Nmea gps px4

I see a pattern:
timestamp: 1532831682 (0.234516 seconds ago)
seconds ago > 0.5 status: NOT OK
Help, what is “timestamp”? from which nmea message?

apart from a few exceptions, here they are:
INFO  [gps] 
INFO  [gps] Secondary GPS
INFO  [gps] protocol: NMEA
INFO  [gps] status: NOT OK, port: /dev/ttyS2, baudrate: 9600
INFO  [gps] sat info: disabled
INFO  [gps] rate reading:            183 B/s
INFO  [gps] rate position:           0.00 Hz
INFO  [gps] rate velocity:           0.00 Hz
INFO  [gps] rate publication:          0.00 Hz
INFO  [gps] rate RTCM injection:      0.00 Hz
 sensor_gps
    timestamp: 850764739 (0.031443 seconds ago)
    time_utc_usec: 0
    device_id: 0 (Type: 0x00, UNKNOWN:0 (0x00))
    lat: 598514186
    lon: 300259473
    alt: 47878
    alt_ellipsoid: 63878
    s_variance_m_s: 0.0000
    c_variance_rad: 0.1000
    eph: 0.0000
    epv: 0.0000
    hdop: 2.2000
    vdop: 0.0000
    noise_per_ms: 0
    jamming_indicator: 0
    vel_m_s: 0.0000
    vel_n_m_s: 0.0000
    vel_e_m_s: 0.0000
    vel_d_m_s: 0.0000
    cog_rad: 0.0000
    timestamp_time_relative: 0
    heading: nan
    heading_offset: 0.0000
    heading_accuracy: 0.0000
    automatic_gain_control: 0
    fix_type: 3
    jamming_state: 0
    vel_ned_valid: False
    satellites_used: 8

INFO  [gps] 
INFO  [gps] Secondary GPS
INFO  [gps] protocol: NMEA
INFO  [gps] status: NOT OK, port: /dev/ttyS2, baudrate: 9600
INFO  [gps] sat info: disabled
INFO  [gps] rate reading:            191 B/s
INFO  [gps] rate position:           0.00 Hz
INFO  [gps] rate velocity:           0.00 Hz
INFO  [gps] rate publication:          0.00 Hz
INFO  [gps] rate RTCM injection:      0.00 Hz
 sensor_gps
    timestamp: 958762682 (0.040315 seconds ago)
    time_utc_usec: 0
    device_id: 0 (Type: 0x00, UNKNOWN:0 (0x00))
    lat: 598514226
    lon: 300259379
    alt: 48857
    alt_ellipsoid: 64857
    s_variance_m_s: 0.0000
    c_variance_rad: 0.1000
    eph: 0.0000
    epv: 0.0000
    hdop: 2.1000
    vdop: 0.0000
    noise_per_ms: 0
    jamming_indicator: 0
    vel_m_s: 0.0000
    vel_n_m_s: 0.0000
    vel_e_m_s: 0.0000
    vel_d_m_s: 0.0000
    cog_rad: 0.0000
    timestamp_time_relative: 0
    heading: nan
    heading_offset: 0.0000
    heading_accuracy: 0.0000
    automatic_gain_control: 0
    fix_type: 3
    jamming_state: 0
    vel_ned_valid: True
    satellites_used: 9

    
INFO  [gps] 
INFO  [gps] Secondary GPS
INFO  [gps] protocol: NMEA
INFO  [gps] status: NOT OK, port: /dev/ttyS2, baudrate: 9600
INFO  [gps] sat info: disabled
INFO  [gps] rate reading:            216 B/s
INFO  [gps] rate position:           0.00 Hz
INFO  [gps] rate velocity:           0.00 Hz
INFO  [gps] rate publication:          0.00 Hz
INFO  [gps] rate RTCM injection:      0.00 Hz
 sensor_gps
    timestamp: 1302832183 (0.685003 seconds ago)
    time_utc_usec: 1686521947000000
    device_id: 11206677 (Type: 0xAB, SERIAL:2 (0x00))
    lat: 598514210
    lon: 300259334
    alt: 47956
    alt_ellipsoid: 63956
    s_variance_m_s: 0.0000
    c_variance_rad: 0.1000
    eph: 0.0000
    epv: 0.0000
    hdop: 2.2000
    vdop: 0.0000
    noise_per_ms: 0
    jamming_indicator: 0
    vel_m_s: 0.0000
    vel_n_m_s: 0.0000
    vel_e_m_s: -0.0000
    vel_d_m_s: 0.0000
    cog_rad: -1.3352
    timestamp_time_relative: 0
    heading: nan
    heading_offset: 0.0000
    heading_accuracy: 0.0000
    automatic_gain_control: 0
    fix_type: 3
    jamming_state: 0
    vel_ned_valid: True
    satellites_used: 8
    
INFO  [gps] 
INFO  [gps] Secondary GPS
INFO  [gps] protocol: NMEA
INFO  [gps] status: NOT OK, port: /dev/ttyS2, baudrate: 9600
INFO  [gps] sat info: disabled
INFO  [gps] rate reading:            237 B/s
INFO  [gps] rate position:           0.00 Hz
INFO  [gps] rate velocity:           0.00 Hz
INFO  [gps] rate publication:          0.00 Hz
INFO  [gps] rate RTCM injection:      0.00 Hz
 sensor_gps
    timestamp: 1336830914 (0.010898 seconds ago)
    time_utc_usec: 1686521981000000
    device_id: 11206677 (Type: 0xAB, SERIAL:2 (0x00))
    lat: 598514179
    lon: 300259324
    alt: 48049
    alt_ellipsoid: 64049
    s_variance_m_s: 0.0000
    c_variance_rad: 0.1000
    eph: 0.0000
    epv: 0.0000
    hdop: 2.2000
    vdop: 0.0000
    noise_per_ms: 0
    jamming_indicator: 0
    vel_m_s: 0.0000
    vel_n_m_s: -0.0000
    vel_e_m_s: -0.0000
    vel_d_m_s: 0.0000
    cog_rad: -1.9268
    timestamp_time_relative: 0
    heading: nan
    heading_offset: 0.0000
    heading_accuracy: 0.0000
    automatic_gain_control: 0
    fix_type: 3
    jamming_state: 0
    vel_ned_valid: True
    satellites_used: 9

How can this work out?