Ground Facility Accesses
We can use the function:
ground_facility_accesses(orbp, [(WGS84)]; kwargs...) -> DataFrameto compute the accesses of a satellite with orbit propagator orbp (see Propagators.init) to the ground facilities defined in the vector [(WGS84)]. The analysis interval begins in the propagator epoch plus initial_time and lasts for duration [s], where both are keywords.
The ground facilities are specified using a vector of tuples with three numbers:
Tuple{T1, T2, T3} where {T1 <: Number, T2 <: Number, T3 <: Number}containing the WGS84 position of each ground facility [(WGS84)]:
(latitude [rad], longitude [rad], altitude [m])Those geodetic information are transformed to an ECEF vector using the function geodetic_to_ecef.
The following keywords are available:
duration::Number: Duration of the analysis [s]. (Default = 86400)f_eci_to_ecef::Function: Function to convert the orbit propagator position represented in the Earth-centered inertial (ECI) reference frame to the Earth-centered, Earth-fixed (ECEF) reference frame. The signature must bef_eci_to_ecef(r_i::AbstractVector, jd::Number) -> AbstractVectorand it must return the position vector
r_irepresented in the ECEF at the instantjd[Julian Day]. By default, we use TEME as the ECI and PEF as the ECEF. (Default:_ground_facility_default_eci_to_ecef)initial_time::Number: Initial time of the analysis after the propagator epoch [s]. (Default = 0)minimum_elevation::Number: Minimum elevation angle for communication between the satellite and the ground facilities [rad]. (Default = 10°)reduction::Function: A function that receives a boolean vector with the visibility between the satellite and each ground facility. It must return a boolean value indicating if the access must be computed or not. This is useful to merge access time between two or more facilities. (Default =v -> |(v...)i.e. compute the access if at least one ground facilities is visible)step::Number: The step [s] used to propagate the orbit. Notice that we perform a cross tuning to accurately obtain the access time. However, if an access is lower than the step, it can be neglected. (Default = 60)unit::Symbol: Select the unit in which the duration will be computed. The possible values are::sfor seconds (Default);:mfor minutes; or:hfor hours.
This function returns a DataFrame with three columns:
access_beginning: Time of the access beginning [UTC] encoded usingDateTime.access_end: Time of the access end [UTC] encoded usingDateTime.duration: Duration of the access [s]. The unit of the columndurationis stored in theDataFrameusing metadata.
Examples
Let's compute the access time of the Amazonia-1 satellite to the INPE's ground station at Cuiabá, MT, Brazil.
First, we need to define an orbit propagator. In this case, we will use the TLE obtained from CelesTrak when this documentation was written:
julia> tle_amz1 = tle""" AMAZONIA 1 1 47699U 21015A 24008.13079366 .00000299 00000+0 10693-3 0 9994 2 47699 98.4120 87.2350 0001570 92.1147 268.0222 14.40836963150331 """TLE: Name : AMAZONIA 1 Satellite number : 47699 International designator : 21015A Epoch (Year / Day) : 24 / 8.13079366 (2024-01-08T03:08:20.572) Element set number : 999 Eccentricity : 0.00015700 Inclination : 98.41200000 deg RAAN : 87.23500000 deg Argument of perigee : 92.11470000 deg Mean anomaly : 268.02220000 deg Mean motion (n) : 14.40836963 revs / day Revolution number : 15033 B* : 0.00010693 1 / er ṅ / 2 : 2.99e-06 rev / day² n̈ / 6 : 0 rev / day³julia> orbp = Propagators.init(Val(:SGP4), tle_amz1)OrbitPropagatorSgp4{Float64, Float64}: Propagator name : SGP4 Orbit Propagator Propagator epoch : 2024-01-08T03:08:20.572 Last propagation : 2024-01-08T03:08:20.572
