-
Notifications
You must be signed in to change notification settings - Fork 1
/
eci2ecef.m
57 lines (49 loc) · 1.9 KB
/
eci2ecef.m
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
%Orbit ECEF ECI Coordinate conversion
% Richard Rieber
% October 1, 2006
%
% Revision 10/1/09: Preallocated memory for ECEF and V_ECEF vectors before
% the for-loop
%
% function [ECEF,V_ECEF] = eci2ecef(ECI, GST, V_ECI)
%
% Purpose: This function rotates Earth Centered Inertial (ECI) coordinates to Earth
% Centered Earth Fixed (ECEF) coordinates via the Greenwich Sideral Time
% hour angle (GST).
%
% Inputs: ECI - A 3 x n matrix of position vectors in km
% GST - A vector of length n providing the Greenwich hour angle for each of
% the above ECI position vectors in radians
% V_ECI - A 3 x n matrix of the velocity vectors in km/s [OPTIONAL]
%
% Outputs: ECEF - A 3 x n matrix of position vectors in the ECEF coordinate system in km
% V_ECEF - A 3 x n matrix of velocity vectors in the ECEF coordinate system in km/s
%
% NOTE: This function requires the use of the subfunction 'R3.m' which creates a
% rotation matrix about the 3-axis (Z-axis).
function [ECEF,V_ECEF] = eci2ecef(ECI, GST, V_ECI)
%Checking number of inputs for errors
if nargin < 2 || nargin > 3
error('Incorrect number of inputs. See help eci2ecef.')
end
[b,n] = size(ECI);
%Checking to see if length of ECI matrix is the same as the length of the GST vector
if n ~= length(GST)
error('Size of ECI vector not equal to size of GST vector. Check inputs.')
end
%Checking to see if the ECI vector has 3 elements
if b ~= 3
error('ECI vector must have 3 elements to the vector (X,Y,Z) coordinates')
end
ECEF = zeros(3,n);
if nargin == 3
V_ECEF = zeros(3,n);
end
for j = 1:n %Iterating thru the number of positions provided by user
% Rotating the ECI vector into the ECEF frame via the GST angle about the Z-axis
ECEF(:,j) = R3(GST(j))*ECI(:,j);
if nargin == 3
V_ECEF(:,j) = R3(GST(j))*V_ECI(:,j);
end
end