diff --git a/Manifest.toml b/Manifest.toml
index cf3ac48..b304107 100644
--- a/Manifest.toml
+++ b/Manifest.toml
@@ -11,9 +11,9 @@ version = "0.8.10"
 
 [[Compat]]
 deps = ["Base64", "Dates", "DelimitedFiles", "Distributed", "InteractiveUtils", "LibGit2", "Libdl", "LinearAlgebra", "Markdown", "Mmap", "Pkg", "Printf", "REPL", "Random", "Serialization", "SharedArrays", "Sockets", "SparseArrays", "Statistics", "Test", "UUIDs", "Unicode"]
-git-tree-sha1 = "84aa74986c5b9b898b0d1acaf3258741ee64754f"
+git-tree-sha1 = "ed2c4abadf84c53d9e58510b5fc48912c2336fbb"
 uuid = "34da2185-b29b-5c13-b0c7-acf172513d20"
-version = "2.1.0"
+version = "2.2.0"
 
 [[Dates]]
 deps = ["Printf"]
@@ -92,9 +92,9 @@ uuid = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 
 [[StaticArrays]]
 deps = ["LinearAlgebra", "Random", "Statistics"]
-git-tree-sha1 = "db23bbf50064c582b6f2b9b043c8e7e98ea8c0c6"
+git-tree-sha1 = "1085ffbf5fd48fdba64ef8e902ca429c4e1212d3"
 uuid = "90137ffa-7385-5640-81b9-e52037218182"
-version = "0.11.0"
+version = "0.11.1"
 
