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soc format

soc format. Ron Muellerschoen Jet Propulsion Laboratory California Institute of Technology. background. Designed to transport 1 Hz GPS data with minimal bandwidth over the Open Internet, July 1999 “soc” from socket, which is the interface at the application layer

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soc format

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  1. soc format Ron Muellerschoen Jet Propulsion Laboratory California Institute of Technology

  2. background • Designed to transport 1 Hz GPS data with minimal bandwidth over the Open Internet, July 1999 • “soc” from socket, which is the interface at the application layer • format stable since January, 2000 (when SNR numbers added) • follows little-endian byte order (PC order) • Transport layer is UDP, so format does not include: • sync bits • checksum or CRC • Efficient compression of 5 GPS observable (CA, P1, P2, L1,L2) down to 14 bytes and a nibble • 1 mm range resolution • .02 mm phase resolution • Total size per station per 1 Hz epoch: • 17+21*n_GPS bytes

  3. soc attributes • contains edit flags (or epochs) for cycle slips • stand alone time-tag per epoch • minimum representation of receiver’s clock solution • 3 SNR numbers • unique site id • modulo 12 hour sequence number • flags: • receiver type • GPS health flag • simple structure • 8 byte header • 9 byte overhead ( timetag, number of gps, etc. ) • 21 data bytes per gps

  4. soc format • header (8 bytes) • unsigned short type max value 2^16 -1 • type 0 - request for replay of packet, to remote site • type 1 - receiver data, from remote site • type 3 - broadcast ephemeris, from remote site • type 4 - heartbeat, among data collectors • type 5 - “will”, among data collectors • unsigned short length max value 65535 • unsigned short seqnum max value 43200 ( 12 hours ) • unsigned short site id max value 65535

  5. type 1 - receiver data • overhead ( 9 bytes ) • unsigned int gps_minutes 4 bytes • minutes past 6-jan-1980, good until year 10151 • unsigned short gps_millisecs 2 bytes • good for 65.535 seconds • short navt 2 bytes • units of 10*meters • range +/-327,680 meters • unsigned char sats_and_status 1 byte • bits 7-4 indicates number of sats less 1 (max of 16) • bits 1-3 indicate receiver type ( max of 7 ) • bit 0 reserved for health status

  6. type 1 - receiver data • data bytes ( 21 per gps ) • unsigned char prn 0 to 255 1 byte • unsigned short epoch_seq 0 to 35999 2 bytes • 10 hour non-ambiguous phase epoch • char ca_range[5] 5 bytes • highest 4 bits of first memory location are status bits • next 36 bits contain the ca_range in mm. • most significant bytes are now in lowest memory location • highest bits of byte are most significant • think big-endian order • 2^36 -1 possible values, min 0, max 68,719.476735 kms • unsigned char snr (for ca_range) 0 to 255 1 byte • L2_block 6 bytes • L1_block 6 bytes

  7. L1 & L2 block: overview • L1 and L2 block ( 6 bytes each ) • first 2 bytes and 2 bits for range • range units in mms • 2*(2^17 - 1) possible values • dynamic range <= 131.071 meters • next 6 bits and 2 bytes for phase • Phase units in 0.02 mms • 2*(2^21- 1) possible values • dynamic range <= 41.94302 meters • extended w/ overflow bits to 83.88606 meters • last byte • unsigned char snr 0 to 255

  8. L1 & L2 block: compression • Align L2/L1 to within integer wavelength of P2/P1 w/ editor • L2_block first: range_2 = P2 - CA_range Like: R + 2.54 I - (R + 1.54 I) ~ I <= 131.071 m. phase_2 = L2 - CA_range + 4.09*range_2 Like: R - 2.54 I - (R + 1.54 I) + 4.09 I ~ 0 <= 83.88606 m. • L1_block second: range_1 = P1 - CA_range Like: R + 1.54 I - (R + 1.54 I) ~ 0 (ca-p code bias) <= 131.071 m. phase_1 = L1 - CA_range + 3.09*range_2 Like: R - 1.54 I - (R + 1.54 I) + 3.09 I ~ 0 <= 83.88606 m.

  9. L1 & L2 block: initial point • Align L1 to integer wavelength of P1 w/ data editor N_L1 = floor( (P1 - L1) / 0.190 + 0.5) at start of phase arc L1 += N_L1 * 0.190 for a phase connected arc But P1-L1 ~ R+ 1.54 I - ( R - 1.54 I + L1_phase_bias ) ~ 3.09 I - L1_phase_bias Must subtract also from N_L1: N_L1 -= floor(3.09*(P2-P1)/0.190 + 0.5) • Likewise for L2 N_L2 = floor( (P2 - L2) / 0.244 + 0.5) at start of phase arc L2 += N_L2 * 0.244 for a phase connected arc But P2-L2 ~ R+ 2.54 I - ( R - 2.54 I + L2_phase_bias ) ~ 5.09 I - L2_phase_bias Must subtract also from N_L2: N_L2 -= floor(5.09*(P2-P1)/0.244 + 0.5)

  10. initial point, for example /* If phase break, then re-adjust the cycle count of the phase to align closer to the range. */ if ( (ll[0] == 1) || (ll[1] == 1) || force_break[SiteIDm1][prnj] ) { adjustct[SiteIDm1][0][prnj] = ((p1 - l1*LAML1) / LAML1) + 0.5; adjustct[SiteIDm1][1][prnj] = ((p2 - l2*LAML2) / LAML2) + 0.5; adjustct[SiteIDm1][0][prnj] = floor( adjustct[SiteIDm1][0][prnj] ); adjustct[SiteIDm1][1][prnj] = floor( adjustct[SiteIDm1][1][prnj] ); ll[0] = 1; ll[1] = 1; force_break[SiteIDm1][prnj] = false; /* further adjust for dynamic range, want only bias portion */ if ( send_to_socket ) { first_iono = p2 - p1; /* all in units of meters */ l1_adjust = floor( ALPHA1*first_iono/LAML1 + .5 ); l2_adjust = floor( ALPHA2*first_iono/LAML2 + .5 ); adjustct[SiteIDm1][0][prnj] -= l1_adjust; adjustct[SiteIDm1][1][prnj] -= l2_adjust; } } /* Add integer cycles to L1 and L2 to align closer to the range. */ l1 += adjustct[SiteIDm1][0][prnj]; l2 += adjustct[SiteIDm1][1][prnj];

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