ootk/src/data/values/Egm96Data.ts

Orbital Object Toolkit including Multiple Propagators, Initial Orbit Determination, and Maneuver Calculations.

/**
 * @author @thkruz Theodore Kruczek
 * @description Orbital Object ToolKit (ootk) is a collection of tools for working
 * with satellites and other orbital objects.
 * @license AGPL-3.0-or-later
 * @copyright (c) 2025 Kruczek Labs LLC
 *
 * Many of the classes are based off of the work of @david-rc-dayton and his
 * Pious Squid library (https://github.com/david-rc-dayton/pious_squid) which
 * is licensed under the MIT license.
 *
 * Orbital Object ToolKit is free software: you can redistribute it and/or modify it under the
 * terms of the GNU Affero General Public License as published by the Free Software
 * Foundation, either version 3 of the License, or (at your option) any later version.
 *
 * Orbital Object ToolKit 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License along with
 * Orbital Object ToolKit. If not, see <http://www.gnu.org/licenses/>.
 */

import { factorial } from '../../utils/functions.js';
import { egm96 } from './egm96.js';

// / EGM-96 entry for l, m indexes and clm, slm values.
export type Egm96Entry = [number, number, number, number];

// / Container for EGM-96 data.
export class Egm96Data {
  private readonly coeffs_: Egm96Entry[];

  // / Create a new [Egm96Data] object.
  constructor(coeffs: Egm96Entry[]) {
    this.coeffs_ = coeffs;
  }

  /**
   * Create a new [Egm96Data] container given a list of EGM-96
   * coefficient tuples [vals].
   * @param vals List of EGM-96 coefficient tuples.
   * @returns A new [Egm96Data] object.
   */
  static fromVals(vals: Egm96Entry[]): Egm96Data {
    const output: Egm96Entry[] = [];

    for (const v of vals) {
      const [l, m, clm, slm] = v;
      const k = m === 0 ? 1 : 2;
      const a = factorial(l + m);
      const b = factorial(l - m) * (k * (2 * l + 1));
      const nFac = Math.sqrt(a / b);
      const normalizedClm = clm / nFac;
      const normalizedSlm = slm / nFac;

      output.push([l, m, normalizedClm, normalizedSlm]);
    }

    return new Egm96Data(output);
  }

  // / Return de-normalized EGM-96 coefficients for a given [l] and [m] index.
  getCoeffs(l: number, m: number): Egm96Entry {
    return this.coeffs_[Egm96Data.index_(l, m)];
  }

  // / Return the EGM-96 index for a given [l] and [m] lookup.
  private static index_(l: number, m: number): number {
    return (((l - 2) * (l + 2) + l) >> 1) - 1 + m;
  }
}

// / De-normalized EGM-96 data container.
export const egm96Data = Egm96Data.fromVals(egm96);