/** Do a blocking acquisition search in two stages : coarse and fine.
* Do a coarse acqusition to find the approximate code phase and carrier
* frequency, and then a more fine grained acquisition to find the code phase
* and carrier frequency more precisely.
*
* \param prn PRN to search (nap_acq_code_wr_blocking must be called prior)
* \param cp Code phase of the acquisition result
* \param cf Carrier frequency of the acquisition result
* \param snr SNR of the acquisition result
*/
u32 acq_full_two_stage(u8 prn, float* cp, float* cf, float* snr)
{
/* Initial coarse acq. */
float coarse_code_phase;
float coarse_carrier_freq;
float coarse_snr;
u32 coarse_count = nap_timing_count() + 1000;
acq_schedule_load(coarse_count);
while(!acq_get_load_done());
acq_start(prn, 0, 1023, -7000, 7000, 300);
while(!acq_get_done());
acq_get_results(&coarse_code_phase, &coarse_carrier_freq, &coarse_snr);
/* Fine acq. */
u32 fine_count = nap_timing_count() + 2000;
acq_schedule_load(fine_count);
while(!acq_get_load_done());
float fine_cp = propagate_code_phase(coarse_code_phase, coarse_carrier_freq, fine_count - coarse_count);
acq_start(prn, fine_cp-20, fine_cp+20, coarse_carrier_freq-300, coarse_carrier_freq+300, 100);
while(!acq_get_done());
acq_get_results(cp, cf, snr);
return fine_count;
}
bool acq_search(gnss_signal_t sid, float cf_min, float cf_max,
float cf_bin_width, acq_result_t *acq_result)
{
acq_set_sid(sid);
/* We have our SID chosen, now load some fresh data
* into the acquisition ram on the Swift NAP for
* an initial coarse acquisition.
*/
u32 sample_count;
do {
sample_count = nap_timing_count() + 20000;
/* acq_load could timeout if we're preempted and miss the timing strobe */
} while (!acq_load(sample_count));
acq_search_begin(cf_min, cf_max, cf_bin_width);
/* Done with the coarse acquisition, check if we have found a
* satellite, if so save the results and start the loading
* for the fine acquisition. If not, start again choosing a
* different PRN.
*/
acq_result->sample_count = sample_count;
acq_get_results(&acq_result->cp, &acq_result->cf, &acq_result->cn0);
return true;
}
/** Do a blocking acquisition search.
* Perform an acquisition for one PRN over a defined code and doppler range.
* Returns the code phase and carrier frequency of the largest peak in the
* search space together with the "SNR" value for that peak defined as
* (peak_magnitude - mean) / std_deviation.
*
* \param prn PRN number - 1 (0..31) to attempt to acquire
* (nap_acq_code_wr_blocking must be called prior).
* \param cp_min Lower bound for code phase search range in chips.
* \param cp_max Upper bound for code phase search range in chips.
* \param cf_min Lower bound for carrier freq. search range in Hz.
* \param cf_max Upper bound for carrier freq. search range in Hz.
* \param cp Pointer to a float where the peak's code phase value will be
* stored in chips.
* \param cf Pointer to a float where the peak's carrier frequency will be
* stored in Hz.
* \param snr Pointer to a float where the "SNR" of the peak will be stored.
*/
void do_acq(u8 prn, float cp_min, float cp_max, float cf_min, float cf_max, float cf_bin_width, float* cp, float* cf, float* snr)
{
acq_start(prn, cp_min, cp_max, cf_min, cf_max, cf_bin_width);
while(acq_state.state == ACQ_RUNNING) {
wait_for_nap_exti();
acq_service_irq();
}
wait_for_nap_exti();
acq_service_irq();
acq_get_results(cp, cf, snr);
}
/** Manages acquisition searches and starts tracking channels after successful acquisitions. */
static void manage_acq()
{
/* Decide which PRN to try and then start it acquiring. */
u8 prn = choose_prn();
if (prn == (u8)-1)
return;
u32 timer_count;
float snr, cp, cf;
acq_set_prn(prn);
/* We have our PRN chosen, now load some fresh data
* into the acquisition ram on the Swift NAP for
* an initial coarse acquisition.
*/
do {
timer_count = nap_timing_count() + 20000;
/* acq_load could timeout if we're preempted and miss the timing strobe */
} while (!acq_load(timer_count));
acq_search(acq_prn_param[prn].dopp_hint_low,
acq_prn_param[prn].dopp_hint_high,
ACQ_FULL_CF_STEP);
/* Done with the coarse acquisition, check if we have found a
* satellite, if so save the results and start the loading
* for the fine acquisition. If not, start again choosing a
* different PRN.
