[~andy/rsl] / wsr88d_m31.c

/*
    NASA/TRMM, Code 613
    This is the TRMM Office Radar Software Library.
    Copyright (C) 2008
            Bart Kelley
            SSAI
            Lanham, Maryland

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/


/* 
 * This file contains routines for processing Message Type 31, the digital
 * radar message type introduced in WSR-88D Level II Build 10.
 */

#include "rsl.h"
#include "wsr88d.h"
#include <string.h>

/* Data descriptions in the following data structures are from the "Interface
 * Control Document for the RDA/RPG", Build 10.0 Draft, WSR-88D Radar
 * Operations Center.
 */

typedef struct {
    short rpg[6]; /* 12 bytes inserted by RPG Communications Mgr. Ignored. */
    unsigned short msg_size;  /* Message size for this segment, in halfwords */
    unsigned char  channel;   /* RDA Redundant Channel */
    unsigned char  msg_type;  /* Message type. For example, 31 */
    unsigned short id_seq;    /* Msg seq num = 0 to 7FFF, then roll over to 0 */
    unsigned short msg_date;  /* Modified Julian date from 1/1/70 */
    unsigned int   msg_time;  /* Packet generation time in ms past midnight */
    unsigned short num_segs;  /* Number of segments for this message */
    unsigned short seg_num;   /* Number of this segment */
} Wsr88d_msg_hdr;

typedef struct {
    char radar_id[4];
    unsigned int   ray_time; /* Data collection time in milliseconds past midnight GMT */
    unsigned short ray_date; /* Julian date - 2440586.5 (1/01/1970) */
    unsigned short azm_num ; /* Radial number within elevation scan */
    float azm;      /* Azimuth angle in degrees (0 to 359.956055) */
    unsigned char compression_code; /* 0 = uncompressed, 1 = BZIP2, 2 = zlib */
    unsigned char spare; /* for word alignment */
    unsigned short radial_len; /* radial length in bytes, including data header block */
    unsigned char azm_res;
    unsigned char radial_status;
    unsigned char elev_num;
    unsigned char sector_cut_num;
    float elev;  /* Elevation angle in degrees (-7.0 to 70.0) */
    unsigned char radial_spot_blanking;
    unsigned char azm_indexing_mode;
    unsigned short data_block_count;
    /* Data Block Pointers */
    unsigned int dbptr_vol_const;
    unsigned int dbptr_elev_const;
    unsigned int dbptr_radial_const;
    unsigned int dbptr_ref;
    unsigned int dbptr_vel;
    unsigned int dbptr_sw;
    unsigned int dbptr_zdr;
    unsigned int dbptr_phi;
    unsigned int dbptr_rho;
} Ray_header_m31;  /* Called Data Header Block in RDA/RPG document. */

typedef struct {
    char dataname[4];
    unsigned int  reserved;
    unsigned short ngates;
    short range_first_gate;
    short range_samp_interval;
    short thresh_not_overlayed;
    short snr_thresh;
    unsigned char controlflag;
    unsigned char datasize_bits;
    float scale;
    float offset;
} Data_moment_hdr;

#define MAX_RADIAL_LENGTH 14288

typedef struct {
    Ray_header_m31 ray_hdr;
    float unamb_rng;
    float nyq_vel;
    unsigned char data[MAX_RADIAL_LENGTH];
} Wsr88d_ray_m31;


enum radial_status {START_OF_ELEV, INTERMED_RADIAL, END_OF_ELEV, BEGIN_VOS,
    END_VOS};


void wsr88d_swap_m31_hdr(Wsr88d_msg_hdr *msghdr)
{
    swap_2_bytes(&msghdr->msg_size);
    swap_2_bytes(&msghdr->id_seq);
    swap_2_bytes(&msghdr->msg_date);
    swap_4_bytes(&msghdr->msg_time);
    swap_2_bytes(&msghdr->num_segs);
    swap_2_bytes(&msghdr->seg_num);
}


void wsr88d_swap_m31_ray(Ray_header_m31 *wsr88d_ray)
{
    int *data_ptr;

