THIS SECTION IS APPLICABLE ONLY FOR GROUP DELAYS PRODUCED BY FOURFIT!!

Using GAMB (for delays derived with Fourfit only)

GAMB is a complicated program which solves simultaneously four problems. It

resolves group delay ambiguities at all baselines at both bands;

determines preliminary parameters of clock polynomial model;

rejects outliers;

computes ionosphere calibration;

Refer to documentation of GAMB for details.

Default GAMB setup is as follows:

(B) Which band ? X-band and S-band both
(G) Use only good or all observations ? All
(S) Save information about ambiguities and outliers? Yes
(I) Calculate group delay ionosphere correction after ? Yes
(M) Minimal acceptable number of good observations at one baseline: 8
(C) Ambiguity spacing constant: as saved in database
(L) Cutoff limit: 8.0 nsec
(Q) Quality code limit: 5
(V) Verbosity mode: 2

Before using GAMB check once more that calibration status for the X-band and S-band databases is exactly the same. Then hit key (A) in order to initiate GAMB. In many cases it is enough for resolving ambiguities. In other cases GAMB issues you a warning. If GAMB doesn't issue a warning you can be sure that ambiguities were resolved correctly. Warning usually means excessive noise or presence of observations with sub-ambiguities. But GAMB may honestly confess that ambiguity resolution was unreliable or it may refuse to process the session at all.

Troubleshooting

It may occur that there are baselines which have less than 8 used observations. Ambiguity resolution becomes unstable for the baselines with few observations since GAMB uses statistical criteria. You can try to reduce the limit of the number of observations, but you cannot set it less than 4. If the number of used observations at some baseline after GAMB processing becomes less than the limit, GAMB suppresses all observations on that baseline and deselects that baseline. As a rule of thumb it is not worth it to try to restore the baseline with such a small number of observations. But if you find that it is necessary to keep those observations you can restore that baseline and then resolve ambiguities on that baseline manually.

Group delay ambiguity spacings are different for different baselines. This situation may occur when some channels were dropped in fringing due to hardware problems. GAMB refuses to process such an experiment. There are two ways to overcome this problem. The easiest way is to reduce the group delay ambiguity spacing by dividing it by an integer number, e.g. 2. Be cautious! It is a dangerous option. An alternative way is to process the session in several steps:
1. Split baselines onto 2 (maybe more) sets with the same group delay ambiguities.
2. First deselect all baselines of the second set and leave the baselines of the first set only. The program SLVEB does it. To call it hit (B) from the OPTIN menu. Then use GAMB. GAMB will resolve ambiguities on the selected baselines and will not see deselected baselines and therefore will not complain.
3. Deselect all baselines of the first set and leave the baselines of the second set only. Then use GAMB again.
4. Then select all baselines, make a solution and check whether the permanent ambiguities at S-band and X-band remain. Since GAMB resolved ambiguities for several subnetworks independently, group delay closure may not be conserved for the baselines from different subnetworks. It is necessary to check first S-band, then the X-band and if the estimates of baseline clocks shows misclosure, then correct ambiguities. ( See below )

Procedure for manual group delay ambiguity resolution suggested by A. Nothnagel (for Fourfit genereated delays only)

Manual group delay ambiguity resolution is an alternative technique. It is rather more time consuming and it is recommended only if you failed to resolve ambiguities automatically.

Steps of manual ambiguities resolution:

Toggle the flag [Use normally (W)eighted delays] to [Down-(W)eighted delays by 1.D9] by hitting (W) in the last SETFL page. This option reduces the weights of the delays by 1.D9 and forces the least squares fit to get its deterministic only from the delay rates which are not affected by the group delay ambiguities. However, the residuals for the group delays are still calculated.