*/
acq_get_results(&cp, &cf, &snr);
/* Send result of an acquisition to the host. */
acq_send_result(prn, snr, cp, cf);
if (snr < ACQ_THRESHOLD) {
/* Didn't find the satellite :( */
/* Double the size of the doppler search space for next time. */
float dilute = (acq_prn_param[prn].dopp_hint_high -
acq_prn_param[prn].dopp_hint_low) / 2;
acq_prn_param[prn].dopp_hint_high =
MIN(acq_prn_param[prn].dopp_hint_high + dilute, ACQ_FULL_CF_MAX);
acq_prn_param[prn].dopp_hint_low =
MAX(acq_prn_param[prn].dopp_hint_low - dilute, ACQ_FULL_CF_MIN);
/* Decay hint scores */
for (u8 i = 0; i < ACQ_HINT_NUM; i++)
acq_prn_param[prn].score[i] = (acq_prn_param[prn].score[i] * 3) / 4;
/* Reset hint score for acuisition. */
acq_prn_param[prn].score[ACQ_HINT_ACQ] = 0;
return;
}
log_info("acq: PRN %d found @ %d Hz, %d SNR\n", prn + 1, (int)cf, (int)snr);
u8 chan = manage_track_new_acq();
if (chan == MANAGE_NO_CHANNELS_FREE) {
/* No channels are free to accept our new satellite :( */
/* TODO: Perhaps we can try to warm start this one
* later using another fine acq.
*/
log_info("No channels free :(\n");
if (snr > ACQ_RETRY_THRESHOLD) {
acq_prn_param[prn].score[ACQ_HINT_ACQ] = SCORE_ACQ + (snr - 20) / 20;
acq_prn_param[prn].dopp_hint_low = cf - ACQ_FULL_CF_STEP;
acq_prn_param[prn].dopp_hint_high = cf + ACQ_FULL_CF_STEP;
}
return;
}
/* Transition to tracking. */
u32 track_count = nap_timing_count() + 20000;
cp = propagate_code_phase(cp, cf, track_count - timer_count);
// Contrive for the timing strobe to occur at or close to a PRN edge (code phase = 0)
track_count += 16*(1023.0-cp)*(1.0 + cf / GPS_L1_HZ);
tracking_channel_init(chan, prn, cf, track_count, snr);
acq_prn_param[prn].state = ACQ_PRN_TRACKING;
nap_timing_strobe_wait(100);
}
/** Manages acquisition searches and starts tracking channels after successful acquisitions. */
static void manage_acq()
{
/* Decide which PRN to try and then start it acquiring. */
u8 prn = choose_prn();
if (prn == (u8)-1)
return;
gnss_signal_t sid = {.sat = prn};
u32 timer_count;
float cn0, cp, cf;
acq_set_prn(prn);
/* We have our PRN chosen, now load some fresh data
* into the acquisition ram on the Swift NAP for
* an initial coarse acquisition.
*/
do {
timer_count = nap_timing_count() + 20000;
/* acq_load could timeout if we're preempted and miss the timing strobe */
} while (!acq_load(timer_count));
/* Check for NaNs in dopp hints, or low > high */
if (!(acq_prn_param[prn].dopp_hint_low
<= acq_prn_param[prn].dopp_hint_high)) {
log_error("Acq: caught bogus dopp_hints (%f, %f)",
acq_prn_param[prn].dopp_hint_low,
acq_prn_param[prn].dopp_hint_high);
acq_prn_param[prn].dopp_hint_high = ACQ_FULL_CF_MAX;
acq_prn_param[prn].dopp_hint_low = ACQ_FULL_CF_MIN;
}
acq_search(acq_prn_param[prn].dopp_hint_low,
acq_prn_param[prn].dopp_hint_high,
ACQ_FULL_CF_STEP);
/* Done with the coarse acquisition, check if we have found a
* satellite, if so save the results and start the loading
* for the fine acquisition. If not, start again choosing a
* different PRN.
*/
acq_get_results(&cp, &cf, &cn0);
/* Send result of an acquisition to the host. */
acq_send_result(sid, cn0, cp, cf);
if (cn0 < ACQ_THRESHOLD) {
/* Didn't find the satellite :( */
/* Double the size of the doppler search space for next time. */
float dilute = (acq_prn_param[prn].dopp_hint_high -
acq_prn_param[prn].dopp_hint_low) / 2;
acq_prn_param[prn].dopp_hint_high =
MIN(acq_prn_param[prn].dopp_hint_high + dilute, ACQ_FULL_CF_MAX);
acq_prn_param[prn].dopp_hint_low =
MAX(acq_prn_param[prn].dopp_hint_low - dilute, ACQ_FULL_CF_MIN);
/* Decay hint scores */
for (u8 i = 0; i < ACQ_HINT_NUM; i++)
acq_prn_param[prn].score[i] = (acq_prn_param[prn].score[i] * 3) / 4;
/* Reset hint score for acquisition. */
acq_prn_param[prn].score[ACQ_HINT_PREV_ACQ] = 0;
return;
}
u8 chan = manage_track_new_acq();
if (chan == MANAGE_NO_CHANNELS_FREE) {
/* No channels are free to accept our new satellite :( */
/* TODO: Perhaps we can try to warm start this one
* later using another fine acq.
