/** Using available almanac and ephemeris information, determine * whether a satellite is in view and the range of doppler frequencies * in which we expect to find it. * * \param prn 0-indexed PRN * \param t Time at which to evaluate ephemeris and almanac (typically system's * estimate of current time) * \param dopp_hint_low, dopp_hint_high Pointers to store doppler search range * from ephemeris or almanac, if available and elevation > mask * \return Score (higher is better) */ static u16 manage_warm_start(u8 prn, gps_time_t t, float *dopp_hint_low, float *dopp_hint_high) { /* Do we have any idea where/when we are? If not, no score. */ /* TODO: Stricter requirement on time and position uncertainty? We ought to keep track of a quantitative uncertainty estimate. */ if (time_quality < TIME_GUESS && position_quality < POSITION_GUESS) return SCORE_COLDSTART; float el = 0; double el_d, _, dopp_hint = 0, dopp_uncertainty = DOPP_UNCERT_ALMANAC; /* Do we have a suitable ephemeris for this sat? If so, use that in preference to the almanac. */ if (ephemeris_good(&es[prn], t)) { double sat_pos[3], sat_vel[3], el_d; calc_sat_state(&es[prn], t, sat_pos, sat_vel, &_, &_); wgsecef2azel(sat_pos, position_solution.pos_ecef, &_, &el_d); el = (float)(el_d) * R2D; if (el < elevation_mask) return SCORE_BELOWMASK; vector_subtract(3, sat_pos, position_solution.pos_ecef, sat_pos); vector_normalize(3, sat_pos); /* sat_pos now holds unit vector from us to satellite */ vector_subtract(3, sat_vel, position_solution.vel_ecef, sat_vel); /* sat_vel now holds velocity of sat relative to us */ dopp_hint = -GPS_L1_HZ * (vector_dot(3, sat_pos, sat_vel) / GPS_C + position_solution.clock_bias); /* TODO: Check sign of receiver frequency offset correction */ if (time_quality >= TIME_FINE) dopp_uncertainty = DOPP_UNCERT_EPHEM; } else if (almanac[prn].valid) { calc_sat_az_el_almanac(&almanac[prn], t.tow, t.wn-1024, position_solution.pos_ecef, &_, &el_d); el = (float)(el_d) * R2D; if (el < elevation_mask) return SCORE_BELOWMASK; dopp_hint = -calc_sat_doppler_almanac(&almanac[prn], t.tow, t.wn, position_solution.pos_ecef); } else { return SCORE_COLDSTART; /* Couldn't determine satellite state. */ } /* Return the doppler hints and a score proportional to elevation */ *dopp_hint_low = dopp_hint - dopp_uncertainty; *dopp_hint_high = dopp_hint + dopp_uncertainty; return SCORE_COLDSTART + SCORE_WARMSTART * el / 90.f; }
static void manage_calc_almanac_scores(void) { double az, el; gps_time_t t = get_current_time(); for (u8 prn=0; prn<32; prn++) { if (!almanac[prn].valid || time_quality == TIME_UNKNOWN || position_quality == POSITION_UNKNOWN) { /* No almanac or position/time information, give it the benefit of the * doubt. */ acq_prn_param[prn].score[ACQ_HINT_ALMANAC] = 0; } else { calc_sat_az_el_almanac(&almanac[prn], t.tow, t.wn-1024, position_solution.pos_ecef, &az, &el); float dopp = -calc_sat_doppler_almanac(&almanac[prn], t.tow, t.wn, position_solution.pos_ecef); acq_prn_param[prn].score[ACQ_HINT_ALMANAC] = el > 0 ? (u16)(SCORE_ALMANAC * 2 * el / M_PI) : 0; acq_prn_param[prn].dopp_hint_low = dopp - ALMANAC_DOPPLER_WINDOW; acq_prn_param[prn].dopp_hint_high = dopp + ALMANAC_DOPPLER_WINDOW; } } }
/** 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); }
/** Using available almanac and ephemeris information, determine * whether a satellite is in view and the range of doppler frequencies * in which we expect to find it. * * \param prn 0-indexed PRN * \param t Time at which to evaluate ephemeris and almanac (typically system's * estimate of current time) * \param dopp_hint_low, dopp_hint_high Pointers to store doppler search range * from ephemeris or almanac, if available and elevation > mask * \return Score (higher is better) */ static u16 manage_warm_start(gnss_signal_t sid, const gps_time_t* t, float *dopp_hint_low, float *dopp_hint_high) { /* Do we have any idea where/when we are? If not, no score. */ /* TODO: Stricter requirement on time and position uncertainty? We ought to keep track of a quantitative uncertainty estimate. */ if (time_quality < TIME_GUESS && position_quality < POSITION_GUESS) return SCORE_COLDSTART; float el = 0; double _, dopp_hint = 0, dopp_uncertainty = DOPP_UNCERT_ALMANAC; bool ready = false; /* Do we have a suitable ephemeris for this sat? If so, use that in preference to the almanac. */ const ephemeris_t *e = ephemeris_get(sid); u8 eph_valid; s8 ss_ret; double sat_pos[3], sat_vel[3], el_d; ephemeris_lock(); eph_valid = ephemeris_valid(e, t); if (eph_valid) { ss_ret = calc_sat_state(e, t, sat_pos, sat_vel, &_, &_); } ephemeris_unlock(); if (eph_valid && (ss_ret == 0)) { wgsecef2azel(sat_pos, position_solution.pos_ecef, &_, &el_d); el = (float)(el_d) * R2D; if (el < elevation_mask) return SCORE_BELOWMASK; vector_subtract(3, sat_pos, position_solution.pos_ecef, sat_pos); vector_normalize(3, sat_pos); /* sat_pos now holds unit vector from us to satellite */ vector_subtract(3, sat_vel, position_solution.vel_ecef, sat_vel); /* sat_vel now holds velocity of sat relative to us */ dopp_hint = -GPS_L1_HZ * (vector_dot(3, sat_pos, sat_vel) / GPS_C + position_solution.clock_bias); /* TODO: Check sign of receiver frequency offset correction */ if (time_quality >= TIME_FINE) dopp_uncertainty = DOPP_UNCERT_EPHEM; ready = true; } if(!ready) { const almanac_t *a = &almanac[sid_to_global_index(sid)]; if (a->valid && calc_sat_az_el_almanac(a, t, position_solution.pos_ecef, &_, &el_d) == 0) { el = (float)(el_d) * R2D; if (el < elevation_mask) return SCORE_BELOWMASK; if (calc_sat_doppler_almanac(a, t, position_solution.pos_ecef, &dopp_hint) != 0) { return SCORE_COLDSTART; } dopp_hint = -dopp_hint; } else { return SCORE_COLDSTART; /* Couldn't determine satellite state. */ } } /* Return the doppler hints and a score proportional to elevation */ *dopp_hint_low = dopp_hint - dopp_uncertainty; *dopp_hint_high = dopp_hint + dopp_uncertainty; return SCORE_COLDSTART + SCORE_WARMSTART * el / 90.f; }