Now, we can compute the access time during one day considering the INPE's station at Cuiabá:
julia> ground_facility_accesses( orbp, [(-(15 + 33 / 60) |> deg2rad, -(56 + 04 / 60) |> deg2rad, 0)]; duration = 1 * 86400, minimum_elevation = 5 |> deg2rad, unit = :m )4×3 DataFrame Row │ access_beginning access_end duration │ DateTime DateTime Float64 ─────┼───────────────────────────────────────────────────────────── 1 │ 2024-01-08T03:08:20.572 2024-01-08T03:09:16.311 0.928983 2 │ 2024-01-08T13:56:48.099 2024-01-08T14:08:50.888 12.0465 3 │ 2024-01-08T15:39:28.281 2024-01-08T15:43:47.366 4.31808 4 │ 2024-01-09T02:18:02.879 2024-01-09T02:30:08.933 12.1009
If we want to change the reference frames used in the analysis, we must provide a function f_eci_to_ecef(r_i, jd) that converts the vector r_i to the desired ECEF frame at the instant jd [Julian Day]. For example, let's use the more precise ITRF instead of the PEF (default):
julia> const eop = fetch_iers_eop(Val(:IAU1980));julia> function f_eci_to_ecef(r_teme, jd) D_itrf_teme = r_eci_to_ecef(TEME(), ITRF(), jd, eop) r_itrf = D_itrf_teme * r_teme return r_itrf endf_eci_to_ecef (generic function with 1 method)julia> ground_facility_accesses( orbp, [(-(15 + 33 / 60) |> deg2rad, -(56 + 04 / 60) |> deg2rad, 0)]; duration = 1 * 86400, f_eci_to_ecef = f_eci_to_ecef, minimum_elevation = 5 |> deg2rad, unit = :m )4×3 DataFrame Row │ access_beginning access_end duration │ DateTime DateTime Float64 ─────┼──────────────────────────────────────────────────────────── 1 │ 2024-01-08T03:08:20.572 2024-01-08T03:09:16.312 0.929 2 │ 2024-01-08T13:56:48.098 2024-01-08T14:08:50.887 12.0465 3 │ 2024-01-08T15:39:28.281 2024-01-08T15:43:47.364 4.31805 4 │ 2024-01-09T02:18:02.880 2024-01-09T02:30:08.935 12.1009
We can also perform analyses using multiple ground facilities. For example, let's find the accumulated access if we consider the INPE's stations at Cuiabá, MT, Brazil, and Alcântara, MA, Brazil:
julia> ground_facility_accesses( orbp, [ (-(15 + 33 / 60) |> deg2rad, -(56 + 04 / 60) |> deg2rad, 0) (-( 2 + 20 / 60) |> deg2rad, -(44 + 24 / 60) |> deg2rad, 0) ]; duration = 1 * 86400, minimum_elevation = 5 |> deg2rad, unit = :m )6×3 DataFrame Row │ access_beginning access_end duration │ DateTime DateTime Float64 ─────┼───────────────────────────────────────────────────────────── 1 │ 2024-01-08T03:08:20.572 2024-01-08T03:09:16.311 0.928983 2 │ 2024-01-08T12:16:55.777 2024-01-08T12:23:20.782 6.41675 3 │ 2024-01-08T13:52:44.135 2024-01-08T14:08:50.888 16.1126 4 │ 2024-01-08T15:39:28.281 2024-01-08T15:43:47.366 4.31808 5 │ 2024-01-09T00:43:23.474 2024-01-09T00:53:25.918 10.0407 6 │ 2024-01-09T02:18:02.879 2024-01-09T02:32:05.994 14.0519
By default, the algorithm computes the accumulated access, i.e., it considers the access active if either station has visibility to the satellite. We can change this logic by overloading the function in the keyword parameter reduction. For example, let's compute only the accesses when the satellite has visibility to both ground stations at the same time:
julia> ground_facility_accesses( orbp, [ (-(15 + 33 / 60) |> deg2rad, -(56 + 04 / 60) |> deg2rad, 0) (-( 2 + 20 / 60) |> deg2rad, -(44 + 24 / 60) |> deg2rad, 0) ]; duration = 1 * 86400, minimum_elevation = 5 |> deg2rad, reduction = v -> (&)(v...), unit = :m )2×3 DataFrame Row │ access_beginning access_end duration │ DateTime DateTime Float64 ─────┼──────────────────────────────────────────────────────────── 1 │ 2024-01-08T13:56:48.099 2024-01-08T14:04:29.601 7.6917 2 │ 2024-01-09T02:21:47.636 2024-01-09T02:30:08.933 8.35495