By typing (B) we now enter the Baseline page where we select the baselines of which data will be used in the least squares fit. All baselines marked by the symbol 'X' will be used. You should include only the independent baselines which are formed with the reference station selected on the Site pages (for example, WETTZELL-MATERA, WETTZELL/MEDICINA, WETTZELL/MATERA ...). All other baselines should be excluded by hitting the space bar on the respective line. NB: There should be a good number of observations on each of these baselines. If there are no observations (e.g. at the baseline KOKEE/HARTRAO with length 98% of Earth's diameter) or only very few on a baseline, you have to select another baseline which connects the remote station to the reference station independently.

Make solution by hitting (Q).

When the adjustment is finished we can analyze the residuals by typing (P) on the OPTIN menu. In the case where there is only one database loaded we see the list of baselines immediately, otherwise we still have to select the database we want to examine before the baselines to be plotted are displayed.
Start with the first of the baselines which contains the reference station which you select from the list of baselines by moving the cursor there and click in the left mouse button. When the plot of residuals appears we may see several vertical lines with observations separated by n * the ambiguity spacing which is indicated in the second dialog window from the top (for example 50, 100 or 200 nsec, this is the so-called ambiguity spacing). Move the mouse pointer onto the dialog window field 'Shift All ambiguities', click the left mouse button and move the cursor to the vertical line of residuals which is closest to 0. Sometimes there may not even be any residuals at this ambiguity level. The vertical position of the cursor does not matter, only the horizontal. Press the left mouse button and all residuals are shifted to that ambiguity level. Click 'CNPLT baseline page' and select the next baseline containing the reference station. Repeat the ambiguity shifting for all baselines with the reference station and run the solution again. Do not bother to look at the non-independent baselines at this stage.

When all the independent baselines are fixed up have a look at the non-independent baselines with the plot routine as well and fix up any ambiguities. Here the residuals must now all be centered around zero! There may, however, still be some drift in the residuals.

If you think you have eliminated all ambiguities press (Q) again to check. Look at the RMS delay residuals which should be below 10 ns at X-band and below about 30 ns at S-band. Repeat the plots and check carefully. The next step is to go to the SETFL last page by hitting (L) from the main OPTIN menu and switching to 'Normally weighted delays' by hitting (W). Repeat the least squares fit and look at the residuals again. They should be much smaller (< 2 - 3 nsec) now and any drifts in the residuals should have vanished.

Now we do the S-band with the same procedure as the X-band. For this switch back to 'Down-weighted delays by 1.D9' in the SETFL last page, then hit (X) on the OPTIN menu and set up the S band database for the solution toggling the yes/no flags. Do not use both, S and X band, in the fit simultaneously! When you want to plot the residuals you have of course to look at the S-band data!
Hit (X) at the OPTIN menu and select the setup as follows:
Include in "Solution Generate residuals"
X-band Yes Yes S-band No Yes In this setup the deterministic of the S-band residuals are not determined by the S-band rates but by the X-band delays which have, of course, to be free of any ambiguities.

Type (Q) for making solution and examine the residuals of the S-band database and shift ambiguities as you did with the X-band data. For a test adjustment after you have shifted the S-band ambiguities switch off the X-band database completely in the "X" page. Do not use both S- and X-band in the fit simultaneously.

After both databases have been corrected for ambiguities manually you have to store the contribution to the group delay due to ionosphere. This contribution is used for some solution types. NB: you don't need to do it when you resolve ambiguities automatically, since GAMB computes and stores the ionosphere contribution for all observations.
To start the computation of S/X ionosphere contribution, return to the main OPTIN page and invoke the program IONO by hitting ([). The program lists the databases which reside in the scratch files. We do not calibrate the S-band database. Normally everything is set up properly and you just type (P) to continue.

Set again databases status. Hit (X) in OPTIN menu. Set YES in fields Include in solution?, Generate residuals? for X-band databases and set NO for S-band database.

Group delay ambiguities are resolved.

The next step is to inspect residuals. Set estimation of baseline-dependent clocks: hit (C) from the menu of the last SETFL page. Menu of the program BCLOK will be displayed. First set all baselines by hitting (W), then deselect a clock reference station by hitting the station code.

