How to obtain the HPAC, OCB, and GAD messages?

[semuadmin]

[reposted from email]

Hello, I am a researcher in the field of GNSS. I came across your libraries, and I must say they are very interesting.
I would need to understand how to obtain the HPAC, OCB, and GAD messages directly from a UBX file received via u-center. I managed to do this by parsing the log obtained using PyGPSclient, but I can’t seem to achieve it directly from the UBX file.
Could you help me with this?
Thank you in advance.

[semuadmin]

Firstly, I'm not in a position to provide support for u-blox's u-center software - for that, you'll need to refer to the u-blox support forum.

Secondly, you may want to refer to this gnsswiki, specifically the sections on RTK and SPARTN.

HPAC, OCB and GAD messages are part of the SPARTN protocol, which is published by u-blox.

• HPAC - High Precision Atmospheric Correction data
• OCB - Orbit, Clock and Bias data
• GAD - Geographic Area Definition data (defines the areas within with the service is available)

While the protocol itself is freely available, in order to receive these message types, you need to be subscribed to a paid SPARTN data service. They are not output by standard u-blox GNSS receivers (e.g. the ZED-F9P) and (as far as I am aware) are not recorded directly in u-center's UBX files.

At time of writing you have three options:

1. Thingstream PointPerfect NTRIP service - available via any NTRIP client but you need a subscription in order to obtain the necessary NTRIP credentials.
2. Thingstream PointPerfect MQTT (IP) service - requires a dedicated MQTT client.
3. L-Band SPARTN reception from a geostationary Inmarsat satellite - requires someting like the u-blox NEO-D9S correction receiver and a suitable L-band antenna (NB: a standard GNSS antenna may not be suitable). The data is transmitted as a series of UBX RXM-PMP messages, but the cumulative payload of those messages contains the individual HPAC, OCB and GAD messages. There is a sample Python script here which illustrates how to extract HPAC, OCB and GAD messages from RXM-PMP payloads.

Note that PointPerfect MQTT and L-Band services are encrypted and require a valid decryption key. Keys are cycled every 4 weeks.

PyGPSClient supports all three modes via its NTRIP and SPARTN clients and the underlying pyspartn library, but you'll need to have a valid subscription in order to receive and decrypt the data.

Hope this helps.