 [[Statistics]]
 deps = ["LinearAlgebra", "SparseArrays"]
diff --git a/src/relative_orbits.jl b/src/relative_orbits.jl
new file mode 100644
index 0000000..430c274
--- /dev/null
+++ b/src/relative_orbits.jl
@@ -0,0 +1,242 @@
+##############
+# RTN | LVLH #
+##############
+
+export rRTNtoECI
+export rECItoRTN
+
+
+"""
+Compute the radial, along-track, cross-track (RTN) to Earth-centered inertial
+rotation matrix. If applied to a position vector in the RTN frame, it will
+transform that vector to into the equivalent position vector in the ECI frame.
+
+The RTN frame is also commonly refered to as the local-vertical, local-horizontal (LVLH) frame.
+
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+
+Returns:
+- `R_rtn2eci::SMatrix{3,3}`: Rotation matrix transforming _from_ the RTN frame _to_ the ECI frame
+"""
+function rRTNtoECI(x::AbstractVector{<:Real})
+    r = x[idx1t3]
+    v = x[idx4t6]
+    n = cross(r, v)
+
+    R = normalize(r)
+    N = normalize(n)
+    T = cross(N, R)
+
+    R_rtn2eci = hcat(R, T, N)
+
+    return R_rtn2eci
+end
+
+
+"""
+Compute the Earth-centered inertial to radial, along-track, cross-track (RTN)
+rotation matrix. If applied to a position vector in the ECI frame, it will
+transform that vector into the equivalent position vector in the RTN frame.
+
+The RTN frame is also commonly refered to as the local-vertical, local-horizontal (LVLH) frame.
+
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+
+Returns:
+- `R_eci2rtn::SMatrix{3,3}`: Rotation matrix transforming _from_ the ECI frame _to_ the RTN frame
+"""
+function rECItoRTN(x::AbstractVector{<:Real})
+    r = x[idx1t3]
+    v = x[idx4t6]
+    n = cross(r, v)
+
+    R = normalize(r)
+    N = normalize(n)
+    T = cross(N, R)
+
+    R_eci2rtn = hcat(R, T, N)'
+
+    return R_eci2rtn
+end
+
+
+export sECItoRTN
+"""
+Compute the radial, along-track, cross-track (RTN) coordinates of a target satellite in the primary satellites RTN frame.
+
+The RTN frame is also commonly refered to as the local-vertical, local-horizontal (LVLH) frame.
+
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+- `xt::AbstractVector{<:Real}`: Inertial state (position and velocity) of the target satellite
+
+Returns:
+- `xrtn::AbstractVector{<:Real}`: Position and velocity of the target relative of the observing satellite in the RTN.
+"""
+function sECItoRTN(x::AbstractVector{<:Real}, xt::AbstractVector{<:Real})
+    if length(xt) >= 6
+        return sECItoRTN(x, xt[idx1t6])
+    else
+        return sECItoRTN(x, xt[idx1t3])
+    end
+end
+
+"""
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+- `xt::SVector{3,<:Real}`: Inertial state (position) of the target satellite
+
+Returns:
+- `xrtn::SVector{3,<:Real}`: Position of the target relative of the observing satellite in the RTN.
+"""
+function sECItoRTN(x::AbstractVector{<:Real}, xt::SVector{3,<:Real})
+    # Create RTN rotation matrix
+    R_eci2rtn = rECItoRTN(x)
+
+    # Transform Position
+    r     = x[idx1t3]
+    rho   = xt[idx1t3] - r
+    r_rtn = R_eci2rtn*rho
+
+    return r_rtn
+end
+
+"""
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+- `xt::SVector{6,<:Real}`: Inertial state (position and velocity) of the target satellite
+
+Returns:
+- `xrtn::SVector{6,<:Real}`: Position and velocity of the target relative of the observing satellite in the RTN.
+"""
+function sECItoRTN(x::AbstractVector{<:Real}, xt::SVector{6,<:Real})
+    # Create RTN rotation matrix
+    R_eci2rtn = rECItoRTN(x)
+
+    # Transform Position
+    r     = x[idx1t3]
+    rho   = xt[idx1t3] - r
+    r_rtn = R_eci2rtn*rho
+
+    v       = x[idx4t6]
+    f_dot   = norm(cross(r, v)) / norm(r)^2
+    omega   = SVector(0, 0, f_dot)
+    rho_dot = xt[idx4t6] - v
+    v_rtn   = R_eci2rtn*rho_dot - cross(omega, r_rtn)
+
+    return vcat(r_rtn, v_rtn)
+end
+
+
+export sRTNtoECI
+"""
+Compute the Earth-center
+
+The RTN frame is also commonly refered to as the local-vertical, local-horizontal (LVLH) frame.
+
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+- `xrtn::AbstractVector{<:Real}`: Inertial state (position and velocity) of the target satellite
+
+Returns:
+- `xt::AbstractVector{<:Real}`: Position and velocity of the target relative of the observing satellite in the RTN.
+"""
+function sRTNtoECI(x::AbstractVector{<:Real}, xrtn::AbstractVector{<:Real})
+    if length(xrtn) >= 6
+        return sRTNtoECI(x, xrtn[idx1t6])
+    else
+        return sRTNtoECI(x, xrtn[idx1t3])
+    end
+end
+
+"""
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+- `xrtn::SVector{3,<:Real}`: Inertial state (position) of the target satellite
+
+Returns:
+- `xt::SVector{3,<:Real}`: Position of the target relative of the observing satellite in the RTN.
+"""
+function sRTNtoECI(x::AbstractVector{<:Real}, xrtn::SVector{3,<:Real})
+    # Create RTN rotation matrix
+    R_rtn2eci = rRTNtoECI(x)
+
+    # Transform position
+    r   = x[idx1t3]
+    r_t = R_rtn2eci*xrtn + r
+
+    return r_t
+end
+
+"""
+Arguments:
+- `x::AbstractVector{<:Real}`: Inertial state (position and velocity) of primary (observing) satellite
+- `xrtn::SVector{6,<:Real}`: Inertial state (position and velocity) of the target satellite
+
+Returns:
+- `xt::SVector{6,<:Real}`: Position and velocity of the target relative of the observing satellite in the RTN.
+"""
+function sRTNtoECI(x::AbstractVector{<:Real}, xrtn::SVector{6,<:Real})
+    # Create RTN rotation matrix
+    R_rtn2eci = rRTNtoECI(x)
+
+    # Transform position
+    r     = x[idx1t3]
+    r_rtn = xrtn[idx1t3]
+    r_t   = R_rtn2eci*r_rtn + r
+
+    # Transform velocity
+    v     = x[idx4t6]
+    v_rtn = xrtn[idx4t6]
+    f_dot = norm(cross(r, v)) / norm(r)^2
+    omega = SVector(0, 0, f_dot)
+    v_t   = R_rtn2eci*(v_rtn + cross(omega, r_rtn)) + v
+
+    return vcat(r_t, v_t)
+end
+
+#######
+# ROE #
+#######
+
+"""
+Convert Keplerian relative orbital element state to ROE relative state
+"""
+function sOSCtoROE(oe_c::SVector{6,<:Real}, oe_d::SVector{6,<:Real})
+end
+
+"""
+Convert ROE relative state and chief relative orbital elements and return
+osculating orbital elements
+"""
+function sROEtoOSC(oe_c::SVector{6,<:Real}, roe_d::SVector{6,<:Real})
+end
+
+"""
+Convert inertial states to relative orbital element 
+"""
+function sECItoROE(x_c::SVector{6,<:Real}, x_d::SVector{6,<:Real})
+end
+
+"""
+Convert ROE relative state and chief absolute inertial state and return
+osculating orbital elements
+"""
+function sROEtoECI(x_c::SVector{6,<:Real}, roe_d::SVector{6,<:Real})
+end
+
+"""
+Convert RTN state and chief absolute inertial state and return relative
+orbital element state
+"""
+function sRTNtoROE(x_c::SVector{6,<:Real}, rtn_d::SVector{6,<:Real})
+end
+
+"""
+Covert relative orbital element state and chief inertial state and return
+RTN state.
+"""
+function sROEtoRTN(x_c::SVector{6,<:Real}, roe_d::SVector{6,<:Real})
+end
\ No newline at end of file
diff --git a/test/test_coordinates.jl b/test/test_coordinates.jl
index eadf671..3633d8f 100644
--- a/test/test_coordinates.jl
+++ b/test/test_coordinates.jl
@@ -315,12 +315,15 @@ let
     azel_enz = sENZtoAZEL(enz, use_degrees=true)
     azel_sez = sSEZtoAZEL(sez, use_degrees=true)
 