Make a solution by hitting (Q). Look at the listing. Normally the total wrms should be in the range [500, 1500] psec. If it exceeds 2000 psec, it means that probably either ambiguity resolution was not successful or there are clock breaks at one or more stations.

You should check estimates of baseline-dependent clocks. If the estimates exceed 1 nsec, it is an indication of remaining permanent ambiguities at that baseline, i.e there are no jumps in ambiguities among observations at all baselines but all observations at some baselines have incorrect ambiguities what causes triangle misclosures to be a multiple of the ambiguity spacing. You have to get rid of permanent ambiguities.

Manual re-distribution of permanent group delay ambiguities

First you have to decide which band is affected by permanent ambiguities. If baseline-dependent clocks has an adjustment by a multiple of the group delay ambiguity spacing at X-band -- then X-band. A permanent ambiguity at S-band will contribute by f_X/f_S = 12 times less. Thus, if you see baseline-dependent clock estimates less than one group delay ambiguity spacing but still larger than 1 nsec, it is an indication that there are S-band permanent ambiguities.

Set solution type X-band only or S-band only on the last SETFL page in accordance with the band affected by permanent ambiguities. Make a solution. Look at the list of baseline-dependent clocks. Find the first baseline with the estimate of the baseline-dependent clocks to be a multiple of group delay ambiguity spacing. Make plot of residuals of that baseline by hitting (P) at the OPTIN menu. Then use the function Set amb. shift. Set the correct sign of ambiguities: the sign should be the same as the sign of the baseline-dependent clock adjustments if the order of stations in the baseline both in the listing and in CNPLT is the same. The sign is opposite if the order of stations in the listing and in CNPLT is different. Then use the function Shift multi pts. CNPLT will ask you to point to the Starting point and then the End point. Click at the point in the very bottom of the plot in order to mark the "Starting point", then click the point in the very top of the plot in order to mark the "end point". Then you make a solution once more. Repeat this procedure if needed with the next baseline. Don't forget to set the solution type back to G-GXS combination after all these manipulations.

Sometime you have observations with ambiguities less than the group delay ambiguity spacing. These are sub-ambiguities and they are caused by a wring choice of maximum in the delay resolution function. There is no way to resolve sub-ambiguities in Solve. You have to suppress all observations with sub-ambiguities. It is still possible to recover at least part of such observations by running the program mk4fit for those observations, but this topic goes beyond this manual.

Inspection of residuals

Now you have to inspect residuals baseline by baseline. The purpose is to check quality of data, check whether the ambiguities were resolved correctly and check whether clock breaks have to be inserted. Call program CNPLT by hitting (P) from the OPTIN menu. (NB: CNPLT conflicts with some X-applications which grabs colors, like Netscape. You should close such applications before running CNPLT.)

If you find a jump in the plot of residuals you may try to insert a clock break. Be sure that it is not a jump in ambiguities at X- or S-band. In order to insert a clock break hit (E) in the main OPTIN menu, then list station's pages by hitting (N) or (P) till you find the station where you are going to insert clock breaks. Then hit (*) several times until you see a line insert. Then hit (C) and enter the time tag of the clock break. Then a new epoch for clock polynomial appears at the SETFL page for that station. Set the first three flags to 1 for the new clock break. Then make a new solution and check the listing and residual plots. Keep in mind that there should be enough observations between the start of the session, clock break(s) and the end of the session. There should be no less than 4 observations, otherwise your solution will be unstable or singular. If it seems to you that the session has many clock breaks, it may indicate another serious problems unrelated to clock behavior.

If you have a station with too few good observations (less than 5 at each baseline), or you have a station with postfit residual scatter larger than 5 nsec you can deselect it. But it is a last resort. In general you should try to keep as many stations/baselines as possible in the initial and intermediary solutions, and to leave the final decision to the time of the final solution. An analyst is able to select/deselect station/baseline in any time during further solutions including batch runs, however the data should be edited properly, otherwise selecting the baselines which have been suppressed during the initial solution might degrade the solution due to the presence of outliers.

This document was prepared by Leonid Petrov

Last update: 01-MAY-2000 12:23:00