'''Processing IPAS data in 2022'''

The IPAS system is mounted on the Owl so that it's viewing angle can be altered throughout a flight.

This is a work in progress by wja and will change as processing progresses.

'''Pre-processing'''

'''Warning: VM is currently writing as whatever user is logged in (instead of the airborne processing user). Be careful with permissions with writing to the Linux network'''

'''Basestations'''

Download basestation data from OS:

SNEO is the closest to Duxford.

Example:

{{{
lftp ftp://rinex.ordnancesurvey.co.uk/gps/rinex
cd 2022/178
# Download SNEO data
mirror --include-glob=sneo* ./
# Download ephemeris data
mirror --include-glob=EPH* ./
exit
}}}

These will need extracting with `unzip`

Annoyingly, the ephemeris file needs its extension renamed from `*.rnx` to `*.rxn`. Probably best to symlink like the example below:


{{{
# In the directory containing the rinex file
ln -sv EPHM00GBR_S_20221800000_01D_MN.rnx EPHM00GBR_S_20221800000_01D_MN.rxn
}}}



Need to combine basestation files into one file or IPAS TC, otherwise IPAS TC loads the first one in and complains that it doesn't match the flight nav times.

Using teqc:


{{{
teqc site257a.22o site257b.22o > site2570.22o
}}}


or with a glob, for example:

{{{
teqc sneo180?.22o > sneo1800.22o
}}}

'''Download from CDDIS:'''

We want to download the precise ephemeris (`*.sp3`) and precise clock (`*.clk`) files.

These are sorted by GPS week. E.g 2022 174 (23/06/22) is GPS week 2215. 2022 178a (27/06/22) is GPS week 2216.

Days of the week are then denoted as 0 - 6. Sunday = 0, Saturday = 6.

FTP example:

{{{
lftp -d -u anonymous,wja@pml.ac.uk -e 'set ftp:ssl-force true' gdc.cddis.eosdis.nasa.gov
}}}

{{{
lftp -d -u anonymous,wja@pml.ac.uk -e 'set ftp:ssl-force true' -e 'get gnss/products/2216/igs22161.sp3.Z ;exit' gdc.cddis.eosdis.nasa.gov
}}}

If the precise ephemeris data was not downloaded from OS (this is the file begging with `EPH*`, then you can download it from: https://cddis.nasa.gov/archive/gnss/data/daily/
and find the file called `brdc*00n`. However please note that IPAS TC doesn't recognise this file unless you click the 'All Files' option in the file browser when it asks you to locate this data.

'''TODO: This isn't working? "Connecting data socket to (198.118.242.43.190.49) oirt 48689" is hanging on 0% "[Making data connection...]"'''

Can download directly from this URL: https://cddis.nasa.gov/archive/gnss/products/2216/ where the last directory of the path is the GPS week.

Info on the orbit products are [here](https://cddis.nasa.gov/Data_and_Derived_Products/GNSS/orbit_products.html)

IGS is the analysis centre name we want. Depending on how NRT the data is required, there are differnt quality versions available. We want the highest quality available, ideally `igsWWWWD.sp3.Z` and `igsWWWWD.clk.Z`.

Extract the `*.Z` files with the `uncompress` command.





'''IPAS TC'''

On Windows VM active on pmp1784 in `~wja/scratch_local/pmpc1483vm10`

New project

Load in the raw navigation data

In the `Setup GNSS Base Station` window, have the `Use Differntial GNSS` butting ticked and click `Add Base Station`.

Load in our basestation file. In our case this is a RINEX file. Select RINEX as the "Receiver Type". '''Note:''' the software sometimes complains that it can't convert the input gpb file to gpb, this seems to be solved by putting the input basesstation RINEX file into a directory on its own.

In the `Setup GNSS Base Station` window, have the `Use Differntial GNSS` button ticked and click `Add Base Station`.

It should read the info from the file and allow you to execute the processing. At the start of the processing it should suggest that you interpolate the basestation data (which should have an interval of 30 seconds) to 1 second. Accept this, create the interpolated data and then run the processing.

If, when you input the basestation `*.gpb` file, everything is greyed out and the information has not been read properly from the basestation, click on the `Advanced` tab and interpolate the  data

'''PPP Processing'''

You can undertake PPP processing in IPAS TC if you download the correct `*.sp3` and `*clk` data. Follow the instructions in the software to do PPP processing once you've converted the  the basestation to`*.gpb` data. This is the "Compute Position via P3" button on the "Coordinate And Antenna Information" tab. You will then be prompted to download data within the software, click yes. This doesn't actually work but on the bottom right of this window you can select "Browse for Downloaded Files", which is when you have to option to input the path to the `*.sp3` (orbit) and `*clk` (clock)  files.

'''Lever Arms'''

The Owl was connected to the old IPAS20 (also referred to as 'Litton' previously). This IMU set up used to use a reference point for a sensor mount called 'PAV80' that we never had so is an arbitrary point below the plane.

We are not using 'PAV80' this year with the Owl, as this was (I belive) needed for the old Leica LiDAR system which isn't flown. Therefore we are using a reference point of 0, 0, 0 (i.e the plane's GNSS antennae) and then the lever arms from the IMU to there. The lever arms from the antennae to the Owl has been input into the APL config file.

In IPAC TC, for GNSS lever arm, input 0.001, 0, 0 (it doesn't let you process if they're all 0).

For the IMU lever arm, input 0.288, -0.638, 1.418

This has been calculated finding the center point of the IPAS20 according to the 2022 survey.