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 basestation RINEX file into a directory on its own.

It might then ask for the navigation data. This is most likely the brdc????.22n file downloaded from CCDIS earlier. But it could be included in the input RINEX file.

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.