*/
if (cn0 > ACQ_RETRY_THRESHOLD) {
acq_prn_param[prn].score[ACQ_HINT_PREV_ACQ] =
SCORE_ACQ + (cn0 - ACQ_THRESHOLD);
acq_prn_param[prn].dopp_hint_low = cf - ACQ_FULL_CF_STEP;
acq_prn_param[prn].dopp_hint_high = cf + ACQ_FULL_CF_STEP;
}
return;
}
/* Transition to tracking. */
u32 track_count = nap_timing_count() + 20000;
cp = propagate_code_phase(cp, cf, track_count - timer_count);
// Contrive for the timing strobe to occur at or close to a PRN edge (code phase = 0)
track_count += 16*(1023.0-cp)*(1.0 + cf / GPS_L1_HZ);
/* Start the tracking channel */
tracking_channel_init(chan, sid, cf, track_count, cn0,
TRACKING_ELEVATION_UNKNOWN);
/* TODO: Initialize elevation from ephemeris if we know it precisely */
acq_prn_param[prn].state = ACQ_PRN_TRACKING;
nap_timing_strobe_wait(100);
}
/** Manages acquisition searches and starts tracking channels after successful acquisitions. */
void manage_acq()
{
/* Decide which PRN to try and then start it acquiring. */
u8 prn = best_prn();
if (prn == (u8)-1)
return;
u32 timer_count;
float snr, cp, cf;
acq_set_prn(prn);
/* We have our PRN chosen, now load some fresh data
* into the acquisition ram on the Swift NAP for
* an initial coarse acquisition.
*/
acq_prn_param[prn].state = ACQ_PRN_ACQUIRING;
do {
timer_count = nap_timing_count() + 20000;
/* acq_load could timeout if we're preempted and miss the timing strobe */
} while (!acq_load(timer_count));
/* Done loading, now lets set that coarse acquisition going. */
if (almanac[prn].valid && time_quality == TIME_COARSE) {
gps_time_t t = rx2gpstime(timer_count);
double dopp = -calc_sat_doppler_almanac(&almanac[prn], t.tow, t.wn, position_solution.pos_ecef);
/* TODO: look into accuracy of prediction and possibilities for
* improvement, e.g. use clock bias estimated by PVT solution. */
/*log_info("Expecting PRN %02d @ %.1f\n", prn+1, dopp);*/
acq_search(dopp - 4000, dopp + 4000, ACQ_FULL_CF_STEP);
} else {
acq_search(ACQ_FULL_CF_MIN, ACQ_FULL_CF_MAX, ACQ_FULL_CF_STEP);
}
/* Done with the coarse acquisition, check if we have found a
* satellite, if so save the results and start the loading
* for the fine acquisition. If not, start again choosing a
* different PRN.
*/
acq_get_results(&cp, &cf, &snr);
/* Send result of an acquisition to the host. */
acq_send_result(prn, snr, cp, cf);
if (snr < ACQ_THRESHOLD) {
/* Didn't find the satellite :( */
acq_prn_param[prn].state = ACQ_PRN_TRIED;
return;
}
log_info("acq: PRN %d found @ %d Hz, %d SNR\n", prn + 1, (int)cf, (int)snr);
u8 chan = manage_track_new_acq(snr);
if (chan == MANAGE_NO_CHANNELS_FREE) {
/* No channels are free to accept our new satellite :( */
/* TODO: Perhaps we can try to warm start this one
* later using another fine acq.
*/
log_info("No channels free :(\n");
acq_prn_param[prn].state = ACQ_PRN_TRIED;
return;
}
/* Transition to tracking. */
u32 track_count = nap_timing_count() + 20000;
cp = propagate_code_phase(cp, cf, track_count - timer_count);
// Contrive for the timing strobe to occur at or close to a PRN edge (code phase = 0)
track_count += 16*(1023.0-cp)*(1.0 + cf / GPS_L1_HZ);
tracking_channel_init(chan, prn, cf, track_count, snr);
acq_prn_param[prn].state = ACQ_PRN_TRACKING;
nap_timing_strobe_wait(100);
}