    swap_4_bytes(&wsr88d_ray->ray_time);
    swap_2_bytes(&wsr88d_ray->ray_date);
    swap_2_bytes(&wsr88d_ray->azm_num);
    swap_4_bytes(&wsr88d_ray->azm);
    swap_2_bytes(&wsr88d_ray->radial_len);
    swap_4_bytes(&wsr88d_ray->elev);
    swap_2_bytes(&wsr88d_ray->data_block_count);
    data_ptr = (int *) &wsr88d_ray->dbptr_vol_const;
    for (; data_ptr <= (int *) &wsr88d_ray->dbptr_rho; data_ptr++)
        swap_4_bytes(data_ptr);
}


void wsr88d_swap_data_hdr(Data_moment_hdr *this_field)
{
    short *halfword;
    halfword = (short *) &this_field->ngates;
    for (; halfword < (short *) &this_field->controlflag; halfword++)
        swap_2_bytes(halfword);
    swap_4_bytes(&this_field->scale);
    swap_4_bytes(&this_field->offset);
}


float wsr88d_get_angle(short bitfield)
{
    short mask = 1;
    int i;
    float angle = 0.;
    float value[13] = {0.043945, 0.08789, 0.17578, 0.35156, .70313, 1.40625,
        2.8125, 5.625, 11.25, 22.5, 45., 90., 180.};

    /* find which bits are set and sum corresponding values to get angle. */

    bitfield = bitfield >> 3;  /* 3 least significant bits aren't used. */
    for (i = 0; i < 13; i++) {
        if (bitfield & mask) angle += value[i];
        bitfield = bitfield >> 1;
    }
    return angle;
}


float wsr88d_get_azim_rate(short bitfield)
{
    short mask = 1;
    int i;
    float rate = 0.;
    float value[12] = {0.0109863, 0.021972656, 0.043945, 0.08789, 0.17578,
        0.35156, .70313, 1.40625, 2.8125, 5.625, 11.25, 22.5};

    /* Find which bits are set and sum corresponding values to get rate. */

    bitfield = bitfield >> 3;  /* 3 least significant bits aren't used. */
    for (i = 0; i < 12; i++) {
        if (bitfield & mask) rate += value[i];
        bitfield = bitfield >> 1;
    }
    if (bitfield >> 15) rate = -rate;
    return rate;
}

#define WSR88D_MAX_SWEEPS 20

typedef struct {
    int vcp;
    int num_cuts;
    float vel_res;
    float fixed_angle[WSR88D_MAX_SWEEPS];
    float azim_rate[WSR88D_MAX_SWEEPS];
    int waveform[WSR88D_MAX_SWEEPS];
    int super_res_ctrl[WSR88D_MAX_SWEEPS];
    int surveil_prf_num[WSR88D_MAX_SWEEPS];
    int doppler_prf_num[WSR88D_MAX_SWEEPS];
} VCP_data;

static VCP_data vcp_data;

void wsr88d_get_vcp_data(short *msgtype5)
{
    short azim_rate, fixed_angle, vel_res;
    short sres_and_survprf; /* super res ctrl and surveil prf, one byte each */
    short chconf_and_waveform;
    int i;
    
    vcp_data.vcp = (unsigned short) msgtype5[2];
    vcp_data.num_cuts = msgtype5[3];
    if (little_endian()) {
        swap_2_bytes(&vcp_data.vcp);
        swap_2_bytes(&vcp_data.num_cuts);
    }
    vel_res = msgtype5[5];
    if (little_endian()) swap_2_bytes(&vel_res);
    vel_res = vel_res >> 8;
    if (vel_res == 2) vcp_data.vel_res = 0.5;
    else if (vel_res == 4) vcp_data.vel_res = 1.0;
    else vcp_data.vel_res = 0.0;
    /* Get elevation related information for each sweep. */
    for (i=0; i < vcp_data.num_cuts; i++) {
        fixed_angle = msgtype5[11 + i*23];
        azim_rate = msgtype5[15 + i*23];
        chconf_and_waveform = msgtype5[12 + i*23];
        sres_and_survprf = msgtype5[13 + i*23];
        vcp_data.doppler_prf_num[i] = msgtype5[23 + i*23];
        if (little_endian()) {
            swap_2_bytes(&fixed_angle);
            swap_2_bytes(&azim_rate);
            swap_2_bytes(&chconf_and_waveform);
            swap_2_bytes(&sres_and_survprf);
            swap_2_bytes(&vcp_data.doppler_prf_num[i]);
        }
        vcp_data.fixed_angle[i] = wsr88d_get_angle(fixed_angle);
        vcp_data.azim_rate[i] = wsr88d_get_azim_rate(azim_rate);
        vcp_data.waveform[i] = chconf_and_waveform & 0xff;
        vcp_data.super_res_ctrl[i] = sres_and_survprf >> 8;
        vcp_data.surveil_prf_num[i] = sres_and_survprf & 0xff;
    }
}


