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Code Issues Releases Wiki Activity

Improve CRC computation of ISM

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This commit is contained in:
Carles Fernandez
2024-09-21 10:22:32 +02:00
parent e23d171b22
commit bc800dea28
4 changed files with 26 additions and 15 deletions
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2
src/core/system_parameters/galileo_inav_message.cc
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@@ -1323,8 +1323,6 @@ int32_t Galileo_Inav_Message::page_jk_decoder(const char* data_jk)
DLOG(INFO) << "Word type 22 arrived";
ism_constellation_id = read_octet_unsigned(data_jk_bits, ISM_CONSTELLATION_ID_BIT);
ism_service_level_id = read_octet_unsigned(data_jk_bits, ISM_SERVICE_LEVEL_ID_BIT);
ism_crc = static_cast<uint32_t>(read_navigation_unsigned(data_jk_bits, ISM_CRC_BIT));
gal_ism.set_ism_crc(ism_crc);
if (gal_ism.check_ism_crc(data_jk_bits))
{
DLOG(INFO) << "I/NAV ARAIM Integrity Support Message CRC OK";

23
src/core/system_parameters/galileo_ism.cc
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@@ -187,20 +187,25 @@ uint16_t Galileo_ISM::get_Tvalidity_hours() const
bool Galileo_ISM::check_ism_crc(const std::bitset<GALILEO_DATA_JK_BITS>& bits)
{
std::bitset<GALILEO_ISM_CRC_DATA_BITS> extracted;
std::bitset<GALILEO_ISM_CRC_DATA_BITS> data_bits;
for (int32_t i = 0; i < GALILEO_ISM_CRC_DATA_BITS; ++i)
{
extracted[i] = bits[i + 32];
data_bits[i] = bits[i + 32];
}
std::bitset<32> crc_bits;
for (int32_t i = 0; i < 32; ++i)
{
crc_bits[i] = bits[i];
}
ism_crc = crc_bits.to_ulong();
std::vector<uint8_t> data_bytes((extracted.size() + 7) / 8);
for (size_t i = 0; i < extracted.size(); i += 8)
std::vector<uint8_t> data_bytes((data_bits.size() + 7) / 8);
for (size_t i = 0; i < data_bits.size(); i += 8)
{
uint8_t byte = 0;
for (size_t j = 0; j < 8 && i + j < extracted.size(); ++j)
for (size_t j = 0; j < 8 && i + j < data_bits.size(); ++j)
{
byte |= (extracted[i + j] << j);
byte |= (data_bits[i + j] << j);
}
data_bytes[i / 8] = byte;
}
@@ -220,5 +225,7 @@ bool Galileo_ISM::check_ism_crc(const std::bitset<GALILEO_DATA_JK_BITS>& bits)
uint32_t Galileo_ISM::compute_crc(const std::vector<uint8_t>& data)
{
crc32_ism.process_bytes(data.data(), data.size());
return crc32_ism.checksum();
const uint32_t crc = crc32_ism.checksum();
crc32_ism.reset();
return crc;
}

4
src/core/system_parameters/galileo_ism.h
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@@ -43,7 +43,7 @@ public:
/*!
* Default constructor
*/
Galileo_ISM() : crc32_ism(0x814141AB, 0, 0, false, false){};
Galileo_ISM() = default;
void set_ism_constellation_id(uint8_t const_id);
void set_ism_service_level_id(uint8_t sl_id);
@@ -74,7 +74,7 @@ public:
private:
uint32_t compute_crc(const std::vector<uint8_t>& data);
boost::crc_basic<32> crc32_ism;
boost::crc_optimal<32, 0x814141AB, 0, 0, false, false> crc32_ism;
// ICD 2.1 Table 97
std::unordered_map<uint8_t, double> ISM_PCONST_MAP = {