nrf_modem_gnss_agps_data_almanac Field Mapping to YUMA Almanac

[ibiglari]

Hi,

I am downloading GPS Almanacs from U.S. Coast Guard Navigation Center web site to inject into the modem using nrf_modem_gnss_agps_write(). Below is current almanac for one satellite:

******** Week 197 almanac for PRN-01 ********
ID:                         01
Health:                     000
Eccentricity:               0.1223945618E-001
Time of Applicability(s):  589824.0000
Orbital Inclination(rad):   0.9890724625
Rate of Right Ascen(r/s):  -0.8114623721E-008
SQRT(A)  (m 1/2):           5153.605957
Right Ascen at Week(rad):  -0.1638998176E+001
Argument of Perigee(rad):   0.938111269
Mean Anom(rad):            -0.1604865606E+001
Af0(s):                     0.2212524414E-003
Af1(s/s):                  -0.3637978807E-011
week:                        197

I cannot map ioda member of nrf_modem_gnss_agps_data_almanac struct to any field in the YUMA (or SEM AL3).

Below is the mapping I have worked out:

nrf_modem_gnss_agps_data_almanac Member	YUMA Field
sv_id	ID
wn	week
toa	Time of Applicability
ioda	???
e	Eccentricity
delta_i	Orbital Inclination
omega_dot	Rate of Right Ascen
sv_health	Health
sqrt_a	SQRT(A)
omega0	Right Ascen at Week
w	Argument of Perigee
m0	Mean Anom
af0	Af0
af1	Af1

I understand that nrf_modem_gnss_agps_data_almanac expects certain values to be in semi-circle and not radians, and am using YUMA format just as a sample, due to it being easier on the eyes compared to SEM AL3.

Could you please advise what is ioda exactly, and how can it be calculated / deducted from GPS Almanac information?

Kind Regards,
Iman

[papatel]

Thank you Helsing for the response.  Totally agree on the issues with google SUPL.  We have done enough testing to conclude that this is absolutely not viable as is.  Nor should we expect it to as Google has zero incentive to make this server good for our use case.

Our data definitely contradicts your statements regarding A-GPS and P-GPS.

I would ask that your engineering team run their own experiments with the latest modem firmware and unmodified SLM on 91x1.  In addition to mediocre A-GPS TTF, we often see a strange bug where the GPS engine will Never get a fix even with a full view of the sky when A-GPS data is injected.  This is extremely disturbing so we moved to de-feature A-GNSS entirely.  Please tell us what we are doing wrong because we are following the guides exactly and running top of tree radio and slm firmware on unmodified 91x1 dev kits...

[helsing]

Hi Patrick, sorry for the long delay.

papatel said:

I would ask that your engineering team run their own experiments with the latest modem firmware and unmodified SLM on 91x1.  In addition to mediocre A-GPS TTF, we often see a strange bug where the GPS engine will Never get a fix even with a full view of the sky when A-GPS data is injected.  This is extremely disturbing so we moved to de-feature A-GNSS entirely.  Please tell us what we are doing wrong because we are following the guides exactly and running top of tree radio and slm firmware on unmodified 91x1 dev kits...

Please create a new ticket and we will look into the details. Please attach logs and modem traces from when this occur.

[papatel]

I was hoping someone else would notice and debug this issue we are seeing with A-GPS performance.  I have a feeling it has to do with Where we are testing from (India and the Philippines). We may have cycles next month to file a ticket with detailed logs if we still see the issue and no one else is complaining.  It is low priority for us as P-GPS alone provides consistent 1-3 second fixes open sky, Read: there isn't much fruit to be had by debugging A-GPS performance.  I hope your engineering team looks at this without our ticket in the meantime.  This is a primary feature in my books.

[Charlie]

Hi Patrick,

Thanks for your feedback. Do you have chance to do more exploration with A-GPS recently?

I see you are located in US, do you have any chance to repeat this issue in US instead of India and Philippines? 

If the problem continues, could you provide modem trace recordings for A-GPS test using SUPL? Just share a link to the new ticket here. 

Best regards,

Charlie