void get_wsr88d_unamb_and_nyq_vel(Wsr88d_ray_m31 *wsr88d_ray, float *unamb_rng,
        float *nyq_vel)
{
    int dbptr, found, ray_hdr_len;
    short nyq_vel_sh, unamb_rng_sh;

    found = 0;
    ray_hdr_len = sizeof(wsr88d_ray->ray_hdr);
    dbptr = wsr88d_ray->ray_hdr.dbptr_radial_const - ray_hdr_len;
    if (strncmp((char *) &wsr88d_ray->data[dbptr], "RRAD", 4) == 0) found = 1;
    else {
        dbptr = wsr88d_ray->ray_hdr.dbptr_elev_const - ray_hdr_len;
        if (strncmp((char *) &wsr88d_ray->data[dbptr], "RRAD", 4) == 0)
            found = 1;
        else {
            dbptr = wsr88d_ray->ray_hdr.dbptr_vol_const - ray_hdr_len;
            if (strncmp((char *) &wsr88d_ray->data[dbptr], "RRAD", 4) == 0)
                found = 1;
        }
    }
    if (found) {
        memcpy(&unamb_rng_sh, &wsr88d_ray->data[dbptr+6], 2);
        memcpy(&nyq_vel_sh, &wsr88d_ray->data[dbptr+16], 2);
        if (little_endian()) {
            swap_2_bytes(&unamb_rng_sh);
            swap_2_bytes(&nyq_vel_sh);
        }
        *unamb_rng = unamb_rng_sh / 10.;
        *nyq_vel = nyq_vel_sh / 100.;
    } else {
        *unamb_rng = 0.;
        *nyq_vel = 0.;
    }
}


int read_wsr88d_ray_m31(Wsr88d_file *wf, int msg_size,
        Wsr88d_ray_m31 *wsr88d_ray)
{
    Ray_header_m31 ray_hdr;
    int n, bytes_to_read, ray_hdr_len;
    float nyq_vel, unamb_rng;

    /* Read ray data header block */
    n = fread(&wsr88d_ray->ray_hdr, sizeof(Ray_header_m31), 1, wf->fptr);
    if (n < 1) {
        fprintf(stderr,"read_wsr88d_ray_m31: Read failed.\n");
        return 0;
    }

    /* Byte swap header if needed. */
    if (little_endian()) wsr88d_swap_m31_ray(&wsr88d_ray->ray_hdr);

    ray_hdr = wsr88d_ray->ray_hdr;
    ray_hdr_len = sizeof(ray_hdr);

    /* Read data portion of radial. */

    bytes_to_read = msg_size - ray_hdr_len;
    n = fread(wsr88d_ray->data, bytes_to_read, 1, wf->fptr);
    if (n < 1) {
        fprintf(stderr,"read_wsr88d_ray_m31: Read failed.\n");
        return 0;
    }

    /* We retrieve unambiguous range and Nyquist velocity here so that we don't
     * have to do it repeatedly for each data moment later.
     */
    get_wsr88d_unamb_and_nyq_vel(wsr88d_ray, &unamb_rng, &nyq_vel);
    wsr88d_ray->unamb_rng = unamb_rng;
    wsr88d_ray->nyq_vel = nyq_vel;

    return 1;
}


void wsr88d_load_ray_hdr(Wsr88d_ray_m31 wsr88d_ray, Ray *ray)
{
    int month, day, year, hour, minute, sec;
    float fsec;
    Wsr88d_ray m1_ray;
    Ray_header_m31 ray_hdr;

    ray_hdr = wsr88d_ray.ray_hdr;
    m1_ray.ray_date = ray_hdr.ray_date;
    m1_ray.ray_time = ray_hdr.ray_time;