-    @test azel_enz[1] == azel_sez[1]
-    @test azel_enz[2] == azel_sez[2]
-    @test azel_enz[3] == azel_sez[3]
-    @test azel_enz[4] == azel_sez[4]
-    @test azel_enz[5] == azel_sez[5]
-    @test azel_enz[6] == azel_sez[6]
+
+    tol = 1e-7
+    @test isapprox(azel_enz[1], azel_sez[1], atol=tol)
+    @test isapprox(azel_enz[2], azel_sez[2], atol=tol)
+    @test isapprox(azel_enz[3], azel_sez[3], atol=tol)
+    @test isapprox(azel_enz[4], azel_sez[4], atol=tol)
+    @test isapprox(azel_enz[5], azel_sez[5], atol=tol)
+    @test isapprox(azel_enz[6], azel_sez[6], atol=tol)
+
 end
 
 let 
diff --git a/test/test_relative_orbits.jl b/test/test_relative_orbits.jl
new file mode 100644
index 0000000..be719a6
--- /dev/null
+++ b/test/test_relative_orbits.jl
@@ -0,0 +1,53 @@
+let
+    epc = Epoch(2018,1,1,12,0,0)
+
+    a   = R_EARTH + 500e3
+    oe  = [a, 0, 0, 0, 0, sqrt(GM_EARTH/a)]
+    eci = sOSCtoCART(oe, use_degrees=true)
+
+    r_rtn   = rRTNtoECI(eci)
+    r_rtn_t = rECItoRTN(eci)
+
+    r = r_rtn * r_rtn_t
+    
+    tol = 1e-8
+    @test isapprox(r[1, 1], 1.0, atol=tol)
+    @test isapprox(r[1, 2], 0.0, atol=tol)
+    @test isapprox(r[1, 3], 0.0, atol=tol)
+
+    @test isapprox(r[2, 1], 0.0, atol=tol)
+    @test isapprox(r[2, 2], 1.0, atol=tol)
+    @test isapprox(r[2, 3], 0.0, atol=tol)
+
+    @test isapprox(r[3, 1], 0.0, atol=tol)
+    @test isapprox(r[3, 2], 0.0, atol=tol)
+    @test isapprox(r[3, 3], 1.0, atol=tol)
+end
+
+let 
+    epc = Epoch(2018,1,1,12,0,0)
+
+    oe  = [R_EARTH + 500e3, 0, 0, 0, 0, 0]
+    eci = sOSCtoCART(oe, use_degrees=true)
+
+    xt = deepcopy(eci) + [100, 0, 0, 0, 0, 0]
+
+    x_rtn = sECItoRTN(eci, xt)
+
+    tol = 1e-8
+    @test isapprox(x_rtn[1], 100.0, atol=tol)
+    @test isapprox(x_rtn[2], 0.0, atol=tol)
+    @test isapprox(x_rtn[3], 0.0, atol=tol)
+    @test isapprox(x_rtn[4], 0.0, atol=tol)
+    @test isapprox(x_rtn[5], 0.0, atol=0.5)
+    @test isapprox(x_rtn[6], 0.0, atol=tol)
+
+    xt2 = sRTNtoECI(eci, x_rtn)
+
+    @test isapprox(xt[1], xt2[1], atol=tol)
+    @test isapprox(xt[2], xt2[2], atol=tol)
+    @test isapprox(xt[3], xt2[3], atol=tol)
+    @test isapprox(xt[4], xt2[4], atol=tol)
+    @test isapprox(xt[5], xt2[5], atol=tol)
+    @test isapprox(xt[6], xt2[6], atol=tol)
+end
\ No newline at end of file