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

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.

GrafNav

TODO: Lets try and move everything to IPAS TC

Use grafnav to convert RINEX to *.gbp which is required by IPAS TC for the processing.

For OS stations you need the extra ephemeris file. It's file extension needs renaming from *.rnx to *.rxn.

Click on 'Convert Raw GPS data to GPB' (5th button from the left on the toolbar).

Add directory containing daily RINEX file (no other files should be in here)

In 'Global Options' add the path the 'External Ephemeris (NAV File).

Hit 'Convert'

PPP is not working - potentially needs updated post-2019 GPS week rollover, skip to IPAS TC section

Load new *.gpb file in as remote.

Settings > PPP, then in the Precise tab, add *.sp3 and *.clk files.

Run processing with Process > PPP

On the Precise tab, add the *.sp3 orbit file downloaded from CDDIS.

Open new *gpb file as remote.

Go to Settings > PPP. On the precise tab download precise clock and ephemeris Note it only recognises the time format of MM/DD/YYYY for the date range

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 *.gpb file of basestation.

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 informaton has not been read properly from the basestation, click on the Advanced tab and interpolate the data