    wsr88d_get_date(&m1_ray, &month, &day, &year);
    wsr88d_get_time(&m1_ray, &hour, &minute, &sec, &fsec);
    ray->h.year = year + 1900;
    ray->h.month = month;
    ray->h.day = day;
    ray->h.hour = hour;
    ray->h.minute = minute;
    ray->h.sec = sec + fsec;
    ray->h.azimuth = ray_hdr.azm;
    ray->h.ray_num = ray_hdr.azm_num;
    ray->h.elev = ray_hdr.elev;
    ray->h.elev_num = ray_hdr.elev_num;
    ray->h.unam_rng = wsr88d_ray.unamb_rng;
    ray->h.nyq_vel = wsr88d_ray.nyq_vel;
    int elev_index;
    elev_index = ray_hdr.elev_num - 1;
    ray->h.azim_rate = vcp_data.azim_rate[elev_index];
    ray->h.fix_angle = vcp_data.fixed_angle[elev_index];
    ray->h.vel_res = vcp_data.vel_res;
    if (ray_hdr.azm_res != 1)
        ray->h.beam_width = 1.0;
    else ray->h.beam_width = 0.5;

    /* For convenience, use message type 1 routines to get some values.
     * First load VCP and elevation numbers into a msg 1 ray.
     */
    m1_ray.vol_cpat = vcp_data.vcp;
    m1_ray.elev_num = ray_hdr.elev_num;
    m1_ray.unam_rng = (short) (wsr88d_ray.unamb_rng * 10.);
    /* Get values from message type 1 routines. */
    ray->h.frequency = wsr88d_get_frequency(&m1_ray);
    ray->h.pulse_width = wsr88d_get_pulse_width(&m1_ray);
    ray->h.pulse_count = wsr88d_get_pulse_count(&m1_ray);
    ray->h.prf = (int) wsr88d_get_prf(&m1_ray);
    ray->h.wavelength = 0.1071;
}


int wsr88d_get_vol_index(char* dataname)
{
    int vol_index = -1;

    if (strncmp(dataname, "DREF", 4) == 0) vol_index = DZ_INDEX;
    if (strncmp(dataname, "DVEL", 4) == 0) vol_index = VR_INDEX;
    if (strncmp(dataname, "DSW",  3) == 0) vol_index = SW_INDEX;
    if (strncmp(dataname, "DZDR", 3) == 0) vol_index = DR_INDEX;
    if (strncmp(dataname, "DPHI", 3) == 0) vol_index = PH_INDEX;
    if (strncmp(dataname, "DRHO", 3) == 0) vol_index = RH_INDEX;

    return vol_index;
}


#define MAXRAYS_M31 800
#define MAXSWEEPS 20

void wsr88d_load_ray(Wsr88d_ray_m31 wsr88d_ray, int data_ptr,
        int isweep, int iray, Radar *radar)
{
    /* Load data into ray structure for this field or data moment. */

    Data_moment_hdr data_hdr;
    int ngates;
    int i, hdr_size;
    float value, scale, offset;
    unsigned char *data;
    Range (*invf)(float x);
    float (*f)(Range x);
    Ray *ray;
    int vol_index, waveform;

    enum waveforms {surveillance=1, doppler_w_amb_res, doppler_no_amb_res,
        batch};

    /* Get data moment header. */
    hdr_size = sizeof(data_hdr);
    memcpy(&data_hdr, &wsr88d_ray.data[data_ptr], hdr_size);
    data_ptr += hdr_size;
    if (little_endian()) wsr88d_swap_data_hdr(&data_hdr);

    vol_index = wsr88d_get_vol_index(data_hdr.dataname);
    if (vol_index < 0) {
        fprintf(stderr,"wsr88d_load_ray: Unknown dataname %s.  isweep = %d, "
                "iray = %d.\n", data_hdr.dataname, isweep, iray);
        return;
    }
    switch (vol_index) {
        case DZ_INDEX: f = DZ_F; invf = DZ_INVF; break;
        case VR_INDEX: f = VR_F; invf = VR_INVF; break;
        case SW_INDEX: f = SW_F; invf = SW_INVF; break;
        case DR_INDEX: f = DR_F; invf = DR_INVF; break;
        case PH_INDEX: f = PH_F; invf = PH_INVF; break;
        case RH_INDEX: f = RH_F; invf = RH_INVF; break;
    }

    waveform = vcp_data.waveform[isweep];
    /* The following somewhat complicated if-statement says we'll do the
     * body of the statement if: 
     * a) the data moment is not reflectivity, or
     * b) the data moment is reflectivity and one of the following is true:
     *   - waveform is surveillance
     *   - waveform is batch
     *   - elevation is greater than highest split cut (i.e., 6 deg.)
     */
    if (vol_index != DZ_INDEX || (waveform == surveillance ||
                waveform == batch || vcp_data.fixed_angle[isweep] >= 6.0)) {
        if (radar->v[vol_index] == NULL) {
            radar->v[vol_index] = RSL_new_volume(MAXSWEEPS);
            radar->v[vol_index]->h.f = f;
            radar->v[vol_index]->h.invf = invf;
        }
        if (radar->v[vol_index]->sweep[isweep] == NULL) {
            radar->v[vol_index]->sweep[isweep] = RSL_new_sweep(MAXRAYS_M31);
            radar->v[vol_index]->sweep[isweep]->h.f = f;
            radar->v[vol_index]->sweep[isweep]->h.invf = invf;
        }
        ngates = data_hdr.ngates;
        ray = RSL_new_ray(ngates);

        /* Convert data to float, then use range function to store in ray.
         * Note: data range is 2-255. 0 means signal is below threshold, and 1
         * means range folded.
         */

        offset = data_hdr.offset;
        scale = data_hdr.scale;
        if (data_hdr.scale == 0) scale = 1.0; 
        data = &wsr88d_ray.data[data_ptr];
        for (i = 0; i < ngates; i++) {
            if (data[i] > 1)
                value = (data[i] - offset) / scale;
            else value = (data[i] == 0) ? BADVAL : RFVAL;
            ray->range[i] = invf(value);
            ray->h.f = f;
            ray->h.invf = invf;
        }
        wsr88d_load_ray_hdr(wsr88d_ray, ray);
        ray->h.range_bin1 = data_hdr.range_first_gate;
        ray->h.gate_size = data_hdr.range_samp_interval;
        ray->h.nbins = ngates;
        radar->v[vol_index]->sweep[isweep]->ray[iray] = ray;
        radar->v[vol_index]->sweep[isweep]->h.nrays = iray+1;
    }
}


void wsr88d_load_ray_into_radar(Wsr88d_ray_m31 wsr88d_ray, int isweep, int iray,
        Radar *radar)
{
    int *data_ptr, hdr_size;
    int i, ndatablocks, nconstblocks = 3;

    hdr_size = sizeof(wsr88d_ray.ray_hdr);
     
    ndatablocks = wsr88d_ray.ray_hdr.data_block_count;
    data_ptr = (int *) &wsr88d_ray.ray_hdr.dbptr_ref;
    for (i=0; i < ndatablocks-nconstblocks; i++) {
        wsr88d_load_ray(wsr88d_ray, *data_ptr-hdr_size, isweep, iray, radar);
        data_ptr++;
    }
}


void wsr88d_load_sweep_header(Radar *radar, int isweep)
{
    int ivolume, nrays;
    Sweep *sweep;
    Ray *last_ray;

    for (ivolume=0; ivolume < MAX_RADAR_VOLUMES; ivolume++) {
        if (radar->v[ivolume] != NULL &&
                radar->v[ivolume]->sweep[isweep] != NULL) {
            sweep = radar->v[ivolume]->sweep[isweep];
            nrays = sweep->h.nrays;
            if (nrays == 0) continue;
            last_ray = sweep->ray[nrays-1];
            sweep->h.sweep_num = last_ray->h.elev_num;
            sweep->h.elev = vcp_data.fixed_angle[isweep];
            sweep->h.beam_width = last_ray->h.beam_width;
            sweep->h.vert_half_bw = sweep->h.beam_width / 2.;
            sweep->h.horz_half_bw = sweep->h.beam_width / 2.;
        }
    }
}


Radar *load_wsr88d_m31_into_radar(Wsr88d_file *wf)
{
    Wsr88d_msg_hdr msghdr;
    Wsr88d_ray_m31 wsr88d_ray;
    short non31_seg_remainder[1202]; /* Remainder after message header */
    int end_of_vos = 0, isweep = 0, iray = 0;
    int msg_hdr_size, msg_size, n;
    Radar *radar = NULL;

    /* Message type 31 is a variable length message.  All other message types
     * are made up of 2432-byte segments.  To handle all types, we read the
     * message header and check the message type.  If it is not 31, we simply
     * read the remainder of the 2432-byte segment.  If message type is 31, we
     * use the size given in message header to determine how many bytes to
     * read.  For more information, see the "Interface Control Document for the
     * RDA/RPG" at the WSR-88D Radar Operations Center web site.
     */  

    n = fread(&msghdr, sizeof(Wsr88d_msg_hdr), 1, wf->fptr);

    /* printf("msgtype = %d\n", msghdr.msg_type); */
    msg_hdr_size = sizeof(Wsr88d_msg_hdr) - sizeof(msghdr.rpg);

    radar = RSL_new_radar(MAX_RADAR_VOLUMES);

    while (! end_of_vos) {
        if (msghdr.msg_type == 31) {
            if (little_endian()) wsr88d_swap_m31_hdr(&msghdr);

            /* Get size of the remainder of message.  The given size is in
             * halfwords, but we want it in bytes, so double it.
             */
            msg_size = (int) msghdr.msg_size * 2 - msg_hdr_size;

            n = read_wsr88d_ray_m31(wf, msg_size, &wsr88d_ray);
            /* Assume error message was issued from read routine */
            if (n <= 0) return NULL;

            /* We need to check radial status for start of new elevation.
             * Sometimes this occurs without an end-of-elevation flag for the
             * previous sweep, which is the trigger for loading the sweep
             * header.  "iray" is set to zero after end-of-elev is received,
             * so that's why we check it.  We issue a warning because when this
             * occurs, the number of rays in the previous sweep is usually less
             * than expected.
             */
            if (wsr88d_ray.ray_hdr.radial_status == START_OF_ELEV &&
                    iray != 0) {
                fprintf(stderr,"Warning: Radial status is Start-of-Elevation, "
                        "but End-of-Elevation was not\n"
                        "issued for elevation number %d.  Number of rays = %d"
                        "\n", isweep+1, iray);
                wsr88d_load_sweep_header(radar, isweep);
                isweep++;
                iray = 0;
            }

            /* Load ray into radar structure. */
            wsr88d_load_ray_into_radar(wsr88d_ray, isweep, iray, radar);
            iray++;
            if (iray >= MAXRAYS_M31) {
                fprintf(stderr,"Error: iray = %d, equals or exceeds MAXRAYS_M31"
                        " (%d)\n", iray, MAXRAYS_M31);
                fprintf(stderr,"isweep = %d\n", isweep);
                RSL_free_radar(radar);
                return NULL;
            }
        }
        else { /* msg_type not 31 */
            n = fread(&non31_seg_remainder, sizeof(non31_seg_remainder), 1,
                    wf->fptr);
            if (n < 1) {
                fprintf(stderr,"Warning: load_wsr88d_m31_into_radar: ");
                if (feof(wf->fptr) != 0)
                    fprintf(stderr, "Unexpected end of file.\n");
                else
                    fprintf(stderr,"Read failed.\n");
                fprintf(stderr,"Current sweep index: %d\n"
                        "Last ray read: %d\n", isweep, iray);
                wsr88d_load_sweep_header(radar, isweep);
                return radar;
            }
            if (msghdr.msg_type == 5) {
                wsr88d_get_vcp_data(non31_seg_remainder);
                radar->h.vcp = vcp_data.vcp;
                /* printf("VCP = %d\n", vcp_data.vcp); */
            }
        }

        /* Check for end of sweep */
        if (wsr88d_ray.ray_hdr.radial_status == END_OF_ELEV) {
            wsr88d_load_sweep_header(radar, isweep);
            isweep++;
            iray = 0;
        }

        /* If not at end of volume scan, read next message header. */
        if (wsr88d_ray.ray_hdr.radial_status != END_VOS) {
            n = fread(&msghdr, sizeof(Wsr88d_msg_hdr), 1, wf->fptr);
            if (n < 1) {
                fprintf(stderr,"Warning: load_wsr88d_m31_into_radar: ");
                if (feof(wf->fptr) != 0) fprintf(stderr,
                        "Unexpected end of file.\n");
                else fprintf(stderr,"Failed reading msghdr.\n");
                fprintf(stderr,"Current sweep index: %d\n"
                        "Last ray read: %d\n", isweep, iray);
                wsr88d_load_sweep_header(radar, isweep);
                return radar;
            }
        }
        else {
            end_of_vos = 1;
            wsr88d_load_sweep_header(radar, isweep);
        }
        if (feof(wf->fptr) != 0) end_of_vos = 1;
    }  /* while not end of vos */

    return radar;
}