getting at about heights possibly interacting with these differential delays. I think the position solution will be insensitive to these delays, just as it is insensitive to receiver clock bias. In any between-satellite difference, these biases will cancel. This holds for L1 and L2 separately, L1-L2, and any other linear combination (such as the iono-free LC).
The phase center of the antenna is a function only of the antenna geometry and frequency; the phase center doesn’t care about the feedline or any other downstream delays. (In some sense, once the incoming signal is homogenized by the antenna into a single TEM mode and shipped out of the antenna connector, all geometry has been forgotten.) Suppose we remove the antenna preamp and connect the antenna’s radiating elements directly to the feedline. Now transmit a carrier from a signal source through the feedline to the antenna. The antenna radiates the signal, and far enough away the wavefront looks like a sphere (or hemisphere). The center of that sphere of constant phase is the phase center. As you know, the position of the phase center will vary slightly with frequency (“longitudinal chromatic aberration”) and also slightly with angular elevation from the ground plane (“spherical aberration”). But it won’t vary with feedline delay. The signal at the antenna connector can have any phase it likes (as a result of varying feedline delay), and the determination of the phase center of the antenna won’t change in any way.
From what I gather, 1 cm is a pretty small systematic error (if indeed you have a systematic error of this magnitude) for ellipsoidal height in a global frame. As was pointed out upthread, OPUS may have biased positions for the CORS stations if the a priori coordinates are not kept current. Maybe AUSPOS or APPS are worth a try: they will be much less local than OPUS-S in terms of reference stations, and may have tighter and more reliable ties to the global IGS solutions. There is also the possibility of genuine mark movement relative to CORS (local subsidence) if the published positions are old.
By the way, you could test whether differential L1/L2 delay is a problem by modifying your RINEX files. Artificially add a few nanoseconds to the L2 code and phase, then resubmit.
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Yesterday’s session on a Height Modernization station got me a (rapid orbits) result 2.5 cm low, instead of being high, with a pk-pk of 0.5 cm. It’s a bit strange to feel good about errors, but I finally got one that’s lower than published, which tends to undermine the idea of a bias.
If that had happened earlier, I wouldn’t have been digging into this.
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I’ve checked the slant-to-vertical height results and they seem fine.
Cable attenuation loss would affect the power not the phase of L1/L2 differently.
I’d recommend splitting the data into hourly sessions and processing each one individually to quantify the repeatability.
I also like to use a pole and collect sessions at three different heights (1.5, 2.0, and 2.5 m) then adding/subtracting half-meter from the bottom/top ones, checking their discrepancies, and finally running a weighted average on the three estimates.
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Felipe G. Nievinski, post: 413655, member: 10769 wrote: Cable attenuation loss would affect the power not the phase of L1/L2 differently
A transmission line having a loss that varies with frequency must also have a phase velocity that differs with frequency. It’s in the math/physics. Whether it is significant in this case is still uncertain.
It is possible that different phase delays and group delays might have different effects depending on the receiver algorithms. The link pmonta gave in the cross correlation thread indicates there are several, and that list may not be all-inclusive.
http://www.bmotion.com/navcom/images/tech_archiv/L2_Phase_Tracking.pdfI’m running some experiments with two different cable lengths to see if I find an effect on measured height as hypothesized above. The method is to alternate every two hours (for OPUS-S files) between my short antenna cable, versus that cable plus 100 ft of RG-213 in series. The additional cable will cause about 12 dB loss and some phase difference between L1 and L2. By alternating, I hope to minimize the effect of ionospheric and other changes on the observed differences.
Preliminary results (rapid orbits, two 8+ hour days with alternation during the day) suggest it does occur, but given the variability I need more data for confidence.
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I ran 5 days, 4 sessions of 2 hours each day, alternating between a short antenna cable versus that cable plus another 100 ft of RG-213 cable. I now have precise orbit OPUS-S data for those.
The results suggest, but don‰Ûªt give great confidence, that there is a small increase in measured height with a longer cable on the Trimble 4000sse receiver. The mean difference (long minus short) of 5.4 mm was 1.69 times the 3.2 mm standard deviation of the mean difference.
Like all good research, the answer is that more research is needed.
More on experiment design:
I plotted the difference between the height measured by each CORS in each session, versus the day‰Ûªs average. If plotted versus the five-day average, the variation is larger but the long-short difference is not much affected.The alternations were intended to minimize the effects of varying ionosphere and other unknowns, and the 2-hour sessions are the shortest that OPUS-S will process.
The first two sessions with rapid orbits suggested a whole cm difference, but that didn‰Ûªt hold up through the rest of the test. None of the varying partial results, as I collected more data and ran rapid orbits, then precise orbits, and added days, ever showed a negative average difference.
The added cable loss didn‰Ûªt seem to affect the spread of the results, as the standard deviation of short cable versus standard deviation for long cable was quite close.
I let OPUS-S choose the CORS, except for one case where it picked a CORS that no other session had used, so I re-ran excluding that one CORS, but it made little difference. This resulted in five of the closest CORS being used in various combinations.
The standard deviation of height for all CORS for all sessions was 16 mm. I excluded one point that was 3 sd out, and an entire session that had points plus and minus over 3 sd. The standard deviations for each CORS across all sessions was mostly 16 to 19 mm with one at only 6 mm.
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Since the beginning of GPS time there have been splitters that enable one to do a zero length baseline test, where one antenna feeds two (or more) receivers. If you could procure one (and have two receivers) you could do the test with data collected at the same time, same point, different antenna lengths. Theoretically when you process the two data sets together the three components of the baseline should be 0.000,0.000,0.000.
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John Hamilton, post: 418526, member: 640 wrote: If you could procure one (and have two receivers)
Indeed, that would be the proper test, but I don’t have the equipment. More time than money for this experiment.
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I just now ran the precise orbit results for two more days of data. The average height difference is now negligible, down from 1.69 standard deviations as mentioned above. Oh, the vagarities of statistics.
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