Soluciones Avanzadas Canarias S.L.

GNSS stations of the IGS are very important, especially when they are located at locations with other space techniques such a Satellite Laser Ranging (SLR) and Very Long Based Interferometry (VLBI), which all provide inputs to the establishment of the Earth’s International Reference Frames. The ‘ties’ between all the different antenna’s reference points is essential to be able to merge all the station position data into a consistent and coherent system.

The problem is when GNSS antennas are covered by uncalibrated Radomes, these are enclosures that cover the top part of the antenna to protect them from rain, snow, animal damage, etc. Below you can see some of the Radomes in use around the IGS network

Most of the Radomes used by station operators around the world are calibrated (more on that in a later post!) like the one pictured on the left. But older stations are still using Radomes that are not calibrated (such a pictured on the right) so we cannot know what effect exists in the signal transmission and the effect on the long term position estimations at these sites.

Covering a GNSS antenna with a radome whose phase center variations (PCVs) have not been calibrated will usually cause an unknown bias in the estimated position for that station. This bias is equivalent to a local tie error for those IGS stations colocated with other techniques and is thus a serious concern for ITRF (International Terrestrial Rotation Frame), which aims for tie accuracies at the 1 to 2 mm level. Radome effects can often be much larger, up to several cm.

At the IGS Infrastructure Committee, which I chair, we have been coordinating with the IGS reference Frame Working group, the IGS Antenna Working group and the IGS Network Coordinator since we think that the net position bias can be measured empirically by comparing solutions with and without the radome present. This requires that the radome can be safely removed temporarily without disturbing the antenna construction and then later be reinstalled. The empirical position offset provides a correction for the local tie vector to other techniques.

Essentially we will organize with each station operator an 8 week period of “radome off” operations over the next 18 months. During this period the station position will continue to be included in the IGS processing and it is hoped that the combined station position before , during and after the radome-off campaign will produce empirical tie corrections to be applied to the ITRF tie vectors.

Best regards,
Nacho Romero

Coming back to this issue we have encountered a significant problem.

As remarked before in this blog P1C1 biases we have to be able to correct the C1 measurements from GPS to mix them correctly with the P1 measurements. This comes from the fact that when the signal is encoded in the satellite there is a delay between the ‘C/A’ and the ‘P’ code encoding.

Of course we detect this offset between signals (differential code bias: DCB) in the RINEX data which is recorded by a receiver after traveling from the satellite to the antenna through the atmosphere, and from the antenna to the receiver through the cable , amplifiers, etc. Internally in the receiver the electronics can also play a role of course. In any case what we see in the IGS is the RINEX data and this is what we have to work with.

As mentioned in the previous blog entry P1C1 biases the Analysis Center CODE (Center for Orbit Determination Europe) at Univ Berne publishes the P1C1 biases as an average over many brands of receivers available in the IGS network, and generally it is accepted that these averages are to be used when doing IGS-type processing or using the IGS products.

We have found that when an IGS-type solution uses a network of receivers with only Trimble receivers (which need the P1C1 bias) the CODE values may not be the best suited since the Trimble receivers do not show the same DCBs as other receiver brands. We have reached this conclusion after MANY internal investigations and testing. It is clear that depending on what receiver types we test the DCBs are slightly different from the accepted CODE averages, but when the receivers are only Trimble then the differences for certain satellites can be very significant:

in the table above you can see the differences of the P1C1 bias from CODE, as calculated using only Ashtech receivers and as calculated using only Trimble receivers, note the differences for satellites such as G07, G12, G29, etc. in plot form this shows the differences more clearly:

This generally means that the clock solutions from using the incorrect P1C1 bias will be biased versus the satellite clock solutions from a mixed network or as published in the IGS clock combination.

Therefore to conclude, be careful when calculating satellite clock solutions when all the receivers are of a certain brands and the data has to be corrected with the P1C1 bias! This is if you are interested in the satellite clock solutions or on resolving the maximum number of ambiguities with fixed satellite clocks!

Best regards to all,
Nacho

The members of the Infrastructure Committee have prepared the following statement which was distributed to the receiver manufacturers late in 2010 by the IGS Governing Board:

”
The IGS urges all GNSS receiver manufacturers to adopt a
standardized phase alignment practice for all GNSS signal
modulations common to a given carrier frequency, in order
to minimize user confusion and to remain consistent with
the long standing and universal practice established for the
GPS L1C/A and L1P(Y) signals.

The issue of phase changes associated with some GPS flex
power modes is a much less pressing concern considering GPS
assurances not to exercise such modes till after 2020 and
considering that the GPS navigation message will be updated
to flag such events when they occur.
”

We have come to this consensus at the IGS to speak with one voice and to send a clear message that the manufacturers need to provide phase measurements on all frequencies that are aligned so that users of the measurements do not have to second guess what they are using.

It is hoped that this statement and that the IGS relationship within the RTCM sc104 will help to resolve this issue for the benefit of all dual frequency GNSS users. In the end we anticipate that either an agreement will be reached for all manufacturers to provide aligned phases on L2 as they already do for L1 and to provide the amount they align them by so that the original measurement can be reconstructed should anyone need to.

To provide the alignment information the RINEX format has had a compulsory line added to the header as follows:

” The applied corrections are to be reported in a new SYS / PHASE SHIFT header record to allow the reconstruction of the original values, if needed. The uncorrected group of observations is not reported in the SYS / PHASE SHIFT records.

In case the reported phase correction of an observation type does not affect all satellites of the same system, the header record allows to also indicate a list of the respective satellites.

The SYS / PHASE SHIFT header record is mandatory. If the file does not contain any observation pairs affected by phase shifts or if (exceptionally) the applied corrections are not known, the observation code field and the rest of the SYS / PHASE SHIFT header record field of the respective satellite system(s) are left blank.

”

This has been added to the latest RINEX format versions:

Rinex 2.12

Rinex 3.01

… more on those versions in an upcoming post!!

Best regards!
Nacho