/** Write to a NAP track channel's INIT register.
* Sets PRN (deprecated), initial carrier phase, and initial code phase of a
* NAP track channel. The tracking channel will start correlating with these
* parameters at the falling edge of the next NAP internal timing strobe.
* Also write CA code to track channel's code ram.
*
* \note The track channel's UPDATE register, which sets the carrier and
* code phase rates, must also be written to before the internal timing
* strobe goes low.
*
* \param channel NAP track channel whose INIT register to write.
* \param prn CA code PRN (0-31) to track. (deprecated)
* \param carrier_phase Initial code phase.
* \param code_phase Initial carrier phase.
*/
void nap_track_init(u8 channel, gnss_signal_t sid, u32 ref_timing_count,
float carrier_freq, float code_phase)
{
struct nap_ch_state *s = &nap_ch_state[channel];
memset(s, 0, sizeof(*s));
u32 track_count = nap_timing_count() + 20000;
float cp = propagate_code_phase(code_phase, carrier_freq,
track_count - ref_timing_count);
/* Contrive for the timing strobe to occur at or close to a
* PRN edge (code phase = 0) */
track_count += (NAP_FRONTEND_SAMPLE_RATE_Hz / GPS_CA_CHIPPING_RATE) * (1023.0-cp) *
(1.0 + carrier_freq / GPS_L1_HZ);
nap_track_code_wr_blocking(channel, sid);
nap_track_init_wr_blocking(channel, 0, 0, 0);
double cp_rate = (1.0 + carrier_freq / GPS_L1_HZ) * GPS_CA_CHIPPING_RATE;
nap_track_update(channel, carrier_freq, cp_rate, 0, 0);
/* Schedule the timing strobe for start_sample_count. */
track_count -= NAP_FRONTEND_SAMPLE_RATE_Hz / (2 * GPS_CA_CHIPPING_RATE);
s->count_snapshot = track_count;
s->carrier_phase = -s->carr_pinc;
s->carr_pinc_prev = s->carr_pinc;
s->code_pinc_prev = s->code_pinc;
COMPILER_BARRIER();
nap_timing_strobe(track_count);
nap_timing_strobe_wait(100);
}
/** Initialises a tracking channel.
* Initialises a tracking channel on the Swift NAP. The start_sample_count
* must be contrived to be at or close to a PRN edge (PROMPT code phase = 0).
*
* \param prn PRN number - 1 (0-31).
* \param channel Tracking channel number on the Swift NAP.
* \param carrier_freq Carrier frequency (Doppler) at start of tracking in Hz.
* \param start_sample_count Sample count on which to start tracking.
*/
void tracking_channel_init(u8 channel, u8 prn, float carrier_freq, u32 start_sample_count)
{
/* Calculate code phase rate with carrier aiding. */
float code_phase_rate = (1 + carrier_freq/L1_HZ) * NOMINAL_CODE_PHASE_RATE_HZ;
/* Adjust the channel start time as the start_sample_count passed
* in corresponds to a PROMPT code phase rollover but we want to
* start the channel on an EARLY code phase rollover.
*/
/* TODO : change hardcoded sample rate */
start_sample_count -= 0.5*16;
/* Setup tracking_channel struct. */
tracking_channel[channel].state = TRACKING_RUNNING;
tracking_channel[channel].prn = prn;
tracking_channel[channel].update_count = 0;
/* Use -1 to indicate an uninitialised value. */
tracking_channel[channel].TOW_ms = -1;
tracking_channel[channel].snr_threshold_count = 0;
comp_tl_init(&(tracking_channel[channel].tl_state), 1e3,
code_phase_rate-1.023e6, 1, 0.7, 1,
carrier_freq, 15, 0.7, 1,
0.005, 1540, 5000);
tracking_channel[channel].I_filter = 0;
tracking_channel[channel].Q_filter = 0;
tracking_channel[channel].code_phase_early = 0;
tracking_channel[channel].code_phase_rate_fp = code_phase_rate*NAP_TRACK_CODE_PHASE_RATE_UNITS_PER_HZ;
tracking_channel[channel].code_phase_rate_fp_prev = tracking_channel[channel].code_phase_rate_fp;
tracking_channel[channel].code_phase_rate = code_phase_rate;
tracking_channel[channel].carrier_phase = 0;
tracking_channel[channel].carrier_freq = carrier_freq;
tracking_channel[channel].carrier_freq_fp = (s32)(carrier_freq * NAP_TRACK_CARRIER_FREQ_UNITS_PER_HZ);
tracking_channel[channel].carrier_freq_fp_prev = tracking_channel[channel].carrier_freq_fp;
tracking_channel[channel].sample_count = start_sample_count;
nav_msg_init(&tracking_channel[channel].nav_msg);
/* Starting carrier phase is set to zero as we don't
* know the carrier freq well enough to calculate it.
*/
/* Start with code phase of zero as we have conspired for the
* channel to be initialised on an EARLY code phase rollover.
*/
nap_track_code_wr_blocking(channel, prn);
nap_track_init_wr_blocking(channel, prn, 0, 0);
nap_track_update_wr_blocking(channel, \
carrier_freq*NAP_TRACK_CARRIER_FREQ_UNITS_PER_HZ, \
tracking_channel[channel].code_phase_rate_fp);
/* Schedule the timing strobe for start_sample_count. */
nap_timing_strobe(start_sample_count);
}