https://ambidata.io/blog/2018/10/12/m5stack_gpswatch/

M5SatRadar.ino

#include <M5Unified.h>

// NMEA文受信用バッファサイズ
#define NMEA_BUFFER_SIZE 128

// 定数化：ヘッダー部分のトークン数
#define GSV_HEADER_SKIP 3   // GSV文：$GPGSV,総メッセージ数,メッセージ番号,視認衛星数 のスキップ
#define GSA_HEADER_SKIP 2   // GSA文：$GPGSA,モード,固定状態 のスキップ
#define GSA_USED_COUNT 12   // GSA文：使用中衛星番号の項目数

// GPSシリアル（UART2：RX=1, TX=2, 9600bps）
HardwareSerial gpsSerial(2);

// M5Canvas は M5Unified.h 経由で M5GFX.h により利用可能
M5Canvas canvas(&M5.Display);

// 衛星情報構造体（GSVパース用）
struct SatData {
	int prn;
	int elevation; // 0～90度
	int azimuth;   // 0～359度
	int snr;       // 信号強度（dB）
	unsigned long tm; // 最終更新時刻（秒）
};

#define MAX_SATELLITES 20
SatData satellites[MAX_SATELLITES];
int satCount = 0;

// 使用中衛星（GSAパース用）
#define MAX_USED 20
int usedSatellites[MAX_USED];
int usedCount = 0;
unsigned long usedUpdateTime = 0;

// VTGパース用構造体
struct VTGData {
	double trueCourse;
	double magneticCourse;
	double speedKnots;
	double speedKmph;
};
VTGData vtgData = {0, 0, 0, 0};

// 描画用中心座標とグリッド半径
int centerX, centerY, gridRadius;

// 共有データ保護用ミューテックス
SemaphoreHandle_t gpsDataMutex;

// ----------------------------------------------------
// チェックサム検証関数
// ----------------------------------------------------
bool verifyChecksum(const char *sentence) {
	if (sentence[0] != '$') return false;
	const char *p = sentence + 1;
	uint8_t cs = 0;
	while (*p && *p != '*') {
		cs ^= *p;
		p++;
	}
	if (*p != '*') return false;
	int provided;
	if (sscanf(p + 1, "%2x", &provided) != 1) return false;
	return (cs == provided);
}

// ----------------------------------------------------
// NMEA文パース
// ----------------------------------------------------
void parseGSV(char *line) {
	char *saveptr;
	char *token = strtok_r(line, ",*", &saveptr);
	for (int i = 0; i < GSV_HEADER_SKIP; i++) {
		token = strtok_r(NULL, ",*", &saveptr);
		if (!token) return;
	}
	while ((token = strtok_r(NULL, ",*", &saveptr)) != NULL) {
		int prn = atoi(token);
		token = strtok_r(NULL, ",*", &saveptr);
		if (!token) break;
		int elev = atoi(token);
		token = strtok_r(NULL, ",*", &saveptr);
		if (!token) break;
		int azim = atoi(token);
		token = strtok_r(NULL, ",*", &saveptr);
		if (!token) break;
		int snr = atoi(token);

		bool found = false;
		for (int i = 0; i < satCount; i++) {
			if (satellites[i].prn == prn) {
				satellites[i].elevation = elev;
				satellites[i].azimuth   = azim;
				satellites[i].snr       = snr;
				satellites[i].tm        = millis() / 1000;
				found = true;
				break;
			}
		}
		if (!found && satCount < MAX_SATELLITES) {
			satellites[satCount].prn = prn;
			satellites[satCount].elevation = elev;
			satellites[satCount].azimuth = azim;
			satellites[satCount].snr = snr;
			satellites[satCount].tm = millis() / 1000;
			satCount++;
		}
	}
}

void parseGSA(char *line) {
	char *saveptr;
	char *token = strtok_r(line, ",*", &saveptr);
	for (int i = 0; i < GSA_HEADER_SKIP; i++) {
		token = strtok_r(NULL, ",*", &saveptr);
		if (!token) return;
	}
	usedCount = 0;
	for (int i = 0; i < GSA_USED_COUNT; i++) {
		token = strtok_r(NULL, ",*", &saveptr);
		if (!token) break;
		if (token[0] != '\0') {
			if (usedCount < MAX_USED)
				usedSatellites[usedCount++] = atoi(token);
		}
	}
	usedUpdateTime = millis() / 1000;
}

void parseVTG(char *line) {
	char *saveptr;
	char *token = strtok_r(line, ",*", &saveptr);
	token = strtok_r(NULL, ",*", &saveptr); // 真方位
	if (!token) return;
	double trueCourse = atof(token);
	token = strtok_r(NULL, ",*", &saveptr); // "T"
	token = strtok_r(NULL, ",*", &saveptr); // 磁方位
	if (!token) return;
	double magneticCourse = atof(token);
	token = strtok_r(NULL, ",*", &saveptr); // "M"
	token = strtok_r(NULL, ",*", &saveptr); // ノット速度
	if (!token) return;
	double speedKnots = atof(token);
	token = strtok_r(NULL, ",*", &saveptr); // "N"
	token = strtok_r(NULL, ",*", &saveptr); // km/h 速度
	if (!token) return;
	double speedKmph = atof(token);

	vtgData.trueCourse = trueCourse;
	vtgData.magneticCourse = magneticCourse;
	vtgData.speedKnots = speedKnots;
	vtgData.speedKmph = speedKmph;
}

// ----------------------------------------------------
// GPS受信処理
// ----------------------------------------------------
void processGPS() {
	char lineBuffer[NMEA_BUFFER_SIZE];
	size_t len = gpsSerial.readBytesUntil('\n', lineBuffer, NMEA_BUFFER_SIZE - 1);
	if (len > 0) {
		lineBuffer[len] = '\0';
		if (!verifyChecksum(lineBuffer)) return;
		if (lineBuffer[0] != '$') return;
		// 文種判定を固定位置（lineBuffer+3, 3文字）で高速化
		if (memcmp(lineBuffer + 3, "GSV", 3) == 0) {
			parseGSV(lineBuffer);
		} else if (memcmp(lineBuffer + 3, "GSA", 3) == 0) {
			parseGSA(lineBuffer);
		} else if (memcmp(lineBuffer + 3, "VTG", 3) == 0) {
			parseVTG(lineBuffer);
		}
	}
}

// ----------------------------------------------------
// 描画処理
// ----------------------------------------------------
void drawGrid() {
	canvas.fillScreen(BLACK);
	for (int r = gridRadius / 3; r <= gridRadius; r += gridRadius / 3) {
		canvas.drawCircle(centerX, centerY, r, DARKGREY);
	}
	for (int angle = 0; angle < 360; angle += 45) {
		float rad = radians(angle);
		int x = centerX + cos(rad) * gridRadius;
		int y = centerY + sin(rad) * gridRadius;
		canvas.drawLine(centerX, centerY, x, y, DARKGREY);
	}
	canvas.setTextSize(2);
	canvas.drawString("N", centerX - 8, centerY - gridRadius - 10);
	canvas.drawString("S", centerX - 8, centerY + gridRadius + 2);
	canvas.drawString("E", centerX + gridRadius + 2, centerY - 8);
	canvas.drawString("W", centerX - gridRadius - 12, centerY - 8);
	canvas.setTextSize(1);
}

void drawSatellites() {
	for (int i = 0; i < satCount; i++) {
		SatData sat = satellites[i];
		int r = map(90 - sat.elevation, 0, 90, 0, gridRadius);
		float rad = radians(sat.azimuth);
		int x = centerX + r * sin(rad);
		int y = centerY - r * cos(rad);
		uint16_t color = BLUE;
		for (int j = 0; j < usedCount; j++) {
			if (sat.prn == usedSatellites[j]) {
				color = GREEN;
				break;
			}
		}
		canvas.fillCircle(x, y, 4, color);
		canvas.drawCircle(x, y, 4, WHITE);
		canvas.drawString(String(sat.prn), x + 8, y - 7);
	}
}

// ----------------------------------------------------
// FreeRTOSタスク
// ----------------------------------------------------
void gpsTask(void *parameter) {
	gpsSerial.begin(9600, SERIAL_8N1, 1, 2);

	while (true) {
		xSemaphoreTake(gpsDataMutex, portMAX_DELAY);

		processGPS();

		xSemaphoreGive(gpsDataMutex);

		vTaskDelay(10 / portTICK_PERIOD_MS);
	}
}

void displayTask(void *parameter) {
	M5.Display.setRotation(1);

	centerX = M5.Display.width() / 2;
	centerY = M5.Display.height() / 2;

	gridRadius = min(M5.Display.width(), M5.Display.height()) / 2 - 16;

	canvas.createSprite(M5.Display.width(), M5.Display.height());

	canvas.fillScreen(BLACK);
	canvas.pushSprite(0, 0);

	while (true) {
		vTaskDelay(1000 / portTICK_PERIOD_MS);

		xSemaphoreTake(gpsDataMutex, portMAX_DELAY);

		canvas.fillScreen(BLACK);

		drawGrid();
		drawSatellites();

		xSemaphoreGive(gpsDataMutex);

		canvas.pushSprite(0, 0);
	}
}

void setup() {
	M5.begin();

	gpsDataMutex = xSemaphoreCreateMutex();

	xTaskCreate(gpsTask, "GPSTask", 4096, NULL, 1, NULL);
	xTaskCreate(displayTask, "DisplayTask", 4096, NULL, 1, NULL);
}

void loop() {
	M5.update();

	vTaskDelay(1000 / portTICK_PERIOD